Proteins are complex nitrogen-containing biopolymers, the monomers of which are amino acids (organic compounds containing carboxyl and amino groups).

Their biological role is diverse.

Proteins perform plastic, catalytic, hormonal, transport and other functions in the body, and also provide specificity, according to Pannochka, an online publication for girls and women aged 14 to 35.. net The value of the protein component of nutrition lies primarily in the fact that it serves as a source of amino acids.

Amino acids are divided into essential and non-essential, depending on whether their formation in the body from precursors is possible.. Essential amino acids include histidine, leucine, isoleucine, lysine, methionine, phenylalanine, tryptophan and valine, as well as cysteine \u200b\u200band tyrosine, synthesized respectively from methionine and phenylalanine..

9 non-essential amino acids (alanine, arginine, aspartic and glutamic acids, glutamine, glycine, proline and series) may be absent in the diet, as they can be formed from other substances. There are also amino acids in the body that are produced by modifying the side chains of the above (for example, a component of collagen - hydroxyproline - and contractile muscle proteins - 3-methylhistidine).

Most amino acids have isomers (D- and L-forms), of which only L-forms are part of the proteins of the human body. D-forms can participate in metabolism, turning into L-forms, but they are utilized much less efficiently.

According to the chemical structure, amino acids are divided into dibasic, diacid and neutral with aliphatic and aromatic side chains, which is important for their transport, since each class of amino acids has specific carriers.. Amino acids with a similar structure usually enter into complex, often competitive relationships..

So, aromatic amino acids (phenylalanine, tyrosine and tryptophan) are closely related to each other.. Although phenylalanine is an essential amino acid, and tyrosine is a non-essential amino acid synthesized from it, the presence of tyrosine in the diet seems to “save” phenylalanine.

If phenylalanine is not enough, or its metabolism is impaired (for example, with vitamin C deficiency), tyrosine becomes an essential amino acid. Similar relationships are also characteristic of sulfur-containing amino acids: indispensable - methionine, and cysteine \u200b\u200bformed from it..

Let's take another example. Tryptophan, during transformations that require vitamin B6 (pyridoxine), is included in the structure of nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP), that is, it duplicates the role of niacin. Approximately half of the normal requirement for niacin is met by tryptophan: 1 mg of dietary niacin is equivalent to 60 mg of tryptophan. Therefore, the state of pellagra can develop not only with a lack of vitamin PP in the diet, but also with a lack of tryptophan or a violation of its metabolism, including due to a deficiency of pyridoxine..

Amino acids are also divided into glucogenic and ketogenic, depending on whether they can, under certain conditions, become precursors of glucose or ketone bodies (Table. 1).

Table 3. Classification of amino acids Amino acids Essential amino acids Non-essential amino acids Aliphatic Valine (G), Leucine (K), Isoleucine (G, K) Glycine (G), Alania (G) Dibasic Lysine (K), Histidine (G, K) * Arginine (G); * - histidine is indispensable in children under one year old; - "

The intake of nitrogen-containing substances with food occurs mainly due to protein and, in less significant quantities, free amino acids and other compounds.. Most of the nitrogen in animal foods is in the form of protein.. In plant products, most of the nitrogen is represented by non-protein compounds, and also contains many amino acids that are not found in the human body and often cannot be metabolized by it..

A person does not need to receive nucleic acids with food. Purine and pyrimidine bases are synthesized in the liver from amino acids, and the excess of these bases ingested with food is excreted as uric acid..

Vitamin B12 is involved in the synthesis of pyrimidine rings; folic acid is necessary for the formation of purine structures.. That is why the deficiency of these nutrients is reflected, first of all, in the organ with a high level of proliferation, where the most intensive synthesis of nucleic acids takes place - hematopoietic tissue..

The typical (but not optimal) daily intake of protein for the average person is approximately 100 g.. Approximately 70 g of protein secreted into the cavity of the gastrointestinal tract joins them.. Of this amount, about 160 g is absorbed. The body itself synthesizes an average of 240-250 g of protein per day.. Such a difference between intake and endogenous transformation indicates the activity of resynthesis processes (Fig.. 1).

Notes: AA - amino acids. In a person weighing 62.5 kg, the total protein content is 10.9 kg (17.5%), 240 g of protein is synthesized and broken down daily. 1 - absorption of free amino acids and peptides after digestion; 2 - supply of amino acids to the liver; 3 - synthesis of liver and plasma proteins, including albumin; 4 - catabolism of excess amino acids; 5 - distribution of amino acids at rest; 6 - entry into the muscles, pancreas, epithelial cells; 7 - excretion of nitrogen in various forms.

A healthy person is characterized by a state of nitrogen balance, when protein losses (with urine, feces, epidermis, etc.). ) correspond to its amount received with food. With the predominance of catabolic processes, a negative nitrogen balance occurs, which is characteristic of low consumption of nitrogen-containing substances (low-protein diets, starvation, impaired protein absorption) and many pathological processes that cause intensification of decay (tumors, burn disease, etc.)..

With the dominance of synthetic processes, the amount of nitrogen introduced prevails over its excretion and a positive nitrogen balance occurs, which is characteristic of children, pregnant women and convalescents after serious illnesses..

After passing the enteric barrier, proteins enter the bloodstream in the form of free amino acids.. It should be noted that gastrointestinal mucosal cells can metabolize certain amino acids (including glutamic acid and aspartic acid to alanine).

The ability of enterocytes to modify these amino acids may help to avoid the toxic effect of their excessive administration..

Amino acids, both those that enter the blood during protein digestion, and those synthesized in cells, form a constantly renewing free pool of amino acids in the blood, which is about 100 g.

75% of the amino acids in the systemic circulation are branched chain amino acids (leucine, isoleucine and valine). Alanine, which is the main precursor of glucose synthesis, and glutamine are released from muscle tissue into the bloodstream.. Many free amino acids undergo transformation in the liver.

Part of the free pool is incorporated into the proteins of the body and, when they are catabolized, enters the bloodstream again.. Others directly undergo catabolic reactions. Some free amino acids are used for the synthesis of new nitrogen-containing compounds (purine, creatinine, adrenaline) and are further degraded, without returning to the free pool, into specific degradation products..

The liver ensures the constancy of the content of various amino acids in the blood. It utilizes approximately 1/3 of all amino acids that enter the body, which helps prevent jumps in their concentration depending on nutrition..

The primary role of the liver in nitrogen and other types of metabolism is ensured by its anatomical location - the products of digestion enter the portal vein directly into this organ. In addition, the liver is directly connected with the excretory system - the biliary tract, which allows some compounds to be excreted in the bile..

Hepatocytes are the only cells that have a complete set of enzymes involved in amino acid metabolism.. Here all the main processes of nitrogen metabolism are performed: the breakdown of amino acids for energy production and ensuring gluconeogenesis, the formation of non-essential amino acids and nucleic acids, the neutralization of ammonia and other end products. The liver is the main site of degradation of most essential amino acids (with the exception of branch chain amino acids).

The synthesis of nitrogen-containing compounds (protein and nucleic acids) in the liver is very sensitive to the intake of their precursors from food.. After each meal, there is a period of increased intrahepatic synthesis of proteins, including albumin.. A similar increase in synthetic processes occurs in the muscles..

These reactions are associated primarily with the action of insulin, which is secreted in response to the introduction of amino acids and/or glucose.. Some amino acids (arginine and branch chain amino acids) enhance insulin production more than others. Others (aspargin, glycine, serine, cysteine) stimulate the secretion of glucagon, which enhances the utilization of amino acids by the liver and affects the enzymes of gluconeogenesis and amino acid catabolism..

Due to these mechanisms, there is a decrease in the level of amino acids in the blood after their intake with food.. The action of insulin is most pronounced for amino acids contained in the bloodstream in a free form (amino acids with branching chains), and is insignificant for those that are transported in bound (tryptophan). Insulin-reverse effect on protein metabolism is exerted by glucocorticosteroids.

The liver has an increased rate of protein synthesis and breakdown compared to other body tissues (except the pancreas). This allows it to synthesize “for export”, as well as quickly provide a labile reserve of amino acids during a period of malnutrition due to the breakdown of its own proteins..

A feature of intrahepatic protein synthesis is that it is enhanced by the action of hormones that produce a catabolic effect in other tissues.. So, during starvation, muscle proteins, to provide the body with energy, undergo decay, and at the same time, the synthesis of proteins, which are enzymes of gluconeogenesis and urea formation, increases in the liver..

Eating too much protein.

Ingestion of food containing excess protein leads to an intensification of synthesis in the liver and muscles, the formation of excess amounts of albumin and the degradation of excess amino acids to precursors of glucose and lipids.. Glucose and triglycerides are utilized as fuel or deposited, and albumin becomes a temporary storage of amino acids and a means of their transportation to peripheral tissues..

During starvation, the level of albumin progressively decreases, and with subsequent normalization of protein intake, it slowly recovers.. Therefore, although albumin is an indicator of protein deficiency, it is insensitive and does not respond promptly to changes in nutrition..

7 out of 10 essential amino acids are degraded in the liver - either forming urea or subsequently used in gluconeogenesis.

Urea is predominantly excreted in the urine, but part of it enters the intestinal lumen, where it is exposed to the urease effect of microflora. Branching chain amino acids are catabolized primarily in the kidneys, muscles, and brain.

Muscles synthesize 75 g of protein daily. In the average person, they contain 40% of the total protein in the body.. Although protein metabolism is somewhat slower here than in other tissues, muscle protein represents the largest endogenous amino acid reserve that can be used for gluconeogenesis during starvation..

8 lack of food albumin and muscle protein synthesis slows down, but amino acid degradation continues. Therefore, at the initial stage of starvation, muscles lose amino acids that go to energy needs.. In the future, the body adapts to the lack of new amino acids (the need for protein-dependent gluconeogenesis decreases due to the use of the energy potential of ketone bodies) and muscle protein loss decreases..

Muscles are the main target of insulin action: here, under its influence, the supply of amino acids increases, muscle protein synthesis increases and decay decreases..

In the process of transformations in the muscles, alanine and glutamine are formed, they can be conditionally considered transport forms of nitrogen.. Alanine directly from the muscles enters the liver, and glutamine first enters the intestines, where it is partially converted into alanine. Since glucose is synthesized from alanine in the liver, partially providing the muscle with energy, the resulting circuit is called the glucose-alanine cycle..

Muscle nitrogen-containing substances also include high-energy creatine phosphate and its degradation product creatinine.. Creatinine excretion is usually considered as a measure of muscle mass..

However, this compound can be ingested with a high-protein diet and affect the results of the study of its content in the urine.. The degradation product of myofibrillar proteins - 3-methylhistidine is excreted in the urine for a short time and is a fairly accurate indicator of the rate of breakdown in the muscles - with muscle wasting, the rate of its output decreases proportionally.

The kidneys not only remove the end products of nitrogen decay (urea, creatinine, etc.). ), but they are also an additional site for the resynthesis of glucose from amino acids, and also regulate the formation of ammonia, compensating for an excess of hydrogen ions in the blood.

Gluconeogenesis and the functioning of acid-base regulation are closely coordinated, since the substrates of these processes appear during the deamination of amino acids: carbon for the synthesis of glucose and nitrogen for ammonia.. There is even an opinion that it is the production of glucose that is the main reaction of the kidneys to acidosis, and the formation of ammonia occurs secondarily..

Nervous tissue has higher concentrations of amino acids than plasma.. This allows you to provide the brain with a sufficient amount of aromatic amino acids, which are precursors of neurotransmitters..

Some non-essential amino acids, such as glutamate (from which gamma-aminobutyric acid (GABA) is formed with the participation of pyridoxine) and aspartate, also have an effect on the excitability of nervous tissue. Their concentration is high here, while nonessential amino acids are able to be synthesized in situ.

Tryptophan, which is a precursor of serotonin, plays a specific role. It is with an increase in the concentration of tryptophan (and, consequently, serotonin) that drowsiness after eating is associated.. This effect is especially pronounced when taking large amounts of tryptophan together with carbohydrate food..

Increased insulin secretion reduces blood levels of branching-chain amino acids, which, when overcoming the blood-brain barrier, have a competitive relationship with aromatic ones, but at the same time does not affect the concentration of albumin-bound tryptophan.. Due to similar effects, tryptophan preparations can be used in psychiatric practice..

Restriction of aromatic amino acids in the diet, due to their influence on the central nervous system, has a preventive value in the management of patients with hepatic encephalopathy..

Elemental amino acid diets with a predominant content of leucine, isoleucine, valine and arginine help to avoid the development of protein deficiency in hepatological patients, and at the same time do not lead to hepatic coma.

The main plastic functions of proteinogenic amino acids are listed in Table.

Table 3. The main functions of amino acids Amino acids The main functions of Alanya Precursor of gluconeogenesis, nitrogen carrier from peripheral tissues to the liver Arginine Immediate precursor of urea Aspartic acid Precursor of gluconeogenesis, a precursor of pyrimidine, is used for the synthesis of urea Glutamic acid Donor of amino groups for many reactions, nitrogen carrier (penetrates through membranes more easily than. Baranovsky.

medbe. en.