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1. Features of protein metabolism.
2. Catabolism of amino acids.
3. Universal processes in the catabolism of amino acids.
4. Methods for the neutralization of ammonia.
5. Protein biosynthesis.
Protein metabolism occupies a central place among the diverse metabolic processes inherent in living matter. All other types of metabolism - carbohydrate, lipid, nucleic, mineral, etc., primarily serve the metabolism of proteins, incl. specific protein biosynthesis. Protein metabolism is very strictly specific, ensures the continuity of reproduction and renewal of the body's protein bodies.
It is protein metabolism that coordinates, regulates and integrates the variety of chemical transformations in an integral living organism, subordinating it to the preservation of the species, the continuity of life. Compared with other types of metabolism, protein metabolism has a number of features.
Features of protein metabolism
One of characteristic features protein metabolism is its extreme branching. In the transformations of more than 20 amino acids of a protein molecule in animals, several hundred intermediate products are involved, closely related to the metabolites of carbohydrate and lipid metabolism. Blocking any specific metabolic pathway, even one amino acid, can lead to the appearance of completely unknown products.
The state of protein metabolism is determined by many factors, both exogenous and endogenous. Great importance at the same time, the biological usefulness of food proteins (feed) plays. Any deviations from the normal physiological state of the body, disturbances in the metabolism of carbohydrates, lipids, etc. are immediately reflected in nitrogen metabolism.
The state of protein metabolism in a living organism can be characterized by nitrogen balance. This term means the quantitative difference between nitrogen introduced with food and excreted in the form of end products expressed in the same units. Since the bulk of food nitrogen is represented by proteins, and most of the excreted end nitrogenous products are the result of protein breakdown, it is generally accepted that for a correct assessment of the state of protein metabolism, the determination of nitrogen balance can be a fairly accurate criterion. In addition, the average nitrogen content in proteins is more or less constant and is 16%. To convert total nitrogen to protein, you need to multiply its total amount found by a factor of 6.25. The problem of protein norms in animal feeding is closely related to the concept of nitrogen balance.
There are 3 types of nitrogen balance in the body: positive, zero (nitrogen balance) and negative.
In clinical biochemistry, the concepts of protein and non-protein nitrogen are distinguished. The amount of non-protein nitrogen in the blood of animals is not large and is in the range of 20-60 mg%. This includes mainly urea nitrogen, amino acids, uric acid, creatine and creatinine, indican, etc. Non-protein blood nitrogen is also called residual nitrogen, that is, remaining in the filtrate after protein precipitation.
In healthy animals, fluctuations in the content of non-protein nitrogen in the blood are insignificant and mainly depend on the amount of proteins supplied with food. However, many pathological conditions are accompanied by a sharp increase in the content of non-protein nitrogen in the blood. This condition is called azotemia.
The main features of protein metabolism are manifested at the stage of intermediate metabolism and can be explained by two factors:
Firstly, the energy value amino acids is not high and perform in the cell, first of all, the functions of building materials. In this regard, in the metabolism of proteins, the central role is played not by the processes of catabolism, but by anabolism, i.e. protein synthesis. Secondly, in a living cell there are no single, universal mechanisms for the breakdown of amino acids. Each amino acid is degraded according to an individual mechanism.
Amino acid catabolism
If 20 protein amino acids are known, then at least 20 pathways of their catabolism function in each cell. However, despite such a variety of catabolic pathways, there are few end products of tissue metabolism of amino acids; The 20 amino acid breakdown pathways merge at certain stages and lead to the formation of only 5 different products, which then enter the tricarboxylic acid cycle and are completely oxidized.
Rice. 21. Ways of transformations of amino acids.
Carbon skeletons of 10 amino acids are cleaved to acetyl-CoA. Moreover, 5 of these 10 amino acids (alanine, cysteine, glycine, serine, threonine) are cleaved to acetyl-CoA through pyruvate. The other 5 (phenylalanine, tyrosine, leucine, lysine, tryptophan) through acetoacetyl-CoA. As is known, acetoacetyl-CoA is a central product in the metabolism of ketone bodies. In the liver, ketone bodies can be formed from these amino acids and therefore they are called ketogenic. The rest - glucogenic, because. glucose is easily synthesized from pyruvate. However, such a division of amino acids is very conditional, because, in general, all amino acids can be called glucogenic, especially since some amino acids can be cleaved, both with the formation of pyruvate and acetoacetyl-CoA.
In addition to acetyl-CoA, α-ketoglutarate, succinyl-CoA, fumarate, and oxaloacetate can be formed during amino acid catabolism (Fig. 21).
Universal processes in catabolism
amino acids.
Each amino acid is degraded according to an individual mechanism. Some catabolic pathways are quite complex, multistage (up to 13 consecutive reactions), with the formation of a large number of metabolites, which in turn can be involved in various biochemical processes. For example, during the breakdown of tryptophan, products are formed that can serve as precursors of the neurohormone serotonin, nicotinic acid and etc.
A number of transformations are known that occur in all amino acid cleavage methods, ie. they are common to all catabolic pathways. These include: deamination, transamination and decarboxylation. In biology, they are better known as universal mechanisms for the breakdown of amino acids.
Deamination - elimination of the amino groups of amino acids. The existence of four types of deamination has been proven. In all cases, the NH2 group of amino acids is released as NH3.
1. Restorative deamination.
2. Hydrolytic deamination.
3. Intramolecular deamination.
4. Oxidative deamination.
The predominant type for animal tissues, plants, and most aerobic microorganisms is the oxidative deamination of amino acids, which proceeds in two stages with the formation of an unstable intermediate product, imino acids. However, it should be noted that most of the enzymes that catalyze the oxidative deamination of amino acids, when physiological values pH is inactive. In animal tissues, the most active is the enzyme catalyzing the oxidative deamination of glutamic acid - glutamate dehydrogenase. The end product of the reaction is α-ketoglutarate.
Transamination (transamination) - reactions of intermolecular transfer of an amino group from an amino acid to an α-keto acid without intermediate formation of ammonia.
Transamination reactions are reversible and universal for all living organisms. They proceed with the participation of specific enzymes - aminotransferases. Any α-amino acid and any α-keto acid can participate in transamination to form a new amino and keto acid. Given the fact that glutamic acid undergoes oxidative deamination at a high rate in animal tissues, it can be assumed that α-ketogutarate is one of the main substrates for transamination. At present, it is considered proven not only that almost all amino acids react with α-ketoglutaric acid to form glutamic acid and the corresponding keto acid, but also that the reactions of transamination and oxidative deamination are coupled in a single process that proceeds according to the scheme:
Rice. 22. Scheme of indirect deamination of amino acids
Since all reactions of this process are reversible, conditions are created for the synthesis of essentially any amino acid, in the presence of the corresponding α-keto acid.
Decarboxylation- Cleavage of the carboxyl group of amino acids in the form of carbon dioxide. The reaction is irreversible and is catalyzed by decarboxylases. There are several types of decarboxylation, among which α-decarboxylation is the most widely used, i.e. cleavage of the –COOH group at the α-carbon of the amino acid. Decarboxylation products are CO2 and amines, and diamines and a new amino acid can also be, depending on the nature of the amino acid being decarboxylated.
Some amines (tryptamine, histamine) have biological activity, poisonous substances are known among diamines (cadaverine, putrescine). There are special mechanisms for the neutralization of such compounds, the essence of which is generally reduced to oxidative deamination with the release of ammonia.
Methods for the neutralization of ammonia.
One of the end products of amino acid metabolism is a highly toxic compound - ammonia. Therefore, the concentration of ammonia in the body should be kept low. Indeed, the level of ammonia in the blood normally does not exceed 60 µmol/l (this is almost 100 times less than the concentration of glucose in the blood). In the human body, about 100 g of amino acids per day undergoes decay, therefore, about 15 g of ammonia is released. Experiments on rabbits have shown that an ammonia concentration of 3 mmol/l is lethal. Thus, ammonia must be subjected to constant neutralization with the formation of non-toxic compounds that are easily excreted in the urine.
There are several main ways to neutralize ammonia.
Formation of amides of dicarboxylic amino acids (reductive amination);
Synthesis of urea;
Formation of ammonium salts;
1. Reductive amination.
One of the ways of binding and neutralizing ammonia in the body, in particular in the brain, retina, kidneys, liver and muscles, is the biosynthesis of amides of glutamic and aspartic acids (glutamine or asparagine).
The formation of glutamine (asparagine) is, firstly, an express method for neutralizing ammonia and, secondly, a method for transferring ammonia from peripheral tissues to the liver and kidneys, where this poison is finally neutralized and excreted from the body.
The neutralization of ammonia by the synthesis of glutamine is also of anabolic importance, since glutamine is used for the synthesis of a number of compounds. The amide group of glutamine can be used for the synthesis of asparagine, glucosamine and other amino sugars, purine and pyrimidine nucleotides. Thus, in these reactions, ammonia nitrogen is included in various structural and functional components of the cell.
2. Formation of ammonium salts.
In general, all ammonia is removed from the body in the urine in two ways:
In the form of urea, which is synthesized in the liver;
In the form of ammonium salts formed in the epithelium of the tubules of the kidneys;
Excretion of ammonia in the urine is normally small - about 0.5 g per day. But it increases several times with acidosis.
The synthesis of ammonium salts occurs in the lumen of the tubules of the kidneys from the ammonia secreted here and the filtered anions of the primary urine.
Ammonia in the kidneys is also formed due to the amide group of blood glutamine, which is not retained in the liver. Glutamine is hydrolyzed by glutaminase present in the epithelial cells of the tubules of the kidney.
The formation of ammonium salts in the renal tubules is an important mechanism for regulating the acid-base state of the body. It sharply increases with metabolic acidosis - the accumulation of acids in the body and decreases with the loss of acids by the body (alkalosis).
3. The main mechanism for the neutralization of ammonia in the body is urea synthesis. Urea is excreted from the body in the urine as the main end product of protein metabolism. Urea accounts for up to 80-85% of all nitrogen excreted from the body. The main site of urea synthesis is the liver. The synthesis of urea is a cyclic metabolic process and is called the ornithine Krebs urea cycle.
The ornithine cycle is closely related to the tricarboxylic acid cycle (Krebs bicycle). The mechanism of the process is quite simple, it is considered only in three stages. However, a feature of the cycle is that the reaction enzymes are distributed between the cytoplasm and mitochondria of cells.
For each turn of the cycle, one urea molecule is synthesized from two ammonia molecules, and three ATP molecules are consumed.
Rice. 23. Scheme of urea biosynthesis.
Protein biosynthesis
Protein synthesis occurs continuously in every living cell. The protein-synthesizing system of a cell involves the coordinated interaction of more than 300 different macromolecules and includes a set of all 20 amino acids that make up protein molecules; at least 20 different tRNAs; a set of at least 20 different enzymes - aminoacyl-tRNA synthetases; ribosomes; protein factors involved in synthesis at different levels of translation; mRNA as the main component of the system that carries information about the structure of the protein synthesized in the ribosome.
Despite this complexity, proteins in the cell are synthesized at a fairly high rate. For example, in E. coli cells, a protein consisting of 100 amino acids is synthesized in 5 seconds.
Rice. 24. Schematic diagram of protein biosynthesis (according to A.S. Spirin). Circles - free amino acids and their residues in the composition of the polypeptide chain.
The amino acid sequence of a protein (primary structure) is known to be encoded in genes. Messenger RNA (mRNA) or messenger RNA (mRNA) serves to transfer genetic information from DNA in the nucleus to the cytoplasm, where it combines with ribosomes and serves as a template on which protein synthesis is carried out. The process of synthesizing messenger RNA is called transcription. After the structural features of the gene became known, the transcription mechanism was completely deciphered. A complete complementary copy of the gene is preliminarily synthesized - pro- and RNA, which then undergoes a maturation process (mRNA processing).
Processing consists in enzymatic cutting of the primary transcript, followed by removal of its intronic regions and reunion (splicing) of exonic regions, forming a continuous coding sequence of mature mRNA, which is further involved in the translation of genetic information. During processing, the 5' and 3' ends of the emerging mature mRNA molecule are also modified.
Translation as the next step in the implementation of genetic information consists in the synthesis of a polypeptide on a ribosome, in which an mRNA molecule is used as a template.
Translation can be thought of as the process of translating the "nucleotide language" of mRNA into the "amino acid" polypeptide chain of a protein molecule. This process occurs due to the fact that in the nucleotide sequence of mRNA there are code "words" for each amino acid - the genetic code. Each consecutive triple combination of nucleotides encodes one amino acid - a codon. The genetic code consists of 64 codons.
The genetic code is degenerate. This means that most amino acids are coded for by more than one codon. The sequence of the first two nucleotides determines the specificity of each codon, i.e. codons encoding the same amino acid differ only in third nucleotides.
Another distinctive feature of the genetic code is its continuity, the absence of "punctuation marks", i.e. signals indicating the end of one codon and the beginning of another. In other words, the code is linear, unidirectional, and continuous. The most significant feature of the code is its universality for all living organisms from bacteria to humans. The code has not undergone significant changes over millions of years of evolution.
Among the 64 codons, 3, namely UAG, UAA, UGA, turn out to be "meaningless". These codons do important function termination signals in polypeptide synthesis in ribosomes.
The translation process can be conditionally divided into three main stages - initiation, elongation and termination.
Translation initiation is provided by the connection of the mRNA molecule with a certain region of the small subunit of the dissociated ribosome and the formation of the initiation complex.
The elongation process is directly related to the large subunit of ribosomes, which has specific sites - A (amino acid) and P (peptidyl). It begins with the formation of a peptide bond between the initiating (first in the chain) and subsequent (second) amino acids. Then the ribosome moves one mRNA triplet in the direction 5" → 3", which is accompanied by detachment of the initiating tRNA from the template (mRNA), from the initiating amino acid and its release into the cytoplasm. In this case, the second aminoacyl-tRNA moves from the A-site to the P-site, and the vacated A-site is occupied by the next (third) aminoacyl-tRNA. The process of sequential movement of the ribosome in "triple steps" along the mRNA strand is repeated, accompanied by the release of tRNA entering the P-site and the increase in the amino acid sequence of the synthesized polypeptide.
Translation termination is associated with the entry of one of the three known mRNA stop triplets into the A site of the ribosome. Since such a triplet does not carry information about any amino acid, but is recognized by the corresponding termination proteins, the process of polypeptide synthesis stops and it is detached from the template (mRNA).
Post-translational modification of a polypeptide is the final stage in the implementation of genetic information in a cell, leading to the transformation of the synthesized polypeptide into a functionally active protein molecule. In this case, the primary polypeptide can undergo processing, which consists in the enzymatic removal of initiating amino acids, the cleavage of other (unnecessary) amino acid residues and the formation of levels of structural organization, etc.
The processes of deamination, transamination and synthesis of amino acids, albumins and most of the blood serum globulins, prothrombin and fibrinogen occur in the liver. It is assumed that albumin and α-globulins are produced by polygonal liver cells, β- and γ-globulins are formed in RES, in particular in Kupffer cells of the liver and plasma cells of the bone marrow.
The leading role of the liver in protein metabolism explains the great interest of clinicians in methods for determining the parameters of this metabolism. These include, first of all, the determination of the total amount of plasma protein and its fractions, including prothrombin. Along with the definition of a proteinogram, one finds practical use and samples indicating only indirectly the presence of changes in blood proteins, including the manifestation of pathological proteins - paraproteins. These include tests for lability and colloidal tests.
Total protein in the plasma of healthy people is 7.0-8.5% (K. I. Stepashkina, 1963). A change in the total amount of protein is observed only in severe disorders of protein metabolism. In contrast, the change in the ratio of individual fractions is a very subtle indicator of the state of protein metabolism.
The most widely used in practice is the determination of protein fractions by electrophoresis on paper. The disadvantage of the latter is fluctuations in the results obtained depending on the method used. Therefore, the literature data on the normal proteinogram are not identical.
Table 7 shows the norm options described by various authors(according to V. E. Predtechensky, 1960).
With liver damage, the synthesis of albumin and α1-globulins in polygonal liver cells decreases, and the synthesis of β- and γ-globulins in Kupffer cells and periportal mesenchymal cells increases (as a manifestation of irritation of reticuloendothelial cells), resulting in quantitative changes in protein fractions - dysproteinemia.
For diffuse liver damage, both acute and chronic during their exacerbation, the following changes in the proteinogram are characteristic: a decrease in the amount of albumin and an increase in globulins. As for the latter, the γ-globulin fraction mainly increases, apparently due to the accumulation of antibodies similar in electrophoretic mobility to γ-globulins. The content of α2- and β-globulins increases less. The degree of change in the proteinogram is directly dependent on the severity of the disease. The exception is agammaglobulinemia in hepatic coma. The total amount of protein is usually slightly increased due to hyperglobulinemia.
When evaluating the proteinogram in patients with liver damage, one should not forget that with a large number of a wide variety of diseases, a significant change in protein fractions is observed, such as, for example, in collagenoses, kidney damage, myelomatosis, etc.
In liver diseases, changes occur in the blood coagulation system, and the determination of various blood coagulation factors is a test to evaluate functional state liver. The most characteristic changes in prothrombin and proconvertin.
Prothrombin(II coagulation factor) is a globulin; in the electrophoretic study of plasma, the prothrombin peak is located between albumins and γ-globulins. Prothrombin is formed in the liver cells with the participation of vitamin K. In the process of blood clotting, prothrombin turns into thrombin. The concentration of prothrombin in plasma is about 0.03%. In practice, it is not the absolute amount of prothrombin that is determined, but the "prothrombin time" and the prothrombin index. The most common method in the Soviet Union for determining the prothrombin index is the method of VN Tugolukov (1952). Normally, the prothrombin index is 80-100%.
The ability of hepatocytes to synthesize prothrombin in liver pathology may be impaired. In addition, liver damage is accompanied by a violation of the deposition of a number of vitamins in it, including vitamin K, which is also the cause of hypoprothrombinemia. Therefore, if a decrease in the prothrombin index is detected, a second study should be carried out after a 3-day vitamin K load - 0.015 vikasol 3 times a day. If the amount of prothrombin remains low, then this indicates damage to the liver parenchyma.
Another factor of the blood coagulation system, which naturally reacts to liver damage, is proconvertin (factor VII, a stable factor). Proconvertin catalyzes the action of thromboplastin, accelerating the formation of thrombin. This factor is formed in the liver, its content in plasma is 0.015-0.03%. The amount of proconvertin, like prothrombin, is expressed as an index. Proconvertin time is normally 30-35 seconds, the index is 80-120%.
When the liver parenchyma is damaged, both the prothrombin index and the proconvertin index decrease. There is a parallelism between these indicators and the severity of liver damage (K. G. Kapetanaki and M. A. Kotovshchikova, 1959; A. N. Filatov and M. A. Kotovshchikova, 1963).
Offered a large number various methods, indirectly determining the presence of dysproteinemia and paraproteinemia. All of them are based on the precipitation of pathological protein with various reagents.
The Takata-Ara test (sublimate test) is based on the precipitation of a flocculent precipitate of coarse proteins under the action of the Takata reagent containing sublimate. The reaction is assessed by the density of the precipitate or by the serum dilution at which turbidity occurred. The sample is considered positive if, in a row of tubes with Takata reagent and a decreasing amount of serum (1.0; 0.5; 0.25; 0.12 ml, etc.), a flocculent precipitate occurs in the first three or more tubes; if only in the first two - weakly positive. The test is positive with an increase in the content of γ-globulins in the blood, in particular with Botkin's disease, with cirrhosis of the liver, but also with a number of other diseases (pneumonia, syphilis, etc.).
One of the modifications of the Takata-Ara test is the Gross test (sublimate-sedimentary reaction), in which the results are expressed in milliliters of the sublimate reagent necessary to obtain a distinct turbidity. The norm is 2 ml or more. In case of liver diseases, the indicators of the Gross test decrease to 1.8-1.6 ml, with severe damage - to 1.4 ml and below.
The Veltman test is based on the coagulation of plasma proteins when heated in the presence of a calcium chloride solution of various concentrations (from 0.1 to 0.01%). Normally, coagulation occurs at a solution concentration higher than 0.04%, i.e., in the first 6-7 test tubes. For liver damage, the appearance of sediment at a lower concentration is characteristic - elongation of the coagulation "tape".
The cephalin test is based on the occurrence of flocculation of the cephalin-cholesterol emulsion in the presence of the patient's blood serum. The test has the advantage over those indicated above that it is sharply positive in the presence of necrosis in the liver parenchyma and therefore can be useful in determining the activity of the process in Botkin's disease and cirrhosis of the liver and in differential diagnosis between obstructive jaundice (in the early stages) and damage to the liver parenchyma.
The thymol turbidity test is based on the determination of the turbidity that occurs when the test serum is combined with the thymol reagent. The degree of turbidity is determined after 30 minutes and evaluated in a spectrophotometer or in a colorimeter. Using the standard turbidity curve, get the result in conventional units. The norm ranges from 0.8 to 5.0 units. With liver damage, the sample index increases, reaching 30-35 units. with Botkin's disease (Popper, Schaffner, 1961).
The thymol turbidity test can be continued as a thymol flocculation test: the flocculation occurring 24 hours after the serum has been combined with the thymol reagent is evaluated.
Residual blood nitrogen is normally 20-40 mg%. Severe azotemia (up to 100 mg% or more) occurs with severe liver damage (acute dystrophy in hepatitis, end-stage cirrhosis, liver failure after surgery on the liver and biliary tract) and indicates the development of liver failure.
serum ammonia is normally 40-100 ү%. Hyperammonemia is observed in liver failure, as well as in the presence of pronounced porto-caval anastomoses (developing naturally or created during surgery), through which blood flows from the intestine, bypassing the liver. The most pronounced increase in the amount of ammonia in the peripheral blood is observed in patients with liver failure after a protein load (eating a large amount of meat, blood entering the intestine during esophageal or gastric bleeding). To identify portal-hepatic insufficiency, a test with a load of ammonia salts can be applied (AI Khazanov, 1968).
Lipoproteins and glycoproteins*. Serum proteins form stable compounds with lipids and carbohydrates: lipo- and glycoproteins. Naturally, when the ratio of different fractions of plasma proteins changes, the content of complexes associated with them also changes.
During electrophoresis, lipoproteins are divided into fractions corresponding to α1-,β and γ-fractions of globulin. The y-fraction (“lipid residue”) includes protein compounds with neutral fat and cholesterol esters that are slightly mobile in an electric field. This fraction is of no practical interest, since the latter does not change under pathological conditions. Healthy individuals have the following percentage of α- and β-fractions, lipoproteins (I. E. Tareeva, 1962): α-lipoproteins - 29.0 ± 4.9; β-lipoproteins - 71.0 ± 4.9; ratio β/α-2.45 ± 0.61.
A relationship has been established between changes in the ratio of α- and β-lipoprotein fractions and the severity of damage to the liver parenchyma. There is no complete parallelism between the change in lipoproteinogram and other functional indicators. However, it should be noted that Botkin's disease and the active phase of liver cirrhosis are characterized by a decrease in the number of α-lipoproteins until they completely disappear on the lipid profile and an increase in β-lipoproteins with a corresponding increase in the β/α ratio by several times. In chronic liver damage, these changes are less pronounced.
Glycoproteins - compounds of various carbohydrates with proteins, mainly with globulins. The electrophoretic method gives the separation of glycoprotein fractions with the corresponding protein fractions. The synthesis of glycoproteins is carried out in the liver, therefore, an attempt to use the definition of glycoproteins with the aim of functional diagnostics. However, the data obtained by various authors in the examination of patients with liver pathology remain very contradictory. More characteristic is the increase in the fraction of α-glycoproteins (N. A. Zaslavskaya, 1961; I. D. Mansurova, V. I. Dronova and M. S. Panasenko, 1962).
* For the determination method, see: A.F. Blyuger. Structure and function of the liver in epidemic hepatitis. Riga, 1964.
Protein metabolism
Protein metabolism is the central link in all biochemical processes that underlie the existence of a living organism. The intensity of protein metabolism is characterized nitrogen balance, since the bulk of the body's nitrogen is in proteins. This takes into account feed nitrogen, body nitrogen and excretion nitrogen. The nitrogen balance can be positive (when there is an increase in the weight of the animal and nitrogen retention in the body), equal to zero, or nitrogen balance is observed (as much nitrogen is excreted from the body as it comes with feed), and negative (protein breakdown is not compensated by feed proteins). The nitrogen balance is characterized protein minimum- the smallest amount of protein in feed, which is necessary to maintain nitrogen balance in the body. The protein minimum, calculated per 1 kg of live weight, has the following average values, g:
| lactating cow | 1 |
| Non-lactating cow | 0,6-0,7 |
| Sheep | 1 |
| Goat | 1 |
| Pig | 1 |
| horse running | 1,24,42 |
| Horse not working | 0,7-0,8 |
Feed proteins are divided into full-fledged and defective. Complete feeds contain residues of essential amino acids that cannot be synthesized by the animal's body: valine, isoleucine, leucine, lysine, methionine, threonine, tryptophan and phenylalanine. The conditionally essential amino acids are
histidine, since its slight deficiency in feed is compensated by the synthesis of microflora in the alimentary canal. The remaining amino acids are non-essential and can be synthesized in the animal's body: alanine, aspartic and glutamic acids, series. Five amino acids are considered partially essential: arginine, glycine, tyrosine, cystine, and cysteine. The imino acids proline and hydroxyproline can be synthesized in the body.
AT various feeds and food products contain an unequal amount of proteins,%:
| Pea beans | 26 | feed yeast | 16 |
| soy beans | 35 | Potato | 2,0-5 |
| grain of wheat | 13 | Cabbage | 1,1-1,6 |
| Corn grain | 9,5 | Carrot | 0,8-1 |
| grain of rice | 7,5 | Beet | 1,6 |
Animal products are rich in complete proteins,%:
| Lean beef meat | 21,5 | Cottage cheese | 14,6 |
| Lean lamb | 19,8 | Cheese | 20-36 |
| Fat lamb | 25 | Chicken egg | 12,6 |
| Pork fat | 16,5 | cow's milk | 3,5 |
| Fish | 9-20 | Cow butter | 0,5 |
The standard of complete protein is most often casein, which contains all the essential amino acids.
Digestion of proteins. In the alimentary canal, proteins are broken down into amino acids and prostatic groups.
AT oral cavity feed containing proteins is mechanically crushed, moistened with saliva and form a food lump, which enters the stomach through the esophagus (in ruminants - into the proventriculus and abomasum, in birds - into the glandular and muscular stomachs). Saliva contains no enzymes capable of breaking down food proteins. The chewed feed masses enter the stomach (in ruminants into the abomasum), are mixed and soaked in gastric juice.
Gastric juice- colorless and slightly opalescent liquid with a density of 1.002-1.010. In a person, about 2 liters are formed during the day, in a large cattle- 30, a horse - 20, a pig - 4, a dog - 2-3, a sheep and a goat - 4 liters of gastric juice. The secretion of gastric juice in the first
(complex-reflex) phase is determined by the type, smell and taste of food, in the second (neurohumoral) phase - by its chemical composition and mechanical irritation of mucosal receptors. The gastric juice contains 99.5% water and 0.5% solids. Dense substances include the enzymes pepsin, rennin, gastrixin, gelatinase, lipase (in pigs and amylase); proteins - serum albumins and globulins, mucoproteins of mucus, Castle factor; from minerals acids (mainly hydrochloric) and salts.
The main enzyme of gastric juice is pepsin, and the acid that creates the conditions for its catalytic action is hydrochloric. The main cells of the glands of the fundus of the stomach are involved in the formation of pepsin, and the parietal cells are involved in the formation of hydrochloric acid. The source of chloride ions is NaCl, H + ions are protons coming from the blood into the cytoplasm of parietal cells due to redox reactions (G. D. Kovbasyuk, 1978).
Hydrochloric acid creates the necessary acidity for the catalytic action of enzymes. So, in humans, the pH of gastric juice is 1.5-2.0, in cattle - 2.17-3.14, in a horse - 1.2-3.1, in a pig - 1.1-2.0 , in a sheep - 1.9-5.6, in birds - 3.8. Hydrochloric acid also creates conditions for the conversion of pepsinogen into pepsin, accelerates the breakdown of proteins into their constituent parts, their denaturation, swelling and loosening, prevents the development of putrefactive and fermentation processes in the stomach, stimulates the synthesis of intestinal hormones, etc. In laboratory practice, determine the total, free and bound acidity of gastric juice.
Rennin (chymosin, or rennet) is produced in young ruminants by glands in the rennet mucosa. Synthesized as prorennin, which at pH
AT stomach hydrolytic breakdown of most feed proteins occurs. So, nucleoproteins under the influence of hydrochloric acid and pepsin break down into
nucleic acids and simple proteins. It also breaks down other proteins. Under the influence of pepsin, peptide bonds are cleaved along the edges of protein molecules. The bonds formed by aromatic and dicarboxylic amino acids are most easily broken. Pepsin easily breaks down animal proteins (casein, myoglobin, myogen, myosin) and some vegetable proteins built mainly from monoamino dicarboxylic acids (gliadin and glutelin of cereals), with the exception of wool keratins, silk fibroins, mucus mucins, ovomucoids, some bone proteins and cartilage.
Some of the proteins are cleaved by other proteolytic enzymes of gastric juice, for example, collagens - by gelatinase, caseins - by rennin.
Under the influence of the components of gastric juice, primarily hydrochloric acid and enzymes, proteins in the stomach are hydrolyzed to prosthetic groups, albumin, peptones, polypeptides, and even amino acids.
Gastric secretion is stimulated by hormones of the mucous membrane of the alimentary canal: gastrin (in the pylorus), enterogastrin (in the intestines), histamine (in the stomach), etc.
Peculiarities of protein digestion in ruminants. In ruminants, the food lump from the esophagus enters the proventriculus, where it undergoes additional mechanical processing, when chewing, it returns to the oral cavity, crushed again, then enters the scar, mesh, book and abomasum, where the first stage of digestion is completed.
In the proventriculus, chemical processing of feed substances occurs under the influence of enzymes of bacteria, ciliates and fungi that symbiote there. Up to 38% of cattle rumen microbes and 10% of sheep rumen microbes have proteolytic activity, 70-80% of these enzymes are concentrated inside the cells, 20-30% in the rumen fluid. Enzymes act similarly to trypsin, cleaving peptide bonds between the carboxyl group of arginine or lysine and the amino group of other amino acids at pH 5.5-6 and pH 6.5-7. Proteins under the influence of peptide hydrolases are cleaved to peptides, peptides by peptidases - to oligopeptides, oligopeptides - to amino acids. So, corn zein is hydrolyzed by 60% to amino acids, and
casein - by 90%. Some of the amino acids are deaminated by bacterial enzymes.
A remarkable feature of digestion in the proventriculus is the synthesis of proteins by microorganisms from non-protein substances of the feed and products of its processing. The bulk of plant foods is represented by carbohydrates, and primarily fiber. Cellulose in the pancreas under the influence of microbial enzymes cellulase and cellobiase is broken down to α-D(+)-glucose and β-D(+)-glucose.
Monoses undergo various types of fermentation, which leads to the formation of low molecular weight fatty acids. So, with lactic acid fermentation caused by Bact. lactis, lactic acid is formed from glucose: C 6 H 12 O 6 → 2CH 3 →CHOH - COOH. During butyric fermentation caused by bacteria of the genus Clostridium, butyric acid is formed: C 6 H 12 O 6 → CH 3 - CH 2 - CH 2 - COOH + 2H 2 + 2CO 2, etc.
The amount of volatile fatty acids in the rumen of a cow can reach 7 kg per day. With a hay-concentrated diet, the rumen of cows contains: acetic acid - 850-1650 g, propionic acid - 340-1160, butyric acid - 240-450 g.
In terms of acetic acid in the rumen of a sheep, 200-500 g of volatile fatty acids are formed per day. Their percentage is as follows:
Some of these acids are used for the synthesis of milk fat, glycogen and other substances (Fig. 22), some serve as material for the synthesis of amino acids and their own protein by the microflora.
The synthesis of amino acids by the microflora in the proventriculus of ruminants occurs due to nitrogen-free fermentation products and ammonia. The source of ammonia is the breakdown products of urea, ammonium salts and
other nitrogen-containing dietary supplements. Thus, urea, under the influence of the urease enzyme produced by the rumen microflora, is broken down to ammonia and carbon dioxide:
The source of nitrogen-free products is most often keto acids, which were formed from fatty acids (see above). This biosynthesis is usually in the nature of reductive amination:
From amino acids, microorganisms synthesize proteins necessary for their existence. Depending on the diet, 300-700 g of bacterial protein per day can be synthesized in the rumen of cows.
From the proventriculus, the feed masses enter the abomasum, where, under the influence of acidic rennet juice, microorganisms die, and their proteins are broken down into amino acids.
From the stomach (abomasum), feed masses in small portions enter the small intestine where protein digestion is completed. It involves proteolytic enzymes of pancreatic secretion and intestinal juice. These reactions take place in a neutral and slightly alkaline medium (pH 7-8.7). In the small intestine, bicarbonates of pancreatic secretion and intestinal juice neutralize hydrochloric acid: HCl + NaHCO 3 → NaCl + H 2 CO 3 .
Carbonic acid, under the influence of the enzyme carbonic anhydrase, is broken down to CO 2 and H 2 O. The presence of CO 2 contributes to the formation of a stable emulsion in chyme, which facilitates digestion.
About 30% of protein peptide bonds are cleaved by trypsin. It is released as an inactive trypsinogen and, under the influence of the intestinal mucosal enzyme enterokinase, turns into active trypsin, losing the hexapeptide that previously closed the active center (Fig. 23). Trypsin cleaves peptide bonds formed by - COOH groups of arginine and lysine and - NH 2 groups of other amino acids.
Almost 50% of peptide bonds are cleaved by chymo-trypsin. It is secreted in the form of chymo-trypsinogen, which under the influence of trypsin is converted into chymo-trypsin. The enzyme cleaves peptide bonds formed by - COOH groups of phenylalanine, tyrosine and tryptophan and - NH 2 groups of other amino acids. The remaining peptide bonds are cleaved by peptidases of intestinal juice and pancreatic juice - carboxypeptidases and aminopeptidases.
Pancreatic juice contains collagenase (breaks down collagen) and elastinase (hydrolyzes elastin). The activity of enzymes is activated by microelements: Mg 2+, Mn 2+, Co 2+, etc. The final stage protein digestion reflects the scheme:
Digestion of proteins occurs in the intestinal cavity and on the surface of the mucous membrane (parietal digestion).
In the intestinal cavity, protein molecules are cleaved, and on the surface of the mucous membrane - their "fragments": albumoses, peptones, polypeptides, tripeptides and dipeptides.
Proteins and their derivatives that have not undergone cleavage in the small intestine, further into colon are subject to decay. Rotting - multi-stage
a process in which various microorganisms participate at certain stages: anaerobic and aerobic bacteria of the genera Bacillus and Pseudomonas, ciliates, etc. Under the influence of bacterial peptide hydrolases, complex proteins are split into proteins and prosthetic groups. Proteins, in turn, are hydrolyzed to amino acids, and they undergo deamination, decarboxylation, intramolecular cleavage, oxidation, reduction, methylation, demethylation, etc. A number of toxic products arise that are absorbed through the intestinal mucosa into the circulatory and lymphatic systems and are carried throughout the body, poisoning its organs, tissues and cells.
Thus, during decay in the colon, amino acids undergo decarboxylation, which leads to the formation of toxic amines, such as cadaverine and putrescine.
During deamination (reductive, intramolecular, hydrolytic, oxidative) ammonia, saturated and unsaturated carboxylic acids, hydroxy acids and keto acids are formed.
Bacterial decarboxylases can cause further decomposition of carboxylic acids with the formation of hydrocarbons, aldehydes, alcohols, etc.: CH 3 -CH 2 - COOH → CH 3 -CH 3 + CO 2;
These processes usually proceed conjugately and in stages, which ultimately leads to the emergence of a wide variety of decay products. So, during the putrefactive decomposition of cyclic amino acids, the following phenols are formed.
Putrefactive decomposition of tryptophan produces skatole and indole.
During the putrefactive decomposition of cystine and cysteine, mercaptans, hydrogen sulfide, methane, carbon dioxide.
The processes of protein decay are intensively developed when animals are fed poor-quality feed, violation of the feeding regimen, in diseases of the alimentary canal (atony of the proventriculus, constipation), infectious (colibacillosis) and parasitic (ascariasis) diseases. This negatively affects the health and productivity of animals.
Absorption of proteins. Proteins are absorbed in the form of amino acids, low molecular weight peptides and prosthetic groups. In newborn animals, part of the unsplit proteins of colostrum and milk is absorbed. Place of absorption - microvilli of the villi of the epithelium of the mucous membrane small intestine. Amino acids penetrate into the cell through the submicroscopic tubules of the microvilli and the exoplasmic membrane due to the processes of diffusion, osmosis, with the help of protein carriers against concentration and electrochemical gradients. First of all, the amino acid binds to the carrier. It is a polyvalent ion that has four sites for
binding with neutral, acidic and basic amino acids, as well as with the Na + ion. After passing through the membrane, the amino acid is cleaved from the carrier and gradually moves along the endoplasmic reticulum and lamellar complex from the apical edge to the basal area of the enterocyte (Fig. 24). Arginine, methionine, leucine are absorbed faster; slower - phenylalanine, cysteine, tyrosine; slowly - alanine, series and glutamic acid.
In the processes of absorption, an important place belongs to the sodium pump, since sodium chloride accelerates absorption.
Mitochondria provide the chemical energy expended in this process.
A protein carrier is involved in the movement of amino acids around the cell. In the basal and lateral parts of the cell, the carrier + amino acid complex is cleaved.
The amino acid diffuses into the intercellular space and enters the circulatory or
the lymphatic system of the villi, and Na + ions return to the cell surface and interact with new portions of amino acids. These processes are regulated by the nervous and humoral systems.
In the colon, decay products are absorbed: phenol, cresol, indole, skatole, etc.
intermediate exchange. The products of protein absorption through the portal vein enter the liver. The amino acids remaining in the blood after passing through the liver from the hepatic vein enter big circle blood circulation and are carried to individual organs, tissues and cells. Some of the amino acids from the intercellular fluid enters lymphatic system, then a large circle of blood circulation.
Blood plasma contains a certain amount of amino acids and polypeptides. Their content increases after feeding.
Blood plasma is rich in glutamine and glutamic acid.
Most of the amino acids are consumed for protein biosynthesis, some - for biosynthesis biologically. active substances(non-protein hormones, peptides, amines, etc.), a part, being deaminated, is used as an energy raw material and material for the biosynthesis of lipids, carbohydrates, nucleic acids, etc.
Protein biosynthesis
Protein biosynthesis occurs in all organs, tissues and cells. The largest number protein is synthesized in the liver. Its synthesis is carried out by ribosomes. By chemical nature, ribosomes are nucleoproteins consisting of RNA (50-65%) and proteins (35-50%).
Ribosomes are formed by self-assembly from pre-synthesized RNA and proteins. They are components of the granular endoplasmic reticulum, where biosynthesis and movement of synthesized protein molecules take place.
Ribosomes in the cell are in the form of accumulations from 3 to 100 units - polysomes (polyribosomes, ergosomes). Ribosomes are usually interconnected by a kind of thread visible under an electron microscope - mRNA (Fig. 25).
Each ribosome is capable of synthesizing
independently one polypeptide chain, a group - several such chains and protein molecules. An example of a large polyribosomal system can be polysomes of muscle tissue synthesizing myosin. The polysome consists of 60-100 ribosomes and carries out the biosynthesis of a protein molecule, which consists of 1800 amino acid residues.
Protein biosynthesis in a cell proceeds through a series of stages.
Amino acid activation. Amino acids enter the hyaloplasm from the intercellular fluid as a result of diffusion, osmosis or active transfer. Each type of amino and imino acids interacts with its activating enzyme - aminoacyl synthetase. The reaction is activated by Mg 2+ , Mn 2+ and Co 2+ cations. An activated amino acid is produced.
Connection of activated amino acids with tRNA. At the second stage of protein biosynthesis, activated amino acids (aminoacyladenylates) from their compounds with
The corresponding enzymes are transferred to the cytoplasmic tRNA. The process is catalyzed by aminoacyl-RNA synthetases.
The amino acid residue is connected by a carboxyl group to the hydroxyl of the second carbon atom of the ribose of the tRNA nucleotide.
Transportation of an activated amino acid complex with tRNA to the cell ribosome. The activated amino acid, combined with its tRNA, is transferred from the hyaloplasm to the ribosome. The process is catalyzed by specific enzymes, of which there are at least 20 in the body,
A number of amino acids are transported by several tRNAs (for example, valine and leucine are transported by three tRNAs). This process uses the energy of GTP and ATP.
Binding of aminoacyl-tRNA to the mRNA-ribosome complex. Aminoacyl-tRNA, approaching the ribosome, interacts with mRNA. Each tRNA has a region that consists of three nucleotides, - antigsodon. In mRNA, it corresponds to a region with three nucleotides - codon. Each codon corresponds to a tRNA anticodon and one amino acid. During biosynthesis, amino acids are attached to the ribosome in the form of aminoacyl-tRNA, which are subsequently combined into a polypeptide chain in the order determined by the placement of ko-dons in mRNA.
Polypeptide chain initiation. After two neighboring aminoacyl-tRNAs have joined the mRNA codons with their anticodons, conditions are created for the synthesis of a polypeptide chain. The first peptide bond is formed. These processes are catalyzed by peptide synthetases, activated by Mg 2+ cations and protein initiation factors - F 1 , F 2 and F 3 . The source of chemical energy is
GTP. The connection arises due to the CO group of the first and NH 2 group of the second aminoacyl-tRNA.
These reactions take place on the free 30S subunit. The 50S subunit attaches to the initiation complex, and they combine to form a ribosome associated with mRNA. Each initiation step requires one molecule of GTP.
elongation of the polypeptide chain. The initiation of the polypeptide chain starts from the N-terminus, since the -NH 2 group of the first amino acid is retained in the resulting dipeptide. The first tRNA, which brought its amino acid, splits off from the mRNA-ribosome complex and "goes" to the hyaloplasm for a new amino acid. The dipeptide associated with the second tRNA (see above) interacts with the third amino-acyl-tRNA, a tripeptide is formed, and the second tRNA descends from the ribosome into the hyaloplasm, etc. The peptide chain lengthens (elongates) as a result of the successive addition of new amino acid residues . The ribosome gradually moves along the mRNA, turning the information encoded in it into a clearly organized polypeptide chain. With each step of the ribosome, a new peptidyl-tRNA is formed, increased by one amino acid residue. The process is catalyzed by peptidyl transferase and activated by Mg 2+ cations and protein factors (EF-Tu, EF-Ts, EF-G). The source of energy is GTP. Several peptide chains are synthesized synchronously on the polysome. This is how the primary structure of the protein molecule is created.
Termination of the polypeptide chain. The ribosome, on the surface of which the polypeptide chain was synthesized, reaches the end of the mRNA chain and "jumps" off it; a new ribosome joins the opposite end of the mRNA in its place, carrying out the synthesis of the next polypeptide molecule. The polypeptide chain is detached from the ribosome and released into the hyaloplasm. This reaction is carried out with the help of a specific release factor (factor R), which is associated with the ribosome and facilitates the hydrolysis of the ester bond between the polypeptide and tRNA. All stages are summarized in the scheme (color, Table III).
In the hyaloplasm, simple and complex proteins are formed from polypeptide chains. Secondary, tertiary and, in some cases, quaternary structures of the protein molecule are formed.
Renewal of proteins in the body. Proteins are in a dynamic state, undergoing constant processes of synthesis and decay. In the course of life, they gradually "wear out" - their quaternary, tertiary, secondary and primary structures are destroyed. Protein functional groups are inactivated and bonds in the protein molecule are destroyed. There is a need to replace "worn out" protein molecules with new ones.
Depending on the degree of damage to the protein molecule, its partial or complete renewal occurs. In the first case, under the influence of special enzymes, small sections of polypeptide chains or individual amino acid residues are renewed (transpeptidation). In the second case, the "worn out" protein molecule is completely replaced by a new one. The damaged protein molecule decomposes under the influence of tissue proteases or cathepsins I, II, III, and IV localized in lysosomes. The protein molecule undergoes the usual transformations for these substances.
Proteins of the human body as a whole are updated within 135-155 days. Proteins of the liver, pancreas, walls of the intestines and blood plasma are updated within 10 days, muscles - 30, collagen - 300 days. The synthesis of a protein molecule in a cell proceeds quickly - within 2-5 s. In the body of an adult, 90-100 g of protein is synthesized daily (1.3 g per 1 kg
masses). The degree of renewal decreases with aging, disease, etc.
Biosynthesis of peptides
Part of the endo- and exogenous amino acids goes to the synthesis of peptides.
Glutathione. It is a tripeptide formed from residues of glutamic acid, cysteine and glycine.
Biosynthesis proceeds in two stages. So, at first under the influence of the enzyme γ -glutamylcysteine synthetase forms a dipeptide-, then with the participation of the tripeptide - synthetase - tripeptide-glutathione:
It is an integral part of many enzymes, protects the SH-groups of proteins from oxidation.
carnosine and anserine. Dipeptides of muscle tissue. Carnosine is formed from histidine and β -alanine, anserine - from 1-methylhistidine and β -alanine.
Peptides are synthesized under the influence of specific enzymes, with the participation of ATP and Mg 2+ ions. The reactions proceed in two stages, for example, the synthesis of carnosine.
Biosynthesis and metabolism of individual amino acids
Non-essential amino acids are synthesized in body tissues; irreplaceable enter the body as part of the feed; conditionally replaceable ones are synthesized in tissues to a limited extent (arginine and histidine) or in the presence of precursors (tyrosine and cysteine). Some amino acids are synthesized by symbiotic microflora in the alimentary canal.
The most common material for the synthesis of amino acids is α -keto- and α -hydroxy acids, which are formed in tissues during the intermediate exchange of carbohydrates, lipids and other compounds. The source of nitrogen is ammonia and ammonium salts, hydrogen - NAD ∙ H 2 or NADP ∙ H 2.
If the source of the amino acid is a keto acid, then it can undergo reductive amination, which proceeds in two stages: first, an imino acid is formed, then an amino acid.
This is how alanine is formed from pyruvic acid, aspartic and glutamic acids from oxaloacetic acid, etc.
Part of glutamic acid can be synthesized from α -ketoglutaric acid by enzyme action L-glutamate dehydrogenase.
Glutamic acid is used by tissues as an amino group donor.
Individual amino acids can be formed from other amino acids by transamination (A. E. Braunshtein and M. G. Kritzman, 1937) under the influence of aminoferase enzymes, which include a derivative of vitamin B 6 - pyridoxal phosphate, which plays the role of a carrier of NH 2 groups (p. 271).
This is how glycine is formed from serine or threonine; alanine - from glutamic and aspartic acids, tryptophan or cysteine; tyrosine from phenylalanine; cysteine and cystine - from serine or methionine; glutamic acid is formed from proline or arginine, etc.
The exchange of individual amino acids has certain features.
Glycine. Participates in a number of important reactions of biosynthesis. So, from it are formed:
In the tissues of the liver, glycine is involved in the process of neutralizing toxic compounds - benzoic,
phenylacetic acids and phenols, forms paired compounds that are excreted in the urine.
Alanine. Formed by transamination of pyruvic acid (see above). Exists in the form α - and β -forms. Participates in biosynthesis.
Aspartic acid. It is usually formed by transamination of oxaloacetic acid (see above). Together with glutamic acid, it provides the relationship between the metabolism of proteins, carbohydrates and lipids. Serves as a donor of amino groups in
transamination reactions. The main reactions are reflected in the scheme.
Glutamic acid. It is found in tissues as part of proteins, in a free state and as an amide. Amino group donator in transamination reactions. The main substances in the synthesis of which acid is involved:
Serine and threonine. Their metabolism is closely related to the metabolism of glycine. Serine in tissues is formed from 3-phosphoglyceric acid. Glycine is formed from serine as a result of the transfer of a one-carbon fragment (C 1) to tetrahydrofolic acid (THFA, see p. 311). Glycine can be formed from threonine. Fragment C 1 is used for the synthesis of histidine and purines. From serine and threonine, pyruvic acid is formed, which, with the help of acetyl-CoA, is included in the TCA.
Part of the transformations reflects the scheme:
The hydroxyl group of serine is part of the active center of many enzymes: trypsin, chymo-trypsin, esterases, phosphorylases.
Methionine. It is a component of many proteins. Serves as a donor for a metal band. The transfer of the methyl group during remethylation occurs under the influence of the corresponding methyl transferases through S-adenosylmethionine:
The precursor of methionine is aspartic acid, which through several stages (homoserine, 0-succinyl-homoserine, cysteine, cystathionine, homocysteine) is converted into methionine.
cysteine and cystine. Components of many proteins, peptides, hormones and other compounds. The SH-group of cysteine is an integral part of the active centers of a number of enzymes. The participation of cysteine in metabolism partially reflects the scheme:
Arginine and ornithine. Arginine is formed in the process of converting carbon dioxide and ammonia into urea.
Both amino acids are involved in the formation of a number of vital substances.
Lysine. Essential amino acid. Participates in the synthesis of many substances.
The Σ-amino group of the lysine residue is involved in the formation of a bond between apo- and coenzymes, especially during the formation of a biotinenzyme. Lysine plays an important role in the binding of phosphorus during mineralization bone tissue and other processes.
Phenylalanine and tyrosine. Their transformations in the body go in the following directions: the biosynthesis of proteins and peptides, the formation
proteinogenic amines, hormones and pigments, oxidation to end products with nuclear rupture, etc.:
Tryptophan. Essential amino acid. Its transformations are illustrated by the diagram:
Histidine. Refers to essential amino acids. Participates in the biosynthesis and metabolism of many vital substances:
Proline and hydroxyproline. Hydroxyproline arises from proline. The process is irreversible. Both imino acids are used for the biosynthesis of proteins, etc.
Transformation of the nitrogen-free residue of amino acids
Part of the amino acids not used in the synthesis of proteins and their derivatives undergoes decomposition processes to ammonia and carboxylic acids. Ammonia is neutralized in the liver in the ornithine cycle. Of several types of deamination, oxidative deamination predominates. The resulting keto acids are used by tissues for various needs. According to the direction of use of the nitrogen-free residue, amino acids are divided into two types: glucoplastic and lipoplastic. From glucoplastic amino acids (alanine, serine, cysteine, etc.), pyruvic acid is usually formed, which serves as the starting material for the biosynthesis of glucose and glycogen.
From lipoplastic amino acids (leucine, isoleucine, arginine, ornithine, lysine, etc.), after deamination, acetoacetic acid is formed - a source of biosynthesis of higher fatty acids.
α -Keto acids formed during the oxidative deamination of amino acids are decarboxylated and simultaneously oxidized to fatty acids.
The resulting fatty acid can undergo β -oxidation, acetyl-CoA appears - a source of chemical energy or raw material for the biosynthesis of many substances.
Features of the intermediate exchange of complex proteins
The biosynthesis of complex proteins proceeds similarly to the biosynthesis of proteins. In this case, the primary, secondary, tertiary and quaternary structures of the protein molecule are formed with the addition of the corresponding prosthetic group.
Exchange of chromoproteins. The body of animals contains a number of chromoproteins: hemoglobin, myoglobin, cytochromes, hemic enzymes, etc.
They are characterized by the presence of heme in the composition of the molecule. The biosynthesis of hemoglobin has been studied in the most detail.
The main components of the hemoglobin molecule are formed in the hematopoietic organs: red bone marrow, spleen, liver. Globin is synthesized from amino acids in the usual way for proteins. Heme formation occurs with the participation of enzymes through a series of stages.
from two molecules δ -aminolevulinic acid, porphobilinogen is formed, which contains a pyrrole ring.
Porphobilinogen then forms a cyclic compound of four pyrrole rings - uroporphyrin.
In further transformations, protoporphyrin is formed from uroporphyrin. Under the influence of the hemosynthetase enzyme, iron (Fe 2+) is included in the protoporphyrin molecule and heme appears, which, through the histidine residue, binds to the simple protein globin, forming a subunit of the hemoglobin molecule.
Hemoglobin makes up 90-95% of the dry mass of red blood cells.
Metabolism of lipoproteins, glycoproteins and phosphoproteins is not much different from the exchange of simple proteins. Their synthesis proceeds similarly to other proteins - with the formation of primary, secondary, tertiary and quaternary structures. The difference lies in the fact that during synthesis, different prosthetic groups are attached to the protein part of the molecules. During the breakdown of a complex protein molecule, the protein part is split into amino acids, and the prosthetic groups (lipid, carbohydrate, phosphoric esters of amino acids) are split into simple compounds.
Final exchange. During the intermediate exchange, a series is formed chemical compounds, which are excreted from the body as protein breakdown products. In particular, carbon dioxide is excreted by the lungs, water - by the kidneys, with sweat, in the feces, with exhaled air. Many other products of protein metabolism, especially nitrogenous ones, are excreted in the form of urea, paired compounds, etc.
Ammonia conversion. Ammonia is formed during the deamination of amino acids, purine and pyrimidine bases, nicotinic acid and its derivatives, and other nitrogen-containing compounds. During the day, 100-120 g of amino acids are deaminated in the human body, 16-19 g of nitrogen or 18-23 g of ammonia are formed. Basically, ammonia in the body of farm animals is neutralized in the form of urea, partially - in the form of allantoin, uric acid and ammonium salts. In birds and reptiles, uric acid is the main end product of nitrogen metabolism.
Urea- the main end product of nitrogen metabolism in most vertebrates and humans. It makes up 80-90% of all nitrogenous substances in urine. Created modern theory the formation of urea in the liver - the ornithine Krebs cycle.
1. NH 3 and CO 2 cleaved off during deamination and decarboxylation under the influence of the enzyme carbamoyl phosphate synthetase combine to form carbamoyl phosphate.
2. Carbamoyl phosphate with ornithine with the participation of ornithinecarbamoyltransferase form citrulline.
3. Under the influence of argininosuccinate synthetase, it interacts with aspartic acid, forming argininosuccinic acid.
4. Argininosuccinic acid is broken down into arginine and fumaric acid under the influence of argininosuccinate lyase.
5. Under the influence of arginase, arginine is broken down into ornithine and urea, which is removed from the body with urine and sweat:
Ornithine reacts with new portions of carbamoyl phosphate, and the cycle repeats.
Part of the ammonia in the tissues is bound in the process formation of amides - asparagine or glutamine that are transported to the liver. In the liver, they are hydrolyzed, after which urea is formed from ammonia. Some ammonia is used by tissues for reductive amination of keto acids, resulting in the formation of amino acids.
In addition, in the tissues of the kidneys, ammonia is involved in the process of neutralizing organic and inorganic acids:
Transformations of other end products of protein metabolism. In the process of protein metabolism, other end products of metabolism are also formed, in particular derivatives of purine and pyrimidine bases, gases (excreted during defecation), phenols, indole, skatole, sulphuric acid and others. Especially many of these substances are formed in the colon during the decay of proteins.
These toxic compounds are neutralized in the liver by the formation of so-called paired acids, which are excreted in urine, partly in sweat and feces.
Indole and skatole, formed during the putrefactive decomposition of tryptophan, are converted into indoxyl and skatoxyl. They form paired compounds with glucuronic or sulfuric acids.
Transformations of the breakdown products of chromoproteins. When splitting chromoproteins, globin and heme are formed. Globin undergoes the usual transformations typical of proteins. Heme serves as a source of education
pigments in bile, urine and feces. Hemoglobin is oxidized to verdohemoglobin(choleglobin). Verdohemoglobin loses the protein part and iron atoms, which leads to the formation of a green substance - biliverdin. Biliverdin is reduced to a red pigment - bilirubin. Formed from bilirubin mesobilirubin, which after the next restoration becomes urobilinogen. Urobilinogen is converted to fecal pigments in the intestines. stercobilinogen and stercobilin, in the kidneys - in the urine pigment urobilin.
The breakdown products of heme are used by the body for various needs. So, iron is deposited in organs as part of ferritins. Biliverdin and bilirubin are bile pigments, the rest of the substances are pigments in urine and feces. The breakdown of myoglobin proceeds in a similar way.
Regulation of protein metabolism. A special place in the regulation belongs to the cortex hemispheres brain and subcortical centers. The hypothalamus contains a center for protein metabolism. Regulation is carried out reflexively, in response to irritations.
The action of hormones on protein biosynthesis is carried out by stimulating the formation of mRNA. Somatotropin enhances the synthetic processes of the protein. Protein biosynthesis is activated by insulin, some
andro- and estrogens, thyroxin. Glucocorticoids of the adrenal cortex stimulate the breakdown of proteins and the release of nitrogenous substances.
The effect of hormones on protein metabolism is associated with a change in the rate and direction of enzymatic reactions. Biosynthesis and, consequently, the activity of enzymes involved in the metabolism of proteins, depends on the presence of a sufficient amount of vitamins in the feed. In particular, pyridoxal phosphate is a coenzyme of amino acid decarboxylases, vitamin B 2 is an integral part of the amino oxidase coenzyme, vitamin PP is the basis of glutamic acid dehydrase, proline and hydroxyproline biosynthesis cannot take place without vitamin C, etc.
Pathology of protein metabolism. Protein metabolism is disturbed during infectious, invasive and non-communicable diseases. The cause of protein metabolism disorders is an incorrectly formulated diet, feeding poor-quality feed, non-compliance with the feeding regimen, etc. This leads to a decrease in the productivity of animals, deterioration in their health, and sometimes death.
The pathology of protein metabolism manifests itself in various forms.
Protein starvation. There are two types of protein starvation: primary, when there is not enough essential amino acids in the feed, and secondary, caused by diseases of the alimentary canal, liver, pancreas. In animals, growth slows down, general weakness, swelling appear, bone formation is disturbed, loss of appetite, and diarrhea are observed. A negative nitrogen balance occurs, hypoproteinemia sets in (the content of proteins in the blood decreases by 30-50%).
Violation of amino acid metabolism. It appears in several forms. So, in some liver diseases (hepatitis, cirrhosis, acute yellow dystrophy), the content of amino acids in the blood and urine increases sharply - alkaptonuria occurs. In particular, if tyrosine metabolism is disturbed, alkaptonuria develops, accompanied by a sharp darkening of the urine after standing in the air. With cystinosis, cystine is deposited in the liver, kidneys, spleen, lymph nodes, intestines and
there is an excess of cystine in the urine (cystinuria). With phenylketonuria, a large amount of phenylpyruvic acid appears in the urine. Often the cause of such violations are beriberi.
Violation of the metabolism of complex proteins. Most often they manifest themselves in the form of disorders of nucleic and porphyrin metabolism. In the latter case, the exchange of hemoglobin, myoglobin and other proteins is disturbed. So, with various liver lesions (hepatitis, fasciolosis, etc.), hyperbilirubinemia occurs - the content of bilirubin in the blood increases to 0.3 - 0.35 g / l. Urine becomes dark, large amounts of urobilin appear in it, urobilinuria occurs. Sometimes there is porphyria - an increase in the blood and tissues of the content of porphyrins. This leads to porphynuria and the urine turns red.
test questions
1. What are proteins, what are their significance, chemical composition, physiochemical properties, structure (primary, secondary, tertiary, quaternary)? Their classification.
2. Give a description of the main groups and subgroups of amino acids, give the structural formulas of the most important of them, analyze their properties.
3. What is nitrogen balance, protein minimum, complete and incomplete proteins, nonessential, conditionally nonessential and irreplaceable amino acids? Write the formulas for essential amino acids.
4. Analyze the main stages of protein metabolism in the body various kinds farm animals - digestion, absorption, intermediate (biosynthesis and decay) and final exchanges.
5. How is protein metabolism regulated in animals and what is the pathology of protein metabolism?
In the body of an adult, nitrogen metabolism in general balanced, that is, the amounts of incoming and outgoing protein nitrogen are approximately equal. If only part of the newly supplied nitrogen is released, the balance positive. This is observed, for example, during the growth of the organism. Negative balance is rare, mainly as a consequence of diseases.
Proteins obtained from food undergo complete hydrolysis in the gastrointestinal tract to amino acids, which are absorbed and distributed in the bloodstream in the body (see). 8 out of 20 protein amino acids cannot be synthesized in the human body (see). These essential amino acids must come with food (see).
Through the intestines and in a small amount also through the kidneys, the body constantly loses protein. In connection with these inevitable losses, it is necessary to receive at least 30 g of protein with food daily. This minimum norm is hardly observed in some countries, while in industrialized countries the protein content in food is most often significantly higher than the norm. Amino acids are not stored in the body, with an excess intake of amino acids in the liver, up to 100 g of amino acids per day are oxidized or used. The nitrogen they contain turns into urea (see) and is excreted in the urine in this form, and the carbon skeleton is used in the synthesis of carbohydrates, lipids (see) or is oxidized to form ATP.
It is assumed that in the body of an adult, 300-400 g of protein is destroyed daily to amino acids ( proteolysis). At the same time, approximately the same number of amino acids is included in the newly formed protein molecules ( protein biosynthesis). A high turnover of protein in the body is necessary because many proteins are relatively short-lived: they begin to renew several hours after synthesis, and the biochemical half-life is 2-8 days. Even shorter lived key enzymes intermediate exchange. They are updated several hours after synthesis. This constant destruction and resynthesis allows the cells to rapidly adjust the levels and activities of the most important enzymes to meet metabolic needs. In contrast, structural proteins, histones, hemoglobin, or components of the cytoskeleton are particularly durable.
Almost all cells are capable of biosynthesis proteins (top left in the diagram). Construction of a peptide chain by broadcasts on the ribosome is considered on in the articles,. However, the active forms of most proteins appear only after a series of further steps. First of all, with the help of auxiliary chaperone proteins, a biologically active conformation of the peptide chain ( clotting, cm. , ). With post-translational ripening in many proteins, parts of the peptide chain are removed or added additional groups such as oligosaccharides or lipids. These processes occur in the endoplasmic reticulum and in the Golgi apparatus (see). Finally, proteins must be transported to the appropriate tissue or organ ( sorting, cm. ).
Intracellular protein breakdown ( proteolysis) occurs partly in liposomes. In addition, there are organelles in the cytoplasm, the so-called proteasomes in which misfolded or denatured proteins are destroyed. Such molecules are recognized using special markers(cm. ).
Articles of the section "Protein metabolism: general information":
- A. Protein metabolism: general information
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Proteins are an essential component of a balanced diet.
The main sources of proteins for the body are food products plant and animal origin. Digestion of proteins in the body occurs with the participation of proteolytic enzymes. gastrointestinal tract. Proteolysis is the hydrolysis of proteins. Proteolytic enzymes are enzymes that hydrolyze proteins. These enzymes are divided into two groups - exopepetidase, catalyzing the breaking of the terminal peptide bond with the release of any one terminal amino acid, and endopeptidase that catalyze the hydrolysis of peptide bonds within the polypeptide chain.
Protein digestion does not occur in the oral cavity due to the absence of proteolytic enzymes. The stomach has all the conditions for the digestion of proteins. Proteolytic enzymes of the stomach - pepsin, gastrixin - exhibit maximum catalytic activity in a strongly acidic environment. The acidic environment is created by gastric juice (pH = 1.0–1.5), which is produced by parietal cells of the gastric mucosa and contains hydrochloric acid as the main component. Under the action of hydrochloric acid of gastric juice, a partial denaturation of the protein occurs, swelling of proteins, which leads to the disintegration of its tertiary structure. Besides, hydrochloric acid converts the inactive proenzyme pepsinogen (produced in the chief cells of the gastric mucosa) into active pepsin. Pepsin
catalyzes the hydrolysis of peptide bonds formed by residues of aromatic and dicarboxylic amino acids (optimum pH = 1.5–2.5). The proteolytic effect of pepsin on proteins is weaker connective tissue(collagen, elastin). Protamines, histones, mucoproteins and keratins (fur and hair proteins) are not cleaved by pepsin.
As the protein food is digested with the formation of alkaline hydrolysis products, the pH of the gastric juice changes to 4.0. With a decrease in the acidity of gastric juice, the activity of another proteolytic enzyme is manifested - gastrixin
(optimum pH = 3.5–4.5).
AT gastric juice children found chymosin (rennin), which breaks down milk caseinogen.
Further digestion of polypeptides (formed in the stomach) and non-digestible food proteins is carried out in the small intestine under the action of pancreatic and intestinal juice enzymes. Proteolytic enzymes of the intestine - trypsin, chymotrypsin - come with pancreatic juice. Both enzymes are most active in a slightly alkaline medium (7.8–8.2), which corresponds to pH small intestine. The trypsin proenzyme is trypsinogen, the activator is enterokinase (produced by the intestinal walls) or previously formed trypsin. trypsin
hydrolyzes peptide bonds formed by arg and lys. The proenzyme of chymotrypsin is chymotrypsinogen, the activator is trypsin. Chymotrypsin cleaves peptide bonds between aromatic amino acids, as well as bonds that have not been hydrolyzed by trypsin.
Due to the hydrolytic action on proteins e ndopeptidase(pepsin, trypsin, chymotrypsin) peptides of various lengths and a certain amount of free amino acids are formed. Further hydrolysis of peptides to free amino acids is carried out under the influence of a group of enzymes - exopeptidase. One of them - carboxypeptidase - synthesized in the pancreas in the form of procarboxypeptidase, activated by trypsin in the intestine, split off amino acids from the C-terminus of the peptide; other - aminopeptidases - synthesized in the cells of the intestinal mucosa, activated by trypsin, split off amino acids from the N - end.
