CYCLIC VOLTAMMETRY AND ELECTROCATALYSIS OF THE BLUE COPPER OXIDASE POLYPORUS VERSICOLOR LACCASE

Cyclic voltammetric cathodic and anodic peaks of monolayers of the blu e four-copper oxidase Polyporus versicolor laccase at edge-plane pyrol ytic graphite electrodes under anaerobic conditions have been observed at low (less than or equal to 1 mV s(-1)) scan rates. The midpoint po tential is approximate to 790 mV (vs. NHE) in accordance with the redu ction potential in homogeneous solution. The peak separation is 210 mV and the halfwidth about 180 mV. The peak areas correspond approximate ly to four-electron transfer. This implies that intramolecular electro n transfer between the type 1 and the type 2 or type 3 centres combine d with interfacial electron transfer at the type 1 or the type 2 and t ype 3 centre must be part of the mechanism. Electrocatalytic dioxygen reduction at edge-plane pyrolytic graphite covered with a monolayer of laccase is almost reversible in the cyclic voltammetric and rotating disk modes. The apparent halfwave potential is 750 mV. The limiting cu rrent variation with dioxygen concentration follows electrochemical Mi chaelis-Menten kinetics with insignificant mass transport limitations. The steady-state rate-determining step follows a bell-shaped PH-profi le. This step could be either one of the steps of intramolecular elect ron transfer between the type 1 and type 2/3 centres, or product relea se. The former gives an average rate constant of 40 s(-1), the latter 10-20 s(-1) or about 10(4) M-1 s(-1) for the maximum second-order rate constant. This accords with values for laccase turnover in homogeneou s solution.