Biochemical and XAS investigation of TsrM, a B12-dependent Radical SAM Enzyme Catalyzing Non-radical reaction

Link to DOI – 10.1002/chem.202200627

Chem. Eur. J. 2022, 28, e2022006

B₁₂-dependent radical SAM enzymes are an emerging enzyme family with approximately 200,000 proteins. These enzymes have been shown to catalyze chemically challenging reactions such as methyl transfer to sp2– and sp3-hybridized carbon atoms. However, to date we have little information regarding their complex mechanisms and their biosynthetic potential. Here we show, using X-ray absorption spectroscopy, mutagenesis and synthetic probes that the vitamin B₁₂-dependent radical SAM enzyme TsrM catalyzes not only C– but also N-methyl transfer reactions further expanding its synthetic versatility. We also demonstrate that TsrM has the unique ability to directly transfer a methyl group to the benzyl core of tryptophan, including the least reactive position C4. Collectively, our study supports that TsrM catalyzes non-radical reactions and establishes the usefulness of radical SAM enzymes for novel biosynthetic schemes including serial alkylation reactions at particularly inert C−H bonds.