Translations:Vitamin B12/58/en

Four of the six coordination sites are provided by the corrin ring, and a fifth by a dimethylbenzimidazole group. The sixth coordination site, the reactive center, is variable, being a cyano group (–CN), a hydroxyl group (–OH), a methyl group (–CH₃) or a 5′-deoxyadenosyl group. Historically, the covalent carbon–cobalt bond is one of the first examples of carbon–metal bonds to be discovered in biology. The hydrogenases and, by necessity, enzymes associated with cobalt utilization, involve metal–carbon bonds. Animals have the ability to convert cyanocobalamin and hydroxocobalamin to the bioactive forms adenosylcobalamin and methylcobalamin by means of enzymatically replacing the cyano or hydroxyl groups.

The structures of the four most common vitamers of cobalamin, together with some synonyms. The structure of the 5'-deoxyadenosyl group, which forms the R group of adenosylcobalamin is also shown.