Using direct electrochemistry to probe rate limiting events during nitrate reductase turnover

Protein film voltammetry of the nitrate reductase Paracoccus pantotrophus NarGH reveals a catalytic response whose shape, magnitude and position reflect intrinsic thermodynamic and kinetic properties of the enzyme. Under steady state conditions the catalytic wave is displaced to more negative potentials as the substrate concentration is increased and reaches a limiting value of 7.5 +/- 10 mV for concentrations exceeding 1 mM nitrate, pH 6. The shape of the wave remains in general agreement with that predicted for an n(app)approximate to1 process across the complete range of nitrate concentrations investigated and the magnitude of the catalytic response varies in agreement with a Michaelis-Menten description of enzyme kinetics. NarGH is a multi-centred redox enzyme containing iron-sulfur clusters in addition to a molybdenum-bis-molybdopterin guanine dinucleotide cofactor, Mo(MGD)(2), capable of accommodating multiple electrons at the enzyme's active site. It is suggested that progression from the semi-reduced to fully-reduced state of the active site, constitutes a rate defining event in the catalytic cycle. Formation of the fully-reduced active site may be limited either by relayed, intramolecular electron delivery to the active site and/or the rates of chemical transformations within the semi-reduced form of the active site.