Electrochemical Oxidation-Reduction Properties of Covalently Bound FAD of Cholesterol Oxidase Adsorbed on Mercury Electrode Surface

Cholesterol oxidase from Schizophyllum commune was studied by cyclic d.c. and a.c. voltammetry with a hanging mercury drop electrode. The enzyme gave d.c. and a.c. waves due to covalently bound FAD of the enzyme adsorbed on an electrode surface. The waves were interpreted in terms of d.c. and a.c. voltammetry of a twostep surface redox reaction. The standard surface oxidation-reduction potential of the enzyme adsorbed on the electrode surface, Es′, was −0.325 V vs. SCE, the semiquinone formation constant being 1.5±1.0 at pH 7.4 (25 °C). The pH dependence of Es′ was −60 mV/pH (pH 2.0–6.4), −30 mV/pH (pH 6.4–8.5), and −60 mV/pH (pH 8.5–12.0). The apparent charge transfer rate constant of the surface redox reaction was 1.6±0.5×103 s−1 at pH 5.4.