The key step of the protonmotive Q-cycle mechanism of the cytochrome bc1 complex is the bifurcated oxidation of ubiquinol at the Qp site. It was postulated that the iron-sulfur protein (ISP) accepts the first electron from ubiquinol to generate ubisemiquinone anion to reduce bL. Because of the difficulty of following the reduction of ISP optically, direct evidence for the early involvement of ISP in ubiquinol oxidation is not available. Using the ultra-fast microfluidic mixer and the freeze-quenching device, coupled with EPR, we have been able to determine the presteady-state kinetics of ISP and cytochrome bL reduction by ubiquinol. The first-phase reduction of ISP starts as early as 100 μs with a t1/2 of 250 μs. A similar reduction kinetic is also observed for cytochrome bL, indicating a simultaneous reduction of both ISP and bL. These results are consistent with the fact that no ubisemiquinone was detected at the Q p site during oxidation of ubiquinol. Under the same conditions, by using stopped flow, the reduction rates of cytochromes bH and c 1 were 403 s-1 (t1/2 1.7 ms) and 164 s -1 (t1/2 4.2 ms), respectively.
|Number of pages||6|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|State||Published - Mar 20 2007|
- Electron transfer
- Rapid freeze-quenching
- Ubiquinol-cytochrome c