Bioelectrocatalytic Oxygen Reduction Reaction by Bilirubin Oxidase Adsorbed on Glassy Carbon and Edge-Plane Pyrolytic Graphite Electrodes: Effect of Redox Mediators

Direct electron transfer (DET) of bilirubin oxidase (BOx, from Myrothecium verrucaria) adsorbed on edge-plane pyrolytic graphite (EPPG) electrode as well as an electron transfer between the EPPG or glassy carbon (GC) electrode and the BOx (adsorbed on these electrodes) via a mediation by redox mediators ([Fe(CN)(6)](3-) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)(ABTS)), i.e., a mediator-assisted electron transfer, have been examined under anaerobic and aerobic conditions by cyclic voltammetry and differential pulse voltammetry. On differential pulse voltammograms on the EPPG electrode two redox processes with formal potentials of 0.50 and 0.29 V vs. Ag/AgCl/NaCl(3M) were clearly observed in 0.05 M phosphate buffer (pH 5.0), corresponding to the redox transformations of the T1 site and the T2/T3 cluster of the BOx, respectively. DET was not observed on the GC electrode. A mediator-assisted electron transfer between the electrode and the BOx adsorbed on it was realized in the absence and the presence of [Fe(CN)(6)](3-) and ABTS through the bioelectrocatalytic oxygen reduction reaction (ORR) by the BOx, and in addition its overall electron transfer mechanism is discussed briefly including the DET, mediator-assisted electron transfer and an intramolecular electron transfer (IET) from the T1 site to the T2/T3 cluster site, in which an uphill IET reaction during the catalytic ORR is suggested.