Enzymatic Pyridine Aromatization during Thiopeptide Biosynthesis.

Thiazole-containing pyritides (thiopeptides) are ribosomally synthesized and post-translationally modified peptides (RiPPs) that have attracted interest owing to their potent biological activities and structural complexity. The class-defining feature of a thiopeptide is a six-membered, nitrogenous heterocycle formed by an enzymatic [4 + 2]-cycloaddition. In rare cases, piperidine or dehydropiperidine (DHP) is present; however, the aromatized pyridine is considerably more common. Despite significant effort, the mechanism by which the central pyridine is formed remains poorly understood. Building on our recent observation of the Bycroft-Gowland intermediate (i.e., the direct product of the [4 + 2]-cycloaddition), we interrogated thiopeptide pyridine synthases using a combination of targeted mutagenesis, kinetic assays, substrate analogs, enzyme-substrate cross-linking, and chemical rescue experiments. Collectively, our data delineate roles for several conserved residues in thiopeptide pyridine synthases. A critical tyrosine facilitates the final aromatization step of pyridine formation. This work provides a foundation for further exploration of the [4 + 2]-cycloaddition reaction and future customization of pyridine-containing macrocyclic peptides.