Open AccessStructural basis for non-radical catalysis by TsrM, a radical SAM methylase
Author(s) -
Hayley Knox,
Percival Yang-Ting Chen,
Anthony J. Blaszczyk,
Arnab Mukherjee,
Tyler L. Grove,
Erica L. Schwalm,
Bo Wang,
Catherine L. Drennan,
Squire J. Booker
Publication year - 2021
Publication title -
nature chemical biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.412
H-Index - 216
eISSN - 1552-4469
pISSN - 1552-4450
DOI - 10.1038/s41589-020-00717-y
Subject(s) - chemistry , stereochemistry , moiety , tryptophan , residue (chemistry) , methyltransferase , cobalamin , cofactor , phosphonate , methylation , amino acid , enzyme , biochemistry , dna , vitamin b12
Tryptophan 2C methyltransferase (TsrM) methylates C2 of the indole ring of L-tryptophan during biosynthesis of the quinaldic acid moiety of thiostrepton. TsrM is annotated as a cobalamin-dependent radical S-adenosylmethionine (SAM) methylase; however, TsrM does not reductively cleave SAM to the universal 5'-deoxyadenosyl 5'-radical intermediate, a hallmark of radical SAM (RS) enzymes. Herein, we report structures of TsrM from Kitasatospora setae, which are the first structures of a cobalamin-dependent radical SAM methylase. Unexpectedly, the structures show an essential arginine residue that resides in the proximal coordination sphere of the cobalamin cofactor, and a [4Fe-4S] cluster that is ligated by a glutamyl residue and three cysteines in a canonical CXXXCXXC RS motif. Structures in the presence of substrates suggest a substrate-assisted mechanism of catalysis, wherein the carboxylate group of SAM serves as a general base to deprotonate N1 of the tryptophan substrate, facilitating the formation of a C2 carbanion.