Vitamin B12 is a generic name for a group of cobalamin vitamers that includes adenosylcobalamin (5’-deoxyadenosylcobalamin), cyanocobalamin, hydroxycobalamin, aquacobalamin, nitritocobalamin and methylcobalamin. Cobalamins have a complex structure, and are referred to chemically as corrinoids. This structure consists of four linked pyrrole rings (a corrin), coordinating with a cobalt atom in the centre. Cobalamins are unique in that they contains a carbon to cobalt bond that is found nowhere else in nature. The different cobalamins are characterised by a variable group attached to the corrinoid structure. Cyano, hydroxy, aquo, nitrito, adenosyl and methylcobalamin have a CN, OH, H2O, 5’deoxyadenosyl and CH3 group, respectively. Only adenosylcobalamin and methylcobalamin are active as coenzymes in humans, but humans can convert the other cobalamins into the active forms. Hydroxycobalamin is found in tissues and is a storage form of the vitamin that is readily converted to adenosyl or methylcobalamin when the valency of the cobalt atom is reduced from three to one.
Vitamin B12 is not found in any plant tissues, and synthesis is confined to microorganisms such as yeast. Animal tissues contains cobalamins, mainly hydroxy and adenosylcobalamin, that the animals have absorbed in their diet. Hydroxycobalamin and cyanocobalamin are the form of the vitamin used in supplements. Although cobalamins are produced by human gut bacteria, they are not absorbed. Absorption of dietary cobalamins occurs when R factor binds to vitamin B12 in the stomach. This complex passes to the small intestine where it is hydrolysed to release free cobalamin. Intrinsic factor then binds to the vitamin which is absorbed in the distal small intestine by a carrier mediated process. Vitamin B12 is required as a cofactor in three reactions. The enzymes that catalyses the conversion of homocysteine to methionine (homocysteine methyltransferase) requires methylcobalamin as a co-factor. Methylmalonyl CoA mutase (converts L-methylmalonyl CoA to succinyl CoA) and leucine aminomutase (isomerises L-leucine and β-leucine) both require adenosylcobalamin as a co-factor.