Electrochemical study of reversible hydrogenase reaction of Desulfovibrio vulgaris cells with methyl viologen as an electron carrier

Abstract

An electrode modified with immobilized whole cells of Desulfovibrio vulgaris (Hildenborough) produces an S-shaped voltammogram with both cathodic- and anodic-catalytic-limiting currents in a methyl viologen-containing buffer saturated with H2. Methyl viologen penetrates into the bacterial cells to serve as an electron carrier in the reversible reaction of hydrogenase in the cells and functions as an electron-transfer mediator between the bacterial cells and the electrode, thus producing the catalytic currents for the evolution and consumption of H2. An equation for the catalytic current that takes into account the reversible hydrogenase reaction explains well the shape of the voltammogram. The potential at null current on the voltammogram agrees with the potential determined by potentiometry with the same electrode, which is equal to the redox potential of the H+/H2 couple in the solution--the standard potential of a hydrogen electrode at the pH of the solution. When D. vulgaris cells are suspended in an argon-saturated buffer containing methyl viologen, the suspension produces a catalytic current at a bare glassy carbon electrode for the evolution of H2. Analysis of the current by a theory for a catalytic current for a unidirectional nonlinear enzyme catalysis allows us to determine the kinetic parameters of the reaction between methyl viologen and hydrogenase in intact D. vulgaris cells. Thus we obtain the apparent Michaelis constant for methyl viologen cation radical, K'MV.+ = 0.16 mM, and the apparent catalytic constant (that is, the turnover number per D. vulgaris cell), zkcat,H+ = 1.2 x 10(7) s-1, for the H2 evolution reaction at pH 5.5 and at 25 degrees C, z being the number of hydrogenases contained in a D. vulgaris cell. The bimolecular reaction rate constant, kcat,H+/K'MV.+, of the reaction between methyl viologen cation radical and oxidized hydrogenase in intact D. vulgaris cells is estimated as 4.2 x 10(7) M-1 s-1. Similarly, the bimolecular reaction rate constant, kcat,H2/K'MV2+, of the reaction between methyl viologen and reduced hydrogenase is estimated to be 1.2 x 10(7) M-1 s-1 at pH 9.5 and 25 degrees C. Both rate constants are large enough for the reactions to be diffusion-limited processes.