Subsystem: Fermentations: Mixed acid

This subsystem's description is:

The products of mixed acid fermentation are succinate, lactate, acetate, ethanol, formate, CO2 and H2 gas. The route to succinate originates at the level of phosphoenolpyruvate (PEP); all other compounds are derived from anaerobic dissimilation of pyruvate. Product patters vary considerably, even in the same species, depending on a number of environmental factors, including the oxidation state of the carbon substrate, the extracellular oxidoreduction potential, and the pH of the external environment. For example, the oxidation state dictates the amount of NADH to be recycled and therefore the composition of the excreted fermentation products. The oxidation of glucose (oxidation state 0) into two molecules of pyruvate produces two NADH molecules (4 reducing equivalents). A more reduced sugar alcohol (e.g., sorbitol; oxidation state -1) produces three NADH, while a highly oxidized sugar acid (e.g., glucuronic acid; oxidation state +2) yields no NADH. Thus, to recycle the larger amount of NADH formed during growth on the more reduced sorbitol, cells must excrete more of the highly reduced ethanol (oxidation state -2). In contrast, cells growing on glucuronic acid are redox balanced and thus do not need to produce ethanol. Instead, they can convert more of their pyruvate to acetate (oxidation state 0). Each of fermentation branches accepts different amounts of reducing equivalents: succinate 4[H], ethanol 4[H], lactate 2[H], acetate 0[H], formate 0[H] (White, 2000). Pathways that utilize 4[H] per PEP/pyruvate molecule are particularly valuable, e.g. succinate branch (it is not encoded in this Subsystem, see SS: “TCA Cycle”).

Similarly, external pH influences the composition of excreted products. Near or above pH 7, the predominant products are acetate, ethanol, and formate, with moderate amounts of succinate. As the pH drops, cells produce lactate and formate (instead of acetate), converting formate to H2 and CO2 (Wolfe, 2005)

Pyruvate formate-lyase (PFL) is the key enzyme for mixed acid fermentation. However, Pyruvate:ferredoxin oxidoreductase (PFOR) and even Pyruvate dehydrogenase complex (PDH) (normally operational only under respiratory conditions) can be can be used for pyruvate cleavage in fermentation as well. This was demonstrated, for example, for Leuconostoc mesenteroides and Oenococcus oeni (Wagner et al., 2005) and B. subtilis (Cruz Ramos et al., 2000). Such cases are not reflected in this SS (impossible to predict from genome sequence alone)

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