The conversion of glucose into two molecules of pyruvate has resulted in the net synthesis of ATP. However, an energy-converting pathway that stopped at pyruvate would not proceed for long, because redox balance has not been maintained. As we have seen, the activity of glyceraldehyde 3-phosphate dehydrogenase, in addition to generating a compound with high phosphoryl-transfer potential, of necessity leads to the reduction of NAD⁺ to NADH. There are limited amounts of NAD⁺ in the cell, which is derived from the vitamin niacin, a dietary requirement in human beings. Consequently, NAD⁺ must be regenerated for glycolysis to proceed. Thus, the final process in the pathway is the regeneration of NAD⁺ through the metabolism of pyruvate. The sequence of reactions from glucose to pyruvate is similar in most organisms and most types of cells. In contrast, the fate of pyruvate is variable. Three reactions of pyruvate are of prime importance: conversion into ethanol, lactic acid, or carbon dioxide (Figure 16.10).

The conversion of glucose into ethanol is an example of alcoholic fermentation. The net result of this anaerobic process is:

Note that NAD⁺ and NADH do not appear in this equation, even though they are crucial for the overall process. NADH generated by the oxidation of glyceraldehyde 3-phosphate is consumed in the reduction of acetaldehyde to ethanol. Thus, there is no net oxidation-reduction in the conversion of glucose into ethanol (Figure 16.12). The ethanol formed in alcoholic fermentation provides a key ingredient for brewing and winemaking.

The overall reaction in the conversion of glucose into lactate is:

As in alcoholic fermentation, there is no net oxidation-reduction. The NADH formed in the oxidation of glyceraldehyde 3-phosphate is consumed in the reduction of pyruvate. The regeneration of NAD⁺in the reduction of pyruvate to lactate or ethanol sustains the continued operation of glycolysis under anaerobic conditions.