Cobalamin (vitamin B12) exists in a number of different chemical forms. The molecule consists of two halves: a 'planar group' and 'nucleotide' set at right angles to it (Figure 5.5). The planar group is a corrin ring and the nucleotide consists of the base, 5,6-dimethylbenziminazole, and a phosphorylated sugar, ribose-s-phosphate. In nature, the vitamin is mainly in the

Figure 5.5 The structure of vitamin B12 (cyanocobalamin).

5'-deoxyadenosyl (ado) form. This is the main form in human tissues and is located in the mitochondria. It serves as the cofac-tor for the enzyme methylmalonyl CoA mutase. The other major natural cobalamin is methylcobalamin, the main form in human plasma, as well as the cytosolic form in cells. It serves as the cofactor for the enzyme methionine synthase. There are also minor amounts of hydroxocobalamin, which is the form to which methyl- and ado-cobalamin are rapidly converted by exposure to light, hydroxocobalamin having its cobalt atom in the fully oxidized Cbl3+ state, whereas the cobalt exists as reduced Cbl1+ in the methyl- and ado-cobalamin forms (Figure 5.2). A glutathionyl cobalamin form has also been identified.

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