BUTYRIC ACID-BUTANOL FERMENTATIONS 49 



acetaldehyde and crotonyl-SCoA. The potential span 

 between the ethanol-aldehyde couple [£'0 (pH 7) = 

 — 0.204 volt] and the crotonyl-SCoA-butyryl-SCoA couple 

 [E' (pH 7) = 0.19 volt], for example, is 0.39 volt which 

 corresponds to a free-energy change of approximately —18 

 kcal. under physiological conditions. This is more than 

 sufficient to provide the energy (8 to 10 kcal.) needed for the 

 synthesis of 1 mole of ATP from ADP and orthophosphate. 

 However, so far attempts to demonstrate a coupling of phos- 

 phorylation with crotonyl-SCoA reduction by a DPNH- 

 generating system have not been successful. 



The enzymatic reactions involved in butyrate formation 

 from acetyl-SCoA in other butyric acid bacteria are prob- 

 ably similar to those demonstrated in CI. kluyveri and 

 animal tissues. Although relatively little specific informa- 

 tion has been obtained with other species, Stern et al. 49 have 

 recently reported that extracts of CI. acetobutylicum and 

 CI. sticklandii (strain HF) show enzymatic activities attrib- 

 utable to crotonase, l(+) -/3-hydroxybutyryl-SCoA dehydro- 

 genase, and thiolase. 



Formation of C 5 — C 7 Fatty Acids. Clostridium kluyveri 

 forms rz-caproate from butyrate and ethanol, and forms 

 n-valerate and n-heptanoate from propionate and ethanol. 57 

 The formation of caproate undoubtedly starts with a con- 

 densation of butyryl-SCoA, derived from butyrate or cro- 

 tonyl-SCoA, with acetyl-SCoA derived from ethanol to give 

 /?-ketocaproyl-SCoA by a reaction analogous to the formation 

 of acetoacetyl-SCoA from acetyl-SCoA. The further enzy- 

 matic steps in the formation of caproate are probably the 

 same as those involved in butyrate synthesis. Experimental 

 evidence for the participation of a C 6 /?-keto acid derivative 

 was obtained by showing that the oxidation of caproate, 

 under conditions which prevent the thiolase reaction, 

 results in the accumulation of /?-ketocaproate. 58 



