BIOELECTROCATALYSIS AT ELECTRODES COATED WITH ALCOHOL-DEHYDROGENASE, A QUINOHEMOPROTEIN WITH HEME-C SERVING AS A BUILT-IN MEDIATOR

Alcohol dehydrogenase (ADH), a bacterial membrane-bound protein contai ning pyrroloquinoline quinone (PQQ) and heme c was held by adsorption on electrodes of gold silver, glassy carbon, or pyrolytic graphite. Al l the electrodes with adsorbed ADH produced anodic currents which oxid ized ethanol, in which the adsorbed ADH catalyzed the electrolysis of ethanol. The electrocatalysis behavior could be described by a theoret ical equation for bioelectrocatalysis at an enzyme-coated electrode, a nd was characterized by two quantities, the Michaelis constant K(m), a nd maximum current density I(max)/A. Using electroreflectance measurem ents with an ADH-coated gold electrode it was revealed that electron t ransfer occurred between heme c of the adsorbed ADH and the electrode. On the basis of these results, the reaction mechanism of the bioelect rocatalysis is discussed and oriented adsorption of ADH is proposed wi th the heme c moiety being in close contact with the electrode and wit h the PQQ moiety, the site reacting with the substrate, facing toward the solution.