STRUCTURAL AND OXIDATION-REDUCTION PROPERTIES OF (4-IRON - 4-SULFUR)³⁺ʼ²⁺ FERREDOXINS.

Abstract

Many biologically important electron transfer reactions involve iron-sulfur centers and flavosemiquinones. Laser flash photolysis and stopped-flow spectrophotometry has been utilized to investigate factors influencing transient kinetic oxidation of free and protein-bound flavosemiquinones by various types of oxidants, but primarily high potential iron-sulfur proteins (HiPIP) and rubredoxin classes of Fe-S proteins. Studies of free flavosemiquinone analog oxidation by Fe-containing redox proteins and inorganic oxidants have found the reactivity of the anionic flavosemiquinone to be greater than that for neutral flavosemiquinone. The second order rate constants of oxidation of various flavosemiquinone analogs by either redox proteins or non-biological oxidants correlate with the difference in redox potential of the reactants according to the Marcus exponential equation. For the protein-flavin analog reactions, deviations from the theoretical Marcus curve are interpreted in terms of the effects of the different exocyclic substitutions on intrinsic anion semiquinone reactivity. Electrostatic effects on FMN and C. pasteurianum flavodoxin semiquinone oxidation by HiPIP can be quantitated yielding the rate constant at infinite ionic strength (k∞) and the charge product for reaction. The magnitude of the electrostatic effects are larger for flavodoxin semiquinone oxidation than for FMN semiquinone oxidation which is consistent with the larger electrostatic charge for flavodoxin. The k∞ values obtained from the electrostatic analysis for FMN semiquinone and flavodoxin semiquinone oxidation indicate a dependence on the redox potential difference between the reactants (ΔE). The ΔE effect is larger in magnitude for the flavodoxin reaction than for the FMN reaction. Deviations from the theoretical curves for the FMN and flavodoxin reactions suggest that factors other than redox potential and electrostatics, such as sterics, could be having an important influence on reactivity. The results suggest that electrostatics, redox potential, and perhaps sterics could have an important role in determining the biological specificity of protein/protein redox reactions. C. pasteurianum rubredoxin forms a 1:1 complex with spinach ferredoxin:NADP+- reductase (FNR) at low ionic strengths. The reduction of Rdₒₓ by lumiflavin semiquinone is unaltered in the complex. Evidence is presented for a rapid (2 x 10³s⁻¹) intracomplex electron transfer from FNR semiquinone to oxidized rubredoxin.