title: Unmediated heterogeneous electron transfer reaction of ascorbate oxidase and laccase at a gold electrode.
abstract: The unmediated electrochemistry of two large Cu-containing proteins, ascorbate oxidase and laccase, was investigated by direct-current cyclic voltammetry. Rapid heterogeneous electron transfer was achieved in the absence of promoters or mediators by trapping a small amount of protein within a solid, electrochemically inert, tributylmethyl phosphonium chloride membrane coating a gold electrode. The problems typical of proteins in solution, such as adsorption on the electrode surface, were avoided by this procedure. In anaerobic conditions, the cyclic voltammograms, run at a scan rate of up to 200 mV/s, showed the electron transfer process to be quasi-reversible and diffusion-controlled. The pH-dependent redox potentials (+360 mV and +400 mV against a normal hydrogen electrode at pH7.0 for ascorbate oxidase and laccase respectively and +390 mV and +410 mV at pH5.5) were similar to those of the free proteins. The same electrochemical behaviour was recorded for the type 2 Cu-depleted derivatives, which contain reduced type 3 Cu, whereas the apoproteins were electrochemically inactive. Under aerobic conditions the catalytic current intensity of holoprotein voltammograms increased up to approx. 2-fold at a low scanning rate, with unchanged redox potentials. The voltammograms of type 2 Cu-depleted proteins and of apoproteins were unaffected by the presence of oxygen. This suggests that electron uptake at the electrode surface involves type 1 Cu and that only in the presence of oxygen is the intramolecular electron transfer to other protein sites rapid enough to be observed. The analogy with available kinetic results is discussed.
authors.affiliation: Department of Experimental Medicine and Biochemical Sciences, University of Rome 'Tor Vergata', Via di Tor Vergata 135, 00133 Rome, Italy.
authors.forename: R
authors.initials: R
authors.lastname: Santucci
authors.forename: T
authors.initials: T
authors.lastname: Ferri
authors.forename: L
authors.initials: L
authors.lastname: Morpurgo
authors.forename: I
authors.initials: I
authors.lastname: Savini
authors.forename: L
authors.initials: L
authors.lastname: Avigliano
mesh_terms: D001204:Ascorbate Oxidase / Q000737:chemistry*; D004563:Electrochemistry; D004566:Electrodes; D004800:Enzymes, Immobilized; D006046:Gold; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D042845:Laccase; D008566:Membranes; D009943:Organophosphorus Compounds; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D004800:Enzymes, Immobilized; D009943:Organophosphorus Compounds; C113117:tributylmethyl phosphonium chloride; D006046:Gold; D010088:Oxidoreductases; D042845:Laccase; D001204:Ascorbate Oxidase