Photoinduced electron transfer between the Rieske iron-sulfur protein and cytochrome c1 in the Rhodobacter sphaeroides cytochrome bc1 complex: Effects of pH, temperature, and driving force

Abstract

Electron transfer from the Rieske iron-sulfur protein to cytochrome c1 (cyt c1) in the Rhodobacter sphaeroides cytochrome bc1 complex was studied using a ruthenium dimer complex, Ru2D. Laser flash photolysis of a solution containing reduced cyt bc1, Ru2D, and a sacrificial electron acceptor results in oxidation of cyt c1 within 1 μs, followed by electron transfer from the iron-sulfur center (2Fe-2S) to cyt c1 with a rate constant of 80,000 s-1. Experiments were carried out to evaluate whether the reaction was rate-limited by true electron transfer, proton gating, or conformational gating. The temperature dependence of the reaction yielded an enthalpy of activation of +17.6 kJ/mol, which is consistent with either rate-limiting conformational gating or electron transfer. The rate constant was nearly independent of pH over the range pH 7 to 9.5 where the redox potential of 2Fe-2S decreases significantly due to deprotonation of His-161. The rate constant was also not greatly affected by the Rieske iron-sulfur protein mutations Y156W, S154A, or S154A/Y156F, which decrease the redox potential of 2Fe-2S by 62, 109, and 159 mV, respectively. It is concluded that the electron transfer reaction from 2Fe-2S to cyt c1 is controlled by conformational gating.