Synthesis of Novel Analogs of Acetyl Coenzyme A: Mimics of Enzyme Reaction Intermediates

An improved method for the synthesis of analogs of coenzyme A (CoA) and its thioesters, which are modified in the thiol or thioester moiety, has been developed using a combination of chemical and enzymatic reactions. The enzymes catalyzing the last two steps of CoA biosynthesis were used to prepare a CoA analog (1c) in which an amide bond is replaced by a thioester bond and the thiol group is replaced by a methyl group. Reaction of 1c with a primary amine in aqueous solution results in aminolysis of the thioester linkage to form the desired CoA analog. Reaction with different amines permits the introduction of a variety of functional groups in place of the normal thiol or thioester group. This methodology has been used in the synthesis of five new analogs of acetyl-CoA in which the thioester sulfur is replaced by a methylene group and the acetyl group is replaced by carboxylate (14a), nitro (14b), carboxamide (14c), methyl sulfoxide (14d), and methyl sulfone (14e) groups. 14a-c were designed to mimic the possible enolate or enol intermediate in the reaction of citrate synthase and related enzymes. 14a and 14c are potent inhibitors of citrate synthase, with K_i values 1000- and 570-fold lower than the K_m for acetyl-CoA, respectively. CD titrations indicate that 14a and 14c have low affinity for citrate synthase in the absence of oxaloacetate, consistent with their recognition as enol or enolate analogs. 14b is a poor inhibitor of citrate synthase, with affinity slightly lower than that for acetyl-CoA. These results are consistent with generation of the enol form of acetyl-CoA as the nucleophilic intermediate in the reaction of citrate synthase. 14d and 14e were designed to mimic the tetrahedral intermediate or transition state in the reaction of chloramphenicol acetyltransferase and related acetyl-CoA-dependent acetyltransferases. Both compounds are poor inhibitors of chloramphenicol acetyltransferase, with affinities slightly lower than that of acetyl-CoA, indicating that these compounds are not good mimics of the enzyme-bound tetrahedral intermediate or transition state.