The molecular structure common to all vitamin B12 or cobalamin compounds is a type of heterocyclic ring known as a corrin ring. It is similar to a porphyrin ring that is the basis of hemoglobin. However, while most porphyrins bind an iron ion (iron with an unbalanced electrical charge) in the center, the corrin binds a cobalt ion.
Cobalamin includes methylcobalamin, cyanocobalamin, adenosylcobalamin and hydroxocobalamin. When a cobalt ion is attached to the corrin ring, four chemical connections, or bonds, are formed, each with a nitrogen atom on the ring. This allows a fifth binding site on the cobalt. The type of cobalamin is determined by which chemical group is attached to the fifth binding site of the cobalt atom.
Methylcobalamin has a methyl group (CH3) at the fifth binding site of the cobalt atom. Cyanocobalamin has a cynanide (CH), adenosylcobalamin has an adenosine (C10H13N5O4) group and hydroxocobalamin has an hydroxide group (OH) attached to the fifth binding site of the cobalt.
Cobalamins are soluble in water. Methylcobalamin and adenosylcobalamin exist naturally and are produced by bacteria, including bacteria that inhabit the human gut. Hydroxycobalamin also is produced by bacteria, but not inside the human body. Such bacteria are used often to manufacture vitamin B12. Cynanocobalamin is a synthetic form of vitamin B12. It is included in many vitamin supplements and also is given therapeutically in higher doses for certain disorders.