198 UNITY AND DIVERSITY IN BIOCHEMISTRY 



operation according to the general equations above, we again lengthen the 

 chain by two carbon atoms. If, for example, we repeat the operation eight 

 times, we shall obtain stearic acid according to the equation : 



9CH3— CO— S— CoA + 32H ^ 



^ C17H35— CO— S— CoA + 8HS— CoA + SHgO 



Since the cycle is completely reversible, it also explains j8-oxidation. 

 Starting from a fatty acid, this acid will be activated by conversion to an 

 acyl-CoA derivative, it will be the object of a dehydrogenation, of a hy- 

 dration and a further dehydrogenation to give a ^-ketoacyl-CoA. The 

 thiolysis of this latter substance will give a molecule of acetyl-CoA and the 

 acyl-CoA of the fatty acid containing two less carbon atoms. 



The overall scheme is summarized in Fig. 42. 



The different enzyme reactions intervening in the fatty acid cycle are as 

 follows : 



1. j3-ketoreductase 



CH3— CH— CH2— CO— S— CoA + DPN ^ CH3— C— CHj— CO— S— CoA + DPNHj 



I II 



OH O 



2. Ethylene-reductase 



CH3— CH = CH— CO— S— CoA + FADHg ^ 



CH3— CHa- CH2— CO— S— CoA + FAD 



3. j8-ketothiolase 



R— CHa- CO— CH2— CO— S— CoA + HS— enz. ^ 



R— CHa- CO— S— enz. + CH3— CO— S— CoA 

 R— CH2— CO— S— enz. + HS— CoA ^ R— CHj- CO— S— CoA + HS— enz. 



The end result being: 



R— CHa- CO— CH2— CO— S— CoA + HS— CoA ^ 



R— CHa- CO— S— CoA h CH3— CO— S— CoA 



Before entering the cycle, a fatty acid must be attached to CoA. This is 

 the general mechanism; it can take several forms : 



1. Reaction with CoA and ATP. This is the mechanism occurring in 

 animal tissues. Three enzymes of differing specificity are known, reacting 

 with — acetate and propionate, C4 to Cjg fatty acids, and acids containing 

 longer chains. 



2. Transfer of CoA. The transfer takes place from an acylated derivative 

 (for example from acetyl-CoA for the Ci to Cg acids in Clostridium 

 kluyveri). In the heart, and probably also in the kidney in vertebrates, such 



