The Enzymatic Carboxylation of Propionyl Coenzyme A

(R)and (S)-Z-deuteriopropionyl coenzyme A of approximately 75% optical purity were prepared by a series of reactions starting from optically pure alanine. With these compounds as substrates for propionyl-CoA carboxylase, the deuterium isotope effect was found to be small and probably secondary. No tritium isotope effect was observed when enzymatically prepared 2-3H-propionyl-CoA was used as substrate. The rate of release of tritium was the same as that of the over-all reaction. Tritium from 3H~0 was not incorporated into propionyl-CoA in the absence of the reverse reaction. The data were consistent with a concerted mechanism in which an or-proton was removed simultaneously with carboxylation of propionyl-CoA. By various techniques the reaction was shown to proceed with retention of configuration about the a-carbon.