The first type of Vitamin B12 coenzyme to be isolated had a cyanide group attached to the cobalt, this was picked up during the purification of the vitamin, and is the form which is still referred to as "Vitamin B12". The commercial form of the vitamin is usually obtained as the cyanide, it metabolises fairly easily to the coenzyme. Recently, the form known as "methylcobalamin" has been made available for pharmaceutical purposes. (See Links). Smokers should note that the cyanide absorbed from the smoke into the blood causes the replacement of the 5'-deoxyadenosyl group by a cyanide.
The principal coenzymatic form of Vitamin B12 is 5'-deoxyadenosylcobalamin. The notable feature of the molecule is the cobalt-carbon bond between the 5' carbon of the 5'-deoxyadenosyl moiety (the sugar part) and the cobalt of cobalamin. In Vitamin B12 as it is extracted, a cyanide replaces this sugar link, this occurs during the final purification with active charcoal. Aquocobalamin and hydroxycobalamin, with water and hydroxide are also known, as is the methylated form, methylcobalamin. A number of other cobalt-carbon bonded have been prepared synthetically, but are not known to occur in vivo

Vitamin B12 in its various forms, and in cooperation with its coenzymes and various different substrates is involved in three different forms of reaction:

  1. several mutases in which a hydrogen and some group on an adjacent carbon exchange places, This can be followed by an elimination of water or ammonia.
  2. Ribonucleotide reductase, a reductase whereby ribose is reduced to deoxyribose.
  3. Methyl group transfers

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."