Spectroelectrochemical study of heme- and molybdopterin cofactor-containing chicken liver sulphite oxidase.

Electron transfer (ET) in sulphite oxidase (SOx), a heme- and molybdopterin cofactor-containing enzyme, was studied spectroelectrochemically using capillary gold electrode modified with aldrithiol. Direct electron exchange between SOx and the surface of modified gold was observed, with a formal potential of -115 mV vs. Agmid R:AgCl, KCl(sat) at pH 7.0. This value agreed well with that previously reported for redox transformation of the heme domain of SOx. However, no bioelectrocatalysis of sulphite oxidation was observed in phosphate buffer solutions. This fact evidently correlated with known inhibition of intramolecular ET in SOx by the presence of bivalent inorganic anions. After changing to a Tris buffer solution, spectra variations and cyclic voltammetry data designated direct ET-based bioelectrocatalysis of sulphite oxidation, upon addition of sulphite. Thus, the bioelectrocatalytic 2e(-) oxidation of sulphite catalysed by SOx due to direct ET exchange with the electrode was attained at aldrithiol-modified gold electrodes and shown to depend essentially on the nature of the buffer solution.