Acceleration of cleavage of the carbon-cobalt bond of sterically hindered alkylcobalamins by binding to apoprotein of diol dehydrase.

Cleavage of the C-Co bond of sterically hindered alkylcobalamins bearing neither an adenine moiety nor functional groups, such as isobutylcobalamin, neopentylcobalamin, and cyclohexylcobalamin, was markedly accelerated by their interaction with apoprotein of diol dehydrase, although these cobalamins do not function as coenzyme. Acceleration of the conversion of alkylcobalamins to enzyme-bound hydroxocobalamin was stoichiometric and obeyed first-order reaction kinetics. These results, together with strong competitive inhibition by these alkylcobalamins with respect to adenosylcobalamin, indicate that acceleration of the C-Co bond cleavage by the apoenzyme is due to labilization of their C-Co bond by binding to the active site of the enzyme. This labilization is considered to be caused by a steric distortion of the corrin ring which is induced by specific tight interaction of the cobalamin moiety with apoprotein. The importance of such a labilizing effect for activation of the C-Co bond of adenosylcobalamin in enzymatic reactions is discussed.