Kinetic analysis of fast hydrogenase reaction of Desulfovibrio vulgaris cells in the presence of exogenous electron acceptors

Desulfovibrio vulgaris (Hildenborough) cells catalyzed the oxidation of hyd rogen with several quinone compounds as exogenous electron accepters, in wh ich hydrogenase existing in the periplasmic space of the bacterial cells fu nctioned as the enzyme to catalyze the reaction. The rates of the hydrogen oxidation and quinone reduction were analyzed by a Michaelis-Menten type eq uation to yield the values of the catalytic constant of a D. vulgaris cell, k(B,cat), and the bimolecular reaction rate constants for hydrogen, k(B,ca t)/K-B,K-H, and for quinone, k(B,cat)/K-B,K-Q. They were in the ranges of k (B,cat) = (1.1-5.3) x 10(7) s(-1), k(B,cat)/K-B,K-H = (1.8-2.2) x 10(12) M- 1 s(-1) and k(B,cat)/K-B,K-Q = (0.97-10) x 10(10) M-1 s(-1) for the reactio ns with four kinds of quinone compounds. The mass transfer process involved in the bacterial cell-catalyzed reaction was considered by a model taking account of the substrate diffusion to and through the cross-membrane channe ls (composed of proteins called porins) distributed in the bacterial outer membrane to reach the periplasmic space. The rate of diffusion of the subst rates toward the whole cell surface was also calculated on the basis of the model of spherical diffusion and compared with the rate of the diffusion t hrough the cross-membrane channels. Calculation based on the model has reve aled that the diffusion toward the cell surface is the slowest step of the mass transfer processes and that the rates of the catalytic reaction are la rge enough to be close to the rates of the substrate diffusion.