page_title: Vitamin B12
section_title: Chemistry
hierarchical_section_title: Vitamin B12 / Chemistry
caption_attribution_description: English: structures of the vitamers, along with some terminology
context_page_description: Vitamin B₁₂, also known as cobalamin, is a water-soluble vitamin involved in the metabolism of every cell of the human body. It is one of eight B vitamins. It is a cofactor in DNA synthesis, and in both fatty acid and amino acid metabolism. It is particularly important in the normal functioning of the nervous system via its role in the synthesis of myelin, and in the maturation of developing red blood cells in the bone marrow.
  Vitamin B₁₂ is the largest and most structurally complex vitamin. The vitamin exists in four near-identical chemical forms: cyanocobalamin, hydroxocobalamin, adenosylcobalamin and methylcobalamin. Cyanocobalamin and hydroxocobalamin are used to prevent or treat vitamin deficiency; once absorbed they are converted into adenosylcobalamin and methylcobalamin, which are the forms which have physiological activity. All forms of vitamin B₁₂ contain the biochemically rare element cobalt positioned in the center of a corrin ring. The only organisms to produce vitamin B₁₂ are certain bacteria and archaea.
context_section_description: B₁₂ is the most chemically complex of all the vitamins. The structure of B₁₂ is based on a corrin ring, which is similar to the porphyrin ring found in heme. The central metal ion is cobalt. 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 C-Co 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.
caption: 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.
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