Search

In this work we present a kinetic study of the superoxide-mediated electron transfer reactions between rubredoxin-type proteins and members of the three different classes of superoxide reductases (SORs). SORs from the sulfate-reducing bacteria Desulfovibrio vulgaris ( Dv) and D. gigas ( Dg) were chosen as prototypes of classes I and II, respectively, while SOR from the syphilis spyrochete Treponema pallidum ( Tp) was representative of class III. Our results show evidence for different behaviors of SORs toward electron acceptance, with a trend to specificity for the electron donor and acceptor from the same organism. Comparison of the different k app values, 176.9±25.0 min−1 in the case of the Tp/ Tp electron transfer, 31.8±3.6 min−1 for the Dg/ Dg electron transfer, and 6.9±1.3 min−1 for Dv/ Dv, could suggest an adaptation of the superoxide-mediated electron transfer efficiency to various environmental conditions. We also demonstrate that, in Dg, another iron–sulfur protein, a desulforedoxin, is able to transfer electrons to SOR more efficiently than rubredoxin, with a k app value of 108.8±12.0 min−1, and was then assigned as the potential physiological electron donor in this organism. [ABSTRACT FROM AUTHOR]

The kinetics of electron transfer between the Desulfovibro gigas hydrogenase and several electron-transfer proteins from Desulfovibrio species were investigated by cyclic voltammetry, square-wave voltammetry and chronoamperometry. The cytochrome C3 from Desulfovibrio vulgaris (Hildenborough). Desulfovibrio desulfuricans (Norway 4), Desulfovibrio desulfuricans (American Type Culture Collection 27774) and D. gigas (NCIB 9332) were used as redox carriers. They differ in their redox potentials and isoelectric point. Depending on the pH, all the reduced forms of these cytochromes were effective in electron exchange with hydrogenase. Other small electron-transfer proteins such as ferredoxin I, ferredoxin II and rubredoxin from D. gigas were tentatively used redox carriers. Only ferredoxin II was effective in mediating electron exchange between hydrogenase and the working electrode. The second-order rate constants k for the reaction between reduced proteins and hydrogenase were calculated based on the theory of the simplest electrocatalytic mechanism [Moreno, C., Costa, C., Moura, I., Le Gall, J., Liu, M. Y., Payne, W.J., van Dijk, C. & Moura. J.J.G. (1193) Eur. J.. Biochem. 212. 79-86] and the results obtained by cyclic voltammetry were compared with those obtained by chronoamperometry. Values for k of 105-106 M&sup-1; s&sup-1; (cytochrome c1 as electron carrier) and 104M&sup-1; s&sup-1; (ferredoxin II as the electron carrier) were determined. The rate-constant values are discussed in terms of the existence of an electrostatic interaction between the electrode surface and the redox carrier and between the redox carrier and a positively charged part of the enzyme. [ABSTRACT FROM AUTHOR]

The electron-transfer kinetics between three different mediators and the hexahemic enzyme nitrite reductase isolated from Desulfovibrio desulfuricans (ATCC 27774) were investigated by cyclic voltammetry and by chronoamperometry. The mediators, methyl viologen, Desulfovibrio vulgaris (Hildenborough) cytochrome c3 and D. desulfuricans (ATCC 27774) cytochrome c3 differ in structure, redox potential and charge. The reduced form of each mediator exchanged electrons with nitrite reductase. Second-order rate constants, k, were calculated on the basis of the theory for a simple catalytic mechanism and the results, obtained by cyclic voltammetry, were compared with those obtained by chronoamperometry. Values for k are in the range 106-108 M-1 s-1 and increase in the direction D. desulfuricans cytochrome c3→D. vulgaris cytochrome c3→methyl viologen. An explanation is advanced on the basis of electrostatic interactions and relative orientation between the partners involved. Chronoamperometry (computer controlled) offers advantages over cyclic voltammetry in the determination of homogeneous rate constants (faster, more accurate and better reproducibility). Direct, unmediated electrochemical responses of the hexaheme nitrite reductase were also reported. [ABSTRACT FROM AUTHOR]