Electrochemical studies of the hexaheme nitrite reductase from Desulfovibrio desulfuricans ATCC 27774

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.