Metal Coordination Dynamics Governs Selective Halogenation in α-KG/Fe-Dependent Halogenase SyrB2.
α-Ketoglutarate (α-KG)-dependent nonheme iron enzymes catalyze a diverse array of oxidative transformations essential for natural product biosynthesis. However, the mechanism by which α-KG/Fe-dependent halogenases achieve selective halogenation while circumventing the thermodynamically favored hydroxylation pathway remains a subject of intense debate. In this study, we elucidate the halogenation mechanism in SyrB2 through extensive computational and crystallographic investigations. Our work reveals that metal coordination dynamics plays a pivotal role in controlling selective C─H bond activation and chlorination in SyrB2. The transformation of the Fe(IV)-oxo species from an equatorial to an axial conformation enables hydrogen atom transfer from the substrate C─H bond. Subsequent re-isomerization of the Fe(III)-OH intermediate to the equatorial conformation is critical for promoting selective chlorination while minimizing competitive hydroxylation. The proposed mechanism is supported by multiple experimental observations, including Mössbauer spectroscopy, nuclear resonance vibrational spectroscopy (NRVS), 2H-HYSCORE spectroscopy, and kinetic analysis of reactions with various substrates.