BUTYRIC ACID-BUTANOL FERMENTATIONS 53 



namely, the reduction of acetoacetyl-SCoA to /?-hydroxy- 

 butyryl-SCoA and the reduction of crotonyl-SCoA to 

 butyryl-SCoA. If the oxidation of triose phosphate is to 

 continue, these reactions must be replaced by other electron- 

 accepting reactions. The only alternative reactions of this 

 type available to these organisms appear to be the reduc- 

 tions of acetyl- and butyryl-SCoA and the corresponding 

 aldehydes to alcohols. Therefore, the formation of acetone 

 is necessarily linked with the formation of alcohols. The 

 trigger mechanism for solvent formation must be a step 

 in the conversion of acetoacetyl-SCoA to acetone. If the 

 conversion of acetoacetyl-SCoA to acetoacetate is in fact a 

 reversible reaction, the irreversible decarboxylation of 

 acetoacetate is probably the key reaction. The observation 

 of Davies 60 that solvent formation is associated with a rapid 

 increase in the level of acetoacetate decarboxylase in 

 CI. acetobutylicum supports this conclusion. 



The material presented in this chapter shows that sub- 

 stantial progress has been made in understanding the 

 complex chemical reactions involved in butyric acid- 

 butanol fermentations. However, despite an early start 

 and rapid initial progress, knowledge of the enzymatic 

 reactions of fatty acid metabolism in bacteria is now less 

 complete than that of similar processes in animal tissues. 

 Only a few bacterial enzymes concerned with fatty acid 

 metabolism have been purified fairly extensively and studied 

 in some detail. More information is needed concerning the 

 enzymes and intermediates functioning between acetoacetyl- 

 SCoA and butyryl-SCoA in butyrate synthesis, and between 

 acetoacetyl-SCoA and acetoacetate in acetone formation. 

 We need to learn something about the mechanisms by which 

 the energy derived from butyrate synthesis and associated 

 reactions is made available to the cell. Also in the carbo- 

 hydrate fermentations, the steps in the conversion of sugars 



