Fates of Pyruvate under Anaerobic Conditions: Fermentation:- Pyruvate Is the Terminal Electron Acceptor in Lactic Acid Fermentation
When animal tissues cannot be supplied with sufficient oxygen to support aerobic oxidation of the pyruvate and NADH produced in glycolysis, NAD is regenerated from NADH by the reduction of pyruvate to lactate. As mentioned earlier, some tissues and cell types (such as erythrocytes, which have no mitochondria and thus cannot oxidize pyruvate to CO2) produce lactate from glucose even under aerobic conditions. The reduction of pyruvate is catalyzed by lactate dehydrogenase, which forms the L isomer of lactate at pH 7:
The overall equilibrium of this reaction strongly favors lactate formation, as shown by the large negative standard free-energy change. In glycolysis, dehydrogenation of the two molecules of glyceraldehyde 3-phosphate derived from each molecule of glucose converts two molecules of NAD+ to two of NADH. Because the reduction of two molecules of pyruvate to two of lactate regenerates two molecules of NAD+, there is no net change in NAD+ or NADH:
The lactate formed by active skeletal muscles (or by erythrocytes) can be recycled; it is carried in the blood to the liver, where it is converted to glucose during the recovery from strenuous muscular activity. When lactate is produced in large quantities during vigorous muscle contraction (during a sprint, for example), the acidification that results from ionization of lactic acid in muscle and blood limits the period of vigorous activity. The best-conditioned athletes can sprint at top speed for no more than a minute.
Although conversion of glucose to lactate includes two oxidation-reduction steps, there is no net change in the oxidation state of carbon; in glucose (C6H12O6) and lactic acid (C3H6O3), the H:C ratio is the same. Nevertheless, some of the energy of the glucose molecule has been extracted by its conversion to lactate—enough to give a net yield of two molecules of ATP for every glucose molecule consumed. Fermentation is the general term for such processes, which extract energy (as ATP) but do not consume oxygen or change the concentrations of NAD+ or NADH. Fermentations are carried out by a wide range of organisms, many of which occupy anaerobic niches, and they yield a variety of end products, some of which find commercial uses.
