Glycolysis

Glycolysis

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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.