The importance of the amide bond nearest the thiol group in enzymatic reactions of coenzyme A

Analogues of coenzyme A (CoA) and of CoA thioesters have been prepared in which the amide bond nearest the thiol group has been modified. An analogue of acetyl-CoA in which this amide bond is replaced with an ester linkage was a good substrate for the enzymes carnitine acetyltransferase, chloramphenicol acetyltransferase, and citrate synthase, with K_m values 2- to 8-fold higher than those of acetyl-CoA and V_max values from 14 to >80% those of the natural substrate. An analogue in which an extra methylene group was inserted between the amide bond and the thiol group showed less than 4-fold diminished binding to the three enzymes but exhibited less than 1% activity relative to acetyl-CoA with carnitine acetyltransferase and no measurable activity with the other two enzymes. Analogues of several CoA thioesters in which the amide bond was replaced with a hemithioacetal linkage exhibited no measurable activity with the appropriate enzymes. The results indicate that some aspects of the amide bond and proper distance between this amide and the thiol/thioester moiety are critical for activity of CoA ester-utilizing enzymes.