title: The Catalytic Mechanism of the Class C Radical S-Adenosylmethionine Methyltransferase NosN.
abstract: S-Adenosylmethionine (SAM) is one of the most common co-substrates in enzyme-catalyzed methylation reactions. Most SAM-dependent reactions proceed through an SN2 mechanism, whereas a subset of them involves radical intermediates for methylating non-nucleophilic substrates. Herein, we report the characterization and mechanistic investigation of NosN, a class C radical SAM methyltransferase involved in the biosynthesis of the thiopeptide antibiotic nosiheptide. We show that, in contrast to all known SAM-dependent methyltransferases, NosN does not produce S-adenosylhomocysteine (SAH) as a co-product. Instead, NosN converts SAM into 5'-methylthioadenosine as a direct methyl donor, employing a radical-based mechanism for methylation and releasing 5'-thioadenosine as a co-product. A series of biochemical and computational studies allowed us to propose a comprehensive mechanism for NosN catalysis, which represents a new paradigm for enzyme-catalyzed methylation reactions.
authors.affiliation: Department of Chemistry, Fudan University, Shanghai, 200433, China.|School of Life Sciences, Lanzhou University, Lanzhou, 730000, China.
authors.forename: Wei
authors.initials: W
authors.lastname: Ding
authors.affiliation: Department of Chemistry, Fudan University, Shanghai, 200433, China.
authors.forename: Yongzhen
authors.initials: Y
authors.lastname: Li
authors.affiliation: Department of Chemistry, Fudan University, Shanghai, 200433, China.|Key Laboratory of Combinatory Biosynthesis and Drug Discovery (Ministry of Education), School of Pharmaceutical Sciences, Wuhan University, Wuhan, 430071, China.
authors.forename: Junfeng
authors.initials: J
authors.lastname: Zhao
authors.affiliation: Department of Chemistry, Fudan University, Shanghai, 200433, China.
authors.forename: Xinjian
authors.initials: X
authors.lastname: Ji
authors.affiliation: Department of Chemistry, Fudan University, Shanghai, 200433, China.
authors.forename: Tianlu
authors.initials: T
authors.lastname: Mo
authors.affiliation: Department of Chemistry, Fudan University, Shanghai, 200433, China.
authors.forename: Haocheng
authors.initials: H
authors.lastname: Qianzhu
authors.affiliation: Department of Chemistry, Fudan University, Shanghai, 200433, China.
authors.forename: Tao
authors.initials: T
authors.lastname: Tu
authors.affiliation: Key Laboratory of Combinatory Biosynthesis and Drug Discovery (Ministry of Education), School of Pharmaceutical Sciences, Wuhan University, Wuhan, 430071, China.
authors.forename: Zixin
authors.initials: Z
authors.lastname: Deng
authors.affiliation: Key Laboratory of Combinatory Biosynthesis and Drug Discovery (Ministry of Education), School of Pharmaceutical Sciences, Wuhan University, Wuhan, 430071, China.
authors.forename: Yi
authors.initials: Y
authors.lastname: Yu
authors.affiliation: Department of Chemistry, Fudan University, Shanghai, 200433, China.
authors.forename: Fener
authors.initials: F
authors.lastname: Chen
authors.affiliation: Department of Chemistry, Fudan University, Shanghai, 200433, China.
authors.forename: Qi
authors.identifier: 0000-0002-8135-2221
authors.initials: Q
authors.lastname: Zhang
mesh_terms: D000900:Anti-Bacterial Agents / Q000096:biosynthesis* / Q000737:chemistry; D002384:Catalysis; D005609:Free Radicals / Q000737:chemistry / Q000378:metabolism; D008745:Methylation; D008780:Methyltransferases / Q000737:chemistry / Q000378:metabolism*; D015394:Molecular Structure; D012436:S-Adenosylmethionine / Q000737:chemistry / Q000378:metabolism*; D013844:Thiazoles / Q000737:chemistry / Q000378:metabolism
chemical_list: D000900:Anti-Bacterial Agents; D005609:Free Radicals; D013844:Thiazoles; D012436:S-Adenosylmethionine; D008780:Methyltransferases; C014134:nosiheptide
keywords: biosynthesis; methyltransferases; natural products; radical SAM enzymes; thiopeptides