From Big Medical Encyclopedia

BIOLOGICAL OXIDATION — the enzymatic processes of oxidation proceeding in organisms of animals and plants, and also in microorganisms. Oxidizing processes are used by a cell for creation and replenishment of resources of energy, for biosynthesis of many connections, essential to a metabolism (sterols, prostaglandins, neurotransmitters, etc.)? For transformation of large organic molecules into simpler and, at last, for the formation of end products of a metabolism and energy which are subject to allocation from an organism. Oxidizing reactions play also large role in neutralization of substances, toxic for an organism.

Beginning of scientific research of processes of O. works of A. Lavoisier who suggested about the slow oxidation of food stuffs in blood of animals and the person which is followed by oxygen absorption and release of carbon dioxide gas were. After Spallantsani (L. Spallanzani) showed that O. occurs not in blood, and in other fabrics. In our country to a problem O. A. N. Bach, V. A. Belitser, V. I. Palladin, S.E. Severin, W. A. Engelgardt's many works, etc. were devoted.

Okislitelno - recovery reactions (see) in organisms proceed in all cells; they are catalyzed by the enzymes belonging to the class oxidoreductases (see). At various diseases and at a number of harmful effects on a live organism there would be a disturbance of processes of O. Big group of hereditary diseases of the person, to a cut belong Fenilketonuriya (see), alkaptonuria (see), etc., it is connected with genetically caused insufficiency or blocking of nek-ry oxidoreductases. At disturbances of oxidation-reduction transformations of vitamin D in kidneys there is no formation of biologically active derivatives of this vitamin that involves disturbance of exchange of calcium in an organism, to-rogo demineralization of bones is a consequence. In the absence of activity catalases (see) or peroxydase (see), napr, glutathione peroxidases (KF, in fabrics there is an accumulation of the hydrogen peroxide activating processes of peroxide oxidation (see. Peroxides ) and oxidizing hemoglobin in a methemoglobin what disturbance of supply of fabrics with oxygen, and also destruction biol is result, of membranes. Leads to similar effects also a hypovitaminosis of E (see. Tokoferola ). Increase in intensity of O. observe at hyperfunction of a thyroid gland, at nek-ry forms of tuberculosis, during the overcooling, at feverish states etc. In fabrics of nek-ry malignant tumors observe activation of anaerobic glycolysis and the inhibition of processes of cellular respiration is frequent. At hypoxias (see) various origins activity of enzymes of a respiratory chain increases (see. Respiratory enzymes ), apparently, due to their strengthened synthesis.

Process of oxidation of any substrate is followed by electron transfer or hydrogen atoms, so-called recovery equivalents, from donor connection to connection acceptor. At heterotrophic organisms, for to-rykh redoxreactions are the only source of obtaining energy, necessary for life activity, as electron donors usually serve various organic compounds (e.g., glucose, fat to - you, amino acids). Much less often this role is carried out by inorganic compounds like hydrogen, hydrogen sulfide, sulfur, ammonia.

At aerobic oxidation final electron sink in a chain of consecutive redoxreactions is oxygen. Such reactions catalyze oxidases (see). Nek-ry bacteria electron sink can have anions rich with oxygen (e.g., NO 3 - , SO 4 2- ) or carbonic acid (CO 2 ). This so-called anaerobic oxidation, i.e. oxidation without use of oxygen.

The oxidation with use of atmospheric oxygen called by also tissue or cellular respiration is a source of the most part of the energy received by aerobic cells. The role of tissue respiration in living cells is extremely big since exactly thanks to it in a cell the stock of the main part of the energy concluded before in complicated organic molecules of various structure and transformed to easily recyclable free energy of phosphatic communication of molecule ATP (is created see. Adenosine triphosphoric acids ). From the natural compounds capable to activate tissue respiration, it should be noted hormones of a thyroid gland, and also free fat to - you.

The initial stage of tissue respiration consider multistage enzymatic process — a cycle Tricarboxylic to - t, to-ry call still a tricarbonic acid cycle or a cycle lemon to - you (see. Tricarboxylic acids cycle ). Early stages of disintegration of carbohydrates, proteins and fats are catalyzed by the most various enzymes and represent a wide range of the reactions specific to each class of substances. However end products carbohydrate metabolism (see), nitrogen metabolism (see) and lipometabolism (see) the small number of the connections involved in these or those ways in the general for all these classes of substances a cycle — a cycle Tricarboxylic to - t is.

Process of tissue respiration is energetically most favorable to an organism. If in process glycolysis (see) — in the transformation of glucose proceeding without oxygen consumption there is a formation of only 2 molecules ATP, and in a cycle Tricarboxylic to - the t is formed 2 molecules ATP on 1 molecule of the spent glucose, at electron transfer in a respiratory chain mitochondrions (see) energy stocks up in vysokoergichesky bonds of 34 molecules ATP on 1 molecule of glucose. Thus, importance of processes of tissue respiration in energy balance of a cell does not raise doubts.

A respiratory chain of mitochondrions, in a cut upon completion of reactions of a cycle Tricarboxylic to - t reokislyatsya recovered OVER and suktsinatde-hydrogenase (KF, represents the unique polifer-mentny complex localized in an inner membrane of mitochondrions. Several groups of enzymes are a part of a respiratory chain: flavinsoderzhashchy dehydrogenases [Over-N-dehydrogenase (KF, succinatedehydrogenase, atsil-KOA-degidrogenaza (KF, etc.]; the proteins containing negemovy iron (zhelezoseroproteida), and also several types tsitokhrom (see) — Tsitokhroma of b, c1, c, an and a3. An obligatory component of a respiratory chain is also the coenzyme of Q, or ubikhinon, apparently, taking part in acceptance of electrons from flavinsoderzhashchy dehydrogenases (see. Coenzymes ). Carriers of electrons are located in a respiratory chain in ascending order of their size redox potential (see). The main function of a respiratory chain is step transfer of recovery equivalents from donor substrates (NAD-N, succinate, atsil-KOA, beta hydroxybutyrate, etc.) to final electron sink — molecular oxygen. Such transfer is resulted by gradual release of free energy of reaction of recovery of oxygen to water. This energy can be partially reserved in the form of energy of phosphatic communication of molecule ATP.

Process of synthesis of molecule ATP due to energy of oxidation of various substrates was opened in the USSR by W. A. Engelgardt in 1930 and received the name of oxidizing or respiratory phosphorylation. At transfer of couple of electrons from recovered OVER to oxygen in a respiratory chain there is a formation of 3 molecules ATP. If oxidation reaction begins at the level of flavinsoderzhashchy dehydrogenases (a succinatedehydrogenase, atsil-KOA — dehydrogenases), only 2 molecules ATP are synthesized. For assessment of efficiency of oxidizing phosphorylation of V. A. Belitser in 1939 entered the size of the relation P/O, i.e. amount of the inorganic phosphate which joined in molecule ATP, in terms of each absorbed oxygen atom. The size of the relation P/O at oxidation of NAD*N is equal 3, and at oxidation amber to - you (succinate) — 2.

The mechanism of transformation of energy of oxidation in energy of chemical communication of ATP so far is completely not found out. Among the existing hypotheses of the most recognized the hemi-osmotic hypothesis of Mitchell is (R. of Mitchell), according to a cut electron transfer in a respiratory chain leads to emergence of electrochemical potential of hydrogen ions on different sides of an inner membrane of mitochondrions, energy of a difference of these potentials is used then in synthesis of molecule ATP.

At nek-ry influences (e.g., during the overcooling of an organism) at animals with constant body temperature the interfaced processes of oxidation and phosphorylations (see) are separated also the free energy which is released at electron transfer does not stock up in molecule ATP, and dissipates in the form of heat. In intact mitochondrions electron transfer in lack of substrates of phosphorylation (ADF and inorganic phosphate) happens to very low speed. In the presence of ADF and inorganic phosphate transport rate of electrons sharply increases. Such rigid interface between oxidation and phosphorylation is characteristic only of intact mitochondrions. Under the influence of nek-ry connections processes of oxidation and phosphorylation can be separated. The substances capable to separate oxidation and phosphorylation, hormones of a thyroid gland, fat to - you, 2,4 dinitrophenol, Dicumarinum, etc. are.

In body tissues of substance, soaked up in blood from intestines (glucose, amino acids, fat to - you, etc.), enter decomposition reaction. Initial stage of a catabolism glucose (see) in tissues of animals the glycolysis representing a certain sequence of anaerobic enzymatic reactions of transformation of glucose in is pyruvic acid (see). The power effect of glycolysis consists in formation of 2 molecules ATP and 2 molecules NAD-N on 1 molecule of glucose. At oxidation of 2 molecules NAD*N in a respiratory chain of mitochondrions there is a formation of 6 more high-energy phosphatic bonds in molecules ATP. The reaction of glycoclastic oxidoreduction catalyzed by glitseraldegidfosfatdegidrogenazy (KF and enolazy is an energy source for formation of ATP in the course of glycolysis (KF 4.2.1. 11). Pyroracemic to - that (pyruvate) is exposed to oxidizing to decarboxylation (see) under the influence of the multifermental piruvatdegidrogenazny complex (KF localized in mitochondrions. A product of this enzymatic reaction is atsetil-KOA, to-ry joins in a cycle Tricarboxylic to - t.

The amino acids which are a part of proteins are exposed to enzymatic oxidizing reactions of disintegration with formation of quite limited number of metabolites, generally atsetil-KOA, alpha and keto-glutaric and oxalacetic to - t. All of them are capable to join in a cycle Tricarboxylic to - t. The central place in a catabolism of amino acids is taken by a trance and a mining (see) and oxidizing deamination (see). At transamination of an alpha amino group practically of all amino acids are transferred to a molecule and - keto-glutaric to - you therefore it is formed glutaminic to - that. In mitochondrions under the influence of enzyme of a glutamatdegidrogenaza (KF; there is an oxidizing deamination glutaminic to - you, followed by education alpha and keto-glutaric to - you and ammonia, toxic for an organism, to-ry are neutralized in a cycle urea (see). As electron sink in glutamatdegidrogenazny reaction serve NAD and NADF, oxidized then in a respiratory chain of mitochondrions.

An important role is played by oxidizing processes and in a lipometabolism. Molecules of free fatty acids (see) in the course of beta oxidation and to a lesser extent α-and ω-oxidations enter cyclic redoxreactions with education as the main intermediate product atsetil-KOA. The enzymes which are taking part in an oxidative breakdown fat to - t at their beta oxidation, are localized preferential in mitochondrions and closely connected with a respiratory chain. The energy output as a result of an oxidative breakdown fat to - t, napr, palmitic to - you, is very big: as a result of β-oxidation of this fat to - you are formed 8 molecules atsetil-KOA, 7 molecules FAD-N2 and 7 molecules NAD-N, oxidation to-rykh in a cycle Tricarboxylic to - t and a respiratory chain of mitochondrions can give up to 130 molecules ATP.

Many redoxreactions proceeding in a human body and animals are not directed to accumulation of a potential energy of fosfoangidridny bonds, but are absolutely necessary for education such vital connection as sterols, prostaglandins, biologically active derivatives

of vitamin D, neurotransmitters, etc. Reactions of this kind are most often catalyzed by oxygenases (KF 1.13.11), to-rye participate as well in a catabolism of many organic matters, including and toxic for an organism.

O.'s intensity. in separate bodies and fabrics it can be studied by means of manometrical methods. Respiratory coefficient (see), quantitatively characterizing O. in the isolated fabrics or their homogenates, represents the size of the relation of volume of the carbon dioxide gas emitted for a certain time term, to volume absorbed for the same time of oxygen. The quantity can be measured by the absorbed cuts of the isolated fabrics or fabric homogenates of oxygen and the emitted carbon dioxide gas in the device of Warburg (see. Mikrorespirometra ).

Great value for studying of processes of O. development of methods of fractionation biol, material has. Such methods as ultracentrifuging, the adsorptive, ion-exchange and affine are applied to allocation of these or those components of living cell chromatography (see), gel filtering (see), electrophoresis (see), etc. By means of these methods it is possible to allocate not only the cleared cellular organellas, napr, mitochondrions, but also individual oxidoreductases in a homogeneous state. Broad application for studying of redoxreactions was found by tracer techniques, spectroscopy (see), electrometric, electrometric and polyarografichesky (see. Polyarografiya ) methods of a research.

Bibliography: Berkovich E. M. Energy balance is normal also of pathology, M., 1964; Lenindzher A. Biochemistry, the lane with English, page 311, M., 1976; The M e c-l of e r D. Biokhimiya, Chemical changes in living cell, the lane with English, t. 2, M., 1980; P equerre E. Biopower mechanisms, new views, the lane with English, M., 1979; Skulachev V. P. Accumulation of energy in a cell, M., 1969; Skulachev V. P. and Kozlov I. A. Proton adenozintrifosfataza, M., 1977; Mitchell P. Keilin’s respiratory chain concept and its chemiosmotic consequences, Science, v. 206, p. 1148, 1979.

V. G. Grivennikova.