Asymmetric and redox-specific binding of quinone and quinol at center N of the dimeric yeast cytochrome bc1 complex. Consequences for semiquinone stabilization.
The cytochrome bc1 complex recycles one of the two electrons from quinol (QH2) oxidation at center P by reducing quinone (Q) at center N to semiquinone (SQ), which is bound tightly. We have analyzed the properties of SQ bound at center N of the yeast bc1 complex. The EPR-detectable signal, which reports SQ bound in the vicinity of reduced bH heme, was abolished by the center N inhibitors antimycin, funiculosin, and ilicicolin H, but was unchanged by the center P inhibitors myxothiazol and stigmatellin. After correcting for the EPR-silent SQ bound close to oxidized bH, we calculated a midpoint redox potential (Em) of approximately 90 mV for all bound SQ. Considering the Em values for bH and free Q, this result indicates that center N preferentially stabilizes SQ.bH(3+) complexes. This favors recycling of the electron coming from center P and also implies a >2.5-fold higher affinity for QH2 than for Q at center N, which would potentially inhibit bH oxidation by Q. Using pre-steady-state kinetics, we show that Q does not inhibit the initial rate of bH reduction by QH2 through center N, but does decrease the extent of reduction, indicating that Q binds only when bH is reduced, whereas QH2 binds when bH is oxidized. Kinetic modeling of these results suggests that formation of SQ at one center N in the dimer allows stabilization of SQ in the other monomer by Q reduction after intradimer electron transfer. This model allows maximum SQ.bH(3+) formation without inhibition of Q binding by QH2.
Covian
Raul
R
Department of Biochemistry, Dartmouth Medical School, Hanover, New Hampshire 03755, USA.
Zwicker
Klaus
K
Rotsaert
Frederik A
FA
Trumpower
Bernard L
BL
D016227:Benzoquinones; D019281:Dimerization; D004579:Electron Transport; D014450:Electron Transport Complex III / Q000378:metabolism*; D006873:Hydroquinones / Q000378:metabolism*; D010084:Oxidation-Reduction; D011809:Quinones / Q000378:metabolism*; D015003:Yeasts
D016227:Benzoquinones; D006873:Hydroquinones; D011809:Quinones; C025232:semiquinone radicals; D014450:Electron Transport Complex III