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 containing pyrroloquinoline quinone (PQQ) and heme c was held by adsorption on electrodes of gold, silver, glassy carbon, or pyrolytic graphite. All the electrodes with adsorbed ADH produced anodic currents which oxidized ethanol, in which the adsorbed ADH catalysed the electrolysis of ethanol. The electrocatalysis behavior could be described by a theoretical equation for bioelectrocatalysis at an enzyme-coated electrode, and was characterized by two quantities, the Michaelis constant K_m, and maximum current density I_max/A. Using electroreflectance measurements with an ADH-coated gold electrode it was revealed that electron transfer occurred between heme c of the adsorbed ADH and the electrode. On the basis of these results, the reaction mechanism of the bioelectrocatalysis is discussed and oriented adsorption of ADH is proposed with the heme c moiety being in close contact with the electrode and with the PQQ moiety, the site reacting with the substrate, facing toward the solution.