Photoinduced electron transfer between the Rieske iron-sulfur protein and cytochrome c(1) in the Rhodobacter sphaeroides cytochrome bc(1) complex. Effects of pH, temperature, and driving force.
Electron transfer from the Rieske iron-sulfur protein to cytochrome c(1) (cyt c(1)) in the Rhodobacter sphaeroides cytochrome bc(1) complex was studied using a ruthenium dimer complex, Ru(2)D. Laser flash photolysis of a solution containing reduced cyt bc(1), Ru(2)D, and a sacrificial electron acceptor results in oxidation of cyt c(1) within 1 micros, followed by electron transfer from the iron-sulfur center (2Fe-2S) to cyt c(1) 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 c(1) is controlled by conformational gating.
Engstrom
Gregory
G
Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, USA.
Xiao
Kunhong
K
Yu
Chang-An
CA
Yu
Linda
L
Durham
Bill
B
Millett
Francis
F
D003575:Cytochromes c1 / Q000737:chemistry*; D014450:Electron Transport Complex III / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D008027:Light; D009154:Mutation; D010084:Oxidation-Reduction; D011487:Protein Conformation; D012242:Rhodobacter sphaeroides / Q000201:enzymology*; D013696:Temperature; D013816:Thermodynamics
D007506:Iron-Sulfur Proteins; C037989:Rieske iron-sulfur protein; D003575:Cytochromes c1; D014450:Electron Transport Complex III