Are carboxylations involving biotin concerted or nonconcerted?

Transcarboxylase and propionyl-CoA carboxylase catalyze the elimination of HF from beta-fluoropropionyl-CoA to form acrylyl-CoA. No carboxylation products of fluoropropionyl-CoA could be detected. The elimination proceeds at approximately the same rate as the normal carboxylation reactions. With propionyl-CoA carboxylase, ATP was required (HCO3- was also present) and was hydrolyzed to ADP and Pi. The rate of ADP formation was equal to that of acrylyl-CoA formation. A previous report (Stubbe, J. A., and Abeles, R. H. (1977) J. Biol. Chem. 252, 8338--8340) that acrylyl-CoA formation is faster than ADP formation is in error. With transcarboxylase, oxalacetate was required for acrylyl-CoA formation, and pyruvate was produced. The rate of pyruvate formation was equal to that of acrylyl-CoA formation. We conclude that the ability of the enzyme to catalyze the elimination of HF from beta-fluoropropionyl-CoA indicates that the enzyme can catalyze the abstraction of the substrate alpha-proton without concomitant carboxylation of the substrate. We also conclude that the normal catalytic reaction, therefore, probably involves a carbanion intermediate and does not proceed through a concerted process as has frequently been proposed.