Conformational Isomerization of the Fe(III)–OH Species Enables Selective Halogenation in Carrier-Protein-Independent Halogenase BesD and Hydroxylase-Evolved Halogenase
Jinyan Zhang, Yifan Li, Weishan Yuan, Xuan Zhang, Yubing Si, Binju Wang
Abstract
Despite extensive studies, how carrier-protein-independent BesD dictates the reaction toward thermodynamically unfavored halogenation is still elusive. Here, we investigated the chlorination versus hydroxylation selectivity in both carrier-protein-independent halogenase BesD and hydroxylase-evolved halogenase Chi-14, employing extensive MD simulations and QM/MM calculations. In BesD, our calculations have shown that 2OG-assisted O 2 activation affords the axial Fe(IV)-oxo species that is responsible for the substrate C–H activation. To facilitate the following Cl-rebound reaction, the nascent axial Fe(III)–OH species has to undergo conformational isomerization to the equatorial one. This can remove the steric effects between the axial Fe(III)–OH and the substrate radical, thereby enhancing the migration of the substrate radical toward the Cl − ligand during the Cl-rebound. Notably, the hydrogen-bond interactions with second-sphere residue Asn are vital to maintain the unsaturated five-coordination shell of the Fe center. This maintenance is essential for enabling the conformational transition of Fe(III)–OH from an axial to an equatorial orientation. Our results are in concordance with existing experimental findings, underscoring the pivotal influence of iron coordination dynamics in governing the catalytic processes of nonheme enzymes.