Orthovanadate cofactor chemistry of marine bromoperoxidases†

Abstract

In the past few years, two models have emerged that describe the cofactor chemistry of vanadate-dependent bromoperoxidases at different levels of theory. The first model, derived from steady-state kinetics, spectroscopy, and X-ray diffraction, describes cofactor bonding as a covalent interaction between orthovanadate and an imidazole nitrogen from a histidine side chain. This imidazole entity, along with side chains from two additional histidines, two arginines, one lysine, and one serine, forms an apparently conserved binding site architecture for the investigated class of enzymes (EC 1.11.1.18). Substrate conversion, according to the first bromoperoxidase reaction model, occurs via oxygen atom transfer from an anionic histidine-bound peroxometavanadate to bromide, assisted by Brønsted-acid catalysis involving a proximate imidazole N–H bond or, alternatively, ammonium from a protonated lysine side chain. A second and more recent approach applies an advanced electronic structure method (B3LYP/6-311++G**) to develop a thermochemistry-based approach for understanding cofactor bonding and reactivity. This is supplemented by natural bond orbital (NBO) analysis to translate results from density functional theory into a molecular orbital-based reaction theory, which describes the bromoperoxidase mechanism. In this approach, the effects of amino acid side chain bonding on the structure and reactivity of orthovanadium compounds and derived peroxoic acids are examined. The role these chemical changes play in controlling cofactor reactivity towards bromide serves as the starting point for aligning elementary steps with the nucleophile-electrophile principle, ultimately leading to a thermochemically consistent catalytic cycle. Accordingly, hydrogen bonding by the guanidinium group from arginine positions dihydrogen orthovanadate for protonation and subsequent transformation into orthovanadium peroxoic acid upon reaction with hydrogen peroxide. According to the density functional theory model, this is the only feasible electrophile capable of mediating oxygen atom transfer to bromide with virtually no activation energy, thereby explaining the remarkable rate of enzymatic bromide oxidation.