BUTYRIC ACID-BUTANOL FERMENTATIONS 41^ 



coenzyme A but not upon orthophosphate. The product 

 of this reaction is acetyl-SCoA which accumulates when 

 substrate amounts of coenzyme A are used. With only 

 catalytic amounts of coenzyme A acetaldehyde oxidation 

 ceases as soon as the coenzyme is mostly converted to the 

 acetylated form. The further addition of orthophosphate 

 in the presence of phosphotransacetylase permits the trans- 

 fer of the acetyl group from SCoA to phosphate, giving 

 acetyl phosphate and regenerating free coenzyme A so that 

 the oxidation of acetaldehyde can continue. Of course, 

 any other reaction that will consume acetyl-SCoA and 

 regenerate coenzyme A will serve the same function. As 

 we shall see, the normal acetyl-SCoA-consuming reaction 

 in the CI. kluyveri fermentation is that leading to the 

 formation of acetoacetyl-SCoA. The formation of acetyl 

 phosphate appears to be a side reaction which may be useful 

 in the storage of acetyl groups and in the generation of ATP 

 by the action of the enzyme acetokinase 31 (reaction 18) . 



CH3COOPO3H2 + ADP =j=± CH3COOH + ATP (18) 



The formation of acetyl-SCoA by oxidation of acetalde- 

 hyde is a peculiarity of CI. kluyveri. Other sugar-fermenting 

 butyric acid bacteria probably form acetyl-SCoA during the 

 phosphoroclastic decomposition of pyruvate (reaction 6) . 

 This reaction in CI. butyricum is evidently quite complex 

 since it requires at least three cofactors: cocarboxylase, 

 coenzyme A, and ferrous ions. 32 The requirement for coen- 

 zyme A, the eventual formation of acetyl phosphate, and 

 the presence of phosphotransacetylase in the enzyme ex- 

 tracts indicate the intermediate formation of acetyl-SCoA. 

 This compound is known to be a product of pyruvate oxida- 

 tion by Escherichia coli. 3i 



The role of coenzyme A in the activation of fatty acids 

 other than acetate has been studied in some detail with 



