Mixed acid fermentation

Mixed acid fermentation is the biological process by which a six-carbon sugar e.g. glucose is converted into a complex and variable mixture of acids. It is an fermentation reaction that is common in bacteria. It is characteristic for members of the Enterobacteriaceae, a large family of Gram-negative bacteria that includes E. coli.

The mixture of end products produced by mixed acid fermentation includes lactate, acetate, succinate, formate, ethanol and the gases H₂ and CO₂. The formation of these end products depends on the presence of certain key enzymes in the bacterium. The proportion in which they are formed varies between different bacterial species. The mixed acid fermentation pathway differs from other fermentation pathways, which produce fewer end products in fixed amounts. The end products of mixed acid fermentation can have many useful applications in biotechnology and industry. For instance, ethanol is widely used as a biofuel. Therefore, multiple bacterial strains have been metabolically engineered in the laboratory to increase the individual yields of certain end products. This research has been carried out primarily in E. coli and is ongoing.

E. coli use fermentation pathways as a final option for energy metabolism, as they produce very little energy in comparison to respiration. Mixed acid fermentation in E. coli occurs in two stages. These stages are outlined by the biological database for E. coli, EcoCyc.

The first of these two stages is a glycolysis reaction. Under anaerobic conditions, a glycolysis reaction takes place where glucose is converted into pyruvate:

      glucose → 2 pyruvate

There is a net production of 2 ATP and 2 NADH molecules per molecule of glucose converted. ATP is generated by substrate-level phosphorylation. NADH is formed from the reduction of NAD.

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