The biological sciences can be characterized as the sciences that study the mechanisms by which molecules carry out their specific functions in living cells.
The mechanism of action of simple inorganic ions and organic molecules in many cases has been explained to some extent. We have, for example, a known idea of the physiological consequences of an increase or decrease in the osmotic pressure of body fluids upon the introduction or removal of sodium chloride. Another example is the violation of the conduction of nerve impulses in the synapses, which occurs after the administration of physostigmine, which can be partially attributed to the effect of this drug on the enzyme cholinesterase. However, even such well-studied systems continue to be an area of exploration and speculation for researchers, indicating the complexity of the cell.
Protein chemists naturally recognize that the easiest way to get closer to understanding cell function is to study the structure and function of protein molecules. This point of view, apparently, is not without foundation. With the exception of those rare phenomena in biology that are purely physical in nature, the "life" of cells is based mainly on a combination of enzymatic catalysis and their regulation.
The field of protein chemistry is now complex enough to think of proteins as organic substances rather than conglomerates of amino acids. Despite the extraordinary complexity of the protein molecule, we can now quantitatively describe phenomena such as denaturation in terms of fairly well-established changes in specific types of chemical bonds. This favorable situation enables us to find reasonable ways to compare the specific features of the covalent and non-covalent structure of proteins with biological activity. Protein molecules, apparently, consist of one or more polypeptide chains, interconnected and held in the form of a helical structure due to the presence of a system of various chemical bonds of varying strength. When any of these bonds change, a substance appears that is not identical to the original native molecule and which, in a sense, can be considered as a denatured protein. However, in terms of function, we can adhere to more stringent criteria. The nativeness of an enzyme, expressed in its ability to catalyze a certain reaction, should not be associated with its entire structure.
The study of the consequences of partial specific destruction of biologically active proteins has begun quite recently. However, more than 20 years ago, it was shown that the replacement of some active groups of proteins or their transformation into any other groups is not accompanied by a loss of activity. Perhaps the best-studied example of this kind of research is the series of works by Herriot and Northrop on the Study of the Activity of Pepsin during the Gradual Acetylation of its Molecule. Pepsin was treated with ketene, and free amino and hydroxyl groups were converted into their acetyl derivatives. Using this method, Herriot was able to obtain a crystalline pepsin acetyl derivative containing 7 acetyl groups per pepsin molecule. Acetylpepsin had 60% of the catalytic activity of the parent enzyme. Herriot showed that the ultraviolet absorption spectrum of this substance, which had 60% activity, changed so much that this change could be explained by the blocking of three hydroxyl groups of tyrosine. Upon careful hydrolysis of acetylated pepsin at pH 0 or at pH 10.0, the elimination of three acetyl groups occurred, accompanied by the restoration of the catalytic activity of the enzyme. These and some other studies have shown that tyrosine residues have something to do with pepsin activity, while acetylation of a number of free amino groups of the protein does not affect its function.
Experiments of this kind have now become relatively common, and there is no doubt that it is possible to somewhat change the structure of many enzymes and hormones without causing their inactivation. Despite these data, until relatively recently it was believed that the structure of biologically active proteins is more or less "inviolable" and that in order to perform their functions, these proteins must retain their three-dimensional structure in all its integrity.
This concept is supported by some theoretical considerations, according to which a protein molecule can have several different resonance configurations. The observations made in the field of immunology also support this concept. It is well known that relatively small changes, for example, in the structure of the hapten, can cause a significant shift in the efficiency of the reaction with a specific antibody.
The idea of the "inviolability" of the protein structure is now being gradually replaced by the idea of the "functional significance of a part of the molecule." Soon after Sanger and his co-workers completed their fundamental research on bovine insulin, Lena showed that a certain disturbance in the structure of the hormone, namely the removal of the C-terminal alanine residue in the B chain, does not lead to a loss of biological activity. The evolutionary significance of this fact at one time was unclear, since this was the first experience of this kind and one could consider it as a separate atypical case. However, at present, many similar observations have accumulated, and it is necessary to tackle the question of why the C-terminal alanine residue was retained as a constant structural element of the insulin molecule, if this residue does not play a role in the biological activity of the hormone.
Insulin has undergone other more detailed studies of this type. However, in order to find out to what extent it is possible to disrupt the structure of proteins without causing their inactivation, we turn to three other examples, about which there is a little more information: 1) the pituitary hormone, ACTH; 2) pancreatic enzyme - ribonuclease and 3) plant enzyme - papain. In the subsequent discussion of these examples, we use, more or less simultaneously, two different approaches to the structural basis of biological activity: first, we will try to show that active polypeptides can be destroyed without disrupting their function, that is, to identify parts of the structure that do not have essential to the function; secondly, it is necessary to determine the essential parts of the structure, that is, the active centers.
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What are proteins in general and what role do they play in the human body. What are the functions of proteins, what is nitrogen balance and what is the biological value of proteins. This is not a complete list of the issues covered in this article.
We continue the series of articles "EXCHANGE OF CARBOHYDRATES IN THE BODY", "EXCHANGE OF FATS IN THE BODY" with the article "EXCHANGE OF PROTEINS IN THE BODY". The information is intended for a wide range of readers, with the approval of the readers, the series of articles on human physiology will continue.
FUNCTIONS OF PROTEINS- Plastic function protein is to ensure the growth and development of the body through biosynthesis processes. Proteins are part of of all cells of the body and interstitial structures.
- Enzymatic activity protein regulates the rate of biochemical reactions. Proteins-enzymes determine all aspects of metabolism and the formation of energy not only from proteins themselves, but from carbohydrates and fats.
- Protective function protein consists in the formation of immune proteins - antibodies. Proteins are able to bind toxins and poisons and also provide blood clotting (hemostasis).
- Transport function consists in the transfer of oxygen and carbon dioxide by erythrocyte protein hemoglobin, as well as in the binding and transfer of certain ions (iron, copper, hydrogen), medicinal substances, toxins.
- Energy role proteins due to their ability to release energy during oxidation. However, at the same time plastic the role of proteins in metabolism surpasses them energy, and plastic the role of other nutrients. The need for protein is especially great during periods of growth, pregnancy, and recovery from serious illnesses.
- In the digestive tract, proteins are broken down to amino acids and simplest polypeptides, of which in the future cells of various tissues and organs, in particular liver, proteins specific to them are synthesized. The synthesized proteins are used for the restoration of destroyed cells and the growth of new cells, the synthesis of enzymes and hormones.
An indirect indicator of the activity of protein metabolism is the so-called nitrogen balance. The nitrogen balance is the difference between the amount of nitrogen ingested with food and the amount of nitrogen excreted from the body in the form of final metabolites. When calculating the nitrogen balance, it is assumed that the protein contains about 16% nitrogen, that is, every 16 g of nitrogen corresponds to 100 g of protein.
- If the amount of supplied nitrogen equals the amount allocated, then we can talk about nitrogenous equilibrium... To maintain nitrogen balance in the body, at least 30-45 g of animal protein per day is required ( physiological minimum of protein).
- A condition in which the amount of incoming nitrogen exceeds highlighted, called positive nitrogen balance... A condition in which the amount of incoming nitrogen smaller allocated, called negative nitrogen balance.
- The nitrogen balance in a healthy person is one of the most stable metabolic indicators. The level of nitrogen balance depends on the conditions of human life, the type of work performed, the functional state of the central nervous system and the amount of fats and carbohydrates supplied to the body.
Proteins of organs and tissues need constant renewal. About 400 g of protein out of 6 kg, which make up the protein "fund" of the body, undergo catabolism every day and must be replaced by an equivalent amount of newly formed proteins. The minimum amount of protein that constantly breaks down in the body is called wear rate... The loss of protein in a person weighing 70 kg is 23 g / day. The intake of protein in a smaller amount leads to a negative nitrogen balance, which does not satisfy the plastic and energy needs of the body.
BIOLOGICAL VALUE OF PROTEINSRegardless of species specificity, all diverse protein structures contain in their composition all 20 amino acids... For normal metabolism, not only the amount of protein received by a person is important, but also its qualitative composition, namely the ratio replaceable and essential amino acids.
- Irreplaceable are 10 amino acids that are not synthesized in the human body, but at the same time are absolutely necessary for normal life. The absence of even one of them leads to a negative nitrogen balance, loss of body weight and other disorders incompatible with life.
- Essential amino acids are valine, leucine, isoleucine, threonine, methionine, phenylalanine, tryptophan, cysteine, irreplaceable conditionally — arginine and histidine... A person receives all these amino acids only with food.
- Essential amino acids are also necessary for human life, but they can be synthesized in the body itself from the metabolic products of carbohydrates and lipids. These include glycocol, alanine, cysteine, glutamic and aspartic acids, tyrosine, proline, serine, glycine; conditionally replaceable — arginine and histidine.
- Proteins containing a complete set of essential amino acids are called full-fledged and have the maximum biological value ( meat, fish, eggs, caviar, milk, mushrooms, potatoes).
- Proteins in which at least one essential amino acid is absent or if they are contained in insufficient quantities are called inferior (vegetable proteins). In this regard, to meet the need for amino acids, the most rational is a varied food with a predominance of animal proteins.
- Daily requirement in proteins in an adult is 80-100 g of protein, including 30 g of animal origin, and during physical exertion - 130-150 g. These amounts on average correspond physiological optimum protein- 1 g per 1 kg of body weight.
- Animal protein food is almost completely converted into the body's own proteins. The synthesis of body proteins from vegetable proteins is less efficient: the conversion factor is 0.6 - 0.7 due to the imbalance of essential amino acids in animal and plant proteins.
- When feeding on plant proteins, works " minimum rule"according to which the synthesis of its own protein depends on an essential amino acid that is supplied with food to minimum quantity.
After a meal, especially protein, an increase in energy exchange and heat production... When eating mixed food, energy metabolism increases by about 6%, with protein nutrition, the increase can reach 30-40% of the total energy value of all protein introduced into the body. An increase in energy exchange begins in 1-2 hours, reaches a maximum after 3 hours and continues for 7-8 hours after a meal.
Hormonal regulation the metabolism of proteins provides a dynamic balance of their synthesis and decay.
- Protein anabolism controlled by hormones of the adenohypophysis ( somatotropin), pancreas ( insulin), male reproductive glands ( androgen). Strengthening the anabolic phase of protein metabolism with an excess of these hormones is expressed in increased growth and increase in body weight. Lack of anabolic hormones causes stunted growth in children.
- Protein catabolism regulated by thyroid hormones ( thyroxine and triiodothyronone), crustal ( clucocorticoids) and cerebral ( adrenalin) substances of the adrenal glands. An excess of these hormones enhances the breakdown of proteins in tissues, which is accompanied by depletion and negative nitrogen balance. Lack of hormones, such as the thyroid gland, is accompanied by obesity.
Proteins are, of course, one of the most important components in the life of the body. And most importantly, they play an extremely important role in human nutrition, since they are the main constituent of the cells of all organs and tissues of the body. It is not without reason that in 2005, according to a bill prepared by the Ministry of Health and Social Development, "in order to improve the quality of food in the new consumer basket, it is proposed to increase the volume of products containing animal protein, while reducing the volume of products containing carbohydrates."
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Proteins are high molecular weight compounds (polymers) consisting of amino acids - monomeric units interconnected by peptide bonds. All 20 amino acids found in proteins are a-amino acids, the common feature of which is the presence of an amino group - NH2 and a carboxyl group - COOH at the a-carbon atom. a-amino acids differ from each other in the structure of the R group and, therefore, in properties. All amino acids can be grouped based on the polarity of the R groups, i.e. their ability to interact with water at biological pH values.
In living organisms, the amino acid composition of proteins is determined by the genetic code; in the synthesis, in most cases, 20 standard amino acids are used. Their many combinations create protein molecules with a wide variety of properties. In addition, amino acid residues in a protein often undergo post-translational modifications, which can occur both before the protein begins to perform its function and during its “work” in the cell. Often in living organisms, several molecules of different proteins form complex complexes, for example, a photosynthetic complex.
Crystals of various proteins grown on the Mir space station and during the flights of NASA shuttles. Highly purified proteins form crystals at low temperatures, which are used to study the spatial structure of a given protein.
The functions of proteins in the cells of living organisms are more diverse than the functions of other biopolymers - polysaccharides and DNA. Thus, enzyme proteins catalyze the course of biochemical reactions and play an important role in metabolism. Some proteins have a structural or mechanical function to form a cytoskeleton that maintains the shape of cells. Proteins also play a key role in cell signaling systems, in the immune response and in the cell cycle.
Proteins are an important part of the nutrition of animals and humans (main sources: meat, poultry, fish, milk, nuts, legumes, grains; to a lesser extent: vegetables, fruits, berries and mushrooms), since their bodies cannot synthesize all the necessary amino acids and some must come with protein foods. In the process of digestion, enzymes break down consumed proteins into amino acids, which are used for the biosynthesis of the body's own proteins or undergo further breakdown for energy.
Determination of the amino acid sequence of the first protein, insulin, by protein sequencing earned Frederick Sanger the Nobel Prize in Chemistry in 1958. The first three-dimensional structures of the proteins hemoglobin and myoglobin were obtained by X-ray diffraction, respectively, by Max Perutz and John Kendrew in the late 1950s, for which they received the Nobel Prize in Chemistry in 1962.
Peptide bonds are formed when the a-amino group of one amino acid interacts with the a-carboxyl group of another amino acid: A peptide bond is an amide covalent bond that connects amino acids in a chain. Therefore, peptides are chains of amino acids.
The depiction of the amino acid sequence in the chain begins with the N-terminal amino acid. The numbering of amino acid residues begins with it. In the polypeptide chain, the group is repeated many times: -NH-CH-CO-. This group forms the peptide backbone. Consequently, the polypeptide chain consists of a backbone (skeleton), which has a regular, repeating structure, and individual side chains of R-groups. The primary structure is characterized by the order (sequence) of alternation of amino acids in the polypeptide chain. Even peptides of the same length and amino acid composition can be different substances because the sequence of amino acids in the chain is different. The sequence of amino acids in a protein is unique and is determined by genes. Even small changes in the primary structure can seriously alter the properties of the protein. It would be wrong to conclude that every amino acid residue in a protein is required to maintain the protein's normal structure and function.
The functional properties of proteins are determined by their conformation, i.e. the location of the polypeptide chain in space. The uniqueness of the conformation for each protein is determined by its primary structure. In proteins, two levels of peptide chain conformation are distinguished - secondary and tertiary structures. The secondary structure of proteins is due to the ability of peptide bond groups to hydrogen interactions: C = O .... HN. The peptide tends to adopt a conformation with a maximum of hydrogen bonds. However, the possibility of their formation is limited by the fact that the peptide bond has a partially double character, therefore, rotation around it is difficult. The peptide chain acquires not an arbitrary, but a strictly defined conformation fixed by hydrogen bonds. There are several known methods of folding the polypeptide chain: a-helix is formed by intrachain hydrogen bonds between the NH-group of one amino acid residue and the CO-group of the fourth residue from it; b -structure (folded sheet) - formed by interchain hydrogen bonds or bonds between sections of one polypeptide chain bent in the opposite direction; a disorderly tangle - these are areas that do not have a regular, periodic spatial organization. But the conformation of these regions is also strictly determined by the amino acid sequence. The content of a-helices and b-structures in different proteins is different: fibrillar proteins have only a-helix or only b-folded sheet; and in globular proteins, individual fragments of the polypeptide chain: either an a-helix, or a b-folded sheet, or a disordered tangle. The tertiary structure of globular proteins represents the spatial orientation of the polypeptide chain containing a-helices, b-structures, and regions without a periodic structure (disordered coil). Additional folding of the coiled polypeptide chain forms a compact structure. This occurs primarily as a result of interactions between the side chains of amino acid residues.
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PROTEIN STRUCTURE
Proteins are natural high molecular weight organic compounds built from 20 amino acids. A protein molecule is a non-branching polymer, the minimum structural unit of which, a monomer, is represented by an amino acid. Amino acids in a protein molecule are linked by a carbamide (polypeptide) bond in long chains. Molecular weight - from several thousand to several million atomic units. Depending on the shape of the protein molecule, globular and fibrillar proteins are distinguished.
Globular proteins are characterized by a spherical shape of the molecule, and are soluble in water and saline solutions. Good solubility is explained by the localization of charged amino acid residues on the surface of the globule, surrounded by a hydration shell, which ensures good contact with the solvent. This group includes all enzymes and most biologically active proteins.
Fibrillar proteins are characterized by a fibrous structure and are practically insoluble in water and saline solutions. Polypeptide chains in molecules are parallel to one another. Participate in the formation of structural elements of connective tissue (collagens, keratins, elastins). A special group is complex proteins, which, in addition to amino acids, include carbohydrates, nucleic acids, etc. In all living organisms, proteins play an extremely important role. They are involved in the construction of cells and tissues, are biocatalysts (enzymes), hormones, respiratory pigments (hemoglobins), protective substances (immunoglobulins), etc. Protein biosynthesis occurs on ribosomes and is determined by the nucleic acid code during translation.
20 amino acids, linked together in value and alternating in different sequences, represent the whole variety of natural proteins. The human body is able to form many amino acids from other substances, but it cannot synthesize 9 amino acids by itself and must necessarily receive them from food. Such acids are called essential, or essential. These are valine, leucine, isoleucine, lysine, methionine, threonine, tryptophan, phenylalanine, histidine. Nonessential amino acids include alanine, asparagine, aspartic acid, arginine, glycine, glutamine, glutamic acid, proline, cysteine, tyrosine, series. If the protein is lacking any essential amino acid, the protein will not be fully absorbed. From this point of view, animal products (meat, fish, milk) are more in line with human needs than plant products.
Primary structure is a concept that denotes the sequence of amino acid residues in a protein. The peptide bond is the main type of bond that determines the primary structure.
The secondary structure characterizes the shape of the protein chain in space. This form varies depending on the set of amino acids and their sequence in the polypeptide chain. There are two main forms of the secondary structure: α-helix and β-configuration. Many proteins have an α-helix shape. It can be imagined as a regular spiral formed on the surface of a cylinder. The stability of the helical configuration is determined by the numerous hydrogen bonds between CO and NH groups of peptide bonds; The β-configuration is characteristic of a small number of proteins. In shape, this structure can be compared with accordion furs (folded structure)
The tertiary structure arises due to the bends of the peptide chain in space. This configuration can be represented as a spiral formed on a cylinder, the axis of which periodically changes direction, which leads to the formation of bends.
PROPERTIES OF PROTEINS
Solubility depends on the pH of the solution, the nature of the solvent (its dielectric constant), the concentration of the electrolyte, i.e. on the ionic strength and type of counterion and on the structure of the protein. Globular proteins are well soluble, fibrillar proteins are much worse. At low ionic strength, ions increase the solubility of the protein by neutralizing its charged groups. So, euglobulins are insoluble in water, but dissolve in weak solutions of sodium chloride. At high ionic strength, ions contribute to the deposition of proteins, as if competing with them for water molecules - the so-called salting out of proteins. Organic solvents precipitate proteins, causing them to denature.Electrolytic properties proteins are due to the fact that in the basic environment the molecules behave like polyanions with a negative, and in an acidic environment - with a positive total charge. This determines the ability of proteins to migrate in an electric field to the anode or cathode, depending on the total charge. The analysis of their mixture - electrophoresis - is based on this property of proteins.
Protein denaturation is a consequence of breaking weak bonds, leading to the destruction of secondary and tertiary structures. The denatured protein molecule is disordered - it takes on the character of a random (statistical) coil. As a rule, protein denaturation is irreversible, but in some cases, after removal of the denaturing agent, renaturation can occur - restoration of secondary and tertiary structures and properties.Denaturing agents: high temperatures (breaking hydrogen and hydrophobic bonds), acids and bases (breaking electrostatic bonds), organic solvents (breaking mainly hydrophobic bonds).
Denaturing agents also include detergents, heavy metal salts, ultraviolet light and other types of radiation.
Denaturation does not violate covalent bonds, but increases their availability for other factors, in particular for enzymes.
FUNCTIONS OF PROTEINS
Catalytic or enzymatic. All chemical transformations in a living organism take place with the participation of catalysts. Biological catalysts (enzymes) by chemical nature are proteins that catalyze chemical transformations in the body, from which metabolism is composed.Transport function. Proteins transport or carry biologically significant compounds in the body. In some cases, the transported compound is sorbed by a protein molecule. This protects them from destruction and ensures their transport with the blood stream. This type of transport is called passive. With the help of membrane proteins, compounds are transferred from zones of low concentration to a zone of high concentration. This is associated with noticeable energy consumption and is called active transport.
Mechanochemical function- the ability of some proteins to change conformation, i.e. reduce the length of the molecule, shrink. These proteins are called contractile proteins (muscle proteins) because they perform mechanical work using the energy of chemical bonds.
Structural The (plastic) function is performed mainly by fibrillar proteins - elements of cell membranes. These proteins in the composition of connective tissues provide their strength and elasticity: hair and hair keratin, collagens of tendons, skin, cartilage, vascular walls and binding tissues.
Hormonal function(control function) is implemented by hormones of a peptide or protein nature. They affect the production or activity of enzyme proteins and change the rate of chemical reactions catalyzed by them, i.e. control metabolic processes
Protective function proteins are realized by antibodies, interferons, fibrinogen.
Antibodies- compounds of a protein nature, the synthesis of which is induced in the course of the immune response - the reaction of the body to the penetration of foreign proteins or other antigenic components into the internal environment (for example, high molecular weight carbohydrates). Antibodies, when combined with the antigen, form an insoluble complex, making the antigen safe for the body.
Interferons- glucoproteins synthesized by the cell after the penetration of the virus into it. Unlike antibodies, interferons do not interact with the antigen, but cause the formation of intracellular enzymes. They block the synthesis of viral proteins, preventing the copying of viral information. This stops the virus from multiplying.
Fibrinogen- a soluble plasma protein, which at the last stage of the blood coagulation process is transformed into fibrin - an insoluble protein. Fibrin forms a skeleton of a thrombus that limits blood loss.
Plasmin- a blood plasma protein that catalyzes the cleavage of fibrin. This ensures the restoration of the patency of the vessel, clogged with fibrin clot.
Energy function protein is provided by a portion of the amino acids released during the breakdown of protein in tissues. In the process of redox decomposition, amino acids release energy and synthesize an energy carrier - ATP (adenosine triphosphoric acid). Protein accounts for about 18% of human energy consumption.
ASSEMBLY OF PROTEINS
Among the organic substances of living matter, proteins occupy a special place in terms of their importance and biological functions. About 30% of all proteins in the human body are found in muscles, about 20% in bones and tendons, and about 10% in skin. But the most important proteins are enzymes. Their number in the body is small, but they control a number of very important chemical reactions. All processes occurring in the body: food digestion, oxidative reactions, the activity of the endocrine glands, muscle activity and the work of the brain are regulated by enzymes. Their variety is enormous. There are many hundreds of them in a single cell.
Proteins, or as they are otherwise called, proteins, have a very complex structure and are the most complex of nutrients. Proteins are an essential part of all living cells. Proteins include carbon, hydrogen, oxygen, nitrogen, sulfur and sometimes phosphorus... Most typical for protein is the presence in it nitrogen.
Other nutrients do not contain nitrogen. Therefore, the protein is called a nitrogen-containing substance. The main nitrogen-containing substances that make up protein are amino acids. The number of amino acids is small - only 28 of them are known. All the enormous variety of naturally occurring proteins is a different combination of known amino acids. The properties and quality of proteins depend on their combination.
When two or more amino acids are combined, a more complex compound is formed - polypeptide... When combined, polypeptides form even larger and more complex particles and, as a result, a complex protein molecule.
In the digestive tract, through a series of intermediate stages (albumoses and peptones), proteins are split into simpler compounds (polypeptides) and further into amino acids. Amino acids, unlike protein, are easily absorbed and assimilated by the body. They are used by the body to form its own specific protein. If, due to the excess intake of amino acids, their breakdown in the tissues continues, then they are oxidized to carbon dioxide and water.
Most proteins are water soluble. Due to their large size, protein molecules hardly pass through the pores of cell membranes. When heated, aqueous solutions of proteins coagulate. There are proteins (such as gelatin) that dissolve in water only when heated.
When absorbed, food first enters the mouth and then through the esophagus into the stomach. Pure gastric juice is colorless, has an acidic reaction, which is due to the presence of hydrochloric acid in a concentration of 0.5%.
Gastric juice has the ability to digest food, which is associated with the presence of enzymes in it. It contains pepsin, an enzyme that breaks down protein into peptones and albumoses. Pepsin is produced by the glands of the stomach in an inactive form; it becomes active when exposed to hydrochloric acid. Pepsin acts only in an acidic environment and becomes inactive when it enters an alkaline environment.
Food, entering the stomach, lingers in it from 3 to 10 hours. The duration of the stay of food in the stomach depends on its nature and physical condition - whether it is liquid or solid. Water leaves the stomach immediately after ingestion. Foods containing more proteins remain in the stomach longer than carbohydrate foods; fatty food remains in the stomach even longer. The advancement of food occurs due to the contraction of the stomach, which contributes to the transition into the pyloric part, and then into the duodenum of the already significantly digested food gruel, where its further digestion takes place. Here, the juice of the intestinal glands is poured onto the food gruel, which is dotted with the intestinal mucosa, as well as the juice of the pancreas and bile. Under the influence of these juices, food substances - proteins, fats, carbohydrates - are further broken down and brought to such a state where they can be absorbed into the blood and lymph.
Pancreatic juice is colorless and has alkaline reaction.One of the main enzymes is trypsin, located in the juice of the pancreas in an inactive state in the form of trypsinogen. Trypsinogen cannot break down proteins unless it is put into an active state, i.e. into trypsin. This occurs under the influence of a substance in the intestinal juice enterokinases... Enterokinase is formed in the intestinal mucosa. In the duodenum, the action of pepsin stops, since pepsin-acts only in an acidic environment. Further digestion of proteins continues under the influence of trypsin.
Trypsin is very active in an alkaline environment. Its action continues in an acidic environment, but the activity decreases. Trypsin acts on proteins and breaks them down to albumosis and peptones and further to amino acids.
In the stomach and duodenum, proteins, fats and carbohydrates are broken down almost completely, only part of them remains undigested. In the small intestines, under the influence of intestinal juice, the final splitting of all nutrients and absorption of products into the blood take place. This takes place through capillaries, each of which approaches a villi located on the wall of the small intestines.
PROTEIN EXCHANGE
After proteins are broken down in the digestive tract, the formed amino acids are absorbed into the bloodstream along with a small amount of polypeptides - compounds consisting of several amino acids. From amino acids, the cells of our body synthesize a protein that is different from the consumed protein and is characteristic of a given human body.
The formation of new protein in the body of humans and animals goes on continuously, since throughout life instead of dying cells of blood, skin, intestinal mucosa, etc. new, young cells are created. Proteins enter the digestive canal with food, where they are broken down into amino acids, and a protein specific for these cells is formed from the absorbed amino acids. If, bypassing the digestive tract, introduce protein directly into the blood, then it not only cannot be used by the human body, but also cause a number of serious complications. The body responds to such an introduction of protein with a sharp increase in temperature and some other phenomena. With repeated introduction of protein after 15-20 days, even death may occur with respiratory paralysis, a sharp violation of cardiac activity and general convulsions.
Proteins cannot be replaced by any other nutrients, since protein synthesis in the body is possible only from amino acids. Therefore, the intake of all or the most important amino acids is so necessary.
Of the known amino acids, not all have the same value for the body. Among them there are those that can be replaced by others or synthesized in the body from other amino acids. Along with this, there are essential amino acids, in the absence of which, or even one of them, protein metabolism in the body is disrupted.
Proteins do not always contain all amino acids, in some - more amino acids needed by the body, in others - less. Different proteins contain different amino acids and different proportions.
Proteins, which include all the amino acids necessary for the body, are called complete proteins. Proteins that do not contain all the essential amino acids are deficient.
For a person, it is important to receive full-fledged proteins, since the body can freely synthesize its specific proteins from them. However, a complete protein can be replaced by two or three defective proteins, which, complementing each other, add up all the necessary amino acids. Therefore, for the normal functioning of the body, it is necessary that the food contains complete proteins or a set of defective proteins, equal in amino acid content to complete proteins.
The intake of complete proteins with food is extremely important for a growing organism, since in a child's body, along with the restoration of dying cells, as in adults, new cells are created in large numbers.
Regular mixed food contains a variety of proteins, which together provide the body's need for amino acids. Not only the biological value of proteins supplied with food is important, but also their quantity. With an insufficient intake of proteins, the normal growth of the body is suspended or delayed, since the needs for protein are not covered due to its insufficient intake.
Complete proteins include mainly proteins of animal origin, except for gelatin, which is a defective protein. Defective proteins are predominantly of plant origin. However, some plants (potatoes, legumes, etc.) contain complete proteins. Of animal proteins, proteins of meat, eggs, milk, etc. are especially valuable for the body.
31. The quaternary structure of a protein is determined by:
a) spiralization of the polypeptide chain
b) the spatial configuration of the polypeptide chain
c) spiralization of several polypeptide chains
d) joining several polypeptide chains.
32. In maintaining the quaternary structure of the protein, the following are not involved:
a) peptide b) hydrogen c) ionic d) hydrophobic.
33. Physicochemical and biological properties of protein are completely determined by the structure:
a) primary b) secondary c) tertiary d) quaternary.
34. Fibrillar proteins include:
a) globulin, albumin, collagen b) collagen, keratin, myosin
c) myosin, insulin, trypsin d) albumin, myosin, fibroin.
35. Globular proteins include:
a) fibrinogen, insulin, trypsin b) trypsin, actin, elastin
c) elastin, thrombin, albumin d) albumin, globulin, glucagon.
36. A protein molecule acquires natural (native) properties as a result of self-assembly of the structure
a) primary b) mostly primary, less often secondary
c) quaternary d) mostly tertiary, less often quaternary.
37. Monomers of nucleic acid molecules are:
a) nucleosides b) nucleotides c) polynucleotides d) nitrogenous bases.
38. The DNA molecule contains nitrogenous bases:
a) adenine, guanine, uracil, cytosine b) cytosine, guanine, adenine, thymine
c) thymine, uracil, thymine, cytosine d) adenine, uracil, thymine, cytosine
39. The RNA molecule contains nitrogenous bases:
a) adenine, guanine, uracil, cytosine b) cytosine, guanine, adenine, thymine c) thymine, uracil, adenine, guanine d) adenine, uracil, thymine, cytosine.
1. What organelles are responsible for protein synthesis?
2. What are the names of the structures of the nucleus that store information about the proteins of the body?
3. What molecule is a template (template) for the synthesis of i-RNA?
4. What is the name of the process of synthesis of the protein polypeptide chain on the ribosome?
5. On which molecule is a triplet called a codon located?
6. On which molecule is there a triplet called anticodon?
7. By what principle does an anticodon recognize a codon?
8. Where does the t-RNA + amino acid complex form in the cell?
9. What is the name of the first stage of protein biosynthesis?
10. Given a polypeptide chain: -VAL - ARG - ASP- Determine the structure of the corresponding DNA chains.
1) The DNA gene fragment has a trace. the nucleotide sequence TCGGTTSAACTTAGCT. Determine the sequence of nucleotides of m-RNA and amino acids in the polypeptide chain of the protein.
2) Determine the sequence of mRNA nucleotides synthesized from the right chain of a section of a DNA molecule, if its left chain has a trace. sequence: -C-G-A-G-T-T-T-G-G-A-T-T-Ts-G-T-G.
3) Determine the sequence of amino acid residues in the protein molecule
-G-T-A-A-G-A-T-T-T-C-T-C-G-T-G
4) Determine the sequence of nucleotides in the mRNA molecule if the part of the protein molecule synthesized from it looks like: - threonine - methionine - histidine - valine - arg. - proline - cysteine -.
5) How will the structure of the protein change if from the DNA region encoding it:
-G-A-T-A-C-C-G-A-T-A-A-A-G-A-C - remove the sixth and thirteenth (left) nucleotides?
6) What changes will occur in the structure of the protein if in the coding region of DNA: -T-A-A-C-A-G-A-G-G-A-C-C-A-A-G -... between 10 and 11 nucleotides, cytosine is included, between 13 and 14 - thymine, and another guanine breaks through at the end next to guanine?
7) Determine the mRNA and the primary structure of the protein encoded in the DNA region: -G-T-T-C-T-A-A-A-A-G-G-C-C-A-T- .. if 5 - the th nucleotide will be deleted, and between the 8th and 9th nucleotide there will be a thymidyl nucleotide?
8) The polypeptide consists of a trace. amino acids located one after the other: valine - alanine - glycine - lysine - tryptophan - valine - sulfur-glutamic acid. Determine the structure of the DNA region encoding the above polypeptide.
9) Asparagine - glycine - phenylalanine - proline - threonine - methionine - lysine - valine - glycine .... amino acids, consistently constitute a polypeptide. Determine the structure of the DNA region encoding this polypeptide.
