If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Locations of Mapped Genes:
|Superclasses:||Generation of Precursor Metabolites and Energy → Fermentation|
Escherichia coli, a facultative anaerobe, is capable of mediating the mixed acid fermentation and thereby, in the absence of any of the exogenous electron acceptors that it can utilize (oxygen, fumarate, nitrate, nitrite, trimethylamine oxide, or dimethyl sulfoxide), able to grow on a variety of compounds including hexoses, hexitols, hexonic acids, and hexuronic acids.
E. coli ferments only when exogenous electron acceptors are not available. In the shift from exogenous electron-dependent growth to the mixed acid fermentation, pyruvate dehydrogenase is no longer synthesized. Instead, pyruvate formate-lyase, which cleaves pyruvate nonoxidatively to formate and acetyl-CoA, is made. Formate is excreted as formic acid. Some of the acetyl-CoA enters the remnant of the TCA cycle, which terminates at 2-oxoglutarate because 2-oxoglutarate dehydrogenase synthesis is repressed. In spite of such repression, all components of the TCA cycle and therefore its essential precursor metabolites, continue to be made by the now two arms of the cycle: one, leading to 2-oxoglutarate, still oxidatively, and the other, leading to succinate, now reductively. Succinate is excreted as succinic acid. The acetyl-CoA not needed for these purposes is metabolized via acetylphosphate to acetate, thereby generating ATP. The acetate so formed is excreted as acetic acid.
During the shift to the mixed acid fermentation, three additional enzymes are made: 1) a lactate dehydrogenase, which reduces pyruvate to lactate that is excreted as lactic acid; 2) an ethanol dehydrogenase, which reduces acetyl-CoA to acetaldehyde and subsequently to ethanol, which is excreted; 3) formic hydrogen lyase, which cleaves formate to CO2 and H2. The multiplicity of possible endproducts of fermentation with differing oxidation states and acidity permits E. coli to maintain redox balance and therefore to grow fermentatively on a broader range of substrates than would otherwise be possible. When growing on a more reduced substrate such as glucitol, greater quantities of more reduced endproducts such as ethanol, acetic acid, and lactic acid are produced; on more oxidized substrates such as glucuronate, more of the oxidized endproducts, succinic acid and formic acid, are made. Conversion of formate to H2 and CO2 is regulated by pH.
Review: Bock, A. and Sawers, G., Chapter 18: Fermentation, in [Neidhardt96]
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