Figure 1.

Structure of vitamin B₁₂ (cobalamin). (a) The ‘R group’ corresponds to substitutions at the upper or β-axial ligand (5′-deoxyadenosyl-, methyl-, hydroxo-, cyano-). The dimethylbenzimidazole constituent (DMB) is shown coordinated to the cobalt in the lower α-axial position (‘base-on’ structure). DMB is linked to the corrin ring through a phosphoribosyl attached to a propionamide side chain. (b) Structure of methylcobalamin (MeCbl) with DMB displaced from the cobalt by a histidine residue in methionine synthase (MS; the ‘base-off/His-on’ structure). A similar configuration is observed for adenosylcobalamin (AdoCbl) bound to methylmalonyl-CoA mutase. Structures are from http://www.genome.jp using the ‘SIMCOMP Search’ utility (query C00576, vitamin B₁₂; C06410, MeCbl-MS).