Structure of Vitamin B12

Vitamin B12 is the member of Vitamin B complex. Vitamin B12 itself is commonly called cynocobalamin. Vitamin B12 is the most structurally complicated vitamin compound among the vitamins, made up of a class of chemically related compounds, and is directly involved in the metabolism of every single cell in the human body, even our DNA.

Vitamin B12 is the only known essential biomolecule with a stable metal-carbon bond, that is an organometallic compound and has corrin ring as a core. The central metal which is cobalt, can link to:
1. A methyl group- methylcobalamin
2. A 5’-deoxyadenosine at the 5’ position- adenosylcobalamin (coenzyme of B12)
3. A cyanide group- vitamin B12

Cyanocobalamin is one such compound that is a vitamin in this B complex, because it can be metabolized in the body to an active co-enzyme form. Since the cyanocobalamin form of B-12 is deeply red colored, easy to crystallize, and is not sensitive to air-oxidation, it is typically used as a form of B-12 for food additives and in many common multivitamins. However, this form is not perfectly synonymous with B-12, inasmuch as a number of substances (vitamers) have B-12 vitamin activity and can properly be labeled vitamin B-12, and cyanocobalamin is but one of them. (Thus, all cyanocobalamin is vitamin B-12, but not all vitamin B-12 is cyanocobalamin).

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. Historically, the covalent C-Co bond is one of first examples of carbon-metal bonds to be discovered in biology.

An important aspect of the corrin ring, when compared to the porphyrin, is the relative flexibility of the corrin system, the corrin ring is also less flat when viewed from the side than is a porphyrin ring. This adds up to some considerable differences between the chemistry of a cobalt porphyrin and a cobalt corrin. In addition, the corrin only has a conjugated chain around part of the ring system, whereas a porphyrin is delocalised around the whole four pyrolle rings.