Mixed acid fermentation

In biochemistry, mixed acid fermentation is the metabolic process by which a six-carbon sugar (e.g. glucose, C₆H₁₂O₆) is converted into a complex and variable mixture of acids. It is a 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. Variations of mixed acid fermentation occur in a number of bacterial species, including bacterial pathogens such as Haemophilus influenzae where mostly acetate and succinate are produced and lactate can serve as a growth substrate.

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:

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.

In the second stage, pyruvate produced by glycolysis is converted to one or more end products via the following reactions. In each case, both of the NADH molecules generated by glycolysis are reoxidized to NAD⁺. Each alternative pathway requires a different key enzyme in E. coli. After the variable amounts of different end products are formed by these pathways, they are secreted from the cell.

Pyruvate produced by glycolysis is converted to lactate. This reaction is catalysed by the enzyme lactate dehydrogenase (LDHA).

Pyruvate is converted into acetyl-coenzyme A (acetyl-CoA) by the enzyme pyruvate dehydrogenase. This acetyl-CoA is then converted into acetate in E. coli, whilst producing ATP by substrate-level phosphorylation. Acetate formation requires two enzymes: phosphate acetyltransferase and acetate kinase.