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Chemical modification of proteins is carried out with the help of acid or alkaline hydrolysis, stabilization of proteins by salt formation, acylation, and plastein reaction.

Alkaline and acid hydrolysis. These methods of protein modification are widely used to solubilize fish proteins in the process of obtaining fish protein concentrates, resulting in increased solubility, emulsification and foaming properties of proteins.

The depth of hydrolysis depends on the type and concentration of alkali or acid, the ratio of substrate and reagent, temperature, duration of treatment. To achieve a certain result, the process must be optimized for a specific protein of a specific raw material.

With the complete hydrolysis of proteins, a mixture of amino acids is formed. This is used in the latest technologies. The degree of hydrolysis can be controlled and adjusted. But it must be taken into account that along with the positive consequences of hydrolysis, there are also negative ones. For example, the formation of racemates of acids - peptides with a bitter taste.

One example of such a modification is the destruction of 11-S-globulin, which is characteristic of legumes, in particular soybeans, and has a globular molecule. Moreover, its quaternary structure is characterized by the fact that several subunits are combined into a globule using intermolecular bonds. Such structures are not capable of gelation, as well as imitation of meat-like systems. Controlled hydrolysis makes it possible to obtain a protein with the properties of a gelling agent, which is more typical for a number of fibrillar proteins, for example, gelatin.

This principle of modification of protein properties has found wide application in the technological process of obtaining structured products by the spinning method.

Similarly, the structure of a protein can be changed by heating. Protein breakdown into subunits, their partial destruction and aggregation lead to the formation of protein jelly. The stability of the resulting gels depends on the formation of disulfide bridges between the polypeptide chains.

Solubilization of proteins by salt formation. The possibility of such modification follows from the basic property of proteins as polymeric ampholytes capable of interacting with both cations and anions. Two types of interaction are possible: the formation of salt bridges and the specific sorption of ions on the protein surface. In this case, proteinates are formed, which are more soluble than native proteins.

The formation of proteinates is widely used in the isolation of proteins from soy (soy proteinates) and from milk (caseinate and sodium coprecipitate).

The most widely used modifier salts are sodium chloride and inorganic phosphates. Thus, by regulating the ability of meat formulations to retain water, sodium chloride, pyro- or sodium tripolyphosphate are used, which increase the solubility of myofibrillar proteins. At the same time, it is known that polyphosphates in relation to proteins are characterized by anti-denaturation, antiseptic, and antioxidant properties.

Every year the use of proteinates in the food industry and catering expands.

Acylation. Acylation with acetic or succinic anhydrides is one of the widely used methods of chemical modification of proteins. The result of this modification is a shift in the isoelectric point of the protein to a more acidic zone. Under the action of succinic anhydride, this process takes place to a greater extent. This makes it possible, even with a low degree of modification, to significantly improve such technological characteristics as solubility, emulsifying and foaming abilities.

The introduction of acyl residues (such as R-COO-) contributes to the deployment and, ultimately, the destruction of the protein globule, which leads to a change in the electrostatic equilibrium characteristic of the native protein due to the blocking of positively charged amino groups in globulins and an increase in the role of electrostatic repulsion of like-charged groups . The consequence of this is a change in protein conformation and its dissociation. At the same time, such technological effects as the ability to gel formation are achieved.

In practice, it has been proven that by acylation it is possible to obtain modified vegetable proteins with improved gel-forming ability, and this ability and the structural and mechanical characteristics of the resulting gels depend on the degree of acylation. So, at a very high degree, its excess of negative charges causes such a strong repulsion of polypeptide chains that the aggregation necessary for gel formation will be impossible. That is, the degree of acylation acts as an indicator of the functional properties of the protein, and acylation itself is a method of regulating these properties.

Acylated milk casein is used as an emulsifier and emulsion stabilizer, thickener for drinks, sauces, fruit and vegetable purees. Fish proteins are used as emulsifiers, binders, as substances that form jelly during heat treatment.

Enzymatic modification of proteins. Using enzymes, it is possible to purposefully change the structure of a protein in the most different directions. Thanks to the partial hydrolysis of the protein, it is possible to provide an increase in solubility, emulsifying activity, foaming ability, stabilization of foams and emulsions. The specificity of enzymes allows you to influence only certain areas or groups of the protein molecule. It is also important that most enzymatic processes take place in an aquatic environment and, as a rule, under conditions close to physiological. However, not all enzymatic changes in proteins are important for food technology.

Yes, in recent times partial hydrolysis of connective tissue proteins by proteases is used, meat tenderization is used to improve its quality indicators. In fish proteins, under the action of enzymes of microbial origin amylosubtilin, protosubtilin, bromelain at pH 6.5-7.0 and a temperature of about 30 ° C, the emulsifying activity increases by 1.5 times, the solubility increases by 20%.

A special effect is achieved by combining the enzymatic process and chemical modification, for example, with succination. Thus, the products of enzymatic hydrolysis of fish proteins, which are characterized by a high foaming ability, lose their characteristic fishy taste as a result of succination, which allows them to be used in the production of confectionery products, ice cream, and drinks.

Very good prospects are given by the recently opened plasticine reaction- a process reverse to enzymatic cleavage, when peptide bonds are re-formed under the action of enzymes. Using this reaction, it is possible to create polypeptide chains with a molecular weight of about 3,000 Daltons from the products of protein hydrolysis. Due to the fact that individual amino acids, including essential ones, are able to react in the form of esters, they can be purposefully incorporated into polypeptides and proteins. By incorporating tryptophan, lysine, and methionine into maize zein, it was possible to obtain plastein with good biological value.

The biological value of soy proteins is low due to their low content of sulfur-containing amino acids. By partial hydrolysis of soy protein with pepsin, mixing it with the same wool keratin hydrolyzate containing many sulfur-containing amino acids, and subsequent plastein reaction under the action of nagarase (Bacilus subtilis protease), plastein with a nutritional value close to casein is obtained.

Plasteins obtained by incorporating glutamic acids obtained from soy proteins into proteins have very good properties. First, these glutamic acids are soluble at all pH values ​​and resistant to thermal coagulation. Secondly, they have a pronounced taste of heat-treated meat.

The plastein reaction has great prospects for the extraction of undesirable amino acids from proteins. The latter include phenylalanine, the presence of which causes serious consequences in patients with phenyloketonuria. Partial enzymatic hydrolysis with pepsin, extraction of phenylalanine peptides by gel filtration, and subsequent plastein synthesis in the presence of ethyl esters of tyrosine and tryptophan under the action of papain plant protease leads to the production of phenylalanine-free plasteins, but balanced in other amino acids.

Physical and chemical methods of modification. Physicochemical methods of influencing protein systems combine the following methods: complex formation with natural polymers (proteins, polysaccharides, etc.), as well as with monomers (carbohydrates, fats), mechanical effects of various kinds, heat treatment, etc.

Complexation by type protein-protein interaction found practical application even earlier, but now there is a scientific interpretation of this phenomenon. So it was found that the joint drying of proteins of different nature - fish and cereals - not only leads to the production of valuable protein mixtures, but also preserves the functional properties of the original proteins. As grain additives to minced fish, wheat, rice or other flour can be used in an amount of from 10% to 30%.

The addition of vegetable proteins to semi-finished meat products, due to complex formation, ensures a minimal decrease in water-holding capacity during heat treatment.

Conjugates of proteins and carbohydrates are characterized by high functional properties, which is traditionally used in technological processes. Thus, the ability of sucrose to increase the coagulation temperature of egg proteins is widely used in the technology of sweet dishes and confectionery. The ability of carbohydrates to stabilize animal proteins to the action of low and high temperature denaturation is known.

When fish proteins are dried together with monosaccharides, highly soluble complexes are formed, the solubility of which depends on the nature of sugars and their concentration in minced fish. In terms of the effect on the solubility of the resulting product, glucose is most effective, and sucrose and fructose are less effective. Similarly, glycerol and modified starch stabilize fish proteins. But it should be borne in mind that in this case, conditions are created for the Maillard reaction to occur, which will lead to a decrease in the nutritional value of proteins.

The addition of glucono-delta-lactone stabilizes minced meats.

There are also known methods of "strengthening" flour gluten in the formation of its complexes with acidic polysaccharides, such as pectin derivatives, as well as in the presence of microbial xanthan polysaccharide in the amount of 0.1-0.5%.

Increase the resistance of proteins to denaturation and lipids, which are also able to form complexes with the former. The nature of this phenomenon has not been sufficiently clarified, but nevertheless it is used in the production of minced sausages, the semi-finished products of which are protein-fat emulsions.

Physical methods of exposure also play a role in modifying the properties of protein substances. Thus, the intensity, method and degree of grinding are key ceteris paribus in shaping the quality of wheat flour. By setting certain temperature conditions, they regulate the water-holding capacity, tenderness, juiciness of meat systems. The temperature and duration of processing regulate the quality indicators of milk curd. Simultaneous mechanical mixing of the mass leads to the formation of "casein grain", which differs significantly in organoleptic characteristics from curd obtained by thermal acid coagulation, but without mixing.

A high degree of minced meat and fish grinding, especially in colloid mills, leads to mechanical degradation of myofibril sarcomeres, resulting in an increase in water-retaining capacity and protein solubility.

Partial thermal coagulation of fish proteins or brewing flour, resulting in denaturation of gluten proteins, changes cohesion, allows you to adjust the ability to form and organoleptic properties of systems.

(from late Latin modificatio-change) biogenic, occurs after completion broadcasts matrix ribonucleic acid, or mRNA, (protein synthesis on an mRNA template) or until it is completed. In the first case, M.b. called post t and n s l a t i n o n y, in the second - to o t r a n c l a t ion n o y. It is carried out due to reactions decomp. functional groups of acid residues, as well as peptide bonds, and determines the final form of the protein molecule, its physiological activity, stability, and movement within the cell.

Extracellular (secreted) proteins, as well as many others. cytoplasmic proteins. membranes and intracellular compartments (isolated parts of the cell) undergo glycosylation, as a result of which glycoproteins. Naib. mannose-containing chains attached to polypeptides by an N-glycosidic bond are complexly organized. The initial stage of the formation of such chains proceeds co-translationally according to the scheme:

Dol-dolichol (polyprenol), Dol-P-P-dolichol pyrophosphate, Glc-glucose, GlcNAc-N-acetyl-D-glucosamine, Man-mannose

Afterbirth. stages are carried out post-translationally with the participation of several. enzymes located in different subcellular compartments. So, for the G-protein of the vesicular stomatitis virus, glycosidic chains to-rogo are built from 15 carbohydrate residues, such a sequence of events has been established. First, in the endoplasmic reticulum occurs in two stages, the separation of terminal glucose residues with the participation of two different glucosidases. Then, mannosidases (I and II) remove 6 mannose residues, and N-acetyl-D-glucosamine transferase adds three GlcNAc residues to the mannose residues of the glycoprotein. Finally, in the Golgi complex, the residues of fucose, galactose, and sialic acid bind to these residues with the participation of the corresponding transferases. Monosaccharide residues can undergo phosphorylation, sulfonation, and other modifications.

Glycosylation of secreted proteins is preceded by proteolytic. processing - separation from the N-terminus of the polypeptide chain "signal" amino acid sequence. In eukaryotic cells (cells of all organisms, with the exception of bacteria and blue-green algae), this process is carried out by translation, in prokaryotes. cells (cells of bacteria and blue-green algae) it can proceed post-translationally. Naib. common signal sequences include 23 amino acid residues. The characteristic features of these sequences are the presence at the end of a short positively charged section, followed by a hydrophobic section containing from 7 to 14 amino acid residues. The signal sequences end with a hydrophilic region, conservative in length (5-7 residues), at the C-terminus of which most often there are residues of alanine, glycine, serine, threonine, cysteine ​​or glutamine.

Almost all functions. classes of extracellular proteins (enzymes, hormones, immunoglobulins, etc.) contain disulfide bonds. They are formed from the cystene SH groups in a multi-step process involving the enzyme disulfide isomerase. Mean appears in its early stages. the number of "wrong" disulfide bridges, to-rye are eliminated as a result of thiol-disulfide exchange, in Krom, apparently, cystamine (H 2 NCH 2 CH 2 S) 2 is involved. It is assumed that such a "enumeration" of connections occurs until the most. stable tertiary structure, in which the disulfide bridges are "buried" and therefore inaccessible to reagents.

To the max. common modifications of intracellular proteins include phosphorylation and dephosphorylation of the OH group of serine, tyrosine and threonine residues, which are carried out with the participation of protein kinase and phosphatase enzymes according to the scheme:

ATP - adenosine triphosphate, ADP - adenosine diphosphate, P - phosphoric acid or its residue

Phosphorylation is accompanied by activation or inactivation of enzymes, for example. glycosyltransferases, as well as a change in the physicochemical properties of non-enzymatic proteins. Reversible protein phosphorylation controls, for example, important processes such as transcription and translation, lipid metabolism, gluconeogenesis, and muscle contraction.

Proteins of mitochondria and chloroplasts, encoded by nuclear DNA, have excess amino acid sequences at the N-terminus, to-rye selectively direct polypeptide chains to certain compartments of organelles, after which they are cleaved off as a result of proteolysis with specific participation. endopeptidases. The excess sequences of precursors of mitochondrial proteins differ significantly in the number of amino acid residues; they can be from 22 to 80. Short sequences are characterized by a high (20-25%) content of positively charged amino acid residues evenly spaced along the polypeptide chain. Long sequences additionally include a section consisting of hydrophobic amino acids, to-ry "anchoring" the precursor in the lipid bilayer of mitochondrial membranes.

Precursors for a number of hormones are known (eg, for gastrin, glucagon and insulin), to-rye pass into an active form by means of splitting of a polypeptide chain in the sites containing two consecutively located remains of the main amino acids (arginine and lysine). Cleavage is carried out with the participation of specific. endopeptidase acting in ensemble with a second enzyme having carboxypeptidase activity. The latter removes the residues of the terminal basic amino acids, completing the transformation. peptide into an active hormone. To proteins undergoing proteolytic. activation, also include proteinases (pepsin, trypsin, chymotrypsin), albumins, procollagen, proteins of the blood coagulation system, etc. In some cases, inactive forms of enzymes (zymogens) are necessary for temporary "preservation" of enzymes. So, the zymogens trypsin and chymotrypsin (respectively, trypsinogen and chymotrypsinogen) are synthesized in the pancreas, secreted into the small intestine, and only there under the action of specific. enzymes convert. into active form.

A wide range of proteins (histones, myosin, actin, ribosomal proteins, etc.) are methylated post-translationally at lysine, arginine and histidine residues (N-methylation), as well as at glutamic and aspartic acid residues (O-methylation) . Usually acts as a methylating agent S-adenosylmethionine.

In some eukaryotic In cells, more than half of p-rimy proteins are acetylated at the N-terminus. This process can be carried out co- and post-translationally (indicated in the diagram respectively. K. T. and P. T.), for example:

HSCoA-coenzyme A, AcCoA - acetyl coenzyme A, Met-methionine, Asp - aspartic acid

For peptides containing from 3 to 64 amino acid residues and secreted in decomp. organs (gastrin, secretin, cholecystokinin, etc.), posttranslations were found. amidation of the C-terminal amino acid residue (with the exception of the terminal residues of arginine and asparagine).

Nek-ry types of modifications are characteristic of separate proteins or small groups of proteins. In particular, in collagen and several. other proteins with similar amino acid sequences have been found to contain 4- and 3-hydroxyproline, as well as 5-hydroxylysine. Hydroxylation of proline and lysine residues proceeds co-translationally and is important for the formation of a unique collagen structure. Hydroxylysine is involved in the formation of covalent crosslinks between the polypeptide chains of collagen according to the scheme:

Nuclear proteins (histones, non-histone proteins) undergo adenosine diphosphate ribosylation and polyadenosine diphosphate ribosylation, during which adenosine diphosphate-tribosyl residues are transferred from the coenzyme nicotinamide adenine dinucleotide (NAD) to acceptor proteins:

These two districts are different in many ways. aspects. In particular, polyadenosine diphosphate ribosylation proceeds in the presence of a day. DNA. Most adenosine diphosphate ribosyl groups are attached to proteins via an ester bond formed by the OH group in position 5 "of the ribose residue and the COOH group of the C-terminal amino acid or glutamic acid, located inside the polypeptide chain.

Of great importance is the carboxylation of glutamine residues to - you with the formation of g-carboxyglutamine to - you in the precursor of prothrombin. This reaction is catalyzed by a vitamin K-dependent carboxylase localized in endoplasmic membranes. reticulum. A similar reaction occurs during the maturation of certain other coagulation factors.

Lit.: Fundamentals of biochemistry, trans. from English, vol. 1, M., 1981, p. 277-80; General organic chemistry, trans. from English, vol. 10, M., 1986, p. 543-70; The enzymology of post-translational modification of proteins, v. 1, L.-N. Y., 1980; The biochemistry of glycoproteins and proteoglycans, N. Y.-L., 1980; cell biology. A comprehensive treatise, v. 4-Translation and the behavior of proteins, N. Y., 1980; Methods in enzymology, v. 106, N.Y., 1984; Hurt E.G., Loon A.P.G.M. van, "Trends in Biochem. Sci.", 1986, v. 11, no. 5. n. 204-07. V. N. Luzikov.