Orthovanadate Cofactor Chemistry of Marine Bromoperoxidases

Abstract

Two models have emerged in the past years describing cofactor chemistry of vanadate-dependent bromoperoxidases on different levels of theory. The first model, derived from steady state kinetics, spectroscopy, and X-ray diffraction describes cofactor bonding as covalent interaction between orthovanadate and an imidazole nitrogen from a histidine side chain. This imidazole entity forms along with side chains from two further histidines, two arginines, one lysine, and one serine an apparently conserved binding site architecture for the investigated class of enzymes (EC 1.11.1.18). Substrate conversion occurs, according to the first of the bromoperoxidase reaction models, via oxygen atom transfer from an anionic histidine-bound peroxometavanadate onto bromide, assisted by Brønsted-acid catalysis involving a proximate imidazole N,H-bond, or alternatively, ammonium from a protonated lysine side chain. A second, more recent approach, applied an assessed electronic structure method (B3LYP/6-311++G**), for developing a thermochemistry-based approach towards understanding cofactor bonding and reactivity, supplemented by Natural Bond Orbital (NBO)-analysis for translating results from density functional theory into a molecular orbital-based reaction theory describing the bromoperoxidase mechanism. Effects of amino acid side chain bonding on structure and reactivity of orthovanadium compounds and derived peroxoic acids, and the role these chemical changes play in controlling cofactor reactivity towards bromide, served in the second approach as starting point for aligning elementary steps in accordance with the nucleophile-electrophile-principle, closing the sequence to a thermochemically consistent catalytic cycle. Accordingly, hydrogen bonding by guanidinium from arginine puts dihydrogen orthovanadate into position for being protonated and subsequently transformed upon reaction with hydrogen peroxide into orthovanadium peroxoic acid, being in the density functional theory model the only feasible electrophile, able mediating oxygen atom transfer to bromide with virtually no activation energy, thus explaining the remarkably rate of enzymatic bromide oxidation.