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Glycolysis

Glycolysis


Glycolysis is the sequence of enzymatic reactions that oxidize the six-carbon sugar glucose into two three-carbon compounds with the production of a small amount of adenosine triphosphate (ATP) . Glycolysis has two basic functions in the cell. First, it metabolizes simple six-carbon sugars to smaller three-carbon compounds that are then either fully metabolized by the mitochondria to produce carbon dioxide and a large amount of ATP or used for the synthesis of fat for storage. Second, glycolysis functions to produce

a small amount of ATP, which is essential for some cells solely dependent on that pathway for the generation of energy.

The glycolytic pathway is nearly ubiquitous, being found in every cell of virtually all living creatures. It is catalyzed by soluble enzymes located in the cytosol of cells. Although the glycolytic pathway is most commonly thought of as metabolizing glucose, other common monosaccharides such as fructose, galactose , and mannose are also metabolized by it. The glycolytic pathway operates in both the presence (aerobic) or absence of oxygen (anaerobic).

The metabolism of fuel molecules in the cell can be thought of as an oxidation process. In glycolysis, glucose is the fuel molecule being oxidized. As the glucose is oxidized by the glycolytic enzymes, the coenzyme nicotinamide adenine dinucleotide (NAD+) is converted from its oxidized to reduced form (NAD+ to NADH). When oxygen is available (aerobic conditions), mitochondria in the cell can reoxidize to NADH to NAD+. However, if either oxygen levels are insufficient (anaerobic conditions) or mitochondrial activity is absent, NADH must be reoxidized by the cell using some other mechanism. In animal cells, the reoxidation of NADH is accomplished by reducing pyruvate , the end-product of glycolysis, to form lactic acid. This process is known as anaerobic glycolysis. During vigorous exercise, skeletal muscle relies heavily on it. In yeast, anaerobic conditions result in the production of carbon dioxide and ethanol from pyruvate rather than lactic acid. This process, known as alcoholic fermentation, is the basis of wine production and the reason why bread dough rises.

Although some cells are highly dependent on glycolysis for the generation of ATP, the amount of ATP generated per glucose molecule is actually quite small. Under anaerobic conditions, the metabolism of each glucose molecule yields only two ATPs. In contrast, the complete aerobic metabolism of glucose to carbon dioxide by glycolysis and the Krebs cycle yields up to thirty-eight ATPs. Therefore, in the majority of cells the most important function of glycolysis is to metabolize glucose to generate three-carbon compounds that can be utilized by other pathways. The final product of aerobic glycolysis is pyruvate. Pyruvate can be metabolized by pyruvate dehydrogenase to form acetyl coenzyme A (acetyl CoA). Under conditions where energy is needed, acetyl CoA is metabolized by the Krebs cycle to generate carbon dioxide and a large amount of ATP. When the cell does not need energy, acetyl CoA can be used to synthesize fats or amino acids.

see also Insulin; Krebs Cycle.

Robert Noiva

Bibliography

Berg, Jeremy M.; Tymoczko, John L.; and Stryer, Lubert (2002). Biochemistry, 5th edition. New York: W.H. Freeman.

Voet, Donald; Voet, Judith G.; and Pratt, Charlotte W. (2002). Fundamentals of Biochemistry, updated edition. New York: Wiley.

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glycolysis

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 glucose. Glycolysis means, literally, the dissolution of sugar. The process is a series of consecutive chemical conversions that require the participation of eleven different enzymes, 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 triphosphate (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 fermentation. 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 cycle; respiration.

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glycolysis

glycolysis The first stage in the production of energy by breakdown of glucose in body cells; a chain of chemical events requiring a specific set of enzymes, and resulting in formation of ATP (adenosine triphosphate). In aerobic metabolism subsequent sequences produce several times more ATP, thereby providing a greater quantity of energy per molecule of glucose, utilizing oxygen, and producing carbon dioxide and water — comparable to burning organic fuels in air. In anaerobic metabolism (metabolism which does not use oxygen) glycolysis is the only means of energy production from glucose, and lactate is the end-product. This occurs in cells which cannot utilize oxygen (red blood cells), predominately in some components of skeletal muscle (fast, ‘white’ fibres), and probably to some extent in all cells when there is a shortage of oxygen. However, it also occurs in the first 1–3 minutes after a sudden increase of demand in cells which will subsequently make all the necessary ATP aerobically, because the glycolytic system can respond within seconds whereas both the biochemical pathways of aerobic metabolism and the systems for supplying them with oxygen take time to adjust; the start of vigorous exercise, even in a trained athlete with large numbers of ‘red’ muscle fibres, is the most obvious example.

Sheila Jennett

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glycolysis

glycolysis (Embden–Meyerhof pathway) The series of biochemical reactions in which glucose is broken down to pyruvate with the release of usable energy in the form of ATP (see illustration). One molecule of glucose undergoes two phosphorylation reactions and is then split to form two triose-phosphate molecules. Each of these is converted to pyruvate. The net energy yield is two ATP molecules per glucose molecule. In aerobic respiration pyruvate then enters the Krebs cycle. Alternatively, when oxygen is in short supply or absent, the pyruvate is converted to various products by anaerobic respiration. Other simple sugars, e.g. fructose and galactose, and glycerol (from fats) enter the glycolysis pathway at intermediate stages. Compare gluconeogenesis.

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glycolysis

glycolysis (Embden-Meyerhof pathway) The stepwise anaerobic degradation of glucose to produce as end-products ethanol and carbon dioxide in the cells of fungi and plants (or lactic acid in animal cells). One mole of glucose yields 1 mole each of ethanol and carbon dioxide in fungi and plants (or 2 moles of lactic acid in animals). In both cases the reaction sequence has a net yield of 2 moles of ATP. However, in most cells, under aerobic conditions, the pathway serves primarily to provide pyruvate, which is oxidized via the citric acid cycle, and intermediate compounds for biosynthetic processes.

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glycolysis

glycolysis (Embden–Meyerhof pathway) The stepwise, anaerobic degradation of glucose to produce as end-products either lactic acid (in cells of animals) or ethanol and carbon dioxide (in those of fungi and plants). In animals, one mole of glucose yields two moles of lactic acid and the reaction sequence has a net yield of two moles of ATP. However, in most cells under aerobic conditions, the pathway serves primarily to provide pyruvate, which is oxidized via the citric-acid cycle, and intermediate compounds for biosynthetic processes.

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glycolysis

gly·col·y·sis / glīˈkäləsis/ • n. Biochem. the breakdown of glucose by enzymes, releasing energy and pyruvic acid. DERIVATIVES: gly·co·lyt·ic / ˌglīkəˈlitik/ adj.

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glycolysis

glycolysis (gly-kol-i-sis) n. the conversion of glucose, by a series of ten enzyme-catalysed reactions, to lactic acid, with the production of energy in the form of ATP.

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glycolysis

glycolysis The first sequence of reactions in glucose metabolism, leading to the formation of two molecules of pyruvic acid from each glucose molecule.

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