Separation of halide oxidation and substrate halogenation chemistries rationalizes site-selective vanadium dependent haloperoxidase catalysis.

Vanadium-dependent haloperoxidases (VHPOs) catalyze the halogenation of organic molecules under mild aqueous conditions. Selective bacterial VHPOs exhibit exquisite regio- and enantiocontrol, however the precise mechanisms dictating selectivity have remained elusive. We have solved the single-particle cryo-electron microscopy (cryo-EM) structure of a selective bromoperoxidase from Enhygromyxa salina (esVHPO). Mutagenesis demonstrates that halide oxidation and substrate halogenation occur in two distinct pockets, with halide transfer mediated by critical lysine residue K329. Isolation of a stable intermediate following bromide oxidation (BrOx) enables single turnover catalysis in the presence of organic substrate; subsequent application of a chemoselective fluorescent probe provides support for an intermediate bromamine involved in selectivity. Cryo-EM of the BrOx state reveals a 'camera shutter' mechanism that compacts the halide entry tunnel and vanadate pocket, minimizing the premature dissociation of hypohalous acid. These findings collectively unveil a multilayered halogen trapping and transfer mechanism and provide a rationale for selective VHPO catalysis.