Asymmetric and Redox-specific Binding of Quinone and Quinol at Center N of the Dimeric Yeast Cytochrome bc sub(1) Complex: Consequences For Semiquinone Stabilization

• Published in: The Journal of biological chemistry Vol. 282; no. 33; pp. 24198 - 24208
• Main Authors: Covian, Raul, Zwicker, Klaus, Rotsaert, Frederik A, Trumpower, Bernard L
• Format: Journal Article
• Language: English
• Published: 01.08.2007
• ISSN: 0021-9258; 1083-351X

The cytochrome bc sub(1) complex recycles one of the two electrons from quinol (QH sub(2)) 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 bc sub(1) complex. The EPR-detectable signal, which reports SQ bound in the vicinity of reduced b sub(H) 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 b sub(H), we calculated a midpoint redox potential (E sub(m)) of similar to 90 mV for all bound SQ. Considering the E sub(m) values for b sub(H) and free Q, this result indicates that center N preferentially stabilizes SQ.b sub(H) super(3+) complexes. This favors recycling of the electron coming from center P and also implies a >2.5-fold higher affinity for QH sub(2) than for Q at center N, which would potentially inhibit b sub(H) oxidation by Q. Using pre-steady-state kinetics, we show that Q does not inhibit the initial rate of b sub(H) reduction by QH sub(2) through center N, but does decrease the extent of reduction, indicating that Q binds only when b sub(H) is reduced, whereas QH sub(2) binds when b sub(H) 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.b sub(H) super(3+) formation without inhibition of Q binding by QH sub(2).