The Mixed Acid Fermentation

The a-oxoacid dehydrogenases yield CoA derivatives which may enter biosynthetic reactions. Alternatively, the acyl-CoA compounds may be cleaved with generation of ATP. The pyruvate formate-lyase system also operates as part of an ATP-generating system for anaerobic organisms, for example, in the "mixed acid fermentation" of enterobacteria such as E. coli (Chapter 17). These two reactions, which are compared in Fig. 15-16, constitute an important pair of processes both of which accomplish substrate-level phosphorylation. They should be compared with the previously considered examples of substrate level phosphorylation depicted in Eq. 14-23 and Fig. 15-16. 

Some of the glucose is converted to D-lactic and to succinic acids (pathway/, Fig. 17-9) hence the name mixed acid fermentation. Table 17-1 gives typical yields of the mixed acid fermentation of E. coli. Among the four major products are acetate, ethanol, H2, and C02, as shown in Eq. 17-25. However, at high pH formate accumulated instead of C02. 

Products of the Mixed Acid Fermentation by E. coli at Low and High Values of pHa... 

One is the formate pathway characteristic of the mixed acid fermentation by bacteria such as Escherichia coli. The pathway was proposed from stoichiometric considerations, such as the molar amount of formate at pH around 7 is equal to the molar amount of C02 and H2 at pH... 

Of course in practice in fermentations such as the mixed acid fermentation carried out by enteric bacteria, acetate and ATP production are less due to the need to balance the fermentation by regenerating NAD through the reduction of acetyl-CoA to ethanol thus providing necessary substrate (NAD) required for further sugar degradation. 

Under anaerobic conditions, in the absence of alternative and much more energetically favorable electron acceptors, such as nitrate or fumarate, Ecoli utilizes the mixed acid fermentation pathway. Since the fermentation products are not only carbon dioxide and... 

The details of the process and the oxidation-reduction balance can be pictured as in Eq. 17-25. Pyruvate is cleaved by the pyruvate formate-lyase reaction (Eq. 15-37) to acetyl-CoA and formic acid. Half of the acetyl-GoA is cleaved to acetate via acetyl-P with generation of ATP, while the other half is reduced in two steps to ethanol using the two molecules of NADH produced in the initial oxidation of triose phosphate (Eq. 17-25). The overall energy yield is three molecules of ATP per glucose. The "efficiency" is thus (3 x 34.5) H- 225 = 46%. Some of the glucose is converted to D-lactic and to succinic acids (pathway/. Fig. 17-9) hence the name mixed acid fermentation. Table 17-1 gives typical yields of the mixed acid fermentation of... 

Ji XJ, Huang H, Li S, Du J, Lian M. (2008). Enhanced 2,3-butanediol production by altering the mixed acid fermentation pathway in Klebsiella oxytoca. Biotechnol Lett, 30, 731—734. 

Vuoristo, K.S., Mars, A.E., Sangra, J.V., Springer, J. et al. (2015) Metabolic engineering of the mixed-acid fermentation pathway of Escherichia coli for anaerobic production of glutamate and itaconate. AAIB Express, 5, 61. 

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