Electrocatalysis by vanadium haloperoxidase

Abstract

Abstract A steady-state potential of an enzyme electrode containing immobilized vanadium haloperoxidase (VHP) on graphite reached a value of 0.46–0.60 V versus SCE in a solution containing hydrogen peroxide, vanadate and halide at pH 6.0. The electrocatalytic current of chloride oxidation was practically constant from −0.1 to 0.25 V versus SCE. The current determined at 0 V versus SCE depended on the hydrogen peroxide, halide and vanadate concentration. An apparent Michaelis–Menten constant ( K m ) of the enzyme electrode was 12–14 μM for hydrogen peroxide at 0.1–0.2 mM of KCl and 0.1 mM of vanadate. K m decreased to 2.3 μM in vanadate free solution. Under the same conditions and at a fixed concentration (0.1 mM) of hydrogen peroxide, K m for chloride changed from 0.6 to 2.5 mM; K m for bromide was 9–16 μM and for iodide K m was 5.5 μM. Chloride oxidation was activated, bromide and iodide inhibited by extra addition of 0.1 mM of vanadate. The enzyme electrode current depended on the solution pH. When iodide was oxidized the maximal current was at pH 6.1; the current of chloride oxidation was largest at pH 5.1–5.3. KNO 3 inhibited the electrode current in a competitive manner when bromide or chloride was oxidized. K i was 0.6 and 0.27 mM, for the respective ions. K i for thiocyanate was 0.25 and 0.08 μM, respectively. The results achieved conclude that the electron exchange rate between the electrode and VHP activated by hydrogen peroxide was slow. The electrode potential was the result of an electrochemical hypohalous acid reduction. Parallel hypohalous acid reduction by hydrogen peroxide lowers the electrode potential in comparison to the potential of the hypohalous acid/halide redox pair.