Vitamin B12, also known as cobalamin, is a water-soluble vitamin that is involved in the metabolism of every cell of the human body. It is essential for the proper function of nerve cells and red blood cells. Vitamin B12 is synthesized only by bacteria and archaea; it is not synthesized by plants or animals.
Vitamin B12 was first isolated from liver in 1948. The name cobalamin comes from cobalt, which is required for its synthesis. Vitamin B12 has a complex structure, consisting of a nitrogen-containing corrin ring with a central metal atom (cobalt in most cases) bound to four coordinated ligands. The most common naturally occurring form of vitamin B12, cyanocobalamin, contains a cyanide ion ligand bound to cobalt. Other natural forms include hydroxocobalamin and methylcobalamin; these two are active as coenzymes in certain biochemical reactions. Various synthetic analogues of vitamin B12 are also available; these are sometimes used to treat vitamin B12 deficiency when natural sources are unavailable or ineffective.
Vitamin B12 plays an important role in DNA synthesis and regulation, as well as energy production and metabolism. Deficiency of vitamin B12 can lead to megaloblastic anaemia and neurological problems such as memory loss and dementia. Vitamin B12 deficiency is relatively common, especially among older adults, vegans and vegetarians who do not consume animal products (which are the main source of dietary vitamin B12). Supplementation with vitamin B12 may be necessary for these groups to prevent deficiency symptoms from developing.