The mechanism of adenosylcobalamin-dependent rearrangements

Adenosylcobalamin (AdoCbl)-dependent rearrangements are a group of reactions with no obvious precedents in organic chemistry. In every case, they are characterized by a mechanism in which a hydrogen atom on one carbon atom exchanges places with a group X on an adjacent carbon: (formula; see text) Much experimental work indicates that an AdoCbl rearrangement is initiated by homolysis of the C-Co bond of the cofactor. The migrating hydrogen is then abstracted from the substrate by the resulting 5'-deoxyadenosyl radical, or by a second radical that is generated elsewhere at the active site, and, after the migration of group X, is returned to the product in a similar reaction. In at least some of the rearrangements, group X migration may occur via a cation radical intermediate that formed by the departure of X with its electrons, a process assisted by the unpaired electron left behind on the adjacent carbon after the abstraction of the migrating hydrogen. Once C-Co bond cleavage has initiated the reaction by producing a free radical at the active site, the corrin ring plays no further role in the rearrangements.