BUTYRIC ACID- BUTANOL FERMENTATIONS 51^ 



very wasteful of energy since the hydrolytic cleavage of the 

 thioester bond involves a free-energy change of about —8 

 kcal. In CI. kluyveri this is unimportant since little or no 

 free acetoacetate is formed during its normal metabolism. 

 A more efficient way of forming acetoacetate would be a 

 transfer of SCoA from acetoacetyl-SCoA to acetate (reaction 

 28) to give acetoacetate and acetyl-SCoA. The latter 



CH3COCH2COSC0A + CH3COOH =^± 



CH3COCH2COOH -f CH3COSC0A (28) 



compound can be used to make more acetoacetyl-SCoA 

 without a further expenditure of energy. The SCoA trans- 

 ferase of animal tissues catalyzes a reversible reaction of 

 this type between acetoacetyl-SCoA and succinate. 61 How- 

 ever, this enzyme probably does not occur in butyric acid 

 bacteria, and the bacterial fatty acid SCoA transphorase, at 

 least that which occurs in CI. kluyveri, does not react with 

 acetoacetate. 35 Nevertheless, it seems probable that bacteria 

 like CI. acetobutylicum which form large amounts of acetone 

 via acetoacetate will be found to conserve the thioester bond 

 energy of acetoacetyl-SCoA by making use of reactions 

 similar to reaction 28. 



The formation of n-butanol from butyrate undoubtedly 

 involves butyryl-SCoA as an intermediate. Butyryl-SCoA 

 can be formed from butyrate and acetyl-SCoA, and probably 

 also by the reaction of butyrate with ATP and HSC0A. 34 



ATP + Butyrate + HSCoA ^±= 



AMP + PP 4- Butyryl-SCoA (29) 



In the first demonstration of the enzymatic formation of 

 butanol with CI. kluyveri, butyryl phosphate was the sub- 

 strate. The path of conversion of butyryl phosphate to 

 butyryl-SCoA has not been determined, but a possible 



