Biotin carboxylations--concerted or not concerted? That is the question!

When beta-fluoropropionyl coenzyme A is used as substrate, propionyl-CoA carboxylase catalyzes the formation of ADP and the elimination of fluoride ion. No F- release occurs in the absence of ATP or in the presence of avidin. ADP formation occurs as rapidly as in the presence of propionyl-CoA, but the rate of F- release is 6 times that of ADP formation. The rate of F- release is indicative of the minimal rate of abstraction of the alpha proton, and the rate of ADP formation is equivalent to the rate of formation of biotin-CO2. The results, therefore, show that hydrogen abstraction can occur without concomitant CO2 transfer from biotin-CO2 to the substrate. Therefore, the concerted mechanism which has been proposed for this, and other biotin enzymes, is not applicable when propionyl-CoA carboxylase acts on beta-fluoropropionyl-CoA. We believe the concerted mechanism is also not involved in the carboxylation of the normal substrate, propionyl-CoA.