The crystal structure of a 27-kilodalton methylcobalamin-containing fragment of methi onine synthase from Escherichia coli was determined at 3.0 Angstrom resolution. This structure depicts cobalamin-protein interactions and reveals that the corrin macrocycle lies between a helical amino-terminal domain and an alpha/beta carboxyl-terminal domain that is a variant of the Rossmann fold. Methylcobalamin undergoes a conformational change on binding the protein; the dimethylbenzimidazole group, which is coordinated to the cobalt in the free cofactor, moves away from the corrin and is replaced by a histidine contributed by the protein. The sequence Asp-X-His-X-X-Gly, which contains this histidine ligand, is conserved in the adenosylcobalamin-dependent enzymes methylmalonyl-coenzyme A mutase and glutamate mutase, suggesting that displacement of the dimethylbenzimidazole will be a feature common to many cobalamin-binding proteins. Thus the cobalt ligand, His(759), and the neighboring residues Asp(757) and Ser(810), may form a catalytic quartet, Co-His-Asp-Ser, that modulates the reactivity of the B-12 prosthetic group in methionine synthase.