Vitamin B12 (Cobalamin)

Vitamin B12 refers to a class of compounds called cobalamins, which consist of a large ring structure with a central cobalt ion. B12 acts as a cofactor for 2 reactions: the production of succinyl-CoA, which is an intermediate in the metabolic citric acid cycle, and the conversion of homocysteine to methionine, an essential amino acid. 

A deficiency of B12 is rare as the body stores relatively large quantities, but can be caused by other medical conditions. One condition related to B12 deficiency is pernicious anemia. Clams, salmon, and beef liver are excellent sources of B12, although many grain products are fortified with B12.