Acid-Thiol Ligases and CoA-Transferases

Thioesters of coenzyme A (D 11) are important intermediates in carboxylic acid metabolism. They are formed by acid-tbiol bgases (thiokinases) either via acyl phosphates  

CoA-transferases catalyze an exchange of the acyl group according to the following equation  

In the thioester grouping rhe resonance of the carboxy group is disturbed and the CO-grouping has a considerable carbonyl character (as is the case in acyl phosphates, C 1.1). 

Because of this activation the oxygen atom carries a negative, and the carbon atom a positive, fractional charge. The positively charged C-atom attracts electrons from the -carbon atom causing weakening of the C—H bonds at this C-atom. Thioesters, therefore, may be easily substituted electrophilically at the (X-carbon atom and nucleophilicaUy at the carbon atom of the carbonyl group  

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