B12

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, 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 B12 forms mentioned below. Historically, the covalent C-Co bond is one of 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.

Vitamin B12 is a generic descriptor name referring to 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 B12 is made, must be synthesized by bacteria. However, after this synthesis is complete, the body has a limited power to convert any form of B12 to another, by means of enzymatically removing certain prosthetic chemical groups from the cobalt atom. The various forms (vitamers) of B12 are all deeply red colored, due to the color of the cobalt-corrin complex.

Vitamin B12 normally plays a significant role in the metabolism of every cell of the body, especially affecting the DNA synthesis and regulation but also fatty acid synthesis and energy production.However, many though not all of the effects of functions of B12 can be replaced by sufficient quantities of folic acid vitamin B9, since B12 is used to regenerate folate in the body. Most vitamin B12 deficiency symptoms are actually folate deficiency symptoms, since they include all the effects of pernicious anemia and megaloblastosis, which are due to poor synthesis of DNA when the body does not have a proper supply of folic acid for the production of thymine. When sufficient folic acid is available, all known B12 related deficiency syndromes normalize, save those narrowly connected with the vitamin B12-dependent enzymes Methylmalonyl Coenzyme A mutase, and 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR), also known as methionine synthase; and the buildup of their respective substrates (methylmalonic acid, MMA) and homocysteine.

Coenzyme B12's reactive C-Co bond participates in three main types of enzyme-catalyzed reactions.
Isomerases Rearrangements in which a hydrogen atom is directly transferred between two adjacent atoms with concomitant exchange of the second substituent, X, which may be a carbon atom with substituents, an oxygen atom of an alcohol, or an amine. These use the adoB12 (adenosylcobalamin) form of the vitamin.
MethyltransferasesMethyl (-CH3) group transfers between two molecules. These use MeB12 (methylcobalamin) form of the vitamin.
Dehalogenases Reactions in which a halogen atom is removed from an organic molecule. Enzymes in this class have not been identified in humans.