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PROTEINS (Article 1)– a class of biological polymers present in every living organism. With the participation of proteins, the main processes that ensure the vital functions of the body take place: respiration, digestion, muscle contraction, transmission of nerve impulses. Bone tissue, skin, hair, and horny formations of living beings consist of proteins. For most mammals, growth and development of the body occurs due to foods containing proteins as a food component. The role of proteins in the body and, accordingly, their structure is very diverse.

Protein composition.

All proteins are polymers, the chains of which are assembled from amino acid fragments. Amino acids are organic compounds containing in their composition (in accordance with the name) an NH 2 amino group and an organic acidic group, i.e. carboxyl, COOH group. Of the entire variety of existing amino acids (theoretically, the number of possible amino acids is unlimited), only those that have only one carbon atom between the amino group and the carboxyl group participate in the formation of proteins. IN general view amino acids involved in the formation of proteins can be represented by the formula: H 2 N–CH(R)–COOH. The R group attached to the carbon atom (the one between the amino and carboxyl groups) determines the difference between the amino acids that form proteins. This group can consist only of carbon and hydrogen atoms, but more often it contains, in addition to C and H, various functional (capable of further transformations) groups, for example, HO-, H 2 N-, etc. There is also an option when R = H.

The organisms of living beings contain more than 100 different amino acids, however, not all are used in the construction of proteins, but only 20, the so-called “fundamental” ones. In table 1 shows their names (most of the names developed historically), the structural formula, as well as the widely used abbreviation. All structural formulas are arranged in the table so that the main amino acid fragment is on the right.

Among these twenty amino acids (Table 1), only proline contains an NH group next to the carboxyl group COOH (instead of NH 2), since it is part of the cyclic fragment.

Eight amino acids (valine, leucine, isoleucine, threonine, methionine, lysine, phenylalanine and tryptophan), placed in the table on a gray background, are called essential, since the body must constantly receive them from protein foods for normal growth and development.

A protein molecule is formed as a result of the sequential connection of amino acids, while the carboxyl group of one acid interacts with the amino group of a neighboring molecule, resulting in the formation of a peptide bond –CO–NH– and the release of a water molecule. In Fig. Figure 1 shows a sequential combination of alanine, valine and glycine.

Rice. 1 SERIES CONNECTION OF AMINO ACIDS during the formation of a protein molecule. The path from the terminal amino group of H 2 N to the terminal carboxyl group of COOH was chosen as the main direction of the polymer chain.

To compactly describe the structure of a protein molecule, abbreviations for amino acids (Table 1, third column) involved in the formation of the polymer chain are used. The fragment of the molecule shown in Fig. 1 is written as follows: H 2 N-ALA-VAL-GLY-COOH.

Protein molecules contain from 50 to 1500 amino acid residues (shorter chains are called polypeptides). The individuality of a protein is determined by the set of amino acids that make up the polymer chain and, no less important, by the order of their alternation along the chain. For example, the insulin molecule consists of 51 amino acid residues (this is one of the shortest chain proteins) and consists of two parallel chains of unequal length connected to each other. The order of alternation of amino acid fragments is shown in Fig. 2.

Rice. 2 INSULIN MOLECULE, built from 51 amino acid residues, fragments of identical amino acids are marked with a corresponding background color. The amino acid cysteine ​​residues contained in the chain (abbreviated CIS) form disulfide bridges –S-S-, which link two polymer molecules, or form bridges within one chain.

Cysteine ​​amino acid molecules (Table 1) contain reactive sulfhydride groups –SH, which interact with each other, forming disulfide bridges –S-S-. The role of cysteine ​​in the world of proteins is special; with its participation, cross-links are formed between polymer protein molecules.

The combination of amino acids into a polymer chain occurs in a living organism under the control of nucleic acids; they provide a strict assembly order and regulate the fixed length of the polymer molecule ( cm. NUCLEIC ACIDS).

Structure of proteins.

The composition of the protein molecule, presented in the form of alternating amino acid residues (Fig. 2), is called the primary structure of the protein. Hydrogen bonds occur between the imino groups HN and the carbonyl groups CO present in the polymer chain ( cm. HYDROGEN BOND), as a result, the protein molecule acquires a certain spatial shape, called a secondary structure. The most common types of protein secondary structure are two.

The first option, called an α-helix, is realized using hydrogen bonds within a single polymer molecule. The geometric parameters of the molecule, determined by bond lengths and bond angles, are such that the formation of hydrogen bonds is possible for groups H-N and C=O, between which there are two peptide fragments H-N-C=O (Fig. 3).

The composition of the polypeptide chain shown in Fig. 3, written in abbreviated form as follows:

H 2 N-ALA VAL-ALA-LEY-ALA-ALA-ALA-ALA-VAL-ALA-ALA-ALA-COOH.

As a result of the constricting effect of hydrogen bonds, the molecule takes on the shape of a spiral - the so-called α-helix, it is depicted as a curved spiral ribbon passing through the atoms forming the polymer chain (Fig. 4)

Rice. 4 3D MODEL OF A PROTEIN MOLECULE in the form of an α-helix. Hydrogen bonds are shown with green dotted lines. The cylindrical shape of the helix is ​​visible at a certain angle of rotation (hydrogen atoms are not shown in the figure). The coloring of individual atoms is given in accordance with international rules, which recommend black for carbon atoms, blue for nitrogen, red for oxygen, yellow for sulfur (for hydrogen atoms not shown in the figure, white is recommended, in this case the entire structure depicted against a dark background).

Another version of the secondary structure, called the β-structure, is also formed with the participation of hydrogen bonds, the difference is that the H-N and C=O groups of two or more polymer chains located in parallel interact. Since the polypeptide chain has a direction (Fig. 1), options are possible when the direction of the chains coincides (parallel β-structure, Fig. 5), or they are opposite (antiparallel β-structure, Fig. 6).

Polymer chains of various compositions can participate in the formation of the β-structure, while the organic groups framing the polymer chain (Ph, CH 2 OH, etc.) in most cases play a secondary role; the relative position of the H-N and C=O groups is decisive. Since relatively polymer chains H-N and C=O groups are directed in different directions (up and down in the figure), simultaneous interaction of three or more chains becomes possible.

The composition of the first polypeptide chain in Fig. 5:

H 2 N-LEY-ALA-FEN-GLY-ALA-ALA-COOH

Composition of the second and third chains:

H 2 N-GLY-ALA-SER-GLY-TRE-ALA-COOH

The composition of the polypeptide chains shown in Fig. 6, the same as in Fig. 5, the difference is that the second chain has the opposite (compared to Fig. 5) direction.

The formation of a β-structure inside one molecule is possible when a chain fragment in a certain area is rotated by 180°; in this case, two branches of one molecule have opposite directions, resulting in the formation of an antiparallel β-structure (Fig. 7).

The structure shown in Fig. 7 in a flat image, shown in Fig. 8 in the form of a three-dimensional model. Sections of the β-structure are usually simply denoted by a flat wavy ribbon that passes through the atoms that form the polymer chain.

The structure of many proteins alternates between α-helix and ribbon-like β-structures, as well as single polypeptide chains. Their mutual arrangement and alternation in the polymer chain is called the tertiary structure of the protein.

Methods for depicting the structure of proteins are shown below using the example of the vegetable protein crambin. The structural formulas of proteins, often containing up to hundreds of amino acid fragments, are complex, cumbersome and difficult to understand, therefore, sometimes simplified structural formulas are used - without symbols of chemical elements (Fig. 9, option A), but at the same time retain the color of the valence strokes in accordance with international rules (Fig. 4). In this case, the formula is presented not in a flat, but in a spatial image, which corresponds to the real structure of the molecule. This method allows, for example, to distinguish disulfide bridges (similar to those found in insulin, Fig. 2), phenyl groups in the side frame of the chain, etc. The image of molecules in the form of three-dimensional models (balls connected by rods) is somewhat more clear (Fig. 9, option B). However, both methods do not allow showing the tertiary structure, so the American biophysicist Jane Richardson proposed depicting α-structures in the form of spirally twisted ribbons (see Fig. 4), β-structures in the form of flat wavy ribbons (Fig. 8), and connecting them single chains - in the form of thin bundles, each type of structure has its own color. This method of depicting the tertiary structure of a protein is now widely used (Fig. 9, option B). Sometimes, for greater information, the tertiary structure and a simplified structural formula are shown together (Fig. 9, option D). There are also modifications of the method proposed by Richardson: α-helices are depicted as cylinders, and β-structures are depicted in the form of flat arrows indicating the direction of the chain (Fig. 9, option E). A less common method is in which the entire molecule is depicted in the form of a rope, where unequal structures are highlighted with different colors, and disulfide bridges are shown as yellow bridges (Fig. 9, option E).

The most convenient for perception is option B, when when depicting the tertiary structure, the structural features of the protein (amino acid fragments, the order of their alternation, hydrogen bonds) are not indicated, and it is assumed that all proteins contain “details” taken from a standard set of twenty amino acids ( Table 1). The main task when depicting a tertiary structure is to show the spatial arrangement and alternation of secondary structures.

Rice. 9 DIFFERENT OPTIONS FOR REPRESENTING THE STRUCTURE OF CRUMBIN PROTEIN.
A – structural formula in spatial image.
B – structure in the form of a three-dimensional model.
B – tertiary structure of the molecule.
D – combination of options A and B.
D – simplified image of the tertiary structure.
E – tertiary structure with disulfide bridges.

The most convenient for perception is the volumetric tertiary structure (option B), freed from the details of the structural formula.

A protein molecule with a tertiary structure, as a rule, takes on a certain configuration, which is formed by polar (electrostatic) interactions and hydrogen bonds. As a result, the molecule takes the form of a compact ball - globular proteins (globules, lat. ball), or filamentous - fibrillar proteins (fibra, lat. fiber).

An example of a globular structure is the protein albumin; the albumin class includes chicken egg white. The polymer chain of albumin is assembled mainly from alanine, aspartic acid, glycine, and cysteine, alternating in a certain order. The tertiary structure contains α-helices connected by single chains (Fig. 10).

Rice. 10 GLOBULAR STRUCTURE OF ALBUMIN

An example of a fibrillar structure is the protein fibroin. They contain a large number of glycine, alanine and serine residues (every second amino acid residue is glycine); There are no cysteine ​​residues containing sulfhydride groups. Fibroin, the main component of natural silk and spider webs, contains β-structures connected by single chains (Fig. 11).

Rice. eleven FIBRILLAR PROTEIN FIBROIN

The possibility of forming a tertiary structure of a certain type is inherent in the primary structure of the protein, i.e. determined in advance by the order of alternation of amino acid residues. From certain sets of such residues, α-helices predominantly arise (there are quite a lot of such sets), another set leads to the appearance of β-structures, single chains are characterized by their composition.

Some protein molecules, while maintaining their tertiary structure, are capable of combining into large supramolecular aggregates, while they are held together by polar interactions, as well as hydrogen bonds. Such formations are called the quaternary structure of the protein. For example, the protein ferritin, consisting mainly of leucine, glutamic acid, aspartic acid and histidine (ferricin contains all 20 amino acid residues in varying quantities), forms a tertiary structure of four parallel α-helices. When molecules are combined into a single ensemble (Fig. 12), a quaternary structure is formed, which can include up to 24 ferritin molecules.

Fig.12 FORMATION OF THE QUATERNARY STRUCTURE OF THE GLOBULAR PROTEIN FERRITIN

Another example of supramolecular formations is the structure of collagen. It is a fibrillar protein, the chains of which are built mainly from glycine, alternating with proline and lysine. The structure contains single chains, triple α-helices, alternating with ribbon-shaped β-structures arranged in parallel bundles (Fig. 13).

Fig.13 SUPRAMOLECULAR STRUCTURE OF FIBRILLAR COLLAGEN PROTEIN

Chemical properties of proteins.

Under the action of organic solvents, waste products of certain bacteria (lactic acid fermentation) or with increasing temperature, the destruction of secondary and tertiary structures occurs without damaging its primary structure, as a result of which the protein loses solubility and loses biological activity, this process is called denaturation, that is, the loss of natural properties, for example, curdling sour milk, coagulated white of a boiled chicken egg. At elevated temperatures, proteins of living organisms (in particular microorganisms) quickly denature. Such proteins are not able to participate in biological processes, as a result, microorganisms die, so boiled (or pasteurized) milk can be preserved longer.

The H-N-C=O peptide bonds that form the polymer chain of a protein molecule are hydrolyzed in the presence of acids or alkalis, causing the polymer chain to break, which can ultimately lead to the original amino acids. Peptide bonds that are part of α-helices or β-structures are more resistant to hydrolysis and various chemical influences (compared to the same bonds in single chains). A more delicate disassembly of the protein molecule into its component amino acids is carried out in an anhydrous environment using hydrazine H 2 N–NH 2 , while all amino acid fragments, except the last one, form so-called carboxylic acid hydrazides containing the fragment C(O)–HN–NH 2 ( Fig. 14).

Rice. 14. POLYPEPTIDE DIVISION

Such an analysis can provide information about the amino acid composition of a particular protein, but it is more important to know their sequence in the protein molecule. One of the methods widely used for this purpose is the action of phenyl isothiocyanate (FITC) on the polypeptide chain, which in an alkaline environment is attached to the polypeptide (from the end that contains the amino group), and when the reaction of the environment changes to acidic, it is detached from the chain, taking with it a fragment of one amino acid (Fig. 15).

Rice. 15 SEQUENTIAL CLEAVATION OF POLYPEPTIDE

Many special techniques have been developed for such analysis, including those that begin to “disassemble” the protein molecule into its constituent components, starting from the carboxyl end.

S-S cross-disulfide bridges (formed by the interaction of cysteine ​​residues, Fig. 2 and 9) are cleaved, converting them into HS groups by the action of various reducing agents. The action of oxidizing agents (oxygen or hydrogen peroxide) again leads to the formation of disulfide bridges (Fig. 16).

Rice. 16. CLEAVATION OF DISULPHIDE BRIDGES

To create additional cross-links in proteins, the reactivity of amino and carboxyl groups is used. The amino groups that are located in the side frame of the chain are more accessible to various interactions - fragments of lysine, asparagine, lysine, proline (Table 1). When such amino groups interact with formaldehyde, a condensation process occurs and cross bridges –NH–CH2–NH– appear (Fig. 17).

Rice. 17 CREATION OF ADDITIONAL CROSS BRIDGES BETWEEN PROTEIN MOLECULES.

The terminal carboxyl groups of the protein are capable of reacting with complex compounds of some polyvalent metals (chromium compounds are more often used), and cross-links also occur. Both processes are used in tanning leather.

The role of proteins in the body.

The role of proteins in the body is varied.

Enzymes(fermentatio lat. – fermentation), their other name is enzymes (en zumh Greek. - in yeast) are proteins with catalytic activity; they are capable of increasing the speed of biochemical processes thousands of times. Under the action of enzymes, the constituent components of food: proteins, fats and carbohydrates are broken down into simpler compounds, from which new macromolecules necessary for a certain type of organism are then synthesized. Enzymes also take part in many biochemical synthesis processes, for example, in the synthesis of proteins (some proteins help synthesize others). Cm. ENZYMES

Enzymes are not only highly efficient catalysts, but also selective (direct the reaction strictly in a given direction). In their presence, the reaction proceeds with almost 100% yield without the formation of by-products, and the conditions are mild: normal atmospheric pressure and temperature of a living organism. For comparison, the synthesis of ammonia from hydrogen and nitrogen in the presence of a catalyst - activated iron - is carried out at 400–500 ° C and a pressure of 30 MPa, the yield of ammonia is 15–25% per cycle. Enzymes are considered unrivaled catalysts.

Intensive research on enzymes began in the mid-19th century; now more than 2000 different enzymes have been studied, this is the most diverse class of proteins.

The names of enzymes are as follows: the ending -ase is added to the name of the reagent with which the enzyme interacts, or to the name of the catalyzed reaction, for example, arginase decomposes arginine (Table 1), decarboxylase catalyzes decarboxylation, i.e. removal of CO 2 from the carboxyl group:

– COOH → – CH + CO 2

Often, to more accurately indicate the role of an enzyme, both the object and the type of reaction are indicated in its name, for example, alcohol dehydrogenase, an enzyme that carries out the dehydrogenation of alcohols.

For some enzymes, discovered quite a long time ago, the historical name (without the ending –aza) has been preserved, for example, pepsin (pepsis, Greek. digestion) and trypsin (thrypsis Greek. liquefaction), these enzymes break down proteins.

For systematization, enzymes are combined into large classes, the classification is based on the type of reaction, the classes are named according to the general principle - the name of the reaction and the ending - aza. Some of these classes are listed below.

Oxidoreductases– enzymes that catalyze redox reactions. Dehydrogenases included in this class carry out proton transfer, for example, alcohol dehydrogenase (ADH) oxidizes alcohols to aldehydes, the subsequent oxidation of aldehydes to carboxylic acids is catalyzed by aldehyde dehydrogenases (ALDH). Both processes occur in the body during the conversion of ethanol into acetic acid (Fig. 18).

Rice. 18 TWO-STAGE OXIDATION OF ETHANOL to acetic acid

It is not ethanol that has a narcotic effect, but the intermediate product acetaldehyde; the lower the activity of the ALDH enzyme, the slower the second stage takes place - the oxidation of acetaldehyde to acetic acid and the longer and stronger the intoxicating effect from ingesting ethanol. The analysis showed that more than 80% of representatives of the yellow race have relatively low ALDH activity and therefore have a noticeably more severe alcohol tolerance. The reason for this congenital reduced activity of ALDH is that some of the glutamic acid residues in the “weakened” ALDH molecule are replaced by lysine fragments (Table 1).

Transferases– enzymes that catalyze the transfer of functional groups, for example, transiminase catalyzes the movement of an amino group.

Hydrolases– enzymes that catalyze hydrolysis. The previously mentioned trypsin and pepsin hydrolyze peptide bonds, and lipases cleave the ester bond in fats:

–RC(O)OR 1 +H 2 O → –RC(O)OH + HOR 1

Lyases– enzymes that catalyze reactions that do not take place hydrolytically; as a result of such reactions, rupture occurs C-C connections, C-O, C-N and the formation of new bonds. The enzyme decarboxylase belongs to this class

Isomerases– enzymes that catalyze isomerization, for example, the conversion of maleic acid into fumaric acid (Fig. 19), this is an example of cis - trans isomerization (see ISOMERIA).

Rice. 19. ISOMERIZATION OF MALEIC ACID to fumaric in the presence of an enzyme.

The work of enzymes is observed general principle, according to which there is always a structural correspondence between the enzyme and the reagent of the accelerated reaction. According to the figurative expression of one of the founders of the doctrine of enzymes, E. Fisher, the reagent fits the enzyme like a key to a lock. In this regard, each enzyme catalyzes a specific chemical reaction or group of reactions of the same type. Sometimes an enzyme can act on one single compound, for example, urease (uron Greek. – urine) catalyzes only the hydrolysis of urea:

(H 2 N) 2 C = O + H 2 O = CO 2 + 2NH 3

The most subtle selectivity is exhibited by enzymes that distinguish between optically active antipodes - left- and right-handed isomers. L-arginase acts only on levorotatory arginine and does not affect the dextrorotary isomer. L-lactate dehydrogenase acts only on levorotatory esters of lactic acid, the so-called lactates (lactis lat. milk), while D-lactate dehydrogenase breaks down exclusively D-lactates.

Most enzymes act not on one, but on a group of related compounds, for example, trypsin “prefers” to cleave peptide bonds formed by lysine and arginine (Table 1.)

The catalytic properties of some enzymes, such as hydrolases, are determined solely by the structure of the protein molecule itself; another class of enzymes - oxidoreductases (for example, alcohol dehydrogenase) can only be active in the presence of non-protein molecules associated with them - vitamins, activating ions Mg, Ca, Zn, Mn and fragments of nucleic acids (Fig. 20).

Rice. 20 ALCOHOL DEHYDROGENASE MOLECULE

Transport proteins bind and transport various molecules or ions across cell membranes (both inside and outside the cell), as well as from one organ to another.

For example, hemoglobin binds oxygen as blood passes through the lungs and delivers it to various tissues of the body, where the oxygen is released and then used to oxidize food components, this process serves as a source of energy (sometimes the term "combustion" is used) food products in organism).

In addition to the protein part, hemoglobin contains a complex compound of iron with the cyclic molecule porphyrin (porphyros Greek. – purple), which causes the red color of blood. It is this complex (Fig. 21, left) that plays the role of an oxygen carrier. In hemoglobin, the porphyrin iron complex is located inside the protein molecule and is held in place through polar interactions, as well as a coordination bond with nitrogen in histidine (Table 1), which is part of the protein. The O2 molecule carried by hemoglobin is attached via a coordination bond to the iron atom on the side opposite to that to which the histidine is attached (Fig. 21, right).

Rice. 21 STRUCTURE OF THE IRON COMPLEX

The structure of the complex is shown on the right in the form of a three-dimensional model. The complex is held in the protein molecule by a coordination bond (blue dotted line) between the Fe atom and the N atom in the histidine that is part of the protein. The O2 molecule carried by hemoglobin is coordinately attached (red dotted line) to the Fe atom from the opposite side of the planar complex.

Hemoglobin is one of the most thoroughly studied proteins; it consists of a-helices connected by single chains and contains four iron complexes. Thus, hemoglobin is like a voluminous package for transporting four oxygen molecules at once. The shape of hemoglobin corresponds to globular proteins (Fig. 22).

Rice. 22 GLOBULAR FORM OF HEMOGLOBIN

The main “advantage” of hemoglobin is that the addition of oxygen and its subsequent elimination during transfer to various tissues and organs occurs quickly. Carbon monoxide, CO (carbon monoxide), binds to Fe in hemoglobin even faster, but, unlike O 2, forms a complex that is difficult to destroy. As a result, such hemoglobin is not able to bind O 2, which leads (if large quantities of carbon monoxide are inhaled) to the death of the body from suffocation.

The second function of hemoglobin is the transfer of exhaled CO 2, but in the process of temporary binding of carbon dioxide, it is not the iron atom that participates, but the H 2 N-group of the protein.

The “performance” of proteins depends on their structure, for example, replacing the single amino acid residue of glutamic acid in the polypeptide chain of hemoglobin with a valine residue (a rare congenital anomaly) leads to a disease called sickle cell anemia.

There are also transport proteins that can bind fats, glucose, and amino acids and transport them both inside and outside cells.

Transport proteins of a special type do not transport the substances themselves, but perform the functions of a “transport regulator”, passing certain substances through the membrane (the outer wall of the cell). Such proteins are more often called membrane proteins. They have the shape of a hollow cylinder and, being embedded in the membrane wall, ensure the movement of some polar molecules or ions into the cell. An example of a membrane protein is porin (Fig. 23).

Rice. 23 PORIN PROTEIN

Food and storage proteins, as the name suggests, serve as sources of internal nutrition, often for the embryos of plants and animals, as well as for early stages development of young organisms. Food proteins include albumin (Fig. 10), the main component of egg white, and casein, the main protein of milk. Under the influence of the enzyme pepsin, casein coagulates in the stomach, which ensures its retention in the digestive tract and effective absorption. Casein contains fragments of all amino acids needed by the body.

Ferritin (Fig. 12), which is found in animal tissues, contains iron ions.

Storage proteins also include myoglobin, which is similar in composition and structure to hemoglobin. Myoglobin is concentrated mainly in the muscles, its main role is to store the oxygen that hemoglobin gives it. It is quickly saturated with oxygen (much faster than hemoglobin), and then gradually transfers it to various tissues.

Structural proteins perform a protective function (skin) or a supporting function - they hold the body together into a single whole and give it strength (cartilage and tendons). Their main component is the fibrillar protein collagen (Fig. 11), the most common protein in the animal world in the body of mammals, accounting for almost 30% of the total mass of proteins. Collagen has high tensile strength (the strength of leather is known), but due to the low content of cross-links in skin collagen, animal skins are of little use in their raw form for the manufacture of various products. To reduce the swelling of leather in water, shrinkage during drying, as well as to increase strength in a watered state and increase elasticity in collagen, additional cross-links are created (Fig. 15a), this is the so-called leather tanning process.

In living organisms, collagen molecules that arise during the growth and development of the organism are not renewed and are not replaced by newly synthesized ones. As the body ages, the number of cross-links in collagen increases, which leads to a decrease in its elasticity, and since renewal does not occur, age-related changes appear - an increase in the fragility of cartilage and tendons, and the appearance of wrinkles on the skin.

Articular ligaments contain elastin, a structural protein that easily stretches in two dimensions. The protein resilin, which is found at the hinge points of the wings of some insects, has the greatest elasticity.

Horny formations - hair, nails, feathers, consisting mainly of keratin protein (Fig. 24). Its main difference is the noticeable content of cysteine ​​residues that form disulfide bridges, which gives high elasticity (the ability to restore its original shape after deformation) to hair, as well as woolen fabrics.

Rice. 24. FRAGMENT OF FIBRILLAR PROTEIN KERATIN

To irreversibly change the shape of a keratin object, you must first destroy the disulfide bridges with the help of a reducing agent, give a new shape, and then create disulfide bridges again with the help of an oxidizing agent (Fig. 16), this is exactly what is done, for example, perm hair.

With an increase in the content of cysteine ​​residues in keratin and, accordingly, an increase in the number of disulfide bridges, the ability to deform disappears, but high strength appears (the horns of ungulates and turtle shells contain up to 18% cysteine ​​fragments). The mammalian body contains up to 30 various types keratin.

The fibrillar protein fibroin, related to keratin, secreted by silkworm caterpillars when curling a cocoon, as well as by spiders when weaving a web, contains only β-structures connected by single chains (Fig. 11). Unlike keratin, fibroin does not have cross-disulfide bridges and is very tensile strength (the strength per unit cross-section of some web samples is higher than that of steel cables). Due to the lack of cross-links, fibroin is inelastic (it is known that woolen fabrics are almost wrinkle-resistant, while silk fabrics wrinkle easily).

Regulatory proteins.

Regulatory proteins, more commonly called hormones, are involved in various physiological processes. For example, the hormone insulin (Fig. 25) consists of two α-chains connected by disulfide bridges. Insulin regulates metabolic processes involving glucose; its absence leads to diabetes.

Rice. 25 PROTEIN INSULIN

The pituitary gland of the brain synthesizes a hormone that regulates the growth of the body. There are regulatory proteins that control the biosynthesis of various enzymes in the body.

Contractile and motor proteins give the body the ability to contract, change shape and move, most notably muscles. 40% of the mass of all proteins contained in muscles is myosin (mys, myos, Greek. – muscle). Its molecule contains both fibrillar and globular parts (Fig. 26)

Rice. 26 MYOSIN MOLECULE

Such molecules combine into large aggregates containing 300–400 molecules.

When the concentration of calcium ions changes in the space surrounding the muscle fibers, a reversible change in the conformation of the molecules occurs - a change in the shape of the chain due to the rotation of individual fragments around valence bonds. This leads to muscle contraction and relaxation; the signal to change the concentration of calcium ions comes from the nerve endings in the muscle fibers. Artificial muscle contraction can be caused by the action of electrical impulses, leading to a sharp change in the concentration of calcium ions; stimulation of the cardiac muscle is based on this to restore heart function.

Protective proteins help protect the body from the invasion of attacking bacteria, viruses and from the penetration of foreign proteins (the general name for foreign bodies is antigens). The role of protective proteins is performed by immunoglobulins (another name for them is antibodies); they recognize antigens that have entered the body and bind firmly to them. In the body of mammals, including humans, there are five classes of immunoglobulins: M, G, A, D and E, their structure, as the name suggests, is globular, in addition, they are all built in a similar way. The molecular organization of antibodies is shown below using the example of class G immunoglobulin (Fig. 27). The molecule contains four polypeptide chains linked by three S-S disulfide bridges (they are shown in Fig. 27 with thickened valence bonds and large S symbols), in addition, each polymer chain contains intrachain disulfide bridges. The two large polymer chains (in blue) contain 400–600 amino acid residues. The other two chains (highlighted green) are almost half as long, they contain approximately 220 amino acid residues. All four chains are arranged in such a way that the terminal H 2 N groups are directed in the same direction.

Rice. 27 SCHEMATIC REPRESENTATION OF THE STRUCTURE OF IMMUNOGLOBULIN

After the body comes into contact with a foreign protein (antigen), cells of the immune system begin to produce immunoglobulins (antibodies), which accumulate in the blood serum. At the first stage, the main work is performed by sections of the chains containing terminal H 2 N (in Fig. 27, the corresponding sections are marked in light blue and light green). These are areas of antigen capture. During the synthesis of immunoglobulin, these areas are formed in such a way that their structure and configuration maximally correspond to the structure of the approaching antigen (like a key to a lock, like enzymes, but the tasks in this case are different). Thus, for each antigen, a strictly individual antibody is created as an immune response. No known protein can change its structure so “plastically” depending on external factors, in addition to immunoglobulins. Enzymes solve the problem of structural correspondence to the reagent in a different way - with the help of a gigantic set of various enzymes, taking into account all possible cases, and immunoglobulins rebuild the “working tool” anew each time. Moreover, the hinge region of the immunoglobulin (Fig. 27) provides the two capture areas with some independent mobility; as a result, the immunoglobulin molecule can “find” at once the two most convenient sites for capture in the antigen in order to securely fix it, this is reminiscent of the actions of a crustacean creature.

Next, a chain of sequential reactions of the body’s immune system is activated, immunoglobulins of other classes are connected, as a result, the foreign protein is deactivated, and then the antigen (foreign microorganism or toxin) is destroyed and removed.

After contact with the antigen, the maximum concentration of immunoglobulin is achieved (depending on the nature of the antigen and the individual characteristics of the organism itself) within several hours (sometimes several days). The body retains the memory of such contact, and with a repeated attack by the same antigen, immunoglobulins accumulate in the blood serum much faster and in greater quantities - acquired immunity occurs.

The above classification of proteins is somewhat arbitrary, for example, the thrombin protein, mentioned among protective proteins, is essentially an enzyme that catalyzes the hydrolysis of peptide bonds, that is, it belongs to the class of proteases.

Protective proteins often include proteins from snake venom and toxic proteins from some plants, since their task is to protect the body from damage.

There are proteins whose functions are so unique that it makes them difficult to classify. For example, the protein monellin, found in an African plant, tastes very sweet and has been studied as a non-toxic substance that could be used instead of sugar to prevent obesity. The blood plasma of some Antarctic fish contains proteins with antifreeze properties, which prevents the blood of these fish from freezing.

Artificial protein synthesis.

The condensation of amino acids leading to a polypeptide chain is a well-studied process. It is possible, for example, to carry out the condensation of any one amino acid or a mixture of acids and, accordingly, obtain a polymer containing identical units or different units alternating in a random order. Such polymers bear little resemblance to natural polypeptides and do not have biological activity. The main task is to combine amino acids in a strictly defined, predetermined order in order to reproduce the sequence of amino acid residues in natural proteins. American scientist Robert Merrifield proposed an original method that made it possible to solve this problem. The essence of the method is that the first amino acid is attached to an insoluble polymer gel, which contains reactive groups that can combine with –COOH – groups of the amino acid. Cross-linked polystyrene with chloromethyl groups introduced into it was taken as such a polymer substrate. To prevent the amino acid taken for the reaction from reacting with itself and to prevent it from joining the H 2 N group to the substrate, the amino group of this acid is first blocked with a bulky substituent [(C 4 H 9) 3 ] 3 OS (O) group. After the amino acid has attached to the polymer support, the blocking group is removed and another amino acid is introduced into the reaction mixture, which also has a previously blocked H 2 N group. In such a system, only the interaction of the H 2 N-group of the first amino acid and the –COOH group of the second acid is possible, which is carried out in the presence of catalysts (phosphonium salts). Next, the entire scheme is repeated, introducing the third amino acid (Fig. 28).

Rice. 28. SCHEME FOR SYNTHESIS OF POLYPEPTIDE CHAINS

At the last stage, the resulting polypeptide chains are separated from the polystyrene support. Now the whole process is automated; there are automatic peptide synthesizers that operate according to the described scheme. This method has been used to synthesize many peptides used in medicine and agriculture. It was also possible to obtain improved analogues of natural peptides with selective and enhanced action. Some small proteins are synthesized, such as the hormone insulin and some enzymes.

There are also methods of protein synthesis that copy natural processes: they synthesize fragments of nucleic acids configured to produce certain proteins, then these fragments are built into a living organism (for example, into a bacterium), after which the body begins to produce the desired protein. In this way, significant quantities of hard-to-reach proteins and peptides, as well as their analogues, are now obtained.

Proteins as food sources.

Proteins in a living organism are constantly broken down into their original amino acids (with the indispensable participation of enzymes), some amino acids are transformed into others, then the proteins are synthesized again (also with the participation of enzymes), i.e. the body is constantly renewed. Some proteins (skin and hair collagen) are not renewed; the body continuously loses them and synthesizes new ones in return. Proteins as food sources perform two main functions: they supply the body with building material for the synthesis of new protein molecules and, in addition, supply the body with energy (sources of calories).

Carnivorous mammals (including humans) obtain the necessary proteins from plant and animal food. None of the proteins obtained from food are incorporated into the body unchanged. In the digestive tract, all absorbed proteins are broken down into amino acids, and from them the proteins necessary for a particular organism are built, while from the 8 essential acids (Table 1), the remaining 12 can be synthesized in the body if they are not supplied in sufficient quantities with food, but essential acids must be supplied with food without fail. The body receives sulfur atoms in cysteine ​​with the essential amino acid methionine. Some of the proteins break down, releasing the energy necessary to maintain life, and the nitrogen they contain is excreted from the body in the urine. Typically, the human body loses 25–30 g of protein per day, so protein foods must always be present in the required quantity. The minimum daily requirement for protein is 37 g for men and 29 g for women, but the recommended intake is almost twice as high. When evaluating food products, it is important to consider protein quality. In the absence or low content of essential amino acids, protein is considered to be of low value, so such proteins should be consumed in larger quantities. Thus, legume proteins contain little methionine, and wheat and corn proteins are low in lysine (both essential amino acids). Animal proteins (excluding collagens) are classified as complete food products. A complete set of all essential acids contains milk casein, as well as cottage cheese and cheese made from it, so a vegetarian diet, if it is very strict, i.e. “dairy-free” requires increased consumption of legumes, nuts and mushrooms to supply the body with essential amino acids in the required quantities.

Synthetic amino acids and proteins are also used as food products, adding them to feed that contain essential amino acids in small quantities. There are bacteria that can process and assimilate oil hydrocarbons; in this case, for complete protein synthesis, they need to be fed with nitrogen-containing compounds (ammonia or nitrates). The protein obtained in this way is used as feed for livestock and poultry. A set of enzymes - carbohydrases - is often added to the feed of domestic animals, which catalyze the hydrolysis of difficult to decompose components of carbohydrate foods (the cell walls of grain crops), as a result of which plant foods are more fully absorbed.

Mikhail Levitsky

PROTEINS (article 2)

(proteins), a class of complex nitrogen-containing compounds, the most characteristic and important (along with nucleic acids) components of living matter. Proteins perform numerous and varied functions. Most proteins are enzymes that catalyze chemical reactions. Many hormones that regulate physiological processes are also proteins. Structural proteins such as collagen and keratin are the main components of bone tissue, hair and nails. Muscle contractile proteins have the ability to change their length by using chemical energy to perform mechanical work. Proteins include antibodies that bind and neutralize toxic substances. Some proteins that can respond to external influences (light, smell) serve as receptors in the senses that perceive irritation. Many proteins located inside the cell and on the cell membrane perform regulatory functions.

In the first half of the 19th century. many chemists, and among them primarily J. von Liebig, gradually came to the conclusion that proteins represent a special class of nitrogenous compounds. The name “proteins” (from the Greek protos - first) was proposed in 1840 by the Dutch chemist G. Mulder.

PHYSICAL PROPERTIES

Proteins in solid state white, and in solution are colorless, unless they carry some chromophore (colored) group, such as hemoglobin. The solubility in water varies greatly among different proteins. It also changes depending on the pH and the concentration of salts in the solution, so it is possible to select conditions under which one protein will selectively precipitate in the presence of other proteins. This "salting out" method is widely used to isolate and purify proteins. The purified protein often precipitates out of solution as crystals.

Compared to other compounds, the molecular weight of proteins is very large - from several thousand to many millions of daltons. Therefore, during ultracentrifugation, proteins are sedimented, and at different rates. Due to the presence of positively and negatively charged groups in protein molecules, they move at different speeds and in electric field. This is the basis of electrophoresis, a method used to isolate individual proteins from complex mixtures. Proteins are also purified by chromatography.

CHEMICAL PROPERTIES

Structure.

Proteins are polymers, i.e. molecules built like chains from repeating monomer units, or subunits, the role of which is played by alpha amino acids. General formula of amino acids

where R is a hydrogen atom or some organic group.

A protein molecule (polypeptide chain) can consist of only a relatively small number of amino acids or several thousand monomer units. The combination of amino acids in a chain is possible because each of them has two different chemical groups: a basic amino group, NH2, and an acidic carboxyl group, COOH. Both of these groups are attached to the a-carbon atom. The carboxyl group of one amino acid can form an amide (peptide) bond with the amino group of another amino acid:

After two amino acids have been linked in this way, the chain can be extended by adding a third to the second amino acid, and so on. As can be seen from the above equation, when a peptide bond is formed, a water molecule is released. In the presence of acids, alkalis or proteolytic enzymes, the reaction proceeds in the opposite direction: the polypeptide chain is split into amino acids with the addition of water. This reaction is called hydrolysis. Hydrolysis occurs spontaneously, and energy is required to connect amino acids into a polypeptide chain.

A carboxyl group and an amide group (or a similar imide group in the case of the amino acid proline) are present in all amino acids, but the differences between amino acids are determined by the nature of the group, or “side chain,” which is designated above by the letter R. The role of the side chain can be played by one a hydrogen atom, like the amino acid glycine, and some bulky group, like histidine and tryptophan. Some side chains are chemically inert, while others are markedly reactive.

Many thousands of different amino acids can be synthesized, and many different amino acids occur in nature, but only 20 types of amino acids are used for protein synthesis: alanine, arginine, asparagine, aspartic acid, valine, histidine, glycine, glutamine, glutamic acid, isoleucine, leucine, lysine , methionine, proline, serine, tyrosine, threonine, tryptophan, phenylalanine and cysteine ​​(in proteins, cysteine ​​can be present as a dimer - cystine). True, some proteins contain other amino acids in addition to the regularly occurring twenty, but they are formed as a result of modification of one of the twenty listed after it has been included in the protein.

Optical activity.

All amino acids, with the exception of glycine, have four different groups attached to the α-carbon atom. From the point of view of geometry, four different groups can be attached in two ways, and accordingly there are two possible configurations, or two isomers, related to each other as an object is to its mirror image, i.e. like the left hand to the right. One configuration is called left-handed, or left-handed (L), and the other is called right-handed, or dextrorotatory (D), because the two isomers differ in the direction of rotation of the plane of polarized light. Only L-amino acids are found in proteins (the exception is glycine; it can only be found in one form because two of its four groups are the same), and all are optically active (because there is only one isomer). D-amino acids are rare in nature; they are found in some antibiotics and the cell wall of bacteria.

Amino acid sequence.

Amino acids in a polypeptide chain are not arranged randomly, but in a certain fixed order, and it is this order that determines the functions and properties of the protein. By varying the order of the 20 types of amino acids, you can create a huge number of different proteins, just as you can create many different texts from the letters of the alphabet.

In the past, determining the amino acid sequence of a protein often took several years. Direct determination is still quite a labor-intensive task, although devices have been created that allow it to be carried out automatically. It is usually easier to determine the nucleotide sequence of the corresponding gene and deduce the amino acid sequence of the protein from it. To date, the amino acid sequences of many hundreds of proteins have already been determined. The functions of the deciphered proteins are usually known, and this helps to imagine the possible functions of similar proteins formed, for example, in malignant neoplasms.

Complex proteins.

Proteins consisting of only amino acids are called simple. Often, however, a metal atom or some chemical compound that is not an amino acid is attached to the polypeptide chain. Such proteins are called complex. An example is hemoglobin: it contains iron porphyrin, which determines its red color and allows it to act as an oxygen carrier.

The names of most complex proteins indicate the nature of the attached groups: glycoproteins contain sugars, lipoproteins contain fats. If the catalytic activity of an enzyme depends on the attached group, then it is called a prosthetic group. Often a vitamin plays the role of a prosthetic group or is part of one. Vitamin A, for example, attached to one of the proteins in the retina, determines its sensitivity to light.

Tertiary structure.

What is important is not so much the amino acid sequence of the protein itself (the primary structure), but the way it is laid out in space. Along the entire length of the polypeptide chain, hydrogen ions form regular hydrogen bonds, which give it the shape of a helix or layer (secondary structure). From the combination of such helices and layers, a compact form of the next order arises - the tertiary structure of the protein. Around the bonds holding the monomer units of the chain, rotations at small angles are possible. Therefore, from a purely geometric point of view, the number of possible configurations for any polypeptide chain is infinitely large. In reality, each protein normally exists in only one configuration, determined by its amino acid sequence. This structure is not rigid, it seems to “breathe” - it fluctuates around a certain average configuration. The circuit is folded into a configuration in which free energy (the ability to produce work) is minimal, just as a released spring compresses only to a state corresponding to the minimum free energy. Often one part of the chain is tightly linked to the other by disulfide (–S–S–) bonds between two cysteine ​​residues. This is partly why cysteine ​​plays a particularly important role among amino acids.

The complexity of the structure of proteins is so great that it is not yet possible to calculate the tertiary structure of a protein, even if its amino acid sequence is known. But if it is possible to obtain protein crystals, then its tertiary structure can be determined by X-ray diffraction.

In structural, contractile and some other proteins, the chains are elongated and several slightly folded chains lying nearby form fibrils; fibrils, in turn, fold into larger formations - fibers. However, most proteins in solution have a globular shape: the chains are coiled in a globule, like yarn in a ball. Free energy with this configuration is minimal, since hydrophobic (“water-repelling”) amino acids are hidden inside the globule, and hydrophilic (“water-attracting”) amino acids are on its surface.

Many proteins are complexes of several polypeptide chains. This structure is called the quaternary structure of the protein. The hemoglobin molecule, for example, consists of four subunits, each of which is a globular protein.

Structural proteins, due to their linear configuration, form fibers that have a very high tensile strength, while the globular configuration allows the proteins to enter into specific interactions with other compounds. On the surface of the globule, when the chains are correctly laid out, cavities of a certain shape appear in which reactive chemical groups are located. If the protein is an enzyme, then another, usually smaller, molecule of some substance enters such a cavity, just as a key enters a lock; in this case, the configuration of the electron cloud of the molecule changes under the influence of the chemical groups located in the cavity, and this forces it to react in a certain way. In this way, the enzyme catalyzes the reaction. Antibody molecules also have cavities in which various foreign substances bind and are thereby rendered harmless. The “lock and key” model, which explains the interaction of proteins with other compounds, allows us to understand the specificity of enzymes and antibodies, i.e. their ability to react only with certain compounds.

Proteins in different types of organisms.

Proteins that perform the same function in different types plants and animals and therefore bear the same name, also have a similar configuration. They, however, differ somewhat in their amino acid sequence. As species diverge from a common ancestor, some amino acids at certain positions are replaced by mutations by others. Harmful mutations that cause hereditary diseases are eliminated by natural selection, but beneficial or at least neutral ones may persist. The closer to each other two biological species, the less differences are found in their proteins.

Some proteins change relatively quickly, others are very conserved. The latter includes, for example, cytochrome c, a respiratory enzyme found in most living organisms. In humans and chimpanzees, its amino acid sequences are identical, but in wheat cytochrome c, only 38% of the amino acids were different. Even when comparing humans and bacteria, the similarity of cytochrome c (the differences affect 65% of the amino acids) can still be noticed, although the common ancestor of bacteria and humans lived on Earth about two billion years ago. Nowadays, comparison of amino acid sequences is often used to construct a phylogenetic (family) tree, reflecting the evolutionary relationships between different organisms.

Denaturation.

The synthesized protein molecule, folding, acquires its characteristic configuration. This configuration, however, can be destroyed by heating, by changing pH, by exposure to organic solvents, and even by simply shaking the solution until bubbles appear on its surface. A protein modified in this way is called denatured; it loses its biological activity and usually becomes insoluble. Well-known examples of denatured protein are: boiled eggs or whipped cream. Small proteins containing only about a hundred amino acids are capable of renaturation, i.e. reacquire the original configuration. But most proteins simply turn into a mass of tangled polypeptide chains and do not restore their previous configuration.

One of the main difficulties in isolating active proteins is their extreme sensitivity to denaturation. Useful Application This property of proteins is found when canning food products: heat irreversibly denatures the enzymes of microorganisms, and the microorganisms die.

PROTEIN SYNTHESIS

To synthesize protein, a living organism must have a system of enzymes capable of joining one amino acid to another. A source of information is also needed to determine which amino acids should be combined. Since there are thousands of types of proteins in the body and each of them consists on average of several hundred amino acids, necessary information must be truly huge. It is stored (similar to how a recording is stored on a magnetic tape) in the nucleic acid molecules that make up genes.

Enzyme activation.

A polypeptide chain synthesized from amino acids is not always a protein in its final form. Many enzymes are synthesized first as inactive precursors and become active only after another enzyme removes several amino acids at one end of the chain. Some of the digestive enzymes, such as trypsin, are synthesized in this inactive form; these enzymes are activated in the digestive tract as a result of the removal of the terminal fragment of the chain. The hormone insulin, the molecule of which in its active form consists of two short chains, is synthesized in the form of one chain, the so-called. proinsulin. The middle part of this chain is then removed, and the remaining fragments bind together to form the active hormone molecule. Complex proteins are formed only after a specific chemical group is attached to the protein, and this attachment often also requires an enzyme.

Metabolic circulation.

After feeding an animal amino acids labeled with radioactive isotopes of carbon, nitrogen or hydrogen, the label is quickly incorporated into its proteins. If labeled amino acids stop entering the body, the amount of label in proteins begins to decrease. These experiments show that the resulting proteins are not retained in the body until the end of life. All of them, with few exceptions, are in a dynamic state, constantly breaking down into amino acids and then being synthesized again.

Some proteins break down when cells die and are destroyed. This happens all the time, for example, with red blood cells and epithelial cells lining the inner surface of the intestine. In addition, the breakdown and resynthesis of proteins also occurs in living cells. Oddly enough, less is known about the breakdown of proteins than about their synthesis. It is clear, however, that the breakdown involves proteolytic enzymes similar to those that break down proteins into amino acids in the digestive tract.

The half-life of different proteins varies - from several hours to many months. The only exception is collagen molecules. Once formed, they remain stable and are not renewed or replaced. Over time, however, some of their properties change, in particular elasticity, and since they are not renewed, this results in certain age-related changes, such as the appearance of wrinkles on the skin.

Synthetic proteins.

Chemists have long learned to polymerize amino acids, but the amino acids are combined in a disorderly manner, so that the products of such polymerization bear little resemblance to natural ones. True, it is possible to combine amino acids in a given order, which makes it possible to obtain some biologically active proteins, in particular insulin. The process is quite complicated, and in this way it is possible to obtain only those proteins whose molecules contain about a hundred amino acids. It is preferable instead to synthesize or isolate the nucleotide sequence of a gene corresponding to the desired amino acid sequence, and then introduce this gene into a bacterium, which will produce by replication a large number of the desired product. This method, however, also has its drawbacks.

PROTEIN AND NUTRITION

When proteins in the body are broken down into amino acids, these amino acids can be used again to synthesize proteins. At the same time, the amino acids themselves are subject to breakdown, so they are not completely reutilized. It is also clear that during growth, pregnancy and wound healing, protein synthesis must exceed breakdown. The body continuously loses some proteins; These are the proteins of hair, nails and the surface layer of skin. Therefore, in order to synthesize proteins, each organism must receive amino acids from food.

Sources of amino acids.

Green plants synthesize all 20 amino acids found in proteins from CO2, water and ammonia or nitrates. Many bacteria are also capable of synthesizing amino acids in the presence of sugar (or some equivalent) and fixed nitrogen, but sugar is ultimately supplied by green plants. Animals have a limited ability to synthesize amino acids; they obtain amino acids by eating green plants or other animals. In the digestive tract, absorbed proteins are broken down into amino acids, the latter are absorbed, and from them proteins characteristic of a given organism are built. None of the absorbed protein is incorporated into body structures as such. The only exception is that in many mammals, some maternal antibodies can pass intact through the placenta into the fetal bloodstream, and through maternal milk (especially in ruminants) can be transferred to the newborn immediately after birth.

Protein requirement.

It is clear that to maintain life the body must receive a certain amount of protein from food. However, the extent of this need depends on a number of factors. The body needs food both as a source of energy (calories) and as material for building its structures. The need for energy comes first. This means that when there are few carbohydrates and fats in the diet, dietary proteins are used not for the synthesis of their own proteins, but as a source of calories. During prolonged fasting, even your own proteins are used to satisfy energy needs. If there are enough carbohydrates in the diet, then protein consumption can be reduced.

Nitrogen balance.

On average approx. 16% of the total mass of protein is nitrogen. When the amino acids contained in proteins are broken down, the nitrogen they contain is excreted from the body in the urine and (to a lesser extent) in feces in the form of various nitrogenous compounds. Therefore, it is convenient for quality assessment protein nutrition use an indicator such as nitrogen balance, i.e. the difference (in grams) between the amount of nitrogen entering the body and the amount of nitrogen excreted per day. With normal nutrition in an adult, these amounts are equal. In a growing organism, the amount of nitrogen excreted is less than the amount received, i.e. the balance is positive. If there is a lack of protein in the diet, the balance is negative. If there are enough calories in the diet, but there are no proteins in it, the body saves proteins. At the same time, protein metabolism slows down, and the repeated utilization of amino acids in protein synthesis occurs with the highest possible efficiency. However, losses are inevitable, and nitrogenous compounds are still excreted in the urine and partly in the feces. The amount of nitrogen excreted from the body per day during protein fasting can serve as a measure of daily protein deficiency. It is natural to assume that by introducing into the diet an amount of protein equivalent to this deficiency, nitrogen balance can be restored. However, it is not. After receiving this amount of protein, the body begins to use amino acids less efficiently, so some additional protein is required to restore nitrogen balance.

If the amount of protein in the diet exceeds what is necessary to maintain nitrogen balance, then there appears to be no harm. Excess amino acids are simply used as an energy source. As a particularly striking example, the Eskimos consume few carbohydrates and about ten times more protein than is required to maintain nitrogen balance. In most cases, however, using protein as an energy source is not beneficial because a given amount of carbohydrate can produce many more calories than the same amount of protein. In poor countries, people get their calories from carbohydrates and consume minimal amounts of protein.

If the body receives the required number of calories in the form of non-protein products, then the minimum amount of protein to ensure the maintenance of nitrogen balance is approx. 30 g per day. About this much protein is contained in four slices of bread or 0.5 liters of milk. A slightly larger number is usually considered optimal; 50 to 70 g is recommended.

Essential amino acids.

Until now, protein was considered as a whole. Meanwhile, in order for protein synthesis to occur, all the necessary amino acids must be present in the body. The animal’s body itself is capable of synthesizing some of the amino acids. They are called replaceable because they do not necessarily have to be present in the diet - it is only important that the overall supply of protein as a source of nitrogen is sufficient; then, if there is a shortage of non-essential amino acids, the body can synthesize them at the expense of those that are present in excess. The remaining, “essential” amino acids cannot be synthesized and must be supplied to the body through food. Essential for humans are valine, leucine, isoleucine, threonine, methionine, phenylalanine, tryptophan, histidine, lysine and arginine. (Although arginine can be synthesized in the body, it is classified as an essential amino acid because it is not produced in sufficient quantities in newborns and growing children. On the other hand, some of these amino acids from food may become unnecessary for an adult person.)

This list of essential amino acids is approximately the same in other vertebrates and even insects. The nutritional value of proteins is usually determined by feeding them to growing rats and monitoring the animals' weight gain.

Nutritional value of proteins.

The nutritional value of a protein is determined by the essential amino acid that is most deficient. Let's illustrate this with an example. The proteins in our body contain on average approx. 2% tryptophan (by weight). Let's say that the diet includes 10 g of protein containing 1% tryptophan, and that there are enough other essential amino acids in it. In our case, 10 g of this incomplete protein is essentially equivalent to 5 g of complete protein; the remaining 5 g can only serve as a source of energy. Note that since amino acids are practically not stored in the body, and in order for protein synthesis to occur, all amino acids must be present at the same time, the effect of the intake of essential amino acids can only be detected if all of them enter the body at the same time.

The average composition of most animal proteins is close to the average composition of proteins human body, so we are unlikely to face amino acid deficiency if our diet is rich in foods such as meat, eggs, milk and cheese. However, there are proteins, such as gelatin (a product of collagen denaturation), that contain very few essential amino acids. Plant proteins, although they are better than gelatin in this sense, are also poor in essential amino acids; They are especially low in lysine and tryptophan. Nevertheless, a purely vegetarian diet cannot be considered harmful at all, unless it consumes a slightly larger amount of plant proteins, sufficient to provide the body with essential amino acids. Plants contain the most protein in their seeds, especially in the seeds of wheat and various legumes. Young shoots, such as asparagus, are also rich in protein.

Synthetic proteins in the diet.

By adding small amounts of synthetic essential amino acids or essential amino acid-rich proteins to incomplete proteins, such as corn, you can significantly increase nutritional value the latter, i.e. thereby increasing the amount of protein consumed. Another possibility is to grow bacteria or yeast on petroleum hydrocarbons with the addition of nitrates or ammonia as a nitrogen source. The microbial protein obtained in this way can serve as feed for poultry or livestock, or can be directly consumed by humans. The third, widely used method uses the physiology of ruminants. In ruminants, in the initial part of the stomach, the so-called. rumen, live special forms bacteria and protozoa, which convert incomplete plant proteins into more complete microbial proteins, and these, in turn, after digestion and absorption, turn into animal proteins. Urea, a cheap synthetic nitrogen-containing compound, can be added to livestock feed. Microorganisms living in the rumen use urea nitrogen to convert carbohydrates (of which there is much more in the feed) into protein. About a third of all nitrogen in livestock feed can come in the form of urea, which essentially means, to a certain extent, the chemical synthesis of protein.

It is known that living matter is based on organic substances - proteins, fats, carbohydrates and nucleic acids. But the most important place Protein is among these substances.

Most substances known to science change from solid to liquid when heated. But there are substances that, on the contrary, turn into a solid state when heated. These substances were combined into a separate class by the French chemist Pierre Joseph Macquet in 1777. By analogy with egg white, which coagulates when heated, these substances were called proteins. Proteins are otherwise called proteins. In Greek, protein (proteios) means “first place.” The protein received this name in 1838, when the Dutch biochemist Gerard Mulder wrote that life on the planet would be impossible without a certain substance, which is the most important of all substances known to science and which is necessarily present in absolutely all plants and animals. Mulder called this substance protein.

Protein is the most complex substance among all nutrients. Chemical reactions occur in every cell of the human body, in which protein plays a very important role.

What does protein consist of?

Proteins contain: nitrogen, oxygen, hydrogen, carbon. But other nutrients do not contain nitrogen.

Protein is a natural polymer. And polymers are substances whose molecules contain a very large number of atoms. Back in the 19th century, Russian chemist Alexander Mikhailovich Butlerov proved that if the structure of a molecule changes, then the properties of the substance also change. Main building material proteins are amino acids. And proteins contain different combinations of amino acids. Consequently, in nature there is a wide variety of proteins with various properties. Through research, approximately 20 amino acids have been discovered that are involved in the creation of proteins.

How does the formation of a protein molecule occur?

Amino acids are added to each other sequentially. As a result of this process, a chain is formed, which is called a polypeptide. Subsequently, the polypeptides can fold into spirals or take on other shapes. The properties of a protein depend on the composition of amino acids, on how many amino acids are involved in synthesis, and in what order these amino acids are added to each other. For example, the synthesis of two proteins involves the same number of amino acids, which also have the same composition. But if these amino acids are located in different sequences, then we will get two completely different proteins.

If peptides contain no more than 15 amino acid residues, then they are called oligopeptides. And peptides containing up to several tens of thousands or even hundreds of thousands of amino acid residues are called proteins. The protein molecule has a compact spatial structure. This structure may be in the form of fibers. Such proteins are called fibrillar. They are building proteins. If a protein molecule has a spherical structure, then the proteins are called globular. These proteins include enzymes, antibodies, and some hormones.

Depending on which amino acids are included in proteins, proteins can be complete or incomplete. Complete proteins contain a full set of amino acids. Incomplete proteins lack some amino acids.

Proteins are also divided into simple and complex. Simple proteins contain only amino acids. In addition to amino acids, complex proteins also include metals, carbohydrates, lipids, and nucleic acids.

The role of proteins in the human body

Proteins perform various functions in the human body.

1.Structural. Proteins are part of the cells of all tissues and organs.

2. Protective. The interferon protein is synthesized in the body to protect against viruses.

3. Motor I. The protein myosin is involved in the process of muscle contraction.

4. Transport. Hemoglobin, which is a protein found in red blood cells, is involved in the transport of oxygen and carbon dioxide.

5. Energy I. As a result of the oxidation of protein molecules, the energy necessary for the functioning of the body is released.

6. Catalytic I. Protein enzymes act as biological catalysts that increase the speed chemical reactions in cells.

7. Regulatory I. Hormones regulate various body functions. For example, insulin regulates blood sugar levels.

There are a huge number of proteins in nature that can perform a wide variety of functions. But the most important function of proteins is to support life on Earth together with other biomolecules.

Squirrels- high molecular weight organic compounds consisting of α-amino acid residues.

IN protein composition includes carbon, hydrogen, nitrogen, oxygen, sulfur. Some proteins form complexes with other molecules containing phosphorus, iron, zinc and copper.

Proteins have a large molecular weight: egg albumin - 36,000, hemoglobin - 152,000, myosin - 500,000. For comparison: the molecular weight of alcohol is 46, acetic acid - 60, benzene - 78.

Amino acid composition of proteins

Squirrels- non-periodic polymers, the monomers of which are α-amino acids. Typically, 20 types of α-amino acids are called protein monomers, although over 170 of them are found in cells and tissues.

Depending on whether amino acids can be synthesized in the body of humans and other animals, they are distinguished: nonessential amino acids- can be synthesized; essential amino acids- cannot be synthesized. Essential amino acids must be supplied to the body through food. Plants synthesize all types of amino acids.

Depending on the amino acid composition, proteins are: complete- contain the entire set of amino acids; defective- some amino acids are missing in their composition. If proteins consist only of amino acids, they are called simple. If proteins contain, in addition to amino acids, a non-amino acid component (prosthetic group), they are called complex. The prosthetic group can be represented by metals (metalloproteins), carbohydrates (glycoproteins), lipids (lipoproteins), nucleic acids (nucleoproteins).

All amino acids contain: 1) carboxyl group (-COOH), 2) amino group (-NH 2), 3) radical or R-group (the rest of the molecule). The structure of the radical is different for different types of amino acids. Depending on the number of amino groups and carboxyl groups included in the composition of amino acids, they are distinguished: neutral amino acids having one carboxyl group and one amino group; basic amino acids having more than one amino group; acidic amino acids having more than one carboxyl group.

Amino acids are amphoteric compounds, since in solution they can act as both acids and bases. In aqueous solutions, amino acids exist in different ionic forms.

Peptide bond

Peptides- organic substances consisting of amino acid residues connected by peptide bonds.

The formation of peptides occurs as a result of the condensation reaction of amino acids. When the amino group of one amino acid interacts with the carboxyl group of another, a covalent nitrogen-carbon bond occurs between them, which is called peptide. Depending on the number of amino acid residues included in the peptide, there are dipeptides, tripeptides, tetrapeptides etc. The formation of a peptide bond can be repeated many times. This leads to the formation polypeptides. At one end of the peptide there is a free amino group (called the N-terminus), and at the other there is a free carboxyl group (called the C-terminus).

Spatial organization of protein molecules

The performance of certain specific functions by proteins depends on the spatial configuration of their molecules; in addition, it is energetically unfavorable for the cell to keep proteins in an unfolded form, in the form of a chain, therefore polypeptide chains undergo folding, acquiring a certain three-dimensional structure, or conformation. There are 4 levels spatial organization of proteins.

Primary protein structure- the sequence of arrangement of amino acid residues in the polypeptide chain that makes up the protein molecule. The bond between amino acids is a peptide bond.

If a protein molecule consists of only 10 amino acid residues, then the number is theoretically possible options protein molecules differing in the order of alternation of amino acids - 10 20. Having 20 amino acids, you can make even more diverse combinations from them. About ten thousand different proteins have been found in the human body, which differ both from each other and from the proteins of other organisms.

It is the primary structure of the protein molecule that determines the properties of the protein molecules and its spatial configuration. Replacing just one amino acid with another in a polypeptide chain leads to a change in the properties and functions of the protein. For example, replacing the sixth glutamic amino acid in the β-subunit of hemoglobin with valine leads to the fact that the hemoglobin molecule as a whole cannot perform its main function - oxygen transport; In such cases, the person develops a disease called sickle cell anemia.

Secondary structure- ordered folding of the polypeptide chain into a spiral (looks like an extended spring). The turns of the helix are strengthened by hydrogen bonds that arise between carboxyl groups and amino groups. Almost all CO and NH groups take part in the formation of hydrogen bonds. They are weaker than peptide ones, but, repeated many times, impart stability and rigidity to this configuration. At the level of secondary structure, there are proteins: fibroin (silk, spider web), keratin (hair, nails), collagen (tendons).

Tertiary structure- packing of polypeptide chains into globules, resulting from the formation of chemical bonds (hydrogen, ionic, disulfide) and the establishment of hydrophobic interactions between the radicals of amino acid residues. The main role in the formation of the tertiary structure is played by hydrophilic-hydrophobic interactions. In aqueous solutions, hydrophobic radicals tend to hide from water, grouping inside the globule, while hydrophilic radicals, as a result of hydration (interaction with water dipoles), tend to appear on the surface of the molecule. In some proteins, the tertiary structure is stabilized by disulfide covalent bonds formed between the sulfur atoms of two cysteine ​​residues. At the tertiary structure level there are enzymes, antibodies, and some hormones.

Quaternary structure characteristic of complex proteins whose molecules are formed by two or more globules. The subunits are held in the molecule by ionic, hydrophobic, and electrostatic interactions. Sometimes, during the formation of a quaternary structure, disulfide bonds occur between subunits. The most studied protein with a quaternary structure is hemoglobin. It is formed by two α-subunits (141 amino acid residues) and two β-subunits (146 amino acid residues). Associated with each subunit is a heme molecule containing iron.

If for some reason the spatial conformation of proteins deviates from normal, the protein cannot perform its functions. For example, the cause of “mad cow disease” (spongiform encephalopathy) is the abnormal conformation of prions, the surface proteins of nerve cells.

Properties of proteins

The amino acid composition and structure of the protein molecule determine it properties. Proteins combine basic and acidic properties, determined by amino acid radicals: the more acidic amino acids in a protein, the more pronounced its acidic properties. The ability to donate and add H + is determined buffering properties of proteins; One of the most powerful buffers is hemoglobin in red blood cells, which maintains blood pH at a constant level. There are soluble proteins (fibrinogen), and there are insoluble proteins that perform mechanical functions (fibroin, keratin, collagen). There are proteins that are chemically active (enzymes), there are chemically inactive proteins that are resistant to various environmental conditions and those that are extremely unstable.

External factors (heat, ultraviolet radiation, heavy metals and their salts, pH changes, radiation, dehydration)

can cause disruption of the structural organization of the protein molecule. The process of loss of the three-dimensional conformation inherent in a given protein molecule is called denaturation. The cause of denaturation is the breaking of bonds that stabilize a certain protein structure. Initially, the weakest ties are broken, and as conditions become stricter, even stronger ones are broken. Therefore, first the quaternary, then the tertiary and secondary structures are lost. A change in spatial configuration leads to a change in the properties of the protein and, as a result, makes it impossible for the protein to perform its inherent biological functions. If denaturation is not accompanied by destruction of the primary structure, then it may be reversible, in this case, self-recovery of the conformation characteristic of the protein occurs. For example, membrane receptor proteins undergo such denaturation. The process of restoring protein structure after denaturation is called renaturation. If restoration of the spatial configuration of the protein is impossible, then denaturation is called irreversible.

Functions of proteins

Enzymes

Enzymes, or enzymes, are a special class of proteins that are biological catalysts. Thanks to enzymes, biochemical reactions occur at tremendous speed. The speed of enzymatic reactions is tens of thousands of times (and sometimes millions) higher than the speed of reactions occurring with the participation of inorganic catalysts. The substance on which the enzyme acts is called substrate.

Enzymes are globular proteins, structural features enzymes can be divided into two groups: simple and complex. Simple enzymes are simple proteins, i.e. consist only of amino acids. Complex enzymes are complex proteins, i.e. In addition to the protein part, they contain a group of non-protein nature - cofactor. Some enzymes use vitamins as cofactors. The enzyme molecule contains a special part called the active center. Active center- a small section of the enzyme (from three to twelve amino acid residues), where the binding of the substrate or substrates occurs to form an enzyme-substrate complex. Upon completion of the reaction, the enzyme-substrate complex breaks down into the enzyme and the reaction product(s). Some enzymes have (except active) allosteric centers- areas to which enzyme speed regulators are attached ( allosteric enzymes).

Reactions of enzymatic catalysis are characterized by: 1) high efficiency, 2) strict selectivity and direction of action, 3) substrate specificity, 4) fine and precise regulation. The substrate and reaction specificity of enzymatic catalysis reactions are explained by the hypotheses of E. Fischer (1890) and D. Koshland (1959).

E. Fisher (key-lock hypothesis) suggested that the spatial configurations of the active center of the enzyme and the substrate must correspond exactly to each other. The substrate is compared to the “key”, the enzyme to the “lock”.

D. Koshland (hand-glove hypothesis) suggested that the spatial correspondence between the structure of the substrate and the active center of the enzyme is created only at the moment of their interaction with each other. This hypothesis is also called induced correspondence hypothesis.

The rate of enzymatic reactions depends on: 1) temperature, 2) enzyme concentration, 3) substrate concentration, 4) pH. It should be emphasized that since enzymes are proteins, their activity is highest under physiologically normal conditions.

Most enzymes can only work at temperatures between 0 and 40°C. Within these limits, the reaction rate increases approximately 2 times with every 10 °C increase in temperature. At temperatures above 40 °C, the protein undergoes denaturation and enzyme activity decreases. At temperatures close to freezing, enzymes are inactivated.

As the amount of substrate increases, the rate of the enzymatic reaction increases until the number of substrate molecules equals the number of enzyme molecules. With a further increase in the amount of substrate, the speed will not increase, since the active centers of the enzyme are saturated. An increase in enzyme concentration leads to increased catalytic activity, since a larger number of substrate molecules undergo transformations per unit time.

For each enzyme, there is an optimal pH value at which it exhibits maximum activity (pepsin - 2.0, salivary amylase - 6.8, pancreatic lipase - 9.0). At higher or lower pH values, enzyme activity decreases. With sudden changes in pH, the enzyme denatures.

The speed of allosteric enzymes is regulated by substances that attach to allosteric centers. If these substances speed up a reaction, they are called activators, if they slow down - inhibitors.

Classification of enzymes

According to the type of chemical transformations they catalyze, enzymes are divided into 6 classes:

1. oxireductases(transfer of hydrogen, oxygen or electron atoms from one substance to another - dehydrogenase),
2. transferases(transfer of methyl, acyl, phosphate or amino group from one substance to another - transaminase),
3. hydrolases(hydrolysis reactions in which two products are formed from the substrate - amylase, lipase),
4. lyases(non-hydrolytic addition to the substrate or detachment of a group of atoms from it, in which case C-C, C-N, C-O, C-S bonds can be broken - decarboxylase),
5. isomerases(intramolecular rearrangement - isomerase),
6. ligases(the connection of two molecules as a result of the formation of C-C, C-N, C-O, C-S bonds - synthetase).

Classes are in turn subdivided into subclasses and subsubclasses. In the current international classification, each enzyme has a specific code, consisting of four numbers separated by dots. The first number is the class, the second is the subclass, the third is the subsubclass, the fourth is the serial number of the enzyme in this subclass, for example, the arginase code is 3.5.3.1.

Go to lectures No. 2"Structure and functions of carbohydrates and lipids"

Go to lectures No. 4"Structure and functions of ATP nucleic acids"

Essential amino acids are used productively. Biological and chemical composition proteins are directly dependent on their amino acid composition.

Chemical composition of proteins

Egg whites do not contain enough body lysine for mammals (lysine deficiency is approximately 6%). The addition of this amino acid accelerates the growth of animals.

Cow's milk proteins contain an excess of lysine, leucine, tryptophan, histidine and threonine and are equal to 20%.

Corn proteins are significantly poorer than the first two groups of food proteins. They are deficient in many amino acids: lysine (60% of the norm), tryptophan, amino acids containing sulfur, valine, isoleucine and threonine. These proteins contain an excess of leucine, histidine, and phenylanine (tyrosine). The biological value of plant proteins can be significantly increased by combining them with milk proteins. Thus, a mixture of 60% corn proteins and 40% milk proteins is almost equivalent in biological value to milk proteins. The combination of plant and animal proteins ensures the best regeneration of hemoglobin components.

Amino acid composition of proteins

In a comparative study of the amino acid composition of proteins and equivalent mixtures of amino acids, the best results were obtained with proteins.

Animal experiments have shown that massive doses of any amino acid can have a toxic effect. The protein composition amino acids studied were added to diets containing varying amounts of protein. Addition of 6-12% methionine to the diet led to high mortality, decreased feed intake, weight loss, liver and spleen atrophy. The toxic effect of methionine increased with diets insufficient in vitamin B8. The addition of glycine reduced the toxic effect of methionine. At the same time, increasing protein in the diet always had a protective effect.

The protein efficiency factor (PEC) is used as an indicator of the nutritional value of protein composition. In practical work, it is customary to determine the BEC at a certain level of protein in the diet, most often at 10%.

Some researchers believe that the maximum biological value is obtained at a level of protein in the diet that covers the endogenous human need, i.e. from 15 to 33 g of protein per day. The values ​​of biological value obtained in this case are proposed to be called absolute values ​​(ABV).

A method for determining the nutritional value of proteins based on the absorption of individual amino acids and their balance has also been proposed. Essential amino acids are usually determined in the blood at various times after eating.

properties of protein

“Life is the form of existence of protein bodies” (F. Engels). The components of the human body realize the properties of proteins (muscles, heart, brain and even bones contain a significant amount of protein), but also the participation of protein molecules in all the most important processes of human life. All enzymes contain at their core the chemical properties of proteins, many hormones are also proteins; Antibodies that provide immunity are proteins.

The importance of the properties of proteins is determined not only by the variety of their functions, but also by their indispensability for other nutrients. Therefore everything properties of proteins are considered the most valuable components of food. Experience has shown that long-term protein-free food leads to the death of the organism.

Chemical properties of proteins

Proteins in foods are highly complex high molecular weight compounds, and these chemical properties of proteins are made up of various amino acids, of which there are up to 80. However, most foods contain about 20 amino acids. The diversity of proteins is determined by the amino acid chain (the primary structure of the protein property), additional amino acid bonds within the polypeptide chain (secondary structure) and the peculiarities of the spatial arrangement of polypeptide chemical chains (tertiary structure).

In the human body under the influence of enzymes proteinases and peptidases protein properties in food they are mainly broken down into free amino acids. This occurs in the intestines and is an important property of proteins. In the oral cavity, crushed food is processed by the enzyme amylase contained in saliva. Amylase breaks down carbohydrates, including carbohydrates in plant foods, associated with the chemical properties of proteins, which releases the proteins for subsequent processing.

General properties of proteins

In the stomach, where hydrochloric acid and pepsin are secreted, under the influence of increased acidity and the enzyme, partial denaturation (change in tertiary structure) of the protein properties occurs and its breakdown into large fragments. In the intestine, partially hydrolyzed proteins are broken down by proteases and peptidases, mainly into amino acids, which are absorbed into the blood and then distributed throughout the body, thereby affecting the ratio that describes the protein norm for humans. Some amino acids are used to build chemical properties of proteins in the body, others are converted into compounds involved in the formation of some important organic substances, such as nucleoproteins, etc.

A certain part of the amino acids is broken down into organic keto acids, from which new amino acids and then proteins are synthesized in the body again; this is an important process when, ultimately, the properties of proteins play an important role. These amino acids are called non-essential. However, 8 amino acids, namely: isoleucine, leucine, lysine, methionine, phenylalanine, tryptophan, trenine and valine - relatively speaking, the properties of protein cannot be formed in the adult body from others.

Minerals

The group of macroelements includes those whose content in dry substance varies from nּ10 -2 to nּ10%. These are C, O, H, N, S and P, which are part of the molecular composition of the basic substances and Ca, Na, Cl, K, Mg, which are part of supporting tissues, blood, lymph and other tissues.

The group of ultramicroelements includes elements whose content is below nּ10 -5% (Sb, Hg, Bi, Pb, etc.).

It has been established that most of the elements are biogenic, of great importance for ensuring the normal development of biochemical life processes, and the most important biogenic elements are included in IV (C); V (W, P) and VI (O, S) groups of the periodic table. Elements VII (Cl, J, Mn) and VIII (Fe, Co) groups are involved in the formation of substances with high biological value.

The microelement composition of raw materials depends on the habitat or growth. Depending on the concentration of individual elements in environment and in food, their availability, as well as the selective ability of individual species of organisms, the degree of use of individual elements during assimilation processes also changes.

Squirrels

Of the organic substances that make up living organisms, the most biologically important and the most complex in structure are proteins. Almost all manifestations of life (digestion, irritability, contractility, growth and reproduction, movement, metabolism, etc.) are associated with protein substances. Proteins play an important role both in the construction of living matter and in the implementation of its vital processes.

Specific catalysts of protein nature - enzymes - accelerate chemical reactions occurring in the body. Various protein compounds perform a transport function, supplying the body with oxygen and nutrients. The breakdown of 1 g of protein into final products provides the body with energy of 4.27 kcal.

When heated, proteins isolated from organs and tissues give a white precipitate and have the same physical properties as chicken egg white. That's why they began to be called squirrels. A synonym for the word “protein” is the word “protein” (from the Greek “proteus” - first, main).

Proteins are high molecular weight polymers of various amino acids. In Fig. 1 shows the formulas of various amino acids.

Fig.1. Formulas of some amino acids.

Amino acids are divided into 2 large groups: replaceable and essential. Most amino acids are formed in the body of animals and humans as a result of hydrolysis of food proteins and biosynthesis. But at least eight amino acids are not synthesized in the body. These are valine, leucine, isoleucine, threonine, lysine, phenylalanine, tryptophan and methionine, called essential. Proteins that lack one or more of these amino acids are called biologically deficient. Animal proteins, including aquatic proteins, contain all the essential amino acids.

The amino acids that make up the protein are connected to each other by peptide bonds formed between the amino group of one amino acid and the carboxyl group of another. The mechanism of this process is shown in Fig. 2.

Rice. 2. Formation of the primary structure of the protein.

The resulting polypeptides are the basis of all proteins, and the specific sequence of amino acids contained in them characterizes the primary structure of the protein.

Thus, since protein macromolecules are built from many hundreds of amino acids, in nature there is an unlimited number of their isomers, and each type of living being can have its own unique protein.

Polypeptide chains, in turn, can be connected to form secondary protein structures, mainly due to the bonds that arise between different groups of polypeptides. This is shown schematically in Fig. 3.

a) formation of hydrogen bonds

b) formation of an a-helix from a polypeptide chain

Rice. 3. Scheme of formation of the secondary structure of the protein.

The spatial arrangement of the polypeptide chains of a protein molecule determines the tertiary structure of the protein molecule.

Proteins themselves are high-molecular compounds of complex structure that differ both in physiological functions and in chemical properties. Proteins of food raw materials are predominantly in a colloidal state - in the form of gels and sols, which determines the instability and variability of the properties (denaturation) of protein substances when environmental conditions change.

When protein solutions are acidified to pH 4.5-5.0 (for example, during pickling), proteins lose solubility and precipitate (coagulate). Many proteins lose solubility when solutions are saturated with sodium chloride (salting). In particular, the main muscle proteins, highly soluble in sodium chloride solutions with a concentration of 7.5-10%, are precipitated (salted out) when its concentration increases to 15%. When heated (during boiling, frying, baking), proteins coagulate (coagulate). Thermal denaturation of proteins begins at 28-35 o C. Denaturation of proteins also occurs during dehydration (dehydration) of their systems (during drying and freezing).

During precipitation (salting out, coagulation) of proteins, their connection with water is disrupted.

As a result of the spatial three-dimensional structure, chemically active groups - NH 2 - appear on the surface of the protein molecule; -COOH; - HE. In an aqueous solution, these groups are in an ionized state with charges of different signs. A protein molecule acquires the corresponding sign and magnitude of charge depending on the ratio of positively and negatively charged groups. The charge of a protein molecule depends on its state. Any change in the structure of a protein molecule leads to a change in its charge; in particular, loss of charge leads to denaturation of the protein. The presence of these charges also determines the hydration properties of proteins. For example, water molecules attach to a positively charged protein molecule with their negatively charged ends, and a structure is formed, the center of which is the protein molecule, and around it is a monomolecular shell of water. Since all the negatively charged ends of the water molecules face the protein molecule, the same charge remains on the surface of the protein-water structure. New groups of water molecules, etc., in turn attach to this surface. In this case, an electrostatically bound hydration layer is formed around each protein molecule. The strength of the binding to the protein decreases in proportion to the square of the distance from the center, i.e. from the protein molecule, and at a sufficiently large distance on the scale of the size of the molecule, this bond is so small that the molecules’ own thermal motion prevents the action of electrostatic forces. This limits the amount of water retained by the surface of the protein.

According to existing views, protein tissue can be considered as a colloidal and capillary-porous colloidal body of a very complex structure, the basis of which is a structural network of proteins in a swollen state, containing viscous solutions containing soluble proteins and other nitrogenous and minerals, which have hydrophilic properties. In this case, part of the water included in the muscle tissue is firmly retained by the proteins of the structural network, as well as by molecules of dissolved proteins and other hydrophilic substances.

Along with the water held by the force field on the outer and inner surface protein particles, muscle tissue contains water retained osmotically and by mechanical bonding forces (capillary retained water). This water is found in liquids (solutions) containing various nitrogenous and organic substances and mineral salts, enclosed in closed cells (micropores) inside protein structures and micro- and macrocapillaries penetrating the latter. According to the data available in the literature, 1 g of protein binds on average 0.3 g of water during hydration.

All processing methods and technological modes are aimed at changing the water in the tissues of the raw material (saturating it with salt, turning it into ice, heating it to a temperature close to the boiling point, evaporation). A change in the internal energy of water leads to a disruption of the equilibrium state between protein and water, which forms a hydration shell. The protein molecule reacts to this by rearranging its own structure and accordingly changing the amount of charge. When these changes result in a sharp decrease or complete disappearance of charge, protein denaturation occurs.

Depending on the intensity and duration of external influence, protein denaturation can be either reversible, partially reversible, or irreversible.

The depth of denaturation can be determined by the ability of muscle tissue to fully or partially restore its connection with water.

Currently used methods for processing food raw materials with high content proteins lead mainly to changes that can be characterized as partial denaturation. The denaturation diagram of a protein molecule is shown in Fig. 4.

Rice. 4. Scheme of denaturation of a protein molecule:

A - initial state, B - beginning reversible development, C - advanced irreversible development.

The most characteristic changes in a protein during thermal denaturation (temperature 70-100°C) are the loss of its native properties (the ability to dissolve in water, solutions of salts and alcohols), as well as a decrease in the ability to swell.

The changes in protein associated with thermal denaturation are more significant, the higher the temperature and duration of heating, the action of pressure, and in an aqueous solution the protein denatures faster than in a dried state.

Denaturation of proteins plays an important role in a number of technological processes: when baking bread, confectionery, when drying meat, fish, vegetables, milk and egg powder, when making canned food, etc.

When the product is brought to full readiness, usually with prolonged exposure to temperatures close to 100°C, proteins undergo further changes associated with the destruction of their macromolecules - hydrolysis.

At the beginning of the process, volatile products can be separated from protein molecules: carbon dioxide, hydrogen sulfide, ammonia, hydrogen phosphide and other substances involved in the formation of the taste and aroma of finished products. With prolonged exposure to water and heat, the formation of water-soluble nitrogenous substances occurs due to depolymerization of the protein molecule, which occurs, for example, during the transition of collagen to glutin.

Protein hydrolysis can be caused with the help of proteolytic enzymes used to intensify certain technological processes (softening tough meat, preparing yeast dough, etc.).