glycolysis(redirected from Anaerobic metabolism)
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the process of anaerobic, enzymatic, nonhydrolytic splitting of carbohydrates (mainly glucose) in animal tissue, accompanied by the synthesis of adenosine triphosphate (ATP) and leading to the formation of lactic acid. Glycolysis is of great importance for muscle cells, spermatozoa, and growing tissues (including tumors), since it provides for energy storage in the absence of oxygen. The products of glycolysis are substrates of the subsequent oxidation transformations. Processes analogous to glycolysis are lactic, butyric-acid, alcoholic, and other varieties of fermentation occurring in vegetable, yeast, and bacterial cells. The rate of the individual stages of glycolysis depends on the acidity, the pH value (the optimum pH is 7-8), the temperature, and the ion composition of the medium. The sequence of the glycolysis reaction (see Figure 1) has been studied in detail, the intermediates have been identified, and the glycolytic enzymes have been isolated in the crystalline or purified state.
Glycolysis starts with the formation of phosphorylated derivatives of sugar, which promotes the transformation of the cyclic substrate form into an acyclic, more reactive form. One of the reactions that control the rate of hydrolysis is reaction (2), which is catalyzed by the enzyme Phosphorylase. An essential regulatory function is also performed by the enzyme phosphofructokinase (reaction ). The activity of this enzyme is inhibited by ATP but stimulated by the products of its decomposition. The central link of the glycolysis cycle consists of glycolytic oxidation-reduction (reactions -), which leads to the oxidation of glyceraldehyde-3-phosphate to 3-phosphoglycerate and to the reduction of the coenzyme nicotinamide adenine dinucleotide (NAD). These transformations are accomplished by 3-phosphoglyceraldyhyde dehydrogenase (PGAD), with the participation of phospholgyceric acid kinase.
Oxidation-reduction results in the evolution of energy, which accumulates in the form of energy-rich compound ATP during the process of substrate phosphorylation. Reaction (13) is a second reaction leading to the formation of ATP. Glycolysis ends with the formation of lactic acid (reaction ) because of the action of lactic acid dehydrogenase with the participation of reduced NAD. Thus, the splitting of one glucose molecule leads to the formation of two molecules of lactic acid and four molecules of ATP. Simultaneously, two molecules of ATP are consumed per molecule of glucose during the initial stages of glycolysis (see reactions  and ). Only 7 percent of the total energy that may be obtained from the complete oxidation of glucose (to CO2 and H2O) is evolved in the process of glycolysis. In addition to glucose, glycerol, some amino acids, and other substrates may be involved in glycolysis. In muscle tissue, where the basic substrate of glycolysis is glycogen, the process starts with reactions (2) and (3) and is called glycogenolysis. The common intermediate of glycogenolysis and glycolysis is glucose-6-phosphate.
All glycolysis reactions except (1), (5,) and (13) are reversible. It is, however, possible to obtain glucose (reaction ) or fructose monophosphate (reaction ) from the corresponding phosphorylated derivatives by the hydrolytic splitting of these compounds to give phosphoric acid. Reaction (13) is apparently virtually irreversible because of the high energy of hydrolysis of the phosphate group (about 13 kilocalories per mole). Therefore, the formation of glucose from the products of hydrolysis proceeds in a different manner.
The rate of glycolysis is reduced in the presence of 02 (the Pasteur effect). In some tissues—for example, tumor cells, the retina, and nonnucleated erythrocytes—intense so-called aerobic glycolysis is possible in the presence of oxygen. In addition, there are examples of the suppression of skin respiration by glycolysis (the Crabtree effect) in certain tissues undergoing intense glycolysis. The mechanisms of interactions between aerobic and anaerobic oxidation processes have not been thoroughly studied.
A. A. BOLDYREV