Two X-ray structures of cobalamin (B12) bound to proteins have now been determined. These structures reveal that the B12 cofactor undergoes a major conformational change on binding to the apoenzymes of methionine synthase and methylmalonyl-coenzyme A mutase: The dimethylbenzimidazole ligand to the cobalt is displaced by a histidine residue from the protein. Two methyltransferases from archaebacteria that catalyze methylation of mercaptoethanesulfonate (coenzyme M) during methanogenesis have also been shown to contain histidine-ligated cobamides. In corrinoid iron-sulfur methyltransferases from acetogenic and methanogenic organisms, benzimidazole is dissociated from cobalt, but without replacement by histidine. Thus, dimethylbenzimidazole displacement appears to be an emerging theme in cobamide-containing methyltransferases. In methionine synthase, the best studied of the methyltransferases, the histidine ligand appears to be required for competent methyl transfer between methyl-tetrahydrofolate and homocysteine but dissociates for reductive reactivation of the inactive oxidized enzyme. Replacement of dimethylbenzimidazole by histidine may allow switching between the catalytic and activation cycles. The best-characterized B12-dependent mutases that catalyze carbon skeleton rearrangement, for which methylmalonyl-coenzyme A mutase is the prototype, also bind cobalamin cofactors with histidine as the cobalt ligand, although other cobalamin-dependent mutases do not appear to utilize histidine ligation. It is intriguing to find that mutases, which catalyze homolytic rather than heterolytic cleavage of the carbon-cobalt bond, can use this structural motif. In methylmalonylCoA mutase a significant feature, which may be important in facilitating homolytic cleavage, is the long cobalt-nitrogen bond linking histidine to the co-factor. The intermediate radical species generated in catalysis are sequestered in the relatively hydrophilic core of an alpha/beta barrel domain of the mutase.