The Catalytic Mechanism of the Class C Radical S ‐Adenosylmethionine Methyltransferase NosN

S ‐Adenosylmethionine (SAM) is one of the most common co‐substrates in enzyme‐catalyzed methylation reactions. Most SAM‐dependent reactions proceed through an S N 2 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.