BACTERIAL FERMENTATIONS 



Although many of the steps in the enzymatic conversion of 

 ethanol and acetate to butyrate and caproate are now known, 

 our understanding of the role of this process in the energy 

 transformation of the bacteria is very inadequate. Presumably 

 the formation of the G4 and Cs fatty acids, which involves a free- 

 energy change of approximately 12 kcal. per mole of ethanol 

 consumed, constitutes the main source of energy for synthetic 

 activities of the organism. This implies that the catabolic 

 reaction should result in a net formation of high-energy phos- 

 phate or other suitable energy donor. But as yet there is no 

 evidence that this occurs. The oxidation of acetaldehyde to 

 acetyl-CoA yields a high-energy thioester bond which can be 

 converted to a high-energy phosphate bond by the phosphotrans- 

 acetylase reaction. However, the thioester bond is not available 

 for this purpose, since it is required for the formation of aceto- 

 acetyl-CoA from acetyl CoA. 



There appear to be two ways in which useful energy might 

 be provided during the C. kluyveri fermentation. One is by an 

 oxidation of acetaldehyde which is not coupled with butyrate 

 synthesis but with the formation of hydrogen gas as illustrated 

 in the following reaction, where Pi represents phosphate: 



CH3CHO + Pi + ADP > GH3COOH + ATP + H2 



It should be noted that hydrogen is formed in appreciable 

 amounts during the C. kluyveri fermentation. The free-energy 

 change is unfavorable (approximately 4 kcal.) for the above 

 reaction to proceed from left to right as written. However, by 

 coupling this reaction with strongly exergonic reactions of ATP, 

 such as the hexokinase reaction, which are involved in the 

 synthesis of cellular constituents, the over-all reaction would be 

 slightly exergonic (approximately —2 kcal.). 



A second possible mechanism for net ATP formation in the 

 C. kluyveri fermentation is an oxidative phosphorylation de- 

 pendent on electron transport from ethanol and acetaldehyde 

 to the unsaturated fatty acid electron acceptors such as crotonyl- 

 CoA. The potential difference between the ethanol-acetalde- 



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