Homolactic fermentation is carried out by bacteria belonging to the genera Lactococcus, Enterococcus, Streptococcus, and Pediococcus, and by some species of the genus Lactobacillus. All of these bacteria can convert sugar to mainly lactic acid, via glycolysis. The enzyme lactate dehydrogenase (LDH) catalyzes the last step of this fermentation; in particular, by transferring hydrogen from NADH to pyruvate, LDH leads to pyruvate reduction and NADH reoxidation, with the production of D- or L-lactate. ATP formation is coupled to pyruvate production and the ATP yield of homolactic fermentation is 2 mol per mole of glucose oxidized (Figure 4). The homolactic behavior is not obligatory, but depends on sugar type, rate ofthe glycolytic flux, and growth conditions. Some homofermentative bacteria can catabolize glucose in a heterofermentative fashion, or carry out mixed-acid fermentation when the glycolytic flux is low, thus decreasing or increasing the ATP yield per mole of glucose, respectively.
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