The mechanism of cobalamin-dependent rearrangements

Adenosylcobalamin-dependent rearrangements are enzyme catalyzed reactions in which a hydrogen atom is transfered from one carbon atom to an adjacent one in exchange for a group X which migrates in the opposite direction. In the hydrogen transfer step, the mechanism of which is reasonably well understood, the cofactor serves as an intermediate hydrogen carrier. The transfer of hydrogen to the cofactor involves homolysis of the carbon-cobalt bond to generate cob(II) alamin and the 5'-deoxyadenos-5'-yl radical, followed by abstraction of a hydrogen atom from the substrate to form 5'-deoxyadenosine and the substrate radical. After migration of group X, the hydrogen atom is returned to the product radical by the reverse of the above reactions to generate the final product and reconstitute the cofactor. In contrast to the transfer of hydrogen, the mechanism of group X migration is poorly understood. Many reactions mechanisms have been proposed on chemical grounds, but there is insufficient biochemical evidence to permit a choice among these propsals. A quantity of negative evidence has accumulated suggesting that group X migration does not involve alkylation of the cobalt of cobalamin by the substrate, but in the absence of firm data supporting an alternative mechanism, even this weak conclusion must be regarded as provisional.