Vanadate-Dependent Peroxidases in Macroalgae: Function, Applications, and Environmental Impact

With an average concentration of 35 nM, vanadium (essentially present in the form of ion pairs NaH2VO4 -) is the second-to-most abundant transition metal in sea water, outclassed only by molybdenum (present as molybdate MoO4 2−, ca. 100 nM). Vanadate-dependent haloperoxidases (VHPOs) can be present in marine brown, green and red algae, in (symbiotic) marine cyanobacteria, in Streptomyces bacteria, and in terrestrial fungi and lichen. Since the discovery of the first representative of these haloperoxidases, the bromoperoxidase VBrPO of the brown alga Ascophyllum nodosum [1], also known as knobbed wreck or pig weed, a plethora of macroalgae has been reported to have available VHPOs, hence haloperoxidases that contain vanadate H2VO4 − coordinated to a histidine residue in the enzyme’s active center, and thus clearly distinct from the more common hemebased and non-heme peroxidases. Contrasting the heme and non-heme peroxidases, VHPOs are rather robust: they withstand comparatively high concentrations of hydrogen peroxide and temperatures up to 70°C, and are also resistant to high concentrations of organic solvents [2]. Depending on the substrate selectivity, one distinguishes between VHPOs that catalyze the oxidation of (i) iodide only (VIPOs), (ii) iodide and bromide (VBrPOs), and (iii) iodide, bromide and chloride (VClPOs). Along with halides, pseudohalides (such as cyanide and thiocyanate) and sulfides (for example dimethylsulfide) can be oxidized. Prochiral sulfides RR’S are oxidized enantioselectively. The general reactions for the enzymatic two-electron oxidation of a substrate X− (halide(1-) and pseudohalide(1-)) or a (prochiral) sulfide are depicted in Eqs. (1a) and (1b), respectively. In addition to halides and sulfides, VHPOs catalyze the oxidation of many other organic substrates either directly or indirectly, i.e. via intermittent bromination. In the absence of a substrate, singlet oxygen is formed, Eq. (1c).