Pyruvate is the end product of glycolysis, where glucose is oxidized to pyruvate, simultaneously reducing NAD+ to NADH. Two molecules of ATP are also produced by substrate-level phosphorylation.
In aerobic organisms, pyruvate is metabolized via the citric acid cycle to produce reduced coenzymes NADH and FADH2. These coenzymes are then oxidized in the electron transport chain to produce ATP and, in the process, regenerate the NAD+ and FAD. As seen in some cell types and organisms, fermentation reduces pyruvate to lactate in the absence of oxygen, oxidizing NADH to NAD+. The regeneration of NAD+ allows glycolysis to continue to make ATP by substrate-level phosphorylation.
Lactic Acid Fermentation
During lactic acid fermentation, pyruvate accepts electrons from NADH and is reduced to lactic acid. Microbes performing homolactic fermentation produce only lactic acid as the fermentation product; microbes performing heterolactic fermentation produce a mixture of lactic acid, ethanol and/or acetic acid, and CO2. Lactic acid production by the normal microbiota prevents the growth of pathogens in certain body regions and is important for the health of the gastrointestinal tract.
The ethanol fermentation of pyruvate by the yeast Saccharomyces cerevisiae is used in the production of alcoholic beverages and also makes bread products rise due to CO2 production. Outside of the food industry, ethanol fermentation of plant products is important in biofuel production.
Several fermentation products are important commercially outside of the food industry. For example, chemical solvents such as acetone and butanol are produced during acetone-butanol-ethanol fermentation. Complex organic pharmaceutical compounds used in antibiotics (e.g., penicillin), vaccines, and vitamins are produced through mixed acid fermentation.
This text is adapted from Openstax, Biology 2e, Section 7.5: Metabolism without oxygen
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