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Revised Manuscript Received May 28, 1993• abstract: Cobalamin-dependent methionine synthase catalyzes methyl group transfer from methyltetrahydrofolate to homocysteine to form tetrahydrofolate and methionine, and the cobalamin prosthetic group serves as an intermediate methyl carrier. Enzyme possessing cobalamin in the cobalt (II) oxidation state is inactive, and this form is activated by one-electron reduction coupled to methylation by S-adenosylmethionine (AdoMet). The enzyme from Escherichia coli has been divided into separable fragments by limited proteolysis with trypsin, and the contribution of each of these fragments to substrate binding and catalysis has been evaluated. The 37.7-kDacarboxyl-terminal domain binds AdoMet, and this was demonstrated through covalent modification with radiolabeled AdoMet during ultraviolet irradiation. Following reductive activation with AdoMet, the enzyme was …