glycolysis

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glycolysis

(glīkŏl`ĭsĭs), term given to the metabolic pathway utilized by most microorganisms (yeast and bacteria) and by all "higher" animals (including humans) for the degradation of glucoseglucose,
 dextrose,
or grape sugar,
monosaccharide sugar with the empirical formula C6H12O6 . This carbohydrate occurs in the sap of most plants and in the juice of grapes and other fruits.
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. Glycolysis means, literally, the dissolution of sugar. The process is a series of consecutive chemical conversions that require the participation of eleven different enzymesenzyme,
biological catalyst. The term enzyme comes from zymosis, the Greek word for fermentation, a process accomplished by yeast cells and long known to the brewing industry, which occupied the attention of many 19th-century chemists.
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, most of which have been crystallized and thoroughly studied. Glycolysis begins with a single molecule of glucose and concludes with the production of two molecules of pyruvic acid. The pathway is seen to be degradative, or catabolic, in that the six-carbon glucose is reduced to two molecules of the three-carbon pyruvic acid. Much of the energy that is liberated upon degradation of glucose is conserved by the simultaneous formation of the so-called high-energy molecule adenosine triphosphateadenosine triphosphate
(ATP) , organic compound composed of adenine, the sugar ribose, and three phosphate groups. ATP serves as the major energy source within the cell to drive a number of biological processes such as photosynthesis, muscle contraction, and the synthesis of
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 (ATP). Two reactions of the glycolytic sequence proceed with the concomitant production of ATP, thus ATP synthesis is said to be coupled to glycolysis. Hundreds of cellular reactions, particularly those involved in the synthesis of cellular components and those that allow the cell to perform mechanical work, require the participation of ATP as a source of chemical energy. While glycolysis is the primary fuel process for some organisms that do not require oxygen, such as yeast, aerobic organisms can only gain a small portion of their needed energy from this process. Glycolysis occurs in two major stages, the first of which is the conversion of the various sugars to a common intermediate, glucose-6-phosphate. The second major phase is the conversion of glucose-6-phosphate to pyruvate. The products of glycolysis are further metabolized to complete the breakdown of glucose. Their ultimate fate varies depending upon the organism. In certain microorganisms lactic acid is the final product produced from pyruvic acid, and the process is referred to as homolactic fermentationfermentation,
process by which the living cell is able to obtain energy through the breakdown of glucose and other simple sugar molecules without requiring oxygen. Fermentation is achieved by somewhat different chemical sequences in different species of organisms.
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. In certain bacteria and in brewer's yeast, lactic acid is not produced in large quantities. Instead pyruvic acid, which is also the precursor of lactic acid, is converted to ethanol and carbon dioxide by an enzyme-catalyzed two-step process, termed alcoholic fermentation. In the tissues of many organisms, including mammals, glycolysis is a prelude to the complex metabolic machinery that ultimately converts pyruvic acid to carbon dioxide and water with the concomitant production of much ATP and the consumption of oxygen. See Krebs cycleKrebs cycle,
series of chemical reactions carried out in the living cell; in most higher animals, including humans, it is essential for the oxidative metabolism of glucose and other simple sugars.
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; respirationrespiration,
process by which an organism exchanges gases with its environment. The term now refers to the overall process by which oxygen is abstracted from air and is transported to the cells for the oxidation of organic molecules while carbon dioxide (CO2
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.

Glycolysis

 

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.

Figure 1

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 [5]). 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 [8]-[10]), 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 [14]) 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 [1] and [5]). 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 [1]) or fructose monophosphate (reaction [5]) 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

glycolysis

[¦glī′käl·ə·səs]
(biochemistry)
The enzymatic breakdown of glucose or other carbohydrate, with the formation of lactic acid or pyruvic acid and the release of energy in the form of adenosinetriphosphate.

glycolysis

Biochem the breakdown of glucose by enzymes into pyruvic and lactic acids with the liberation of energy
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