Enzymes of biological oxidation.
Biological oxidationthe aggregate of oxidation reactions that proceed in all living cells. The primary function of biological oxidation is to provide the organism with usable energy. Biological oxidation is catalyzed by enzymes called oxidoreductases.
In the 18th century, A. Lavoisier was the first to study oxidation in organisms. Subsequently, major developments in the field have included the localization of biological oxidation within the living cell, the relationship between biological oxidation and other metabolic processes, the elucidation of enzymatic oxidation-reduction reaction mechanisms, and the discovery of how a cell stores and converts energy. Abroad, these significant contributions were made by O. Warburg and H. Wieland in Germany; D. Keilin, H. Krebs, and P. Mitchell in Great Britain; and D. Green, A. Lehninger, B. Chance, and E. Racker in the USA. The major Soviet researchers of biological oxidation include A. N. Bakh, V. I. Palladin, V. A. Engel’gardt, S. E. Severin, V. A. Belitser, and V. P. Skulachev.
Biological oxidation in cells is related to the transfer of reducing equivalents—hydrogen atoms or electrons—from a donor compound to an acceptor. In aerobic organisms, including most animals and plants and many microorganisms, oxygen is the final acceptor of reducing equivalents, which are supplied by an organic or inorganic compound (see Table 1).
Most of the energy that is liberated in biological oxidation is stored in high-energy compounds, for example, adenosine triphosphoric acid (ATP). Biological oxidation, which is accompanied by the synthesis of ATP from adenosine diphosphoric acid (ADP) and inorganic phosphate, takes place in glycolysis, in the oxidation of α-ketoglutaric acid, and in oxidative phosphorylation—the transfer of reducing equivalents over a chain of oxidative, or respiratory, enzymes.
During respiration, carbohydrates, fats, and proteins are oxidized in a multistage process that results in the reduction of the major donors of reducing equivalents for the metabolic chain— flavins, nicotinamide-adenine dinucleotide (NAD), nicotinamide-adenine dinucleotide phosphate (NADP), and lipoic acid. These donor compounds are almost completely reduced in the tricarboxylic acid cycle, which completes the major metabolic pathway for the oxidative cleavage of carbohydrates, fats, and amino acids (for carbohydrates, this pathway starts with glycolysis). The coenzymes flavin adenine dinucleotide (FAD) and NAD are reduced in the oxidation of fatty acids; NAD is also reduced in the oxidative deamination of glutamic acid, and NADP, in the pentose phosphate cycle.
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