Primary steps in the energy conversion reaction of the cytochrome bc(1) complex Q(o) site

The primary energy conversion (Q(O)) site of the cytochrome bc(1) complex i s flanked by both high- and low-potential redox cofactors. the [2Fe-2S] clu ster 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 propo sed a double-occupancy model for the Q(O) site by ubiquinone in Rhodobacter capsulatus membrane vesicles containing the cytochrome bc(1) complex. Biop hysical and biochemical experiments have confirmed the double occupancy mod el and from a combination of these results and the available cytochrome bc( 1) crystal structures we suggest that the two ubiquinone molecules in 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 remai ns tightly associated with the Q(O) site during turnover, serving as a cata lytic cofactor; and the weaker bound ubiquinone, Q(OW), is distal to the [2 Fe-2S] cluster and can-exchange with the membrane Q(pool) on a time scale m uch 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 subu nit is proximal to the Q(O) site. On the basis of this, we also report prel iminary results modeling the electron transfer reactions that can occur in the cytochrome bc(1) complex and show that because of the strong distance d ependence 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.