Mechanism of Oxidative Ring‐Closure as Part of the Hygromycin Biosynthesis Step by a Nonheme Iron Dioxygenase

Nonheme iron dioxygenases catalyze vital reactions for biosystems including the biosynthesis of antibiotics. One such enzyme, namely the hygromycin biosynthesis enzyme (HygX), performs an oxidative ring-closure reaction to form an ortho−ester product, which is a relevant reaction step for drug synthesis and biotechnology. To understand the selective reaction mechanism of oxidative ring-closure to form ortho−ester products in HygX, we investigated its catalytic reaction mechanism leading to various products. Large active site cluster models were set-up and various pathways for substrate activation have been calculated. The work identifies a high-valent iron(IV)-oxo species in the quintet spin state as the active oxidant that selectively abstracts a proton of an alcohol group of the substrate, which is followed by a hydrogen atom abstraction from a tertiary C−H group and rapid electron transfer. The latter-formed biradical intermediate rearranges to form the desaturated ring-closed product. The calculations show that an active site Lys residue donates positive charge to the metal−oxo group and guides the reaction to a chemoselective desaturation pathway.