Coenzyme A Hemithioacetals as Easily Prepared Inhibitors of CoA Ester-Utilizing Enzymes

Abstract

Hemithioacetals are formed by reactions of coenzyme A (CoA) with aldehydes in aqueous solution. Equilibria for hemithioacetal formation with four commercially available aldehydes and rate constants for hemithioacetal dissociation have been studied. The hemithioacetals are viewed as acyl-CoA analogs having a tetrahedral center in place of the planar trigonal thioester carbonyl carbon. These compounds may serve as mimics of the tetrahedral intermediate or transition state in the reactions of acyl-CoA dependent acyltransferase enzymes. The hemithioacetal generated by reaction of CoA with formaldehyde is a poor inhibitor of chloramphenicol acetyltransferase, with a Ki more than 6-fold higher than the Km for the substrate acetyl-CoA. The hemithioacetals formed by reaction of CoA with acetaldehyde and trifluroacetaldehyde are substantially better inhibitors, with Ki values approximately 2.4-fold and 10-fold lower than the Km values for acetyl-CoA, respectively. The hemithioacetal formed by reaction of CoA with succinic semialdehyde inhibits succinic thiokinase, with a Ki 4-fold lower than the Km for the substrate succinyl-CoA. The CoA hemithioacetals provide a novel readily accessible new class of acyl-CoA analogs for use in mechanistic and structural studies of CoA ester-utilizing enzymes.