Glycolysis is the major pathway for glucose metabolism, and it occurs in the cytosol of all cells. Glycolysis breaks down glucose (a 6-carbon molecule) into pyruvate (a 3-carbon molecule). Glycolysis can function either aerobically or anaerobically, depending on the availability of oxygen and the electron transport chain. The ability of glycolysis to provide energy in the form of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) in the absence of oxygen allows tissues to survive anoxia.
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| Glycolysis |
Glucose enters glycolysis by phosphorylation to glucose 6-phosophate, an irreversible reaction catalyzed by hexokinase, and ATP serves as the phosphate donor. Glucose 6-phosphate is converted to fructose- 6-phosphate by phosphohexose isomerase. This intermediate is then phosphorylated to yield fructose- 1,6-diphosphate. At this stage, the hexose molecule is cleaved by aldolase into two 3-carbon compounds: glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Dihydroxyacetone phosphate is quickly converted to glyceraldehyde 3-phosphate. The aldehyde group (CHO) of glyceraldehyde 3-phosphate is oxidized by a nico tinamide adenine dinucleotide (NAD)–dependent enzyme, and a phosphate group is attached, yielding 1, 3-bisphosphoglycerate. The energy of this oxidative step now rests in the phosphate bond at position 1. This energy is transferred to a molecule of ADP, forming ATP.
📖 Wilson and Gisvold’s Textbook of Organic Medicinal and Pharmaceutical Chemistry
The above reaction yields energy that is not immediately given off as heat but is stored in the form of ATP. Because two molecules of glyceraldehyde 3-phosphate are produced for every molecule of glucose, two molecules of ATP are formed at this step per molecule of glucose undergoing glycolysis. An ensuing transformation of phosphoenolpyruvate to pyruvate (catalyzed by pyruvate kinase) gives rise to another ATP (2 molecules of ATP per molecule of glucose oxidized). When a tissue possesses the systems for further oxidation of pyruvate, provided oxygen is present, pyruvate is cleaved to acetyl coenzyme A (CoA), and it enters the tricarboxylic acid cycle (see Plate 5-7). However, when the oxidative systems are absent (e.g., in erythrocytes that lack mitochondria) or if oxygen is excluded or is present in insuffi cient amounts (e.g., under anaerobic conditions), pyruvate is reduced to lactic acid by the enzyme lactate dehydrogenase. This system provides for the reoxidation of NADH and thus enables its participation again in oxidizing glyceraldehyde 3-phosphate; otherwise, the latter reaction would stop as soon as all the molecules of NAD were reduced.
The coupling of these two reactions allows the provision of energy by carbohydrates in the absence of oxygen, albeit at the expense of considerable amounts of carbohydrate. Under aerobic conditions, approximately 30 molecules of ATP are generated per molecule of glucose that is oxidized to CO2 and H2O, but only two molecules of ATP when oxygen is absent. Glycolysis is regulated by the three enzymes that catalyze nonequilibrium reactions: hexokinase, phosphofructokinase, and pyruvate kinase.
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