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Mechanism of a Class C Radical S-Adenosyl-l-methionine Thiazole Methyl Transferase

Zhang, Zhengan, Mahanta, Nilkamal, Hudson, Graham A., Mitchell, Douglas A., van der Donk, Wilfred A.
Journal of the American Chemical Society 2017 v.139 no.51 pp. 18623-18631
S-adenosylmethionine, antibiotics, biosynthesis, carbon, hydrogen, isotope labeling, methylation, methyltransferases, moieties, protons, solvents, thiazoles
The past decade has seen the discovery of four different classes of radical S-adenosylmethionine (rSAM) methyltransferases that methylate unactivated carbon centers. Whereas the mechanism of class A is well understood, the molecular details of methylation by classes B–D are not. In this study, we present detailed mechanistic investigations of the class C rSAM methyltransferase TbtI involved in the biosynthesis of the potent thiopeptide antibiotic thiomuracin. TbtI C-methylates a Cys-derived thiazole during posttranslational maturation. Product analysis demonstrates that two SAM molecules are required for methylation and that one SAM (SAM1) is converted to 5′-deoxyadenosine and the second SAM (SAM2) is converted to S-adenosyl-l-homocysteine (SAH). Isotope labeling studies show that a hydrogen is transferred from the methyl group of SAM2 to the 5′-deoxyadenosine of SAM1 and the other two hydrogens of the methyl group of SAM2 appear in the methylated product. In addition, a hydrogen appears to be transferred from the β-position of the thiazole to the methyl group in the product. We also show that the methyl protons in the product can exchange with solvent. A mechanism consistent with these observations is presented that differs from other characterized radical SAM methyltransferases.