Primary Steps in the Energy Conversion Reaction of theCytochrome bc ₁ Complex QO Site

Abstract

The primary energy conversion (Q_O) site of the cytochrome bc ₁ complex is flanked by bothhigh- and low-potential redox cofactors, the [2Fe–2S] cluster and cytochrome b _L, respectively.From the sensitivity of the reduced [2Fe–2S] cluster electron paramagnetic resonance (EPR)spectral g _x-band and line shape to the degree and type of Q_O site occupants, we have proposeda double-occupancy model for the Q_O site by ubiquinone in Rhodobacter capsulatus membranevesicles containing the cytochrome bc ₁ complex. Biophysical and biochemical experimentshave confirmed the double occupancy model and from a combination of these results and theavailable cytochrome bc ₁ crystal structures we suggest that the two ubiquinone molecules inthe Q_O site serve distinct catalytic roles. We propose that the strongly bound ubiquinone,termed Q_OS, is close to the [2Fe–2S] cluster, where it remains tightly associated with the Q_Osite during turnover, serving as a catalytic cofactor; and the weaker bound ubiquinone, Q_OW,is distal to the [2Fe–2S] cluster and can exchange with the membrane Q_pool on a time scalemuch faster than the turnover, acting as the substrate. The crystallographic data demonstratesthat the FeS subunit can adopt different positions. Our own observations show that theequilibrium position of the reduced FeS subunit is proximal to the Q_O site. On the basis of this, wealso report preliminary results modeling the electron transfer reactions that can occur in thecytochrome bc ₁ complex and show that because of the strong distance dependence of electrontransfer, significant movement of the FeS subunit must occur in order for the complex to beable to turn over at the experimental observed rates.