Biological Oxidation

the 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.

Interaction and localization of reaction mechanisms. In glycolysis, one molecule of glucose yields two molecules of ATP, while in oxidative phosphorylation, 34 molecules of ATP are produced. All eukaryotes rely on glycolysis, the tricarboxylic acid cycle, and the respiratory chain. The oxidation of fatty acids in vertebrates is almost totally absent in brain cells but supplies half the energy that is required by the liver, kidneys, heart muscles, and resting skeletal muscles. Oxidation in the pentose phosphate cycle occurs in the liver and lactating mammary glands but almost not at all in cardiac and skeletal muscles.

All the glycolytic enzymes are found in solution in the liquid phase of the cytoplasm. The phosphorylating enzymes of the electron-transfer chain are located in the inner membrane of mitochondria, in the cell membrane of bacteria, and in the thylakoids, which are one of the internal structures of chloroplasts. The enzymes that oxidize fatty acids, the enzymes of the tricarboxylic acid cycle, and glutamate dehydrogenase are located in the mitochondrial matrix. Enzymes that oxidize succinic and β-hydroxybutyric acids are found in the inner membrane of mitochondria, while the outer membrane contains those that participate in amino-acid metabolism, namely, monoamine oxidase and kynurenine hydroxylase.

Flavin oxidase—which oxidizes amino acids, glycolic acid, and other substrates with the evolution of hydrogen peroxide— is found in the peroxisomes, or microbodies, which account for up to 20 percent of the overall oxygen consumption in the liver. The hydrogen peroxide that is produced is subsequently decomposed by catalase or used by peroxidases in oxidation reactions. Hydroxylases and oxygenases are present in membranes of the endoplasmic reticulum of cells, where they are arranged into short, nonphosphorylating electron-transfer chains.

Oxidation reactions do not always result in the storage of energy: in several cases, they are involved in the conversion of compounds, for example, in the formation of bile acids and steroid hormones and in the conversion pathways of amino acids. Foreign and poisonous compounds, including aromatic compounds and incompletely oxidized respiratory products, are detoxified by oxidation. Free oxidation produces heat and is not connected with the storage of energy. Apparently, in homeothermic animals the electron-transfer system that effects oxidative phosphorylation is also capable of free oxidation in response to an increased heat requirement.

Mechanism of use of oxidation energy. The mechanism of the transformation of energy that is liberated in the transfer of reducing equivalents along the oxidative enzyme chain was not understood for a long time. According to the chemosmotic theory, which was developed in the 1960’s by—among others—the British biochemist P. Mitchell, energy is first used to create a positive field on one side of the membrane, a negative field on the other, and a difference in the concentration of H⁺ ions across the membrane. Both the electric field and the difference in H⁺ conventration

can serve as the driving force for the enzyme ATP synthetase, which carries out the synthesis of ATP. A portion of the energy field may be used directly by the cell for the transfer of ions through the membrane, for the reduction of electron carriers, and for the production of heat without the mediation of ATP.

Evolution of energy supply. The earliest organisms were anaerobes and heterotrophs that presumably existed in a primitive atmosphere that did not contain oxygen. Energy was supplied to the cells by processes similar to glycolysis. An oxidation mechanism that is known in some modern microorganisms may have existed by which reducing equivalents were transferred along a respiratory chain to nitrate (NO₃^-) or sulfate (SO₄⁼). An important evolutionary step that is related to the appearance of oxygen in the earth’s atmosphere was the development of photosynthesis in primitive unicellular organisms. Oxygen, which has a high oxidation-reduction potential, became the final acceptor of electrons in the respiratory chain. This occurred with the appearance of the enzyme cytochrome oxidase, which reduces oxygen, and resulted in the modern type of biochemical respiration. The supply of energy in all aerobes (whose cells contain mitochondria) is based on this type of respiration.

Cells have also retained the enzyme system for glycolysis. Pyruvic acid, which is formed during glycolysis, is subsequently oxidized in the tricarboxylic acid cycle, which, in turn, supplies electrons to the respiratory chain. Thus, the evolution of energy metabolism apparently proceeded along a path that involved the use and elaboration of already-existing mechanisms of energy supply. It has been proposed that chloroplasts and mitochondria developed from primitive symbiotic microorganisms, since the enzyme systems in modern organisms for glycolysis, respiration, and photosynthesis can be found intact in the cytoplasm, mitochondria, and chloroplasts, respectively. Another line of evidence draws on the striking similarity between the mechanisms for the transformation of energy in mitochondria and chloroplasts, on the one hand, and microorganisms, on the other.