Primary Steps in the Energy Conversion Reaction of the Cytochrome bc_1 Complex Q_o Site

摘要

The primary energy conversion (Q_o) site of the cytochrome bc_1 complex is flanked by both high- 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 proposed a double-occupancy model for the Q_o site by ubiquinone in Rhodobacter capsulatus membrane vesicles containing the cytochrome bc_1 complex. Biophysical and biochemical experiments have confirmed the double occupancy model and from a combination of these results and the available cytochrome bc_1 crystal structures we suggest that the strongly bound ubiquinone, the 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_o site 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 scale much faster than the turnover, acting as the substrate. The crystallographic data demonstrates that the FeS subunit can adopt different positions. Our own observations show that the equilibrium position of the reduced FeS subunit is proximal to the Q_o site. On the basis of this, we also report preliminary results modeling the electron transfer reactions that can occur in the cytochrome bc_1 complex and show that because of the strong distance dependence of electron transfer, significant movement of the FeS subunit must occur in order for the complex to be able to turn over at the experimental observed rates.