Reactions between diaphorase and quinone compounds in bioelectrocatalytic redox reactions of NADH and NAD+

Spectroelectrochemical study of cellobiose dehydrogenase and diaphorase in a thiol-modified gold capillary in the absence of mediators

A spectroelectrochemical cell was constructed from a gold capillary with 200 microm inner diameter as a working electrode. This allowed spectroelectrochemical study of liquid samples with available volumes less than 5 microl. The optical measurements were accomplished with an optical fibre spectrometer. The optical path of the cell was about 1 cm. To facilitate electrochemistry of biomolecules, the surface of the gold capillary was modified with thiols. The formal potential, E degrees', of the heme in cellobiose dehydrogenase (CDH) from Phanerochaete chrysosporium was determined by spectroelectrochemistry in the absence of redox mediators. The number of electrons per redox conversion of heme in CDH was found to be equal to 0.98 + 0.04 corresponding well to a theoretical value representing the redox reaction Fe3+ + e-= Fe2+. Similar spectroelectrochemical experiments with diaphorase from Bacillus stearothermophilus showed the redox conversion of the flavin mononucleotide in diaphorase in the absence of external redox mediators.

Strategies for the improvement of an amperometric cholesterol biosensor based on electropolymerization in flow systems: use of charge-transfer mediators and platinization of the electrode

Different configurations based on an amperometric biosensor with cholesterol oxidase entrapped in a polypyrrole film have been developed with a view to improving the analytical properties of this biosensor. The alternatives considered involve the simultaneous entrapment of the enzyme and a charge-transfer mediator as well as previous platinization of the surface of the Pt electrode. Both artificial (a ferrocene derivative) and natural (flavin nucleotides) mediators were studied as constituents of the charge-transfer process between the enzyme and the electrode. The comparative study of these biosensors, which were prepared in situ in a continuous flow system, made it possible to determine the advantages and disadvantages of each configuration when applied to flow-injection determination of cholesterol.

Role of 2-amino-3-carboxy-1,4-naphthoquinone, a strong growth stimulator for bifidobacteria, as an electron transfer mediator for NAD(P)(+) regeneration in Bifidobacterium longum

2-Amino-3-carboxy-1,4-naphthoquinone (ACNQ) is a novel growth stimulator for bifidobacteria. The role of ACNQ as a mediator of the electron transfer from NAD(P)H to dioxygen (O(2)) and hydrogen peroxide (H(2)O(2)), proposed in our previous paper, was examined using the cell-free extract and whole cells of Bifidobacterium longum. Continuous monitoring of ACNQ, O(2) and H(2)O(2) by several amperometric techniques has revealed that ACNQ works as a good electron acceptor of NAD(P)H diaphorase and that the reduced form of ACNQ is easily autoxidized and also acts as a better electron donor of NAD(P)H peroxidase than NAD(P)H. The generation of H(2)O(2) by B. longum under aerobic conditions is effectively suppressed in the presence of ACNQ. These ACNQ-mediated reactions would play roles as NAD(P)(+)-regeneration processes. The accumulation of ACNQ in the cytosol has been also suggested. These characteristics of ACNQ seem to be responsible for the growth stimulation of bifidobacteria. Vitamin K(3), which has an extremely low growth-stimulating activity and was used as a reference compound, exhibits much lower activity as an electron transfer mediator. The difference in the activity is discussed in terms of the redox potential and partition property of the quinones.

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

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.

Mechanistic study on the roles of a bifidogenetic growth stimulator based on physicochemical characterization

2-Amino-3-carboxy-1,4-naphthoquinone, discovered as a novel bifidogenetic growth stimulator (BGS), has been characterized by determination of redox and acid-base equilibria, partition properties, and UV-vis and electron spin resonance spectral properties. BGS is proposed to function as an electron transfer mediator from NADH to O2. BGS is reduced by NADH-reduced diaphorase (or related enzymes) and the reduced BGS is reoxidized by autoxidation and a peroxidase-catalyzed reaction. The proposed reaction would spare pyruvate as an important metabolic intermediate, and minimize the cytotoxic effects of H2O2 generated by the autoxidation. Kinetic studies were performed in model enzymatic systems using 2-methyl-1,4-naphthoquinone (VK3) as a reference compound with a very weak growth-stimulating effect. The results support our proposal and reveal the superiority of BGS to VK3 as an electron transfer mediator in the proposed reactions.