SATURATED FATTY ACIDS 29 



(a) The Function of Phosphotransacetylase. Phosphotransacetylase is an 

 enzyme which activates the acetyl group in acetylphosphate (Figure 2), 

 both for methyl transfer in the synthesis of citrate, and for carboxyl reac- 

 tions in acetylation.^- This enzyme has been detected by Stadtman^"^ 

 only in extracts of the Clostridium kluyverii. It requires potassium and 

 ammonium ions for its activity."" Stern and co-workers"^ explain the 

 dual action of acetyl-CoA as due to the fact that it may exist as an equi- 

 librium of tautomeric forms as follows : 



CH3-C0-SR , CH2:C(0H)SR 



Tautomerism of Acetyl-CoA^'* 



(b) The Role of the Condensing Enzyme. The so-called condensing en- 

 zyme functions in the reversible reaction:'"^ 



Condensing 

 enzyme 



acetyl-CoA + oxaloacetate ^ ^^ CoA + citric acid 



When citrate and phosphate are combined with the condensing enzyme 

 and with highly purified phosphotransacetylase, acetylphosphate is formed. 

 The condensing enzyme, which is widely distributed in animal tissues, was 

 purified by Stadtman,"" as well as by Stern, Ochoa, and collabo- 

 rators^°®'"^~"^ and, in fact, has been separated in crystalline form."^ 



b. The Mechanism of Transfer of Acyl Groups by Coenzyme A. The 

 reaction by which acetyl-CoA functions to bring about oxidation with 

 long-chain acids indicates that the co-factor can condense with acyl groups, 

 as well as with acetate, to form a condensing compound. This either 

 requires ATP or it functions by the simultaneous mediation of active 

 acetate. Thus, Chantrenne"^ demonstrated the synthesis of hippuric acid 

 from glycine and benzoic acid in the presence of CoA and ATP. According 

 to Barker,"^ the activation of butyric acid with acetyl-CoA follows the 

 following pathway: 



Ac-CoA + butyrate ^ ^ butyryl-CoA + acetate 



The Mechanism of Activation of Butyric Acid with CoA^^^ 



The activation of the higher fatty acids by this method has been demon- 

 strated in the case of extracts of the anaerobe, Clostridium kluyverii. ''■^^ 



!»' E. R. Stadtman, Federation Proc, 11, 291 (1952). 



"» E. R. Stadtman, /. Biol. Chem., 196, 527-534, 535-546 (1952). 



111 J. R. Stern, B. Shapiro, E. R. Stadtman, and S. Ochoa, /. Biol. Chem., 1.93, 703-720 

 (1951). 



112 J. R. Stern and S. Ochoa, /. Biol. Chem., 191, 161-172 (1951). 



118 J. R. Stern, S. Ochoa, and F. Lvnen, /. Biol. Chem., 198, 313-321 (1952). 



11^ H. Chantrenne, J. Biol. Chem., 189, 227-233 (1951). 



11* H. A. Barker, Phosphorus Metabolism, 1, 204-245 (1951). 



