The metabolism of proteins in the human body is characterized by one important feature - neither proteins nor amino acids can be stored for future use, such as lipids in adipose tissue or carbohydrates in the form of glycogen.
Nonessential amino acids can be synthesized in the human body. There are several ways for this: amination of unsaturated acid, reductive amination and transamination.
Aluminization of unsaturated acid Asp is formed from fumaric acid under the influence of aspartate:ammonia lyase(see Fig. 6.40). The reaction is reversible and therefore Asp, turning into fumaric acid, can be completely oxidized in the Krebs cycle.
Reductive amination- a process reverse to oxidative deamination (see Fig. 3.14 and 12.1). But only Ala and Glu are formed in this way, since the activity of their dehydrogenases is significant.
Thus, Ala, Asp and Glu believe primary, and all other non-essential amino acids are formed in transamination reactions (see Fig. 3.15).
Dietary amino acids (formed during the digestion of proteins) are carried from the blood to different organs and tissues where they are used for protein synthesis. It is estimated that in the body of an adult, 1.3 g of protein per 1 kg of body weight is synthesized daily (on average 90-100 g). At the same time, using isotope methods, it was established that food amino acids make up only 1/4 of the total. This indicates that proteins in the tissues of the body undergo constant renewal. Different proteins are renewed at different rates. For example, the functioning time of insulin is 20-30 minutes, intestinal mucosal proteins - 2-4 days, hemoglobin - 100-120 days, collagen - 6-8 months.
Protein molecules that have served their useful life are exposed to tissue peptide hydrolases and are broken down into free amino acids according to the following scheme:
Protein -? High molecular weight -? Low molecular weight -? Amino acids, polypeptides polypeptides
Protein breakdown occurs similarly outside the body, in various biological tissues, liquids and food systems. For example, when cheeses are ripened, all the components presented in this diagram are always present in the finished product. The ratio of breakdown products: peptides, amino acids, amines significantly affects the taste and aroma. Medium and low molecular weight peptides that have a bitter taste give some cheeses a characteristic bitter taste.
The processes of protein metabolism in the human body are regulated with the participation of a number of hormones (Table 12.4).
Table 12.4
Regulation of protein and amino acid metabolism
|
Organ |
Synthesized hormones and their effect |
|
Pituitary |
Somatotropin enhances protein synthetic processes |
|
Thyroid |
Thyroxine increases the rate of protein biosynthesis |
|
Pancreas |
Insulin ensures the predominance of protein synthesis over their breakdown; stimulates the binding of mRNA to ribosomes |
|
Adrenal medulla |
Adrenaline increases the rate of breakdown of proteins in tissues and the release of nitrogenous metabolic products in the urine |
|
Adrenal cortex |
Cortisone inhibits protein synthesis, increases their breakdown and the release of nitrogenous metabolic products in the urine |
|
Testes |
Testosterone stimulates protein biosynthesis in muscle tissue causing nitrogen accumulation in the body |
As a result of protein metabolism, some amino acids undergo breakdown. A mandatory step in this case is deamination. or reamii-rovaiy(see paragraph 3.2).The most common option is oxidative deamination. In Fig. Figure 3.14 shows the summary equation. In reality, the reaction occurs in two stages: dehydrogenation and hydrolysis (see Fig. 12.1). When oxidized by the action of a specific NAD-dehydrogenase an imino acid is formed. During hydrolysis, the double bond in the imino group is cleaved and NH 3 is released.
This transformation has great importance for protein metabolism, since both of its stages are reversible and thus an amino acid can be formed from a keto acid.
Based on the direction of use of the nitrogen-free residue, amino acids are divided into two groups: ketogenic and glycogenic (Table 12.5).
Simultaneously ketogenic and glycogenic - Ile, Liz, Fen, Tyr, Tri.
Currently, the breakdown pathways of all proteinogenic amino acids are known.
Examples of ketogenic and glycogenic amino acids
Metabolism of individual amino acids
Glycine- the simplest amino acid. It is synthesized mainly from Ser, the hydroxymethyl group of which is removed by an enzyme containing vitamin By. Like GABA, Gly is an inhibitory neurotransmitter. Gly is involved in the synthesis of purine nitrogenous bases (see Fig. 13.9) and pyrrole cycles. Participates in the neutralization of toxic aromatic compounds that are formed from plant products, if they predominate in the diet. Gly forms water-soluble compounds with benzoic, fsnilacetic acids and phenols, which are excreted through the kidneys. For example, a complex of Gly with benzoic acid is called hyinuric acid (Fig. 12.2).
Rice. 12.2.
With cholic acid, Gly forms glycocholic acid (Fig. 12.3), which has surfactant properties and is involved in the emulsification of fats during digestion.
Gly deamination is carried out according to the oxidative type by NAD-dependent dehydrogenase with the formation of glyoxylic acid (Fig. 12.4).
Rice. 12.4.
Serin - replaceable hydroxyamino acid. Its skeleton is formed from 3-PGA, the source of which is glucose, and NH 2 -rpynna is introduced by transamination. Ser is necessary for the synthesis of phospholipids (see Fig. 11.42 and 11.43), and is a precursor of aminoethanol (Fig. 12.5) and choline.
Rice. 12.5.
The Ser hydroxy group is part of the active sites of many enzymes, such as trypsin, chymotrypsin, esterases, phosphorylases, phosphatases.
During decomposition, Ser is first freed from the alcohol hydroxyl, and then hydrolytically from the amino group (Fig. 12.6). As a result, PVC is formed, which is easily involved in the TCA cycle and is oxidized there to H 2 0 and C0 2.
Rice. 12.6.
Methionine - essential sulfur-containing amino acid. Transfers a methyl group to other compounds. As a result, choline, creatine, adrenaline, and nitrogenous bases are formed.
After being freed from the methyl group, Met sulfur mainly transforms into Cys sulfur.
In fact, all transformations occur when Met is in its active form - in the form of 8 + -adenosylmethionine (see Fig. 6.31).
Although Met is an essential amino acid, it can be regenerated from homocysteine into reversible reaction, shown in Fig. 12.7. The transformation is catalyzed by enzymes that contain vitamins B 9 and B 12. By-
Rice. 12.7.
Since Met is the only source of homocysteine, the body’s supply of this amino acid depends solely on its content in food.
Cysteine- a non-essential sulfur-containing amino acid, since it can be synthesized from two amino acids: Ser and Met (see Fig. 12.7). Cys contains a highly reactive sulfhydryl group that can be easily oxidized to form a disulfide bond. This transformation occurs between different polypeptide chains or within one polypeptide chain during the formation of the tertiary structure of the protein and is called post-translational modification of the protein. This is how the molecules of insulin, chymotrypsin and other proteins are stabilized in the tertiary structure.
The activity of the sulfhydryl group is manifested in enzymatic catalysis. For example, many enzymes contain SH groups in the active site, which are necessary for the catalytic reaction. It is known that the activity of such enzymes is lost upon oxidation of SH-rpynn.
Experiments with animals have proven that cysteine is transformed into the tripeptide glutathione, which has redox properties. It is believed that glutathione maintains the active reduced form of enzymes due to its own oxidation. The positive antioxidant effect of glutathione has been proven:
- in improving the processes of neutralizing heavy metals and toxins;
- reducing the unwanted effects of radiation and chemotherapy during treatment oncological diseases;
- in slowing down the aging process.
In tissues, cysteine can be decarboxylated to form aminoethanethiol (Fig. 12.8), which is necessary for the synthesis of Co A or is oxidized to taurine (Fig. 12.9).
Thus, cysteine is a precursor of taurine, which plays the role of a neurotransmitter and has anticonvulsant activity. Taurine improves energy metabolism, stimulates recovery processes, for example, in the tissues of the eye.
In the liver, taurine forms taurocholic acid, similar to glycocholic acid (see Fig. 12.3), which contributes to the emulsification of fats in the intestine.
Rice. 12.9.
Often, complexes of bile acids with taurine and glycine are called conjugates or paired compounds.
Aspartic And glutamic acid play an important role in protein metabolism and carry out trans- and deamination of amino acids. They can accept NH 3 not only in free form, but also as part of proteins. As a result, the corresponding amides are formed: aspragine (Asi) and glutamine (Gln). Thus, Asi and Glu participate in the neutralization of NH 3.
The metabolism of most amino acids passes through the stage of formation of aspartic and glutamic acids in transamination reactions.
Both amino acids are involved in the synthesis of nitrogenous bases (see Fig. 13.8 and 13.9).
Decarboxylation of aspartic acid leads to the formation of a- or (3-alapine (Fig. 12.10). The latter can be included in the synthesis of pantothepic acid (see Fig. 6.47).
Rice. 12.10.
During α-decarboxylation of glutamic acid, γ-aminobutyric acid is formed (Fig. 12.11), which inhibits excitation processes in the gray matter of the cerebral cortex and is used as medicine for some diseases of the central nervous system.
Phenylalanine- an essential aromatic amino acid. It is oxidized to tyrosine, which is further converted to quinone (Fig. 12.12). Quinones are part of melanonroteins - complex proteins that give color to skin, hair, and fur.
Rice. 12.12.
1 - the reaction is catalyzed by phenylalanine hydroxylase;2 - the reaction is catalyzed
tyrosinase
In the Fen metabolism, a hereditary failure can be observed - the synthesis of a number of defective enzymes. For example, with a synthesis defect phenylalanine hydroxylase disease is observed fensketonuria. In this case, it is not Tyr that is formed, but phenyl lactate, phenylpyruvate and phenylacetate, which accumulate in the blood and are excreted in the urine. These foods are toxic to the brain and cause severe developmental delays in children. mental development(phenylpyruvic oligophrenia), the development of which can be prevented by following a diet that does not contain Fen. In particular, glycomacropeptide, which is cleaved off during the enzymatic hydrolysis of casein and passes into whey, does not contain Fen, which means it can be used in the nutrition of such children.
Another violation occurs when there is a defect tyrosinase and is called albinism(from lat. albus- white). Due to a failure in the synthesis of the melanin pigment, a person’s skin and hair are poorly pigmented, and the pupils of the eyes are red, as the vessels of the fundus are visible due to the lack of pigments in the iris.
Tyrosine is a non-essential amino acid, as it is synthesized from Phen (see Fig. 12.12). However, the oxidation of Phen to Tyr, catalyzed phenylalanine hydroxylase - an irreversible process, therefore, if there is a deficiency of Fen in products, Tyr cannot replace it.
Tyr is a precursor to a number of important compounds. Firstly, hormones are synthesized from Tyr thyroid gland: tetraiodothyronine (T) and triiodothyronine (T 3).
Secondly, Tyr, with the participation of tyrosinase, is oxidized to dioxyphenylalanine (DOPA), and then to DOPA-quinone, which is necessary for the synthesis of colored proteins - melanonroteins.
Finally, dioxyphenylalanine can undergo decarboxylation to form dopamine (dioxyphenylethylamine), which is a precursor to the catecholamines (neurotransmitters) norepinephrine and epinephrine (see Fig. 8.3).
Rice. 12.13.
Tryptophan is an essential amino acid for humans and animals. From it biologically active compounds such as serotonin (Fig. 12.14) and nicotinic acid ribonucleotide are synthesized. Serotonin is a highly active biogenic amine with vasoconstrictor action. It regulates arterial pressure, body temperature, respiration, renal filtration and is a mediator of nervous processes in the central nervous system.
Rice. 12.14.
Normally, no more than 1% of Tri is converted into serotonin. More than 95% of Tri is oxidized through a pathway that leads to the formation of NAD, reducing the body's need for vitamin B5.
Prolyl is a non-essential amino acid, so in the animal body there is the possibility of its synthesis: either from the γ-semialdehyde of glutamic acid (a-amino-γ-oxopentanoic acid) or from ornithine, which is formed during the hydrolysis of Apr (Fig. 12.15).
Rice. 12.15.
During the decomposition, Pro is first oxidized by the same NLD dehydrogenase to 5-pyrroline-2-carboxylic acid, in which the cycle at the site of the double bond is hydrolytically destroyed. As a result, γ-semialdehyde is formed. Its aldehyde group is oxidized to a carboxyl group. This is how Glu arises, the ways of using it depend on the needs of the cell.
Coursework: 34 pp., 12 sources, 5 drawings
Object of study– Protein metabolism in the human body.
Goal of the work– study of protein metabolism disorders in the human body.
Research method– descriptive
valine, threonine, phenylalanine, arginine, cystine, tyrosine, alanine, serine, Protein, amino acids, hemoglobin,purines, inacin, hydrophilicity, urates, creatinine
Introduction
1. Protein metabolism
1.1 Intermediate protein metabolism
1.2 The role of the liver and kidneys in protein metabolism
1.3 Metabolism of complex proteins
1.4 Balance of nitrogen metabolism
1.5 Protein standards in nutrition
1.6 Regulation of protein metabolism
2. Tissue metabolism of amino acids
2.1 Participation of amino acids in biosynthesis processes
2.2 Participation of amino acids in catabolic processes
2.3 Formation of end products of simple protein metabolism
3 Tissue nucleotide metabolism
3.1 DNA and RNA synthesis
3.2 Catabolism of DNA and RNA
4 Regulation of nitrogen metabolism processes
5 Radioisotope research nitrogen metabolism
6 Pathology of nitrogen metabolism
6.1 Protein deficiency
6.2 Pathology of amino acid metabolism
7 Nitrogen metabolism in an irradiated body
8 Changes in nitrogen metabolism during aging
Literature
INTRODUCTION
The human body consists of proteins (19.6%), fats (14.7%), carbohydrates (1%), minerals (4.9%), water (58.8%). It constantly consumes these substances to produce the energy necessary for functioning. internal organs, maintaining warmth and carrying out all life processes, including physical and mental work.
At the same time, the restoration and creation of cells and tissues from which the human body is built occurs, and the energy consumed is replenished from substances supplied with food. Such substances include proteins, fats, carbohydrates, minerals, vitamins, water, etc., they are called food substances. Consequently, food for the body is a source of energy and plastic (building) materials.
These are complex organic compounds of amino acids, which include carbon (50-55%), hydrogen (6-7%), oxygen (19-24%), nitrogen (15-19%), and may also include phosphorus, sulfur , iron and other elements.
Proteins are the most important biological substances of living organisms. They serve as the main plastic material from which cells, tissues and organs of the human body are built. Proteins form the basis of hormones, enzymes, antibodies and other formations that perform complex functions in human life (digestion, growth, reproduction, immunity, etc.), and contribute to the normal metabolism of vitamins and mineral salts in the body. Proteins are involved in the formation of energy, especially during periods of high energy expenditure or when there is an insufficient amount of carbohydrates and fats in the diet. The energy value of 1 g of protein is 4 kcal (16.7 kJ).
With a lack of proteins in the body, serious disorders occur: slower growth and development of children, changes in the liver of adults, the activity of the endocrine glands, blood composition, weakening of mental activity, decreased performance and resistance to infectious diseases.
Protein in the human body is continuously formed from amino acids entering cells as a result of digestion of food protein. For human protein synthesis, food protein is required in a certain amount and a certain amino acid composition. Currently, more than 80 amino acids are known, of which 22 are the most common in foods. Based on their biological value, amino acids are divided into essential and non-essential.
Eight amino acids are essential - lysine, tryptophan, methionine, leucine, isoleucine, valine, threonine, phenylalanine; For children, histidine is also needed. These amino acids are not synthesized in the body and must be supplied with food in a certain ratio, i.e. balanced. Particularly valuable are the essential amino acids tryptophan, lysine, methionine, found mainly in products of animal origin, the ratio of which in the diet should be 1: 3: 3.
Nonessential amino acids (arginine, cystine, tyrosine, alanine, serine, etc.) can be synthesized in the human body.
The nutritional value of protein depends on the content and balance of essential amino acids. The more essential amino acids it contains, the more valuable it is. Sources of complete protein include meat, fish, dairy products, eggs, legumes (especially soybeans), oatmeal and rice.
The daily protein consumption rate is 1.2-1.6 g per 1 kg of human weight, i.e. only 57-118 g, depending on the gender, age and nature of the person’s work. Animal proteins should be 55% daily norm. In addition, when compiling a diet, you should take into account the balance of the amino acid composition of food. The most favorable amino acid composition is presented in a combination of products such as bread and porridge with milk, meat pies, and dumplings.
1 Protein metabolism
Biological significance and specificity of proteins. Proteins are the main substance from which the protoplasm of cells and intercellular substances are built. Life is a form of existence of protein bodies (F. Engels). Without proteins there is no and cannot be life. All enzymes, without which metabolic processes cannot occur, are protein bodies. Phenomena are associated with protein bodies - myosin and actin muscle contraction. The carriers of oxygen in the blood are pigments of a protein nature, in higher animals - hemoglobin, and in lower animals - chlorocruorin and hemocyanin. The blood owes its ability to clot to a plasma protein, fibrinogen. Some plasma proteins, so-called antibodies, are associated with the body's immune properties. One of the retinal proteins - visual purple, or rhodopsin - increases the sensitivity of the retina to the perception of light. Nuclear and cytoplasmic nucleoproteins take a significant part in the processes of growth and reproduction. The phenomena of excitation and its propagation are associated with the participation of protein bodies. Among the hormones involved in the regulation of physiological functions, there are a number of protein substances.
The structure of proteins is very complex. When hydrolyzed by acids, alkalis and proteolytic enzymes, protein is broken down into amino acids, total number of which there are more than twenty-five. In addition to amino acids, various proteins also contain many other components (phosphoric acid, carbohydrate groups, lipoid groups, special groups).
Proteins are highly specific. Every organism and every tissue contains proteins that are different from the proteins that make up other organisms and other tissues. High protein specificity can be detected using the following biological assay. If you introduce protein from another animal or plant protein into the blood of an animal, the body responds to this with a general reaction, which consists of changing the activity of a number of organs and increasing the temperature. At the same time, special protective enzymes are formed in the body that can break down the foreign protein introduced into it.
Parenteral (i.e., bypassing the digestive tract) administration of a foreign protein makes the animal, after a certain period of time, extremely sensitive to repeated administration of this protein. So, if a guinea pig is not administered parenterally a large number of(1 mg or even less) of foreign protein (whey proteins of other animals, egg whites, etc.), then after 10-12 days ( incubation period) repeated administration of several milligrams of the same protein causes a violent reaction in the guinea pig’s body. The reaction manifests itself in convulsions, vomiting, intestinal hemorrhages, low blood pressure, respiratory distress, and paralysis. As a result of these disorders, the animal may die. This increased sensitivity to a foreign protein is called anaphylaxis (C. Richet, 1902), and the body’s reaction described above is called anaphylactic shock. A significantly larger dose of foreign protein, administered for the first time or before the end of the incubation period, does not cause anaphylactic shock. Increasing the body's sensitivity to a particular effect is called sensitization. Sensitization of the body caused by parenteral administration of a foreign protein persists for many months and even years. It can be eliminated if the same protein is reintroduced before the incubation period expires.
The phenomenon of anaphylaxis is also observed in humans in the form of so-called “serum sickness” with repeated administration of medicinal serums.
The high specificity of proteins is understandable if we consider that by various combinations of amino acids it is possible to form countless numbers of proteins with various combinations amino acids. The breakdown of proteins in the intestine provides not only the possibility of their absorption, but also supplies the body with products for the synthesis of its own specific proteins.
The main importance of proteins is that they build cells and intercellular substance and substances involved in the regulation of physiological functions are synthesized. To a certain extent, proteins, however, along with carbohydrates and fats, are also used to cover energy costs.
1.1 Intermediate protein metabolism
Proteins in the digestive canal are broken down by proteolytic enzymes (pepsin, trypsin, chymotrypsin, polypeptidases and dipeptidases) until the formation of amino acids. Amino acids received from the intestines into the blood are distributed throughout the body and proteins are synthesized from them in the tissues.
As studies using the heavy nitrogen isotope (N18) have shown, the body is constantly restructuring protein bodies with amino acids leaving them and reincorporating them back into their composition. Body proteins are in a state of constant exchange with those amino acids that are part of the non-protein fraction. Conversions of some amino acids into others also occur in the body. Such transformations include transamination, which involves the transfer of an amino group from amino acids to keto acids (A.E. Braunshtein and M.G. Kritsman). During the oxidative breakdown of amino acids, deamination occurs first. Ammonia, which is cleaved off as one of the final products of protein metabolism, in higher animals, in a significant part, undergoes further conversion into urea. In humans, urea nitrogen makes up on average 85% of the total urine nitrogen.
In birds and reptiles, the main end product of protein metabolism is not urea, but uric acid. Even urea introduced into the body is converted into uric acid in the body of birds. This feature of nitrogen metabolism is due to the fact that the embryonic period of bird life occurs in a confined space, inside the egg. Uric acid has very low solubility and poorly penetrates animal membranes. Therefore, the accumulation in the cavity of the allantois and embryos of such a product of nitrogen metabolism as uric acid does not harm the embryos.
In mammals, uric acid is also one of the end products excreted in urine. It is formed only from purine bodies, which are part of nucleoproteins and nucleotides, which are coenzymes of some enzymatic systems.
In dogs, uric acid undergoes further breakdown, and the end product of purine body metabolism is allantoin.
Important end products of nitrogen metabolism also include creatinine and hippuric acid. Creatinine is creatine anhydride. Creatine is found in muscles and brain tissue in a free state and in combination with phosphoric acid (phosphocreatine).
Creatinine is formed from phosphocreatinine by the elimination of phosphoric acid. The amount of creatinine excreted from the body in urine is relatively constant (1.5 g in daily urine) and depends little on the amount of protein taken with food. Only with meat foods rich in creatine, the amount of creatinine in the urine increases.
Hippuric acid is synthesized from benzoic acid and glycocol (in dogs, mainly in the kidneys, in most animals and in humans, mainly in the liver and, to a lesser extent, in the kidneys).
This synthesis appears to be aimed at detoxifying benzoic acid. Hippuric acid is especially abundant in herbivores due to the fact that plant foods contain substances that are converted into benzoic acid in the animal body. An increase in the content of hippuric acid in urine is also observed in humans when switching to a plant-based diet.
Protein breakdown products, sometimes having a large physiological significance, are amines (for example, histamine).
1.2 The role of the liver and kidneys in protein metabolism
As blood flows through the liver, amino acids are partially retained in it and from them “reserve” protein is synthesized, which is easily consumed by the body with limited protein intake. A small supply of protein, apparently, can be deposited in the muscles (A. Ya. Danilevsky).
Figure 1.1 – Scheme of Ecc-Pavlovian fistula.
I - diagram of the course of blood vessels before surgery; II - Ecc-Pavlovian fistula. An anastomosis is created between the portal vein and the inferior vena cava; the portal vein between the anastomosis and the liver is ligated; III - “inverted” Ecc-Pavlovian fistula. After applying an anastomosis between the portal vein and the inferior vena cava, the latter is ligated above the anastomosis - in this case, collaterals develop between v. porta n v. azygos.
The formation of proteins probably also occurs in the liver. Thus, after blood loss, the normal content of albumin and globulin in the blood plasma is quickly restored. If liver function is impaired by phosphorus poisoning, then the restoration of the normal protein composition of the blood is extremely slow. The formation of albumin in the liver was shown in experiments with crushed liver tissue. The liver also plays a central role in intermediate protein metabolism. In it in large volume deamination processes take place, as well as urea synthesis. In the liver, a number of toxic products of intestinal protein putrefaction (phenols, indole) are neutralized. Removal of the liver causes the death of the animal after some time, even with repeated administration of glucose. Obviously, this is due to poisoning by intermediate protein metabolism products, in particular, the accumulation of ammonia. The method of applying an anastomosis between veins (Eck-Pavlov fistula) played a very important role in the study of liver function.
The Eck-Pavlovian fistula represents the anastomosis between the portal vein and the inferior vena cava (Fig. 157), and the portion of the portal vein near the liver is ligated. As a result of such an operation, the blood flowing from the intestines and entering the portal vein cannot flow from it into the liver, but flows into the inferior vena cava, bypassing the liver. This operation keeps the liver viable, since the latter is supplied with blood through the hepatic artery. But this eliminates the possibility of the liver retaining toxic substances absorbed by the intestines. This difficult operation was first performed by N.V. Ekk in the laboratory of I.R. Tarakhanov. However, Eck was unable to keep dogs with such a fistula alive. I.P. Pavlov operated on about 60 dogs in 1892, and about a third of them remained alive and were studied. The biochemical part of the research was carried out by M. V. Nenetsky and his colleagues. It turned out that dogs with Eck-Pavlovian fistula can live for a considerable period of time, as long as their food contains little protein. When eating protein foods, in particular, when giving dogs a large amount of meat, the body is poisoned with toxic breakdown products of proteins. The animal becomes agitated, coordination of movements is impaired, convulsions occur and then death. In this case, an increased level of ammonia is detected in the blood. The organ that takes a significant part in protein metabolism is the kidneys. In the kidneys, ammonia is separated from amino acids, and the released ammonia is used to neutralize acids. The latter are excreted in the urine in the form of ammonium salts.
Through the kidneys, the body is liberated from the formed nitrogenous end products of protein metabolism (urea, creatinine, uric acid, hippuric acid, ammonia). When kidney function is impaired as a result of their disease, all these products are retained in the tissues and in the blood, which leads to the accumulation of non-protein (so-called residual) nitrogen in the blood (azotemia and uremia). If the accumulation of nitrogen-containing metabolic products in the blood progresses, the person dies.
1.3 Metabolism of complex proteins
Nucleoproteins take part in the phenomena of growth and reproduction. In tissues that no longer increase their mass, the role of nucleoproteins appears to be reduced to participation in the reproduction of protein substances in the tissue. The exchange of cytoplasmic nucleoproteins (ribonucleoproteins) occurs more intensively than the exchange of nuclear nucleoproteins, deoxyribonucleoproteins. Thus, the rate of phosphorus renewal in the ribonucleic acid of the liver is 30 times higher, and in the ribonucleic acid of the brain 10 times higher than in the deoxyribonucleic acid of these tissues. The metabolism of nucleoproteins in the human body is judged by the excretion of purine bodies, in particular, uric acid. IN normal conditions of nutrition it is released 0.7 g per day. When eating meat, its formation in the body is increased. When there is a metabolic disorder, expressed in the disease gout, poorly soluble uric acid is deposited in the tissues, in particular, in the circumference of the joints.
The body continuously breaks down and synthesizes hemoglobin. Glycocol and acetic acid are used in the synthesis of the heme group. Adequate intake of iron into the body is also necessary.
The intensity of the breakdown of hemoglobin in the body can be obtained from the formation of bile pigments, the appearance of which is associated with the cleavage of the porphyrin ring of the hemin group and the elimination of iron. Bile pigments enter the intestines with bile and are reduced to stercobilinogen or urobilinogen in the colon. Some urobilinogen is lost with feces, and part is absorbed in the large intestines and then enters the liver, from which it again enters the bile. In some liver diseases, urobilinogen is not completely retained in the liver and ends up in the urine. Urobilinogen contained in urine in the presence of oxygen is oxidized into urobilin, causing the urine to darken.
1.4 Balance of nitrogen metabolism
The study of protein metabolism is facilitated by the fact that protein contains nitrogen. The nitrogen content in various proteins ranges from 14 to 19%, with an average of 16%. Every 16 g of nitrogen corresponds to 100 g of protein, air nitrogen, therefore, 6.25 g of protein. Therefore, by studying the nitrogen balance, i.e., the amount of nitrogen introduced with food and the amount of nitrogen excreted from the body, protein metabolism can be characterized in total. Nitrogen absorption by the body is equal to dietary nitrogen minus fecal nitrogen, excretion is the amount of nitrogen excreted in the urine. By multiplying these amounts of nitrogen by 6.25, the amount of protein consumed and decomposed is determined. The accuracy of this method is affected by the body's loss of proteins from the skin surface (sloughing cells of the stratum corneum of the epidermis, growing hair, nails). The processes of protein breakdown in the body and the removal of metabolic products, as well as the absorption of ingested proteins, require many hours. Therefore, to determine the amount of protein breakdown in the body, it is necessary to collect urine throughout the day, and in critical studies, even for many days in a row.
During the growth of the body or weight gain due to the assimilation of an increased amount of proteins (for example, after fasting, after infectious diseases, etc.), the amount of nitrogen introduced with food is greater than the amount excreted. Nitrogen is retained in the body in the form of protein nitrogen. This is referred to as a positive nitrogen balance. During fasting, in diseases accompanied by a large breakdown of proteins, there is an excess of released nitrogen over the input, which is referred to as a negative nitrogen balance. When the amount of nitrogen input and output is the same, we speak of nitrogen equilibrium.
Protein metabolism differs significantly from the metabolism of fats and carbohydrates in that in adults healthy body There is almost no deposition of easily used reserve protein. The amount of reserve protein deposited in the liver is insignificant, and this protein is not retained for a long time. An increase in the total mass of proteins in the body is observed only during the period of growth, during the period of recovery from infectious diseases or fasting, and to a certain extent during the period of increased muscle training when there is some increase in total muscle mass. In all other cases, excessive protein intake causes an increase in protein breakdown in the body.
If, therefore, a person who is in a state of nitrogen equilibrium begins to take a large amount of protein with food, then the amount of nitrogen excreted in the urine also increases. However, the state of nitrogen equilibrium is more high level It is not installed immediately, but within a few days. The same thing happens, but in reverse order, if you move to more low level nitrogen balance. As the amount of nitrogen introduced with food decreases, the amount of nitrogen excreted in the urine also decreases, and after a few days it settles at a lower level.
Under normal nutritional conditions, nitrogen balance is established when 14-18 g of nitrogen is excreted in the urine. When the amount of protein in food is reduced, it can be set at 8-10 g. A further decrease in the amount of protein in food leads to a negative nitrogen balance. The minimum amount of protein nitrogen introduced with food (6-7 g), at which it is still possible to maintain nitrogen balance, is called the protein minimum. The amount of nitrogen excreted in the urine during protein fasting depends on whether other nutrients are introduced or not. If all the energy expenditure of the body can be provided by others nutrients, then the amount of nitrogen excreted in the urine can be reduced to 1 g per day or even lower.
When proteins enter the body in quantities less than what corresponds to the protein minimum, the body experiences protein starvation: protein losses by the body are insufficiently replenished. For a more or less long period, depending on the degree of fasting, a negative protein balance does not threaten dangerous consequences. Observations are described of “artists of fasting” who did not take food, limiting themselves to only a small amount of water, for 20-50 days. However, if the fast does not stop, death occurs.
With prolonged general fasting, the amount of nitrogen excreted from the body decreases sharply in the first days, then settles at a constant low level (Fig. 158). Experiments on animals have shown that shortly before death, nitrogen breakdown in the body increases again. This is due to the exhaustion of the last remnants of other energy resources, in particular fats.
Figure 1.2 - The effect of complete fasting on the daily urinary excretion of gross nitrogen (according to Benedict).
1.5 Protein standards in nutrition
Due to the fact that when different conditions The nutritional minimum may vary, and the meaning of large quantities of proteins in food is not clear, protein standards are not certain. Voith, based on statistical figures, proposed 118 g of protein as the daily requirement. The Chittenden (50-60 g) and Hindhede (25-35 g) norms, as shown by a large number of observations, are completely insufficient and, as a rule, lead to a negative nitrogen balance.
The focus on minimum daily protein standards abroad is an indicator of the desire of the ruling classes in capitalist countries to justify the attack on the living standards of the working masses, doomed to a half-starved existence as a result of increased exploitation. Research by Soviet scientists (O.P. Molchanov and others) allows us to consider 100-120 g of protein per day as the most reasonable minimum. Eating large amounts of protein is not harmful for healthy people.
It should be borne in mind that quantitative standards in protein nutrition retain their significance only if the composition of food proteins is proper. The intake of a number of amino acids from food, the synthesis of which is impossible in the animal body, is absolutely necessary in order to ensure the synthesis of body proteins. On the contrary, some amino acids can be synthesized from other amino acids and even from
nitrogen-free bodies and ammonia, and their intake into the body with food is not necessary. Research in recent years has shown that the number of such amino acids is greater than previously thought.
Of the 20 amino acids listed below, only 8 are vital for humans.
Essential amino acids
Isoleucine
Methionine
Phenylalanine
Tryptophan
Nonessential amino acids
Glycocol
Citrulline
Aspartic acid
Glutamic acid
Oxyproline
Histidine
When one of the essential amino acids is removed from food, the processes of protein synthesis in the body are disrupted. A growing organism experiences growth retardation and then weight loss. Thus, the “law of the minimum” is applicable to protein nutrition, according to which protein synthesis in the body is limited to that of the essential amino acids that is introduced with food in a minimal amount.
Those proteins that contain the necessary amino acids in the proportion most favorable for protein synthesis in the body are used most fully by the body. Therefore, it turns out that to maintain the normal growth of an animal, unequal amounts of different proteins are required, i.e., the biological value of proteins, depending on their amino acid composition, is not the same. The biological value of proteins is measured by the amount of body protein that can be formed from 100 g of food protein. It turns out that animal proteins (meat, eggs and milk) have a high biological value (70-95%), and most proteins plant origin(rye bread, oats, corn) - lower biological value (60-65%). There are, however, proteins of animal origin (for example, gelatin) that do not contain some valuable amino acids (tryptophan, tyrosine, cystine) and are therefore incomplete.
1.6 Regulation of protein metabolism
The intensity of protein metabolism largely depends on the humoral influences of the thyroid gland. The thyroid hormone, thyroxine, increases the intensity of protein metabolism. In Graves' disease, which is characterized by increased secretion of thyroid hormones (hyperthyroidism), protein metabolism is increased. On the contrary, with hypofunction of the thyroid gland (hypothyroidism), the intensity of protein metabolism decreases sharply. Since the activity of the thyroid gland is under control nervous system, then the latter is the true regulator of protein metabolism (p. 480).
The nature of food has a great influence on the course of protein metabolism. When eating meat, the amount of uric acid, creatinine and ammonia produced is increased. With plant foods, these substances are formed in significantly smaller quantities, since plant foods are low in purine bodies and creatine. The amount of ammonia formed in the kidneys depends on the acid-base balance in the body - with acidosis more of it is formed, with alkalosis - less. A significant amount of alkaline salts of organic acids is introduced with plant foods. Organic acids are oxidized to carbon dioxide, which is excreted through the lungs. The corresponding proportion of the base, remaining in the body and then excreted in the urine, shifts acid-base balance towards alkalosis. Therefore, with a plant diet, there is no need for the formation of ammonia in the kidneys to neutralize excess acids, and in this case its content in the urine is negligible.
Proteins are an essential component of a balanced diet.
The main sources of proteins for the body are food products of plant and animal origin. Digestion of proteins in the body occurs with the participation of proteolytic enzymes of the gastrointestinal tract. Proteolysis is the hydrolysis of proteins. Proteolytic enzymes are enzymes that hydrolyze proteins. These enzymes are divided into two groups - exopepetidases, catalyzing the cleavage of the terminal peptide bond with the release of one terminal amino acid, and endopeptidases, catalyzing the hydrolysis of peptide bonds within the polypeptide chain.
In the oral cavity, protein breakdown does not occur due to the lack 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 the parietal cells of the gastric mucosa and contains hydrochloric acid as its main component. Under the influence of hydrochloric acid of gastric juice, partial denaturation of the protein occurs, swelling of the proteins, which leads to the disintegration of its tertiary structure. In addition, hydrochloric acid converts the inactive proenzyme pepsinogen (produced in the main cells of the gastric mucosa) into active pepsin. Pepsin
catalyzes the hydrolysis of peptide bonds formed by aromatic and dicarboxylic amino acid residues (optimum pH = 1.5–2.5). The proteolytic effect of pepsin on proteins is weaker connective tissue(collagen, elastin). Protamines, histones, mucoproteins and keratins (wool and hair proteins) are not broken down by pepsin.
As protein foods are digested with the formation of alkaline hydrolysis products, the pH of gastric juice changes to 4.0. With a decrease in the acidity of gastric juice, the activity of another proteolytic enzyme manifests itself - gastricsin
(optimum pH = 3.5–4.5).
Chymosin (rennin), which breaks down milk caseinogen, was found in the gastric juice of children.
Further digestion of polypeptides (formed in the stomach) and undigested food proteins is carried out in the small intestine under the action of enzymes of pancreatic and intestinal juices. Intestinal proteolytic enzymes - trypsin, chymotrypsin - come with pancreatic juice. Both enzymes are most active in a slightly alkaline environment (7.8–8.2), which corresponds to pH small intestine. The proenzyme of trypsin 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 were not hydrolyzed by trypsin.
Due to the hydrolytic effect on proteins, ndopeptidases(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 - exopeptidases. One of them - carboxypeptidases – synthesized in the pancreas in the form of procarboxypeptidase, activated by trypsin in the intestine, cleaves off amino acids from the C-terminus of the peptide; other - aminopeptidases – synthesized in the cells of the intestinal mucosa, activated by trypsin, cleave amino acids from the N-end.
In the adult human body, nitrogen metabolism in general balanced, that is, the amounts of incoming and outgoing protein nitrogen are approximately equal. If only a portion of the newly supplied nitrogen is released, the balance positive. This is observed, for example, during the growth of an organism. Negative balance is rare, mainly as a consequence of disease.
Proteins obtained from food undergo complete hydrolysis into 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 be supplied with food (see).
The body constantly loses protein through the intestines and, to a small extent, also through the kidneys. Due to these inevitable losses, it is necessary to obtain at least 30 g of protein from food daily. This minimum standard is hardly met in some countries, while in industrialized countries the protein content of food is most often significantly higher than the norm. Amino acids are not stored in the body; with an excess supply of amino acids in the liver, up to 100 g of amino acids per day are oxidized or used. The nitrogen they contain is converted into urea (see) and in this form is excreted in the urine, 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 adult body, 300-400 g of protein is broken down to amino acids daily ( proteolysis). At the same time, approximately the same amount of amino acids is included in the newly formed protein molecules ( protein biosynthesis). High protein turnover in the body is necessary because many proteins are relatively short-lived: they begin to renew a few hours after synthesis, and the biochemical half-life is 2-8 days. They turn out to be even shorter-lived key enzymes intermediate exchange. They are updated several hours after synthesis. This constant breakdown and resynthesis allows cells to quickly adjust the levels and activity of the most important enzymes to meet metabolic needs. In contrast, structural proteins, histones, hemoglobin, or cytoskeletal components are particularly durable.
Almost all cells are capable of carrying out biosynthesis proteins (in the diagram above on the left). Construction of a peptide chain by broadcasts on the ribosome is discussed in the articles. However, the active forms of most proteins arise 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 must be formed ( clotting, cm. , ). With post-translational maturation many proteins have parts of the peptide chain removed or added additional groups, such as oligosaccharides or lipids. These processes occur in the endoplasmic reticulum and in the Golgi apparatus (see.
Protein metabolism
Protein metabolism is the central link of 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 comes from proteins. This takes into account the nitrogen of the feed, the nitrogen of the body and the nitrogen of excretory products. 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 a nitrogen balance is observed (as much nitrogen is removed from the body as is supplied with feed), and negative (the breakdown of proteins is not compensated by feed proteins). 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 |
| Working horse | 1,24,42 |
| The horse is not working | 0,7-0,8 |
Feed proteins are divided into full-fledged And inferior. 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. Conditionally essential amino acids include
histidine, since its slight deficiency in feed is compensated by synthesis by microflora in the alimentary canal. The remaining amino acids are replaceable 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.
Different feeds and food products contain different amounts of proteins, %:
| Pea beans | 26 | Feed yeast | 16 |
| Soybeans | 35 | Potato | 2,0-5 |
| wheat grain | 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 | 21,5 | Cottage cheese | 14,6 |
| Lean lamb | 19,8 | Cheeses | 20-36 |
| Fatty lamb | 25 | Chicken egg | 12,6 |
| Pork is fatty | 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.
IN oral cavity feed containing proteins is mechanically crushed, moistened with saliva and forms a food bolus, which enters the stomach through the esophagus (in ruminants - into the proventriculus and abomasum, in birds - into the glandular and muscular stomachs). Saliva does not contain enzymes capable of breaking down food proteins. The chewed feed enters the stomach (in ruminants, into the abomasum), mixed and soaked in gastric juice.
Gastric juice- colorless and slightly opalescent liquid with a density of 1.002-1.010. A person produces about 2 liters per day, a large cattle- 30, for a horse - 20, for a pig - 4, for a dog - 2-3, for a sheep and goat - 4 liters of gastric juice. Secretion of gastric juice in the first
(complex reflex) phase is determined by the appearance, smell and taste of the food, in the second (neurohumoral) phase - by its chemical composition and mechanical irritation of the receptors of the mucous membrane. The composition of gastric juice includes 99.5% water and 0.5% solid substances. Dense substances include the enzymes pepsin, rennin, gastrixin, gelatinase, lipase (in pigs and amylase); proteins - serum albumins and globulins, mucoproteins, Castle factor; from mineral substances, 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 acid. The main cells of the fundus glands of the stomach participate in the formation of pepsin, and the parietal cells participate in the formation of hydrochloric acid. The source of chloride ions is NaCl, H + ions - protons coming from the blood into the cytoplasm of the parietal cells due to redox reactions (G. D. Kovbasyuk, 1978).
Hydrochloric acid creates the necessary acidity for the catalytic action of enzymes. Thus, in humans the pH of gastric juice is 1.5-2.0, in cattle - 2.17-3.14, in horses - 1.2-3.1, in pigs - 1.1-2.0 , in 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 component 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, total, free and bound acidity of gastric juice.
Rennin (chymosin, or rennet enzyme) is produced in young ruminants by the glands of the abomasum mucosa. It is synthesized in the form of prorennin, which at pH
IN stomach Hydrolytic breakdown of most feed proteins occurs. Thus, nucleoproteins under the influence of hydrochloric acid and pepsin break down into
nucleic acids and simple proteins. The breakdown of other proteids also occurs here. Under the influence of pepsin, peptide bonds at the edges of protein molecules are cleaved. The bonds formed by aromatic and dicarboxylic amino acids are the easiest to break. Pepsin easily breaks down proteins of animal origin (casein, myoglobin, myogen, myosin) and some plant proteins, built mainly from monoaminodicarboxylic acids (gliadin and glutelin of cereals), with the exception of wool keratins, silk fibroins, mucus mucins, ovomucoids, some bone proteins and cartilage.
Some proteins are broken down by other proteolytic enzymes of gastric juice, for example, collagens - gelatinase, kasenny - rennin.
Under the influence of the constituents 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 hormonoids of the mucous membrane of the alimentary canal: gastrin (in the pylorus), enterogastrin (in the intestines), histamine (in the stomach), etc.
Features of protein digestion in ruminants. In ruminants, the food bolus from the esophagus enters the proventriculus, where it undergoes additional mechanical processing; when chewing the cud, it returns to oral cavity, is crushed again, then enters the rumen, 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 from 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 such enzymes are concentrated inside cells, 20-30% in rumen fluid. The 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 broken down into peptides, peptides by peptidases into oligopeptides, oligopeptides into amino acids. Thus, corn zein is hydrolyzed by 60% to amino acids, and
casein - 90%. Some 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 feed and its processed products. The bulk of plant foods is represented by carbohydrates, and primarily fiber. Fiber in the forestomach, under the influence of microbial enzymes cellulase and cellobiase, is broken down into α-D(+)-glucose and β-D(+)-glucose.
Monoses undergo various types of fermentation, which leads to the formation of low molecular weight fatty acids. Thus, during lactic fermentation caused by Bact. lactis, lactic acid is formed from glucose: C 6 H 12 O 6 → 2CH 3 → CHOH - COOH. During butyric acid 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 a cow's rumen 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 composition is as follows:
Some of these acids are used for the synthesis of milk fat, glycogen and other substances (Fig. 22), while some serve as material for the microflora to synthesize amino acids and its own protein.
The synthesis of amino acids by microflora in the forestomach 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 additives to diets. Thus, urea, under the influence of the urease enzyme produced by the rumen microflora, is broken down into ammonia and carbon dioxide:
The source of nitrogen-free products most often are keto acids, which are formed from fatty acids (see above). This biosynthesis is usually of 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 enter in small portions into small intestine, where protein breakdown is completed. It involves proteolytic enzymes of pancreatic secretions and intestinal juice. These reactions take place in a neutral and slightly alkaline environment (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 into 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 the peptide bonds of proteins are cleaved by trypsin. It is released in the form of inactive trypsinogen and, under the influence of the enzyme of the intestinal mucosa, enterokinase, it is converted into active trypsin, losing the hexapeptide that previously covered 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 chymotrypsin. It is released in the form of chymotrypsinogen, which, under the influence of trypsin, is converted into chemotrypsin. 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 The digestion of proteins is reflected in the diagram:
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 broken down, and on the surface of the mucous membrane - their “fragments”: albumoses, peptones, polypeptides, tripeptides and dipeptides.
Proteins and their derivatives that have not undergone breakdown in the small intestine are subsequently colon subject to rotting. 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 broken down 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 large intestine, amino acids undergo decarboxylation, which leads to the formation of toxic amines, for example, 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 occur in tandem and in stages, which ultimately leads to the emergence of a wide variety of rotting products. Thus, during the putrefactive decomposition of cyclic amino acids, the following phenols are formed.
During the putrefactive decomposition of tryptophan, skatole and indole are formed.
During the putrefactive decomposition of cystine and cysteine, mercaptans, hydrogen sulfide, methane are formed, carbon dioxide.
The processes of protein putrefaction develop intensively when animals are fed poor-quality feed, the feeding regime is violated, in diseases of the alimentary canal (atony of the proventriculus, constipation), infectious (colibacillosis) and invasive (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 undigested proteins of colostrum and milk is absorbed. Site of absorption - microvilli of the villous epithelium of the mucous membrane small intestine. Amino acids enter 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 transporter. It is a polyvalent ion that has four sites for
binding to neutral, acidic and basic amino acids, as well as to the Na + ion. Having passed the membrane, the amino acid is cleaved from the carrier and gradually moves through the endoplasmic reticulum and lamellar complex from the apical edge to the basal region of the enterocyte (Fig. 24). Arginine, methionine, leucine are absorbed faster; slower - phenylalanine, cysteine, tyrosine; slowly - alanine, serine and glutamic acid.
The sodium pump plays an important role in absorption processes, since sodium chloride accelerates absorption.
The chemical energy consumed in this process is provided by mitochondria.
A protein carrier is involved in the movement of amino acids throughout the cell. In the basal and lateral regions of the cell, the transporter + amino acid complex is cleaved.
The amino acid diffuses into the intercellular space and enters the blood 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, rotting products are absorbed: phenol, cresol, indole, skatole, etc.
Intermediate exchange. The products of protein absorption enter the liver through the portal vein system. The amino acids remaining in the blood after passing through the liver from the hepatic vein enter the big circle blood circulation and are carried to individual organs, tissues and cells. Some of the amino acids from the intercellular fluid enter lymphatic system, then the systemic 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 spent on protein biosynthesis, some - on biological biosynthesis active substances(non-protein hormones, peptides, amines, etc.), some of them, being deaminated, are used as energy raw materials and material for the biosynthesis of lipids, carbohydrates, nucleic acids, etc.
Protein biosynthesis
Protein biosynthesis occurs in all organs, tissues and cells. The largest amount of 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 the biosynthesis and movement of synthesized protein molecules occurs.
Ribosomes in the cell are found in the form of a cluster of 3 to 100 units - polysomes (polyribosomes, ergosomes). Ribosomes are usually connected to each other 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 is the polysomes of muscle tissue that synthesize myosin. A 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 number 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 acid interacts with its own activating enzyme - aminoacyl synthetase. The reaction is activated by Mg 2+, Mn 2+ and Co 2+ cations. An activated amino acid appears.
Compound 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 tRNA of the cytoplasm. The process is catalyzed by aminoacyl-RNA synthetases.
The amino acid residue is connected by a carboxyl group to the hydroxyl group of the second carbon atom of the ribose nucleotide of tRNA.
Transport of a complex of activated amino acid with tRNA to the cell ribosome. The activated amino acid, coupled 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 - 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 has a tRNA anticodon and one amino acid. During biosynthesis, amino acids are added 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 the mRNA.
Initiation of a polypeptide chain. After two neighboring aminoacyl-tRNAs have joined the mRNA codons with their anticodons, conditions are created for the synthesis of the polypeptide chain. The first peptide bond is formed. These processes are catalyzed by peptide synthetases and activated by Mg 2+ cations and protein initiation factors - F 1, F 2 and F 3. The source of chemical energy is
GTF. The connection occurs due to the CO group of the first and the NH 2 group of the second aminoacyl-tRNA.
These reactions occur on the free 30S subunit. The 50S subunit joins the initiation complex and they combine to form a ribosome bound to the mRNA. Each initiation step requires one GTP molecule.
Elongation of a polypeptide chain. The initiation of the polypeptide chain begins from the N-terminus, since the -NH 2 -group of the first amino acid is retained in the resulting dipeptide. The first tRNA that brings its amino acid is split off from the mRNA-ribosome complex and “sent” 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 leaves the ribosome into the hyaloplasm, etc. The peptide chain lengthens (elongates) as a result of the sequential addition of new amino acid residues . The ribosome gradually moves along the mRNA, transforming 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 synchronously synthesized on a polysome. This creates the primary structure of the protein molecule.
Polypeptide chain termination. The ribosome, on the surface of which the polypeptide chain was synthesized, reaches the end of the mRNA chain and “jumps off” from it; a new ribosome attaches to the opposite end of the mRNA in its place, synthesizing the next polypeptide molecule. The polypeptide chain is detached from the ribosome and released into the hyaloplasm. This reaction is carried out by a specific release factor (R factor), which is associated with the ribosome and facilitates the hydrolysis of the ester bond between the polypeptide and tRNA. All stages are summarized by a diagram (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 dynamic state, undergoing constant processes of synthesis and decay. During their 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, it is partially or completely renewed. 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 with a new one. The damaged protein molecule breaks down 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 in the human body are generally renewed within 135-155 days. The proteins of the liver, pancreas, intestinal wall and blood plasma are renewed within 10 days, muscles - 30 days, collagen - 300 days. The synthesis of a protein molecule in a cell occurs quickly - within 2-5 s. In the adult body, 90-100 g of protein are synthesized daily (1.3 g per 1 kg
masses). The degree of renewal decreases with aging, illness, etc.
Peptide biosynthesis
Some endo- and exogenous amino acids are used for the synthesis of peptides.
Glutathione. It is a tripeptide formed from glutamic acid, cysteine and glycine residues.
Biosynthesis occurs in two stages. So, initially, under the influence of the enzyme γ -glutamylcysteine synthetase forms a dipeptide-, then with the participation of tripeptide synthetase - tripeptide-glutathione:
It is an integral part of many enzymes and protects the SH groups of proteins from oxidation.
Carnosine and anserine. Muscle tissue dipeptides. 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 occur in two stages, for example the synthesis of carnosine.
Biosynthesis and metabolism of individual amino acids
Nonessential amino acids are synthesized in body tissues; essential ones enter the body as part of food; conditionally essential are synthesized in tissues to a limited extent (arginine and histidine) or in the presence of precursors (tyrosine and cysteine). A certain amount of amino acids is synthesized by symbiotic microflora in the alimentary canal.
The most common material used for the synthesis of amino acids is α -keto- and α -hydroxy acids that are formed in tissues during the intermediate metabolism of carbohydrates, lipids and other compounds. The source of nitrogen is ammonia and ammonium salts, and the source of hydrogen is 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 occurs 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.
Some glutamic acid can be synthesized from α -ketoglutaric acid under the action of the enzyme 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. Braunstein and M.G. Kritsman, 1937) under the influence of aminoferase enzymes, an integral part of which is 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 metabolism of individual amino acids has certain characteristics.
Glycine. Participates in a number of important biosynthetic reactions. So, from it are formed:
In liver tissues, glycine participates in the process of neutralizing toxic compounds - benzoin,
phenylacetic acids and phenols, forms paired compounds that are excreted in the urine.
Alanin. 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 a 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 diagram.
Glutamic acid. Contained in tissues as part of proteins, in a free state and in the form of an amide. Amino group donor in transamination reactions. The main substances in the synthesis of which acid is involved:
Serine and threonine. Their metabolism is closely related to glycine metabolism. 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. The C1 fragment is used for the synthesis of histidine and purines. Pyruvic acid is formed from serine and threonine, which is included in the TCA cycle with the help of acetyl-CoA.
Some of the transformations are reflected in the diagram:
The hydroxyl group of serine is part of the active center of many enzymes: trypsin, chemo-trypsin, esterases, phosphorylases.
Methionine. It is a component of many proteins. Serves as a donor for the metal group. The transfer of the methyl group during the process of 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 is partially reflected in the diagram:
Arginine and ornithine. Arginine is formed during the conversion of carbon dioxide and ammonia into urea.
Both amino acids are involved in the formation of a number of vital substances.
Lysine. The most important amino acid. Participates in the synthesis of many substances.
The Σ-amino group of the lysine residue is involved in the formation of the bond between apo- and coenzymes, especially during the formation of the biotin enzyme. Lysine plays an important role in binding phosphorus during bone tissue mineralization and other processes.
Phenylalanine and tyrosine. Their transformations in the body go in the following directions: biosynthesis of proteins and peptides, formation
proteinogenic amines, hormones and pigments, oxidation to end products with core rupture, etc.:
Tryptophan. The most important 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.
Conversion of nitrogen-free residue of amino acids
Some of the amino acids not used in the synthesis of proteins and their derivatives undergo decomposition processes to ammonia and carboxylic acids. Ammonia is neutralized in the liver in the ornithine cycle. Of the several types of deamination, oxidative deamination predominates. The resulting keto acids are used by tissues for various needs. Based on the direction of use of the nitrogen-free residue, amino acids are divided into two types: glucoplastic and lipoplastic. Glucoplastic amino acids (alanine, serine, cysteine, etc.) usually form pyruvic acid, 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 into fatty acids.
The resulting fatty acid can be subjected to β -oxidation, acetyl-CoA appears - a source of chemical energy or raw material for the biosynthesis of many substances.
Features of intermediate metabolism 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.
Chromoprotein metabolism. The animal body contains a number of chromoproteins: hemoglobin, myoglobin, cytochromes, hemin enzymes, etc.
They are characterized by the presence of a heme molecule. The biosynthesis of hemoglobin has been studied in 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 number of stages.
Of two molecules δ -aminolevulinic acid produces porphobilinogen, 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 enzyme hemosynthetase, iron (Fe 2+) is incorporated into the protoporphyrin molecule and heme is formed, which, through a 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 not much different from the metabolism of simple proteins. Their synthesis proceeds similarly to other proteins - with the formation of primary, secondary, tertiary and quaternary structures. The difference is that during synthesis, different prosthetic groups are attached to the protein part of the molecules. When a complex protein molecule decomposes, the protein part is broken down into amino acids, and prosthetic groups (lipid, carbohydrate, phosphorus esters of amino acids) into simple compounds.
The ultimate 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 released by the lungs, water by the kidneys, with sweat, in feces, and 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, the main end product of nitrogen metabolism is uric acid.
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 formation of urea in the liver - ornithine Krebs cycle.
1. NH 3 and CO 2 that are split off during deamination and decarboxylation combine under the influence of the enzyme carbamoyl phosphate synthetase to form carbamoyl phosphate.
2. Carbamoyl phosphate with ornithine with the participation of ornithine carbamoyltransferase forms citrulline.
3. Under the influence of argininosuccinate synthetase, it interacts with aspartic acid, forming argininosuccinic acid.
4. Argininosuccinic acid, under the influence of argininosuccinate lyase, is broken down into arginine and fumaric acid.
5. Arginine, under the influence of arginase, 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 tissues is bound during the process formation of amides - asparagine or glutamine which are transported to the liver. They are hydrolyzed in the liver, after which urea is formed from ammonia. Some ammonia is used by tissues for the reductive amination of keto acids, which leads to the formation of amino acids.
In addition, in kidney tissue, ammonia is involved in the process of neutralizing organic and inorganic acids:
Conversions of other products of final protein metabolism. In the process of protein metabolism, other end-metabolic products are also formed, in particular derivatives of purine and pyrimidine bases, gases (released during bowel movements), phenols, indole, skatole, sulfuric acid etc. Especially a lot of such substances are formed in the large intestine during the decay of proteins.
These toxic compounds are neutralized in the liver by the formation of so-called paired acids, which are released 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 pair compounds with glucuronic or sulfuric acids.
Transformations of breakdown products of chromoproteins. When chromoproteins are broken down, globin and heme are formed. Globin undergoes the usual transformations typical of proteins. Heme serves as a source of formation
pigments of bile, urine and feces. Hemoglobin, when oxidized, turns into verdohemoglobin(choleglobin). Verdohemoglobin loses its protein part and iron atoms, which leads to the formation of a green substance - biliverdin. Biliverdin is reduced to a red pigment - bilirubin. Bilirubin is formed from mesobilirubin, which after the next restoration becomes urobilinogen. Urobilinogen in the intestine is converted into stool pigments - stercobilinogen And stercobilin, in the kidneys - into urine pigment urobilin.
Heme breakdown products are used by the body for various needs. Thus, iron is deposited in organs as ferritins. Biliverdin and bilirubin are bile pigments, the remaining substances are urine and feces pigments. The breakdown of myoglobin proceeds in a similar way.
Regulation of protein metabolism. A special place in regulation belongs to the cortex cerebral hemispheres brain and subcortical centers. The hypothalamus contains a center for protein metabolism. Regulation is carried out reflexively, in response to irritation.
The effect of hormones on protein biosynthesis is carried out by stimulating the formation of mRNA. Somatotropin enhances protein synthetic processes. Protein biosynthesis is activated by insulin, some
andro- and estrogens, thyroxine. Glucocorticoids from the adrenal cortex stimulate the breakdown of proteins and the release of nitrogenous substances.
The effect of hormones on protein metabolism is associated with changes in the speed and direction of enzymatic reactions. The biosynthesis and, consequently, the activity of enzymes involved in protein metabolism depends on the presence of sufficient vitamins in the feed. In particular, pyridoxal phosphate is a coenzyme of amino acid decarboxylases, vitamin B 2 is a component of the coenzyme of amino oxidases, vitamin PP is the basis of glutamic acid dehydrase, without vitamin C the biosynthesis of proline and hydroxyproline cannot take place, etc.
Pathology of protein metabolism. Protein metabolism is disrupted during infectious, invasive and non-communicable diseases. Protein metabolism disorders can be caused by an incorrectly formulated diet, feeding with poor-quality feed, non-compliance with the feeding regime, etc. This leads to a decrease in the level of animal productivity, a deterioration in their health, and sometimes even death.
Pathology of protein metabolism manifests itself in various forms.
Protein fasting. 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, and pancreas. In animals, growth slows down, general weakness and swelling appear, bone formation is impaired, loss of appetite, and diarrhea are observed. A negative nitrogen balance occurs, hypoproteinemia occurs (the protein content in the blood decreases by 30-50%).
Amino acid metabolism disorder. It appears in several forms. Thus, with some liver diseases (hepatitis, cirrhosis, acute yellow dystrophy), the content of amino acids in the blood and urine sharply increases - alkaptonuria occurs. In particular, when tyrosine metabolism is disrupted, 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. Vitamin deficiencies are often the cause of such disorders.
Violation of the metabolism of complex proteins. Most often they manifest themselves in the form of disorders of nucleic acid and porphyrin metabolism. In the latter case, the exchange of hemoglobin, myoglobin and other proteins is disrupted. Yes, when various lesions liver (hepatitis, fascioliasis, etc.), hyperbilirubinemia occurs - the bilirubin content in the blood increases to 0.3 - 0.35 g/l. The urine becomes dark, large amounts of urobilin appear in it, and urobilinuria occurs. Sometimes porphyria is observed - an increase in the content of porphyrins in the blood and tissues. This results in porfinuria and the urine turns red.
Control questions
1. What are proteins, what is their significance, chemical composition, physicochemical characteristics, structure (primary, secondary, tertiary, quaternary)? Their classification.
2. Give a description of the main groups and subgroups of amino acids, give structural formulas the most important of them, analyze their properties.
3. What is nitrogen balance, protein minimum, complete and incomplete proteins, nonessential, conditionally essential and essential amino acids? Write the formulas of essential amino acids.
4. Analyze the main stages of protein metabolism in the body of various types of farm animals - digestion, absorption, intermediate (biosynthesis and breakdown) and final metabolism.
5. How is protein metabolism regulated in the body of animals and how does the pathology of protein metabolism manifest itself?
