The oxidation of D-glucose and nicotinic acid by intact cells of Gluconobacter industrious and Pseudomonas fluorescens, respectively, is successfully measured by an amperometric method using such compounds as Fe(CN)6(3-), p-benzoquinone, and dichlorophenolindophenol as electron acceptors. Analysis of the experimental results reveals that the intact cells behave like oxidoreductases whose kinetics follows a Michaelis-Mententype equation. The catalytic behavior is explained by a model which treats the bacterial cells as bags of enzymes and assumes distribution equilibrium in the concentrations of both the substrate and the electron acceptor between the test solution and the medium within the cells. The catalytic activity can be characterized by three quantities: the maximum reaction rate (vB) and the ratios of the Michaelis constant to the distribution constant for the substrate (Ks,cell/Ks,p) and to that for the electron acceptor (KM,cell/KM,p). Advanced modification of the model to involve the membrane permeability reveals that the three quantities are effective for explaining the catalytic behavior even when the permeability effect is significant. Thus, the three quantities should be regarded as the parameters which can reflect the permeability effect.
A mathematical model of a biocatalyst electrode with entrapped mediator is presented. Differential equations describing the diffusion of the substrate and mediator coupled with the enzyme reaction in the immobilized enzyme layer adjacent to the electrode surface are numerically solved to simulate the dependence of the steady-state current, I, on the concentration of the substrate at the surface of the enzyme layer, ics, and on the concentration of the mediator entrapped in the enzyme layer, CM. The numerical solutions are expressed by Michaelis-Menten type equations with three parameters which characterize the I vs. acs and I'vs. CM curves. The diffusional resistance of a semipermeable membrane covering the enzyme layer on the electrode surface is also taken into consideration. The dependence of the current on the potential applied to the electrode, E, is also derived and the parameters characterizing the I vs. E curve are discussed.
D-Gluconate dehydrogenase isolated from Pseudomonas fluorescens was immobilized on the surfaces of carbon and gold electrodes by irreversible adsorption. The electrodes with the adsorbed enzyme produced anodic currents in solutions containing D-gluconate. The currents were at
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