Specific Mutagenesis of the Rieske Iron−Sulfur Protein in Rhodobacter sphaeroides Shows That both the Thermodynamic Gradient and the pK of the Oxidized Form Determine the Rate of Quinol Oxidation by the bc1 Complex

Abstract

In the Rieske iron−sulfur protein (ISP) of the ubiquinol:cytochrome c₂ oxidoreductase (bc₁ complex) of Rhodobacter sphaeroides, residue Tyr 156 is located close to the iron−sulfur cluster. Previous studies of the equivalent residue in both Saccharomyces cerevisiae [Denke, E., Merbitz-Zahradnik, T., Hatzfeld, O. M., Snyder, C. H., Link, T. A., and Trumpower, B. L. (1998) J. Biol. Chem. 273, 9085−9093] and Paracoccus denitrificans [Schroter, T., Hatzfeld, O. M., Gemeinhardt, S., Korn, M., Friedrich, T., Ludwig, B., and Link, T. A. (1998) Eur. J. Biochem. 255, 100−106] have indicated that mutations at this site can lead to modifications in the redox potential of the ISP. To study the effect of similar modifications on the thermodynamic behavior and kinetics of partial reactions of the bc₁ complex upon flash activation, we have constructed four mutant strains of Rb. sphaeroides where Tyr 156 was mutated to His, Leu, Phe, or Trp. The bc₁ complex was assembled and able to support photosynthetic growth in all mutants. Three substitutions (Leu, Phe, Trp) led to alteration of the midpoint potential (E_m) of the ISP and a slowing in rate of quinol oxidation, suggesting that electron transfer from quinol to the oxidized ISP controls the overall rate and that this step includes the high activation barrier. The Trp mutation led to an increase of ∼1 pH unit in the pK value of the oxidized ISP. The pH dependence of the rate of quinol oxidation in this mutant was also shifted up by ∼1 pH unit, showing the importance of the protonation state of the ISP for quinol oxidation. This provides support for a model in which the dissociated form of the oxidized ISP is required for formation of the enzyme−substrate complex [Ugulava, N., and Crofts, A. R. (1998) FEBS Lett. 440, 409−413].