vitamin B12

Vitamin B-12 is the name for a class of chemically-related compounds, all of which have vitamin activity. It is structurally the most complicated vitamin.

Biosynthesis of the basic structure of the vitamin can only be accomplished by bacteria, but conversion between different forms of the vitamin can be accomplished in the human body.

Structure

Vitamin B-12 is a collection of cobalt and corrin ring molecules which are defined by their particular vitamin function in the body. All of the substrate cobalt-corrin molecules from which B-12 is made must be synthesized by bacteria.

However, after this synthesis is complete, the body has a limited power to convert any form of B-12 to another, by means of enzymatically removing certain prosthetic chemical groups from the cobalt atom.

B-12 is the most chemically complex of all the vitamins. The structure of B-12 is based on a corrin ring, which is similar to the porphyrin ring found in heme, chlorophyll, and cytochrome.

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 center of reactivity, is variable, being a cyano group (-CN), a hydroxyl group (-OH), a methyl group (-CH3) or a 5’-deoxyadenosyl group (here the C5’ atom of the deoxyribose forms the covalent bond with Co), respectively, to yield the four B-12 forms mentioned above.

The covalent C-Co bond is one of first examples of carbon-metal bonds in biology. The hydrogenases and, by necessity, enzymes associated with cobalt utilization, involve metal-carbon bonds.

Function

Vitamin B-12 is a vitamin which is important for the normal functioning of the brain and nervous system, and for the formation of blood. It is normally involved in the metabolism of every cell of the body, especially affecting DNA synthesis and regulation, but also fatty acid synthesis and energy production.

See also

 cyanocobalamin