Vitamin B12 in its various forms, and in cooperation with its coenzymes and various different substrates is involved in three different forms of reaction:
The first two reactions involve a Co(II) oxidation state intermediate, the third probably involves a Co(I), and in both in its 'resting state' the cobalt is a Co(III). Central to the catalytic rôle of is the relative weakness of the cobalt-carbon bond, with a dissociation energy of around 120 kJmol-1. In several of the rearrangements, and in the reductase, EPR signals have been observed for the deoxyadenosyl radical and for a cobalt(II) corrin.
In the hydrogen migrations in all of the Vitamin B12 coenzyme-dependent rearrangement reactions, there is no exchange of the hydrogen with water protons, and the 1,2 intramolecular shifts are stereospecific for both the hydrogen atom and the exchanging group. Different reactions also proceed with either retention or inversion of the configuration.
The mechanistic picture which emerges from this is that the rearrangement involves a kind of 'reaction in a bottle', between the Vitamin B12 and the substrate, with the large coenzyme acting as the bottle. Co(II) and free radicals are very oxygen sensitive, so that the whole process must be kept anaerobic.
Around 1994 we entered a fascinating stage in the development of our understanding of the mechanisms of action of the B12, enzyme-coenzyme systems. (and it is necessary to use the plurals!). Thus all of the genes in the B12 biosynthesis have been cloned, sequenced and expressed. Thus, an X-ray crystal structure of a 27-kilodalton methyalcobalamin-containing fragment of methionine synthase fromEscherichia coli was obtained. "This structure depicts cobalamin-protein interactions and reveals that the corrin macrocycle lies between a helical amino-terminal domain and an a/b carboxyl terminal domain that is a variant of the Rossmann fold. Methylcobalamin undergoes a conformational change on binding the protein; the dimethylbenzamidazole group, which is coordinated to the free cofactor, moves away from the corrin and is replaced by a histadine contributed by the protein. The sequence Asp-X-His-X-X-Gly, which contains this histadine ligand, is conserved in the adenosylcobalamin-dependent enzymes methylmalonyl-coenzyme A mutase and glutamate mutase, suggesting that displacement of the dimethylbenzamidazole will be a feature common to many cobalamin-binding proteins."