Of the two parts of anaerobic metabolism, glycolysis and fermentation, all organisms carry out the former. Glycolysis, which occurs in the cytosol of a cell, involves the degradation of glucose to carbon dioxide, water, and pyruvic acid. Because glucose has little tendency to break down into simpler products, it must be made more reactive by adding a small amount of energy. Thus, the energy of two ATP molecules must be spent in readying a single molecule of glucose to begin releasing the much greater amount of energy locked in its molecular structure. During the first reaction of glycolysis, an ATP molecule gives its phosphate to a glucose molecule.
During the second reaction, the glucose is converted into fructose, which is phosphorylated in the third reaction. Thus, after the first three glycolytic reactions one molecule of fructose diphosphate and two molecules of ADP have been produced. Each of the first three reactions is controlled by an allosteric enzyme that is affected by both positive and negative modulators. A build up of ADP in a cell induces conformational changes in the enzymes that make them more reactive, whereas a build up of ATP induces changes in the enzymes that make them less reactive. During the fourth glycolytic reaction, the fructose diphosphate is broken into two phosphorylated three-carbon sugars, dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P). Only G3P is used in subsequent reactions; an enzyme converts
DHAP to G3P.
In the sixth reaction, each G3P is both oxidized by a electron-accepting compound
