Capturing Intermediates in the Reaction Catalyzed by NosN, a Class C Radical S-Adenosylmethionine Methylase Involved in the Biosynthesis of the Nosiheptide Side-Ring System.

Nosiheptide is a ribosomally synthesized and post-translationally modified thiopeptide natural product that possesses antibacterial, anticancer, and immunosuppressive properties. It contains a bicyclic structure composed of a large macrocycle and a unique side-ring system containing a 3,4-dimethylindolic acid bridge connected to the side chains of Glu6 and Cys8 of the core peptide via ester and thioester linkages, respectively. In addition to the structural peptide, encoded by the nosM gene, the biosynthesis of the side-ring structure requires the actions of NosI, -J, -K, -L, and -N. NosN is annotated as a class C radical S-adenosylmethionine (SAM) methylase, but its true function is to transfer a C1 unit from SAM to C4 of 3-methyl-2-indolic acid (MIA) with concomitant formation of a bond between the carboxylate of Glu6 of the core peptide and the nascent C1 unit. However, exactly when NosN performs its function during the biosynthesis of nosiheptide is unknown. Herein, we report the syntheses and use of three peptide mimics as potential substrates designed to address the timing of NosN's function. Our results show that NosN clearly closes the side ring before NosO forms the pyridine ring and most likely before NosD/E catalyzes formation of the dehydrated amino acids, although the possibility of a more random process (i.e., NosN acting after NosD/E) cannot be ruled out. Using a substrate mimic containing a rigid structure, we also identify and characterize two reaction-based adducts containing SAM fused to C4 of MIA. The two SAM adducts are derived from a consensus radical-containing species proposed to be the key intermediate-or a derivative of the key intermediate-in our proposed catalytic mechanism of NosN.