COBALAMIN-DEPENDENT METHIONINE SYNTHASE - THE STRUCTURE OF A METHYLCOBALAMIN-BINDING FRAGMENT AND IMPLICATIONS FOR OTHER B-12-DEPENDENT ENZYMES

Cobalamin-dependent methionine synthase is a large enzyme composed of structurally and functionally distinct regions. Recent studies have be gun to define the roles of several regions of the protein. In particul ar, the structure of a 27 kDa cobalamin-binding fragment of the enzyme from Escherichia coli has been determined by X-ray crystallography, a nd has revealed the motifs and interactions responsible for recognitio n of the cofactor. The amino acid sequences of several adenosylcobalam in-dependent enzymes, the methylmalonyl coenzyme A mutases and glutama te mutases, show homology with the cobalamin-binding region of methion ine synthase and retain conserved residues that are determinants for t he binding of the prosthetic group, suggesting that these mutases and methionine synthase share common three-dimensional structures.