Proteins (synonym proteins) — the high-molecular nitrogenous organic compounds which are polymers of amino acids. B. — the main and necessary component of all organisms.
Nonvolatile solid of most bodies and tissues of the person and animals, and also the most part of microorganisms consist hl. obr. from B. Proteic matters are the cornerstone of the most important processes of life activity. So, e.g., processes of a metabolism (digestion, breath, allocation, etc.) are provided with activity enzymes (see), B. K B. which are by the nature also the sokratitelny structures which are the cornerstone of the movement, napr, sokratitelny protein of muscles (actomyosin), basic body tissues (collagen of bones, cartilages, sinews), covers of an organism (skin, hair, nails, etc.), the consisting hl belong. obr. from kollagen, elastin, keratins, and also toxins, antigens and antibodies, many hormones and other biologically important substances.
The role of proteins in a live organism is emphasized with their already name «proteins» (Greek protos first, primary) offered by Mulder (G. J. Mulder, 1838) who found out that tissues of animals and plants contain the substances reminding ovalbumin on the properties. It was gradually established that B. represent an extensive class of the various substances constructed according to the identical plan. Noting paramount value B. for processes of life activity, Engels defined that life is the way existence of proteinaceous bodies consisting in continuous self-updating of chemical components of these ph.
Chemical composition and structure of proteins
B. contain on average apprx. 16% of nitrogen. At full hydrolysis of B. break up with accession of water to amino acids (see). Molecules B. represent polymers which consist of the remains apprx. 20 various amino acids relating to a natural L-row, i.e. having an identical configuration of alpha and carbon atom though their optical rotation can be unequal and not always sent to one party. The amino-acid structure of different B. is not identical and serves as the most important characteristic of each B., and also criterion of its value in food (see the section of the Squirrel in food). Some B. can be deprived of these or those amino acids. E.g., B. of corn — zein does not contain a lysine and tryptophane. Other B., on the contrary, are very rich with separate amino acids. So, protamin of a salmon — salmin supports over 80% of arginine, fibroin of silk — apprx. 40% of glycine (the amino-acid structure of some B. is presented in tab. 1).
Table 1. AMINO-ACID COMPOSITION of SOME PROTEINS (in weight percents of amino acids of protein)
At incomplete (usually enzymatic) hydrolysis of B., in addition to free amino acids, a number of the substances with rather small molecular weights called is formed peptides (see) and polypeptides. In B. and peptides the amino-acid remains are connected among themselves the thane naz. the peptide (acid and amide) bond formed by a carboxyl group of one amino acid and an amino group of other amino acid:
Depending on number of amino acids such connections call di - three - by tetrapeptides etc., e.g.:
Long peptide chains (polypeptides) consisting of tens and hundreds of the amino-acid remains form a basis of structure of a proteinaceous molecule. Many B. consist of one polypeptide chain, in other B. there are two or more polypeptide chains connected among themselves and forming more complex structure. Long polypeptide chains of identical amino-acid structure can give huge number of isomers due to various sequence of the separate amino-acid remains (just as it is possible to make a set of various words and their combinations of 20 letters of the alphabet). As various amino acids can be a part of polypeptides in different ratios, the number of possible isomers becomes almost infinite, and for each individual B. the sequence of amino acids in polypeptide chains is characteristic and unique. This sequence of amino acids defines primary structure of B., edges in turn is defined by the corresponding sequence of deoxyribonucleotides in structural genes of DNA of this organism. To a crust. time primary structure of many B., hl is studied. obr. proteinaceous hormones, enzymes and some other biologically active B. Posledovatelnost of amino acids define by enzymic hydrolysis of B. and obtaining so-called peptide maps by means of two-dimensional chromatography (see) and electrophoresis (see). Each peptide is investigated on trailer amino acids before processing by aminopolypeptidase — the specific enzyme which is consistently chipping off aminotrailer (N-trailer) amino acids and the carboxypolypeptidase which is chipping off carboxytrailer (S-trailer) amino acids. Apply the reactants connecting to a free amino group of trailer amino acid to definition of N-trailer amino acids. Usually use dinitrofluobenzene (1-fluorine-2,4-dinitrobenzene) giving dinitrophenylic derivative with N-trailer amino acid which then can be identified after hydrolysis and chromatographic fractionation of a hydrolyzate. Along with the dinitrofluobenzene offered by F. Sanger also processing by phenyl isothiocyanide according to P. Edman is applied. At the same time with trailer amino acid it is formed feniltiogidantoin which is easily chipped off from a polypeptide chain and it can be identified. Apply heating of peptide in acetic anhydride with ammonium thiocyanate to definition of S-trailer amino acids. As a result of condensation the ring of thiohydantoin including a radical of trailer amino acid which then is easy for chipping off from peptide turns out and to establish character of S-trailer amino acid. The sequence of amino acids in protein is established on the basis of the sequence of the peptides received using different enzymes and taking into account specificity of each enzyme splitting B. on the peptide bond formed by a certain amino acid. Thus, definition of primary structure of B. represents very laborious and long work. Various methods of direct definition of the sequence of amino acids found successful application with the help X-ray crystallographic analysis (see) or way mass spectrometry (see) derivative the peptides received at B.'s hydrolysis by different enzymes.
Spatially polypeptide chains often form the spiral configurations withheld by means of hydrogen bindings and forming secondary structure B. Most often meets so-called and - a spiral, in a cut 3,7 amino-acid remains are the share of one round.
The separate amino-acid remains in same or in different polypeptide chains can be connected among themselves by means of disulfide or radio bridges. So, in a molecule of monomer of insulin (fig. 1) disulfide bridges connected among themselves the 6 and 11 remains of cysteine of the A-chain and the 7 and 20 remains of cysteine of the A-chain according to the 7 and 19 the remains of cysteine of the V-chain. Such bonds give to the polypeptide chain having usually spiralizovanny and nespiralizovanny sites, the certain conformation called by tertiary structure B.
Mean formation of complexes from monomeric proteinaceous molecules by quarternary structure of B. So, e.g., the molecule of hemoglobin consists of four monomers (two alpha chains and two beta chains). The quarternary structure of enzyme of a lactate dehydrogenase represents the tetramer consisting of 4 monomeric molecules. These monomers happen two types: N characteristic of a cardiac muscle, and M characteristic of skeletal muscles. Respectively 5 different isoenzymes of a lactate dehydrogenase representing tetramers from different combinations of these two monomers — HHHH, HHHM, HHMM, HMMM and MMMM meet. B.'s structure defines its biological properties, and even small disturbance of conformation can affect enzymatic activity or other biological properties B very significantly. Primary structure of B. defined genetically and in turn often defining the highest structures this B nevertheless most is important. Replacement even of one amino-acid rest in the polypeptide chain consisting of hundreds of amino acids can completely change very significantly this B.'s properties and even to deprive of it biological activity. So, e.g., the hemoglobin which is found in erythrocytes at drepanocytic anemia differs from normal hemoglobin A only in replacement of the rest of glutamic acid in the 6th provision of a r-chain for the rest of valine, i.e. replacement of only one of 287 amino acids. However this replacement is enough in order that the changed hemoglobin had sharply broken solubility and considerably lost the main function of transfer of oxygen to fabrics. On the other hand, in strictly certain structure of insulin (fig. 1) character of the amino-acid remains in the 8, 9 and 10 provisions of a chain And (between two remains of cysteine), apparently, has no essential value as these three rests have specific specificity; in insulin of a bull they are presented by the sequence is scarlet, at a sheep — is scarlet - gli - a shaft, at a horse — tre-gli-silt, and in insulin of the person, a pig and whale — tre.........
Physical and chemical properties
Pier. the weight of most of B. makes from 10 — 15 thousand to 100 thousand, however there are B. about a pier. weighing 5 — 10 thousand and several millions. Conditionally polypeptides about a pier. weighing below 5 thousand carry to peptides. B.'s most of liquids and body tissues (e.g. B. blood, eggs, etc.) rastvorima in water or in solutions of salts. B. usually give opalescent solutions which behave as colloid. Incorporating many hydrophilic groups, B. easily connect water molecules and are in fabrics in the hydrated state, forming solutions or gels. Many B. are rich with the hydrophobic remains and are insoluble in usual solvents B. Such B. (e.g., collagen and elastin of connecting fabric, fibroin of silk, keratins of hair and nails) have fibrillar character, and their molecules are extended in long fibers. Soluble B. are usually presented by molecules klubkoobrazny, globular, forms. However B.'s division a pas globular and fibrillar is not absolute as some B. (e.g., actin of muscles) are capable to turn reversibly from a globular configuration in fibrillar depending on conditions of the environment.
Like amino acids. B. are typical ampholytes (see. Ampholytes ), i.e. change the electric charge depending on pH of the environment. In electric field of B. move to the anode or to the cathode depending on a sign of electric charge of a molecule which is defined by both this B.'s properties, and pH of the environment. This movement in electric field called by an electrophoresis is applied to analytical and preparative division of B. which are usually differing on the electrophoretic mobility. At the certain pH called isoelectric point (see), unequal for different B., number of positive and negative charges of a molecule equally each other, and a molecule in general the elektroneytralna also does not move in electric field. This property B. is used for their allocation and cleaning with method of isoelectric focusing, B. consisting in an electrophoresis in the gradient of pH created by system of buffered solutions. At the same time it is possible to pick up such pH value, at Krom the necessary B. drops out in a deposit (as B.'s solubility in an isoelectric point the smallest), and most of the «contaminating» B. will remain in solution.
In addition to pH, B.'s solubility significantly depends from presence and salt content at solution. High salt contents of monovalent cations (most often apply ammonium sulfate) besiege the majority B. Mekhanizm of such sedimentation (salting-out) consists in binding by ions of salts of the water forming a hydrated cover of proteinaceous molecules. Owing to dehydration B.'s solubility goes down and they drop out in a deposit. The mechanism of sedimentation of B. alcohols and acetone is same. B.'s sedimentation by the salting-out or organic liquids which are mixing up with water is applied to division and B.'s allocation with preservation of their natural (native) properties. Under certain conditions B.'s sedimentation it is possible to receive in a crystal look and it is good to clear of other B. and nonprotein impurity. A number of procedures such apply to receiving crystal drugs of many enzymes or others B. Heating of solutions B. to high temperature, and also B.'s sedimentation by salts of heavy metals or concentrated acids, especially trichloroacetic, sulphosalicylic, chloric, leads to coagulation (coagulation) of B. and formation of an insoluble residue. At such influences labile molecules B. denature, lose the biological properties, in particular enzymatic activity, become insoluble in initial solvent. At a denaturation the native configuration of a proteinaceous molecule is broken, and polypeptide chains form chaotic balls.
At B.'s ultracentrifuging are besieged in the field of acceleration of centrifugal force with a speed depending hl. obr. from the sizes of proteinaceous particles. Respectively apply definition of sedimentation constants in the ultracentrifuge to determination of molecular weights of B., and also diffusion rate of B., their filtering through molecular sieves, definition of electrophoretic mobility at an electrophoresis in special conditions and some other methods.
Methods of detection and definition of proteins
Qualitative tests are based on B. on their physical. - hnm. properties or on reactions of certain chemical groups in a molecule B. However, as a large number of various chemical groups is a part of a molecule B., reactivity of B. is very big and any of qualitative tests on proteins is not strictly specific. The conclusion about presence of protein can be made only on the basis of set of a number of reactions. In the analysis of biological liquids, e.g. urine where only certain B. can appear and it is known what substances can prevent reaction, there is enough even one reaction for establishment of presence or absence B. Reactions to B. subdivide into precipitation reactions and staining reactions. Sedimentation by concentrated acids concerns to the first, and in clinical practice most often apply sedimentation nitric to - that. Characteristic reaction is also B.'s sedimentation by sulphosalicylic or trichloroacetic acids (the last is often applied not only to B.'s detection, but also to release of liquids from B.). B.'s presence can be revealed also but to coagulation at boiling in the subacidic environment, sedimentation by alcohol, acetone and some other reactants. From staining reactions it is very characteristic biuret reaction (see) — violet coloring with copper ions in alkaline condition. This reaction depends on presence at B. of the peptide bonds forming the painted complex connection with copper. The name of biuret reaction comes from a product of heating of urea of ureido formamide (H 2 N-CO-NH-CO-NH 2 ), being the elementary connection giving this reaction. Xanthoproteic reaction (see) consists in yellow coloring of a deposit of B. at influence concentrated nitric to - that. Coloring appears owing to formation of nitrates of the aromatic amino acids which are a part of a proteinaceous molecule. Millon's reaction gives bright red coloring with mercury salts and nitrogenous to - that in acid medium. In practice usually use nitric to - that, edges always contains small impurity of nitrogenous. Reaction is specific to the phenolic radical of tyrosine and therefore it turns out only with B. containing tyrosine. The Adamkiewicz's test is caused by the radical of tryptophane. It gives violet coloring in the concentrated chamois to - those with acetic to - that (see. Adamkevich reaction ). Reaction turns out during the replacement acetic to - you on various aldehydes. During the use acetic to - you reaction is caused glyoxylic to - that, contained in acetic as impurity. Quantitatively B. determine usually by protein nitrogen, i.e. by the content of the general nitrogen in B.'s deposit washed from low-molecular substances, soluble in a precipitator. Nitrogen in biochemical researches and in clinical analyses is usually determined by Kyeldal's method (see. Kyeldalya method ). The general maintenance of B. in liquids is often determined by colorimetric methods which cornerstone different modifications of biuret reaction are. Often use Lauri's method, in Krom Folin's reactant on tyrosine in combination with biuret reaction is applied (see. Lauri method ).
Classification of proteins
Because of rather big sizes of proteinaceous molecules, complexity of their structure and absence enough exact data on structure of most of B. is not present rational chemical classification B yet. The existing classification considerably is conditional and constructed by hl. obr. on the basis of physical. - chemical properties B., sources of their receiving, biological activity and others, quite often accidental, signs. So, on physical. - to chemical properties B. divide on fibrillar and globular, on hydrophilic (soluble) and hydrophobic (insoluble), etc. On a source of receiving a squirrel subdivide on animal, vegetable and bacterial; on B. muscular, nervous tissue, blood serum, etc.; on biological activity — on proteins-enzymes. proteins-hormones, structural. B., sokratitelny B., antibodies etc. It must be kept in mind, however, that because of imperfection of the classification, and also owing to exclusive variety of B. many of separate B. cannot be carried to one of the groups described here.
All B. can be shared on simple, or proteins (actually B.), and difficult, or proteids (complexes B. with nonprotein connections). Simple B. are polymers only of amino acids; difficult, in addition to the remains of amino acids, contain also nonprotein, so-called prosthetic groups.
Albumine — easily soluble globular B. (e.g., albumine of blood serum or ovalbumin); are dissolved in water p saline solutions with settling-out only at saturation of solution ammonium sulfate.
Globulins differ from albumine in insolubility in water and settling-out at semi-saturation of solution ammonium sulfate. Globulins possess higher, than albumine, molecular weight and sometimes supports carbohydrate groups in the structure.
Phytalbumins also belong to proteins — prolamines (see), meeting usually together with glutelins (see) in seeds of cereals (rye, wheat, barley, etc.), forming the ground mass of a gluten. These B. of a rastvorima in 70 — 80% alcohol are also water-insoluble; they are rich with the remains of proline and glutaminic to - you. Also gliadine of wheat, zein of corn, hordein of barley belong to prolamines.
Scleroproteins (proteinonda, albuminoids) — structural B., water-insoluble, in divorced alkalis, acids and saline solutions. Fibrillar B. of hl concern to them. obr. animal origin, very steady against digestion by digestive enzymes. These B. subdivide into B. of connecting fabric: collagen (see) and elastin (see); B. covers — hair, nails and hoofs, epidermis — keratins (see) of which the high content of sulfur in the form of the rest of amino acid — cystine is characteristic; B. cocoons and other secrets of shelkootdelitelny glands of insects (e.g., webs) — fibroin (see), consisting more than half of the remains of glycine and alanine.
The special group of proteins is made protamins (see) — rather low-molecular B. of the main character (unlike albumine, globulins and other fabric B. having an isoelectric point usually in the subacidic environment). Protamins meet in sperm of some fishes and other animals and consist more than half of diaminomonocarboxylic acids. So, protamins of a herring — clupein and a salmon — salmin contain apprx. 80% of arginine. Other protamins contain, in addition to arginine, also a lysine or a lysine and a histidine.
Histones (see) — nuclear B. of less alkaline character as well as protamins, usually are in a complex with deoxyribonucleic acid (see), have a little higher molecular weight, contain less, than protamins, diaminomonocarboxylic acids.
Difficult B. divide into a number of classes depending on character of prosthetic group. Nucleoproteids (see) represent complexes B. with nucleic acids. These are high-molecular, often supermolecular features, e.g. chromatin (see), ribosomes (see), many viruses (see), the carrying-out major vital signs connected with transfer of hereditary information, B.'s biosynthesis and regulation of these processes.
Mukoproteida (see) contain mucopolysaccharides of acid character. Group-specific substances of blood, mucins and mucoids of slizy, synovial fluid concern to them.
Phosphoproteins (see) include the remains phosphoric to - you, the amino acids of serine which are usually connected by an ester group with the rest. Phosphoproteins meet in cellular kernels, in milk (see. Caseins ), in egg yolk.
Metalloproteins (see) — the difficult B. containing these or those metals or containing organometallic (prosthetic) groups. Many enzymes, in particular oxidoreductases concern to them.
In special group allocate chromoproteids (see) — B. supporting the painted groups. Many of them contain metals, e.g. hemoglobin (see) or chlorophyll (see) and others pigments (see). Presence of metal and chromoproteids is not obligatory (e.g., rhodopsin of a retina of an eye).
Biosynthesis of proteins
B. come to a human body and animals with poor and are the main source of food nitrogen. In the course of B.'s digestion are exposed to hydrolysis to amino acids in the form of which are soaked up in blood and are exposed to further transformations. A special role is played by irreplaceable amino acids which define nutrition value B. Since absorption in blood, B.'s exchange is, in essence, exchange of amino acids. Protein metabolism, or amino-acid exchange, represents the main part nitrogen metabolism (see).
B.'s biosynthesis proceeds in all cells of live organisms and provides B.'s obnovlenets of an organism, processes of a metabolism and their regulation, and also growth and a differentiation of bodies and fabrics. B. are synthesized in fabrics from free amino acids with participation nucleic acids (see). Process of biosynthesis of B. proceeds with consumption of the energy accumulated in the form of ATP (see. Adenozinfosforny acids ). At B.'s biosynthesis certain B.' education of strictly specific structure is provided, edges it is coded in structural genes (cistrons) deoxyribonucleic to - you, being hl. obr. in chromatin of kernels of cells (see. Genetic code ). Information defining primary structure of B. is transferred to special type of the RNA (RNA) called information, or matrix, by RNA (MRNK) in the form of the complementary sequence of nucleotides. This process received the name of a transcription. MRNK connects with ribosomes (see), representing ribonukleoproteidny granules, more than half consisting of the special ribosomal RNA (RRNK) synthesized also on special cistrons (genes) of DNA. Ribosomes consist of two subparticles on which they are capable to dissociate reversibly at decrease in ion concentration of magnesium. Big and small subparticles of ribosomes contain but one molecule RNA with molecular weight respectively apprx. 1,7×10 6 and 0,7×10 6 and but some tens of molecules B. Having connected to ribosomes, MRNK forms polyribosomes, or polysom on which there is a synthesis of the polypeptide chains forming primary structure B. Before connecting to ribosomes. amino acids are activated, then connect to low-polymeric RNA carriers, or acceptor RNA (TRNK) in the form of complexes, with to-rymi they and come to ribosomes. The general scheme of biosynthesis of B. is submitted in fig. 2.
Activation of amino acids happens at their interaction to ATP with formation of aminoacyladenylate and release of a pyrophosphate: amino acid + ATP = aminoacyladenylate + pyrophosphate. Aminoatsiladenilat represents the mixed anhydride formed by the rest phosphoric to - you adenosinemonophosphate and a carboxyl group of amino acid, and is the activated form of amino acid. From aminoacyladenylate the rest of amino acid is postponed to TRNK specific to each amino acid, and in a look aminoacyl-TRNK comes to ribosomes. Formation of aminoacyladenylate and transfer of the amino-acid rest on TRNK are catalyzed by the same enzyme (aminoatsiladenilatsintetazy, or aminoacyl-TRNK-sintetazoy), strictly specific to each amino acid and each TRNK. In total TRNK have rather small molecular weight (apprx. 25 000) and contain apprx. 80 nucleotides. They have the crosswise configuration reminding a clover leaf, and the nucleotide chain forms the two-filamentous structure withheld by the complementary bases and passes in one-filamentous only in the field of loops. The beginning of a nucleotide chain which is usually presented 5' - a guanylic nucleotide, is near the trailer, often exchanging group from two remains cytidylic to - you and adenosine with free 3 '-OH - group, to a cut and the rest of amino acid joins. On the loop which is at the opposite end of molecule TRNK there is a triplet of the bases, complementary to the triplet coding this amino acid (codon), and called by an anti-codon. The nucleotide sequence of many TRNK is already established, known also their full structure.
A certain sequence of amino acids in primary structure of a synthesizable polypeptide chain is provided with information which is written down in the sequence of nucleotides of MRNK reflecting the corresponding sequence in DNA cistrons. Each amino acid is coded by certain triplets of nucleotides of MRNK. These triplets (codons) are presented in tab. 2. Their interpretation allowed to establish the RNA nucleotide code, or an amino-acid code, i.e. a way, with the help to-rogo there is a broadcasting, or transfer of information which is signed up in the sequence of nucleotides of RNA in primary structure of B., or the sequence of the amino-acid remains in a polypeptide chain.
Table 2. RNK-AMINOKISLOTNYY CODE
Note: At — uridilovy acid, C — cytidylic acid, And — adenylic acid — guanylic acid. Three letters designate the corresponding amino-acid rest: e.g. The hair dryer — phenylalanine. Silt — an isoleucine, Gloux — glutamic acid, Gln — a glutamine, etc. Triplets of UAA, UAG, UGA do not code amino acids, but define termination of a polypeptide chain.
Apparently from the table, from 64 possible triplets (61 code certain amino acids, i.e. are «semantic». Three triplets — UDA, UAG and UGA — do not code amino acids, however their role consists in completion (termination) of synthesis of the growing polypeptide chain. The code is degenerate, i.e. almost all amino acids are coded more than one triplet of nucleotides. So, 3 amino acids — a leucine, arginine and series — are coded by six codons, 2 — methionine p tryptophane — the others have only on one codon, and 15 — from 2 to 4. Process of broadcasting is carried out by means of TRNK loaded with amino acids. Aminoatsil-TRNK joins the complementary triplet (anti-codon) a codon of MRNK in a ribosome. Another joins the next codon of MRNK aminoacyl-TRNK. The first TRNK at the same time attaches the amino-acid rest the carboxyl end to an amino group of the second amino acid, with formation of dipeptide, and itself is released and separates from a ribosome. Further, in process of advance of a ribosome but a chain of MRNK from 5' - the end to Z '-to the end, the third joins aminoacyl-RNA; there is a compound of dipeptide the carboxyl end to an amino group of the third amino acid with formation of tripeptide and release of the second TRNK and so until the ribosome does not pass all site coding this protein on MRNK corresponding to DNA cistron. Then there is a termination of synthesis of B., and the formed polypeptide is exempted from a ribosome. The first ribosome in polysom is followed by the second, third Etc., K-rye consistently read out information on the MRNK same thread in polysom. Thus, growth of a polypeptide chain happens since the N-end to carboxyl (With-) to the end. If to suppress B.'s synthesis, e.g., by means of an antibiotic of puromycin, then it is possible to receive unfinished polypeptide chains from the S-end, incomplete at different stages. Aminoatsil-TRNK joins at first a small ribosomal subparticle, and then it is transferred to a big subparticle, on a cut and there is a growth of a polypeptide chain. According to A.S. Spirin's hypothesis in operating time of a ribosome at B.'s biosynthesis there is a repeating smykaniye and disconnection of subparticles of ribosomes. Reproduction of synthesis of B. out of an organism, in addition to ribosomes, MRNK and aminoacyl-TRNK, requires presence of a guanozintrifosfat (GTF) which is split to GDF and again regenerates in the course of growth weed a peptide chain. Also presence of several proteinaceous factors which are carrying out, apparently, an enzymatic role is necessary. These so-called transfer factors interact with each other and for the activity demand presence of sulphhydryl groups and ions of magnesium. In addition to actually broadcasting (i.e. growth of a polypeptide chain in the certain sequence corresponding to a structural gene of DNA and transmitted by the sequence of nucleotides to MRNK), a special role is played by the beginning (or initiation) broadcastings and end (or termination) it. Initiation of proteinaceous synthesis in a ribosome, at least in bacteria, begins with special codons — initiators in MRNK — АУГ and GUG. At first the small subparticle of a ribosome contacts such codon. then formylmethionyl-TRNK joins it, about a cut and synthesis of a polypeptide chain begins. Owing to special characteristics of this aminoacyl-TRNK it is capable to be transferred to a big subchastntsug is similar peptidil-TRNK, and thus to begin growth of a polypeptide chain. GTF and proteinaceous initiating factors are necessary for initiation (it is known three). Termination of growth of a polypeptide chain happens on «senseless» codons of UAA, UAG or UGA. Apparently, these codons contact a special proteinaceous termination factor which in the presence of one more factor promotes release of polypeptide.
Components of system of biosynthesis of B. are synthesized by hl. obr. in a cellular kernel. On a matrix of DNA in the course of a transcription there is a synthesis of all RNA types. participating: in this process: RRNK, MRNK and TRNK. So, RRNK and MRNK are synthesized in the form of very big molecules and in a cellular kernel undergo process of «maturing», in the course to-rogo a part (very considerable for MRNK) molecules is chipped off and exposed to disintegration, without leaving in cytoplasm, and the functioning molecules being a part originally synthesized, come to cytoplasm to places of proteinaceous synthesis. Before getting into structure the policy, MRNK, apparently, from the moment of synthesis contacts special proteinaceous particles, «informoferes», and in the form of a ribonukleoproteidny complex is transferred to ribosomes. Ribosomes, obviously, also «ripen» in cytoplasm, a part of proteins joins the predecessors of ribosomes leaving a kernel, already in cytoplasm. It should be noted that at the lowest, nuclear-free organisms (prokariot), to the Crimea bacteria, blue-green seaweed and viruses belong, there are some differences from the higher organisms in components of system of biosynthesis of B., and in particular in its regulation. Ribosomes at prokariot slightly less by the sizes also differ on structure, process of a transcription and broadcasting is directly connected in a single whole. At the same time at the higher nuclear organisms (eukaryotes) formation of RNA happens also in the organellas of cytoplasm, mitochondrions and chlorolayers (at plants) possessing own system of synthesis of protein and own genetic information in the form of DNA. On the device the system of proteinaceous synthesis in mitochondrions and chlorolayers is similar that at prokariot and significantly differs from the system which is available in a kernel and cytoplasm of the highest animals and plants.
Regulation of biosynthesis of B. represents very complex system and allows a cell quickly and to react accurately to changes in the pericellular environment by the termination or induction of synthesis of various B. which often have enzymatic activity. At bacteria suppression of synthesis of B. is carried out by hl. obr. by means of special B. — repressor (see. Operon ), synthesized by special regulator genes. Interaction of a repressor with the metabolite arriving from the environment or synthesized in a cell can suppress or, on the contrary, activate it, regulating thus synthesis of one B. or several interconnected B., in particular the enzymes which are synthesized is also interconnected on one operon. At the higher organisms in the course of a differentiation of fabric lose ability to synthesis of a row B. and specialize in synthesis of smaller number B., necessary for function of this fabric, e.g. muscles. Such blocking of synthesis of a row B. happens, apparently, at the level genome (see) by means of nuclear proteins — histones (see), DNA connecting nonfunctional sites. However at the regeneration, malignant growth and other processes connected with dedifferentiation, such blocked sites can derepressirovatsya and deliver to MRNK for synthesis unusual to this fabric B. Nevertheless and at the higher organisms regulation of synthesis of B. in response to these or those incentives takes place. So, action of a number of hormones consists in induction of synthesis of B. in the fabric which is «target» of this hormone. Such induction, apparently, happens by linkng of hormone with special B. of this fabric and activation of a gene by means of an educated complex.
Process of biosynthesis of B. and its regulation demand the extraordinary clearness, accuracy and coordination of work of all components of system. Even small disturbances of this accuracy lead to disturbance of primary structure of B. and serious pathological consequences. Genetic disorders, napr, replacement or loss of one nucleotide in a structural gene, lead to synthesis of the changed B. which is quite often deprived of biological activity. Such changes are the cornerstone of inborn disbolism, to the Crimea, in essence, all belong hereditary diseases (see). On the other hand, a number of B. and enzymes can differ not only at different species, but also at different individuals, keeping at the same time the biological activity. Quite often such B. have different immunological and electrophoretic properties. In human populations many examples of so-called polymorphism of B. when at different individuals, and sometimes and it is possible to find two or several unequal B. possessing the same function as, e.g., in the same individual are described hemoglobin (see), gaptoglobin (see) and some other.
Proteins in food
Among numerous feedstuffs B. the most important role belongs. They are sources of the irreplaceable amino acids and so-called nonspecific nitrogen necessary for synthesis for B. of a human body. The expressed B.'s insufficiency in food leads to heavy dysfunctions of an organism (see. Nutritional dystrophy ). The state of health, physical development and - efficiency of the person, and at children of early age in a certain measure and intellectual development to a large extent depends on the level of supply of B. If to consider all vegetable and animal B. made for food, then on average on each inhabitant of Earth it is necessary apprx. 58 g a day. Actually more than a half of the population, especially developing countries, does not receive this amount of protein. Global deficit of food B. shall be referred to number of the most acute economic and social issues of the present (see. Crisis proteinaceous ). In this regard establishment of optimum levels of maintenance of B. in diets gains paramount importance.
In the greatest numbers of B. are required during the periods of intensive growth. However and in the organism which reached a maturity, processes of life activity are connected with continuous expenditure of proteic matters and, therefore, need vospol a neniya of these losses with food. According to recommendations of Expert group of FAO/WHO calculation of need for protein nitrogen should be carried out on a formula: R = 1,1 (U b + F b + S+G), where R — the need for protein nitrogen; U b — release of nitrogen with urine; F b — release of nitrogen with a stake; S — loss of nitrogen at the expense of desquamation of epidermis, growth of hair, nails, releases of nitrogen with then at not intensive sweating; G — deduction of nitrogen in the course of growth (calculation is conducted on 1 kg of weight a day).
The coefficient 1,1 reflects the additional expenditure of B. (on average 10%) resulting from stressful reactions and adverse effects on an organism. Borders of individual variations of needs for B. are accepted equal ±20%. Official recommendations of expert group of FAO/WHO are reflected in tab. 3.
Table 3. The AVERAGE DAILY NEED FOR PROTEINS (on condition of its full assimilation) *
- The size of need for nitrogen is increased by coefficient 6,25.
It is obvious that the specified sizes but correspond to optimum supply of the person B. and shall be referred to the minimum level of their contents in a diet, at non-compliance to-rogo inevitably rather bystry development of serious effects of proteinaceous insufficiency. The actual consumption of B. in the majority of economically developed countries in 1,5 and even 2 times is higher than the provided figures. According to the concept of the balanced food, optimum need of the person for B. depends on many factors, including physiological features of an organism, the qualitative characteristic of food B. and contents in a diet of other feedstuffs.
In the USSR sizes of needs of the population for B. are recorded in the physiological norms of food which are officially approved by the Ministry of Health which are periodically reconsidered and specified. Physiological norms food are the average approximate sizes reflecting optimum needs of separate groups of the population for the main feedstuffs and energy (tab. 4).
Table 4. The RECOMMENDED SIZES of DAILY CONSUMPTION of PROTEIN, gr. (according to recommendations M3 of the USSR, 1968)
Are provided in them differentiation of needs for proteins, depending on a sex, age, the nature of work etc. The recommended sizes are calculated on the basis of studying of features of protein metabolism and nitrogenous balance at the corresponding groups of the population, and they are much higher than the minimum needs for B. necessary for maintenance of nitrogen equilibrium. B.'s surplus is necessary plant louses of ensuring the additional expenditure of an organism connected with exercise and nervous tension, adverse effects of external environment and also for maintenance of the optimum immunological status. Are specially allocated in norms of size of consumption of the most valuable B. of animal origin.
Physiological norms of food are a basis of planning of production of this or that foodstuff. At assessment of usefulness of separate proteinaceous products their amino-acid structure, degree of digestibility by enzymes of a digestive tract and the integral indicators of comprehensibility established as a result of biological experiments is considered. In practice with a certain degree of convention proteinaceous products divide into two groups. Carry animal products to the first: milk, meat, eggs, fish which proteins are easily and completely acquired by a human body; to the second — the majority of products of plant origin, in particular wheat, rice, corn and other cereals which proteins are acquired by an organism not completely. Convention of similar division is emphasized with the high biological value of a number B. of a plant origin (potatoes, a buckwheat, soy, sunflower) and the low biological value of B. of some animal products (gelatin, skin, sinews, etc.). The reasons of low comprehensibility of fibrillar B. (keratin, elastin and kollagen) consist in features of their tertiary structure and difficulty of digestion by enzymes of a digestive tract. On the other hand, assimilation of a number B. of a plant origin can depend on structure of plant cells and the arising difficulties in B.'s engagement with digestive enzymes.
Completeness of use of separate proteins by the person or their biological value and the first stage are defined by degree of compliance of their amino-acid structure to the differentiated requirements of an organism and to some extent to amino-acid body composition. A huge variety of the proteins which are found in the nature is generally constructed of 20 amino acids, 8 of them (tryptophane, a leucine, an isoleucine, valine, threonine, a lysine, methionine and phenylalanine) are irreplaceable for the person since cannot be synthesized in body tissues (see. Amino acids ). Plant louses of small children the ninth irreplaceable amino acid is the histidine. Other amino acids are among replaceable and can be regarded in food of hl. obr. as suppliers of nonspecific nitrogen. It is established that the best digestion of proteins of food is reached during the balancing of its amino-acid structure with «ideal» amino-acid scales. As a similar scale in 1957 the so-called preliminary amino-acid scale of FAO was offered. Later it was proved that content of a number of amino acids in it, especially tryptophane and methionine, is determined not quite precisely. According to results of biological researches as optimum scales of amino-acid composition of proteins of eggs and women's milk are recommended in recent years. B. these two products are intended by the nature for food of the developing organisms and are almost completely utilized both in experiences on experimental animals, and during the use in food of children of early age.
For definition of compliance of amino-acid structure of B. to needs of the person a number of indexes is offered, each of which has only limited value. Among them it is necessary to mention an index N / Oh, reflecting the relation of the sum of irreplaceable amino acids (N in mg) to the general content of nitrogen of proteins (About in) which helps definition a ratio of nitrogen irreplaceable, or essential, amino acids and nonspecific nitrogen. The below size N / O the is higher the content of nonspecific nitrogen. For proteins of milk and eggs this index is rather high — 3,1 — 3,25, for meat — 2,79 — 2,94; for wheat — 2. The great value is attached to an indicator of amino-acid is fast, allowing to receive fuller judgment of the biological value of protein on the basis of its chemical structure.
A method it is fast it is based on calculation in the studied product of percent of providing each of irreplaceable amino acids in comparison with ideal amino-acid scales.
For this purpose for each of essential amino acids of the studied B. size I is calculated issl , equal And issl /Н issl , reflecting the relation of each irreplaceable amino acid (And in mg) to the sum of irreplaceable amino acids (N in); the received figure is compared with size I St , equal And St /Н St for the same amino acid calculated on a standard scale. As a result of division of the sizes Iissl into Ist and multiplication on 100 receive an indicator of amino-acid it is fast for each of irreplaceable amino acids. Amino acid is limiting the biological value of the studied B., an indicator amino-acid it is fast for a cut is the smallest. As standard scales along with a preliminary scale of FAO use amino-acid scales of eggs and women's milk (tab. 5).
Table 5. STANDARD AMINO-ACID SCALES
According to indicators of amino-acid it is fast (tab. 6) the smallest biological value B. of a number of cereals, especially wheat possess (50%; the limiting amino acids — a lysine and threonine); corn (45%; the limiting amino acids — a lysine and tryptophane); millet (60%; the limiting amino acids — a lysine and threonine); peas (60%; the limiting amino acids — methionine and cystine). Indicator of amino-acid it is fast the limiting amino acid sets a limit of use of nitrogen of this type of B. for the plastic purposes. Excess of other amino acids which are contained in B. can be used only as a source of nonspecific nitrogen or for power needs of an organism. The method of studying of amino-acid structure is one of the main ways of evaluation test of B. Obychno he allows to receive indicators of comprehensibility, relatives in relation to results of longer and expensive methods of biological determination of value B. At the same time establishment in some cases of reliable discrepancies between the specified indicators forces to resort at a research of new proteinaceous products to integral methods biol. estimates as on laboratory animals, and directly in public. These methods are based on studying in balance experiences of completeness of use of separate B. the growing animals (an indicator of proteinaceous efficiency of a diet), ratios of the nitrogen withheld by an organism to the nitrogen adsorbed from intestines (an indicator biol. value), the relations of the adsorbed nitrogen to the general nitrogen of food (an indicator of true digestibility), etc. At statement of researches on studying biol, values of protein rather caloric providing a diet, its balancing on all irreplaceable factors of food is obligatory (see. Balanced food ) and rather low level of B. — within 8 — 10% of the general caloric content (see. Metabolism and energy ). Comparison of indicators of amino-acid it is fast also the utilization of protein defined in experiences on experimental animals for some products it is presented in tab. 6.
Table 6. COMPARISON of INDICATORS of AMINO-ACID it is FAST ALSO UTILIZATION of PROTEIN
Important advantage of biological methods of assessment of B. is their integralnost giving the chance to consider all complex of properties of the products influencing comprehensibility of B. Izuchaya entering them biol. separate B.' value, it is worth to remember that practically in all food allowances not separate B., but their complexes are used, and, as a rule, various B. mutually supplement each other, providing some average values of digestion of protein nitrogen. At enough various mixed diets the indicator of digestibility of B. of food allowances is rather constant and approaches 85% that is quite often used during the carrying out practical calculations.
See also Food .
Histochemical methods of identification of proteins in fabrics
The biochemical methods adapted for definition of proteins in thin fabric sections are the cornerstone of histochemical methods of identification of B., as a rule. It must be kept in mind that biochemical reaction can be used as histochemical if reaction product has steady color coloring, drops out in a deposit and has no the expressed tendency to diffusion. Histochemical methods of identification of B. in fabrics are based on identification of the certain amino acids which are B.'s part (e.g., Millon's reaction to tyrosine, Sakagusha's reaction to arginine, Adams's reaction to tryptophane, reaction of a tetrazoniyevy combination to a histidine, tyrosine, tryptophane etc.), on identification of certain chemical groups (NH 2 =, COOH — SH =, SS =, etc.), on use of some physical. - chemical methods (tsvetn. fig. 1 — 3), definition of an isoelectric point etc. At last, it is possible to find out existence in a fabric cut of some amino acids an indirect way, having defined existence in fabrics of the enzymes connected with these amino acids (e.g., oxidases of D-amino acids). Some simple B. (collagen, elastin, reticuline, fibrin) come to light in cuts by means of numerous histologic methods among which so-called polikhromny methods (Mallori's method and his modifications, an orseinpikrofuksinovy method of Romeys, etc. are preferable. B. come to light and during the use of methods of luminescent microscopy. B.'s localization in fabrics (myosins, albumine, globulins, fibrin etc.) can be received by means of a method of marked antibodies according to Koons, etc. These methods and their modifications allow to identify and define rather precisely localization of the separate B. differing from each other in the content of these or those amino acids. Methods of quantitative definition of B., napr, a method of definition of B. by indirect reaction of marked antibodies, and also definitions of SH-group by Barnett and Seligman's method are developed (see. Amino acids , histochemical methods of identification of amino acids). All methods of identification of B. mentioned above in fabrics have sufficient specificity and yield quite reliable results. Fixing of fabric material during the use of the called methods is various. The most suitable fixers in most cases should be considered ethyl or methyl alcohol, the dehydrated acetone, mix of alcohol with formalin, solution trichloroacetic to - you on alcohol, in certain cases are applied (to proteids of a front share of a hypophysis) formalin. The choice of the fixer depends on a method, time of fixing — on total quantity and character of fabric. It is possible to use cryostately or paraffin sections.
Radioactive proteins — proteic matters which molecule contains one or several atoms of radioisotopes of any elements. At radioactive marking of B. it is necessary to ensure durability and perhaps big safety of a proteinaceous molecule. As a radioactive label of B. for biochemical pilot studies hl are used. about river isotopes 3 N and 14 With; during the receiving radio pharmaceuticals on the basis of B. apply isotopes of iodine — 125 I and 131 I, and also isotopes 111 In, 113m In, 99 m Tc, etc. Administration of isotopes of iodine in B. is based on electrophilic substitution of hydrogen on iodine in a phenolic kernel of tyrosine of a molecule B. or peptide. Marked B. is cleared of untied iodide and other impurity (by a gelfiltration, dialysis, adsorption, ion exchange, isoelectric sedimentation, etc.). If B. does not contain tyrosine, for carrying out iodination the deputies containing a radioiodine enter into it, or use the tyrosinecontaining analogs, or resort to a tag others radioisotopes (see).
Radioactive B. are important in studying of a catabolism and metabolism of proteic matters in pilot biochemical studies. Besides, they are used in radio isotope diagnosis of in vivo and in vitro during the studying of a functional condition of many bodies and systems of an organism in case of various diseases. In the researches in vivo the greatest application is found by albumine of blood serum of the person, marked radioisotopes of iodine ( 125 I and 131 I), and also received on its basis by a thermal denaturation and aggregation with the same tag micro and macrounits of albumine. Indicators of a hemodynamics and regional blood circulation, volume of the circulating blood and plasma can be defined by marked albumine, scanning of heart and large vessels is made (see. Scanning ), and also tumors of a brain. Microunits of albumine use for scanning of a liver and a stomach, definition of a blood-groove of a liver, and macrounits — for scanning of lungs.
Radioactive B. found broad application during the determination of microamounts of hormones, enzymes and other proteic matters in fabrics and environments of an organism of animals and the person in the researches in vitro.
Bibliography: Proteins, under the editorship of G. Neyrat and K. Bailey, lane with English, t. 1 — 3, M., 1956 — 1959, bibliogr.; Biosynthesis of protein and nucleic acids, under the editorship of A.S. Spirin, M., 1965; Gaurovnts F. Chemistry and functions of proteins, the lane with English. M, 1965; Ichas M. A biological code, the lane with English, M., 1971; Kiselyov L. L., etc. Molecular bases of biosynthesis of proteins. M, 1971; Poglaaov B. F. Structure and functions of sokratitelny proteins, M., 1965; Spirin A. S. and Gavrilova of L. P. Ribosom, M., 1971; Chemistry and biochemistry of nucleic acids, under the editorship of. And. B. Zbarsky and S. S. Debov, L., 1968; Advances in protein chemistry, ed. by M. L. Anson a. J. T. Edsall, v. 1—28, N. Y., 1944 — 1974; Hess G. P. a. R up ley J. A. Structure and function of proteins, Ann. Rev. Biochcm., v. 40, p. 1013, 1971; In vitro procedures with radioisotopes in mcdlcinc, Proceedings of the symposium, Vienna, 1970; M a r g-l (n A. Nerrif ieldR.B. Chemical synthesis of peptides and proteins, Ann. Rev. Biochem., v. 39, p. 841, 1970; Proteins, composition, structure, and function, ed. by H. Neurath, v. 1 — 5, N. Y. — L., 1963 — 1970.
B. in food — Lavrov B. of A. Textbook of physiology of food, page 92, M., 1935; Molchanova O. P. Value of protein in food for the growing and adult organism, in book: Vopr. pitas., under the editorship of O. P. Molchanova, century 2, page 5, M., 1950; P about to r ovsky A. A. K to a question of needs of various groups of the population for energy and the main feedstuffs, Vestn. USSR Academy of Medical Sciences, No. 10, page 3, 1966, bibliogr.; it, Fieiologo-biokhimichesky bases of development of products of baby food, M., 1972; Energy
Histochemical methods of identification of B. in fabrics — Kissels D. Prakticheskaya of the microtechnician and a histochemistry, the lane with a veyager., page 119, 152, Budapest» 1962; L and l-l I am ruble the Patogistologichesky equipment and the Actual histochemistry, the lane with English, page 509, M., 1969; P and r with E. Gistokhimiya, the lane e English. M, 1962; The Principles and methods of gp-ggo-cytochemical analysis in pathology, aod an edition of A. P. Avtsyn, etc., page 238, JI., '.971; R e and of s e A. G. E. Histochemistry, t. 1 — 2, Edinburgh — L., 1969 — 1972.
I. B. Zbarsky; A. A. Pokrovsky (pitas.), V. V. Sedov (I am glad.), R. A. Simakova (gist.).