Biological oxidation

The biological oxidation (cell or tissue respiration) - redox reactions occurring in the cells of the body, resulting in complex organic substances are oxidized with the specific enzymes oxygen delivered by the blood. The end products of biological oxidation are water and carbon dioxide. Released in the process of biological oxidation energy partly out in the form of heat, the main part of it goes to the formation of complex molecules of organophosphorus compounds (mainly ATP - ATP), which are sources of energy necessary for the human body.
This oxidation process is taking away from oxidizable substance (substrate) electrons and equal number of protons. Substrates biological oxidation products are transformations of fats, proteins and carbohydrates. Biological oxidation substrata to the final products is carried out by a chain of consecutive reactions, including intermediate products which include tricarboxylic acids - citric, isakanova and solimana acid, so the whole chain of reactions is called the tricarboxylic acid cycle, or Krebs cycle (on behalf of the researcher established this cycle).
The initial reactions of the Krebs cycle is the condensation of oxaloacetic acid with an activated form of acetic acid (acetate), which represents a connection with coenzyme acetylation - acetyl-COA. The reaction is formed citric acid, which after four of dehydrogenization (removal from 2 molecules of hydrogen atoms) and double decarboxylation (dissociation of molecules of CO2) forms salewoman acid. Sources of acetyl-COA, which is used in the Krebs cycle, are acetic acid, pyruvic acid is one of the products of glycolysis (see), fatty acids (see), etc. Along with oxidation acetyl-COA in the Krebs cycle can undergo oxidation and other substances, able to turn into intermediate products of this series, for example, many amino acids formed in the decay of protein. Due to the reversibility of the majority of the reactions of the Krebs cycle products of decomposition of proteins, fats and carbohydrates (intermediates) it can not only be oxidized, but to get at his conversion. So is the relationship between the exchange of fats, proteins and carbohydrates.
Taking place in the cycle Krebsa oxidation reactions are not accompanied, as a rule, education, energy-rich compounds. The exception is the transformation of succinyl-COA in succinate (see Amber acid), which is accompanied by a GTP-independent. A large part of the ATP is formed in the respiratory chain enzymes (see), where the electron transfer (as at the first stages and protons) to oxygen accompanied by release of energy.
The reactions of detachment of hydrogen are enzymes class dehydrogenases, and the atoms of hydrogen (i.e., protons and electrons) are connected to the coenzymes: dinucleotide (NAD), adenine dinucleotide-phosphate (NADP), flavine adenine dinucleotide (FAD), and other
The process of biological oxidation associated with the Krebs cycle and respiratory chain enzymes, there are mainly in the mitochondria and localized in their membranes.
Thus, the process of biological oxidation associated with the Krebs cycle, have the meaning as in the formation of compounds rich in energy and communication of carbohydrate, fat and protein metabolism. Other types of biological oxidation, apparently, have a narrower meaning, for example the supply of cells. This is the stage of glycolysis, which consists in oxidation number of phosphoric compounds with simultaneous restoration of the ABOVE and the formation of ATP or reactions pantotenova cycle (i.e., oxidative transformation of glucose-6-phosphate), accompanied by the formation of fotopatos and restored NADP. Pentony cycle plays an important role in the tissues, characterized by intensively expanding the synthesis of nucleic acid -, fatty acids, cholesterol , etc. Cm. also the Metabolism and energy.

The biological oxidation - set of redox reactions in biological objects. Under the process of oxidation understand the loss of the substance of electrons or of electrons and protons at the same time (loss of hydrogen atoms) or accession oxygen. The reaction of the opposite direction characterize the recovery process. Reducers known substances that loses electrons, oxidising substances, gaining electrons. The biological oxidation is the basis of tissue or cellular respiration (process, in which tissues and cells absorb oxygen and produce carbon dioxide and water, the main source of energy for the body. The substance of the host (accepterait) electrons, i.e. recovering, is molecular oxygen, turning it into oxygen anion O--. Hydrogen atoms, otdalennye from organic matter - of substrate oxidation (SH2), turn the loss of electrons into protons or positively charged cations of hydrogen:
SH2→S→2H; 2N→2H+ + 2e: ½O2→ABOUT; ABOUT→2nd→O--; 2H+ + O--→H2O+55 kcal. As a result of reaction between hydrogen cations and anions oxygen water is formed, and the reaction is accompanied by release of a significant amount of energy on every 18 g of water). As a by-product of bio-oxidation produces carbon dioxide. Some of the reactions O. B. lead to the formation of hydrogen peroxide, under the influence of catalase decaying into H2O and O2.
Suppliers of energy in the human body are food - proteins, fats and carbohydrates. However, these substances can serve as substrates O. B. They previously are split in the digestive tract, where proteins are formed amino acids, fats, fatty acids and glycerine, from complex carbohydrates - monosaccharides, primarily hexose. All these compounds are absorbed and receives (directly or through the lymphatic system) in the blood. Together with analogous substances, educated in organs and tissues, they are "metabolic Fund, from which the body draws on material for biosynthesis and to meet energy demands. The main substrates O. B. are the products of tissue metabolism of amino acids, carbohydrates and fats, called substances citrate cycle". These include acid:
lemon, isakanova, solimena, safelevitra, α ketoglutarova, amber, fumaric, Apple, savelovkaya.
Pyruvic acid CH3-WITH-COOH not included directly in the citrate cycle, but plays a significant role as the product of its decarboxylation - active form acetic acid CH3Socoa (acetyl-coenzyme A).
The processes that make up the citrate cycle" ("the Krebs cycle, tricarboxylic acid cycle"), flow under the action of enzymes, the prisoners in the cell organelles called mitochondria. The elementary act of oxidation of any substance included in the citrate cycle,is taking away from the substance of hydrogen, i.e. the act of dehydrogenization caused by the activity of the corresponding specifically applicable dehydrogenase enzyme (Fig. 1).

Fig. 1. The scheme citrate Krebs cycle.

If the process begins with pyruvic acid, the removal of two hydrogen atoms (2N) in the Krebs cycle is repeated 5 times, and is accompanied by three sequential phases decarboxylation. The first act - dehydrogenase - occurs when the transformation of pyruvic acid acetyl-COA, condensing with Savelevoj acid in the lemon. The second time dehydrogenase leads to the formation safelevitra acid from solimano. The third act, the removal of two hydrogen atoms is connected with the transformation of ketoglutarova acid in succinyl-COA; the fourth - degidrogenaza succinic acid and finally the fifth : the transformation of malic acid in salewoman, which again can join the condensation with acetyl-COA and to provide education citric acid. The disintegration of succinyl-COA formed rich in energy link (about R) is the so - called substrate phosphorylation: Succinyl-COA + N3RO4 + ADP → amber acid + COA + ATP.

Fig. 2. Scheme of dehydrogenization substrates citrate cycle specific enzymes consisting of dissociative complexes: proteins - B1, B2, B3 and B4 with OVER and ADN and protein B5, forming a complex with FAD (succenderunt); CAC - isakanova acid.


Four of these acts of dehydrogenization are carried out with the specific dehydrogenases, coenzyme which is dinucleotide (NAD). One act - the transformation of amber acid fumaric - occurs under the influence of succenderunt - flavoprotein I. In this case, the coenzyme is flavine adenine dinucleotide (FAD). Five of repeated acts of dehydrogenization (Fig. 2) when the reactions occurring in the citrate cycle, formed restored form of coenzymes: 4-NADN 1-FADN. Dehydrogenase restored OVER, i.e., receiving hydrogen with ADN, also belongs to flavins enzymes is flavoprotein II. However, it differs from succenderunt structure as protein and flavins component. Further oxidation of reduced forms of flavoproteins I and II, containing FADN occurs with the participation of cytochromes (see), a complex proteins chromoproteids, containing in its composition galataperform - gems.
At oxidation FADN way protons and electrons differ: protons coming into the environment in the form of hydrogen ions, and electrons through a series of cytochromes (Fig.3) is transferred to oxygen, turning it into oxygen anion O-- . Between FDN and cytochromes, apparently, participates another factor - coenzyme Q. the next link in the respiratory chain from NADN to oxygen is characterized by a high redox potential (see). Throughout the respiratory chain from NADN to ½O2 potential changes to 1.1 in (-0,29V to+0,V). When the complete oxidation, such as pyruvic acid, accompanied by five times the removal of hydrogen energy efficiency of the process will be approximately 275 kcal (55X5). This energy is not completely dissipated as heat; about 50% of it is accumulated in the form of rich energy
phosphoric compounds, mainly of adenosine triphosphate (ATP).
The process of transformation of energy of oxidation in energy-rich communications (about R) end-phosphate residue ATP molecules localized in the inner mitochondrial membranes and associated with certain stages of the transport of hydrogen and electrons in the respiratory chain (Fig. 4). It is considered that the first phosphorylation is connected with the transport of hydrogen from NADN to FAD, the second involves the transfer of electrons on the cytochrome c1 and, finally, the third, least understood, is located between cytochrome c and a.
The mechanism of formation of rich energy ties have not yet been deciphered. Explained, however, that the process is composed of several intermediate reactions (Fig. 4 - from J~X to ATP), only the latter of which is the creation of a rich energy phosphate residue of ATP. Energy-rich communication end-phosphate groups in ATP is estimated at 8.5 kcal per gram-molecule (in physiological conditions - about 10 kcal). When the transfer of hydrogen and electrons in the respiratory chain, starting with NADN to the formation of water is released 55 kcal and accumulates in the form of ATP not less than 25.5 kcal (8,5X3). Therefore, the energy efficiency of the process of biological oxidation is about 50%.

Fig. 3. The scheme of transfer of hydrogen and electrons in the respiratory chain; E0 redox potential.
Fig. 5. Scheme of using energy phosphate bonds between the ATP (AMF-R~R) for various physiological functions.

Biological sense fosfauriliruetsa oxidation clear (Fig. 5): all life processes (muscle work, nervous activity, biosynthesis) require energy region is provided by gap-energy-rich phosphate relations (about R). Biological sense defosfauriliruetsa - free - oxidation can be seen in numerous oxidation reactions that are not associated with the citrate cycle and transport of hydrogen and electrons in the respiratory chain. These include, for example, all unemotionally the oxidation, oxidative destruction of toxic active ingredients and many acts regulating the quantitative content of biologically active compounds (some amino acids, biogenic amines, adrenaline, histidine, serotonin, and so on, aldehydes and other) by more or less intensive oxidation. The ratio of free oxidation and phosphorylation pathway is also one of the ways of thermoregulation in humans and warm-blooded animals. Cm. also the Metabolism and energy.