Theoretical study on the mechanism of tryptophan methylation catalyzed by the cobalamin-dependent radical enzyme TsrM

Abstract

TsrM is a radical S-adenosylmethionine (SAM) methylase that catalyzes the C2 methylation of L-tryptophan to produce 2-methyltryptophan, an intermediate in the biosynthesis of the antibiotic thiostrepton A. The mechanism of TsrM is investigated through the quantum chemical cluster approach. The calculations suggested an expanded ‘ping-pong’ mechanism that requires the participation of two SAM molecules. In the first half-reaction, one SAM molecule binds to the active site of TsrM and delivers its methyl group to the CoI ion of the cobalamin, leading to the generation of methylcobalamin. With the subsequent dissociation of a SAH molecule, another SAM molecule binds into the active site, which facilitates the transfer of the methyl group from methylcobalamin to the C2 of tryptophan via a non-radical mechanism. The resulting carbon cation intermediate then readily undergoes facile deprotonation to yield the final product 2-methyltryptophan. Our proposed mechanism also rationalizes the reactivity trends observed among four substrates with different substitutions.