Evidence for supercomplexes between reaction centers, cytochrome c2 and cytochrome bc1 complex in Rhodobacter sphaeroides whole cells

Light-induced oxidation of the primary electron donor P, the secondary electron donors cytochrome c 2 , cytochrome c 1 , and the FeS Rieske protein was studied in whole cells of Rhodobacter sphaeroides in the presence of myxothiazol which slows down their reduction. During continuous illumination, the primary and secondary donors are not in thermodynamic equilibrium, despite the rate of electron transfer being light-limited. After a short saturating flash excitation, part of the photooxidized P is not rereduced by the secondary electron donors, although more than 50% of them are still in the reduced state. Theseresults are interpreted in terms of a model where reaction centers, cytochrome c 2 and cytochrome bc 1 , complexes form supercomplexes. About70% of these supercomplexes include two reaction centers, one cytochrome c 2 and one cytochrome bc 1 , complex, while the remaining ones include only two reaction centers and one cytochrome c 2 . During the course of illumination, the redox equilibrium is achieved only within each supercomplex, considered as an isolated entity. On the other hand, equilibration between redox components included in different supercomplexes occurs slowly in the dark via diffusing electron carriers. Our model also readily explains the biphasic kinetics of reduction of the primary donor and of the oxidation of the secondary donor. According to our hypothesis, only half of the reaction centers is tightly associated with cytochrome c 2 . Following a saturating flash excitation, in this fraction of the reaction centers the reduction of the primary donor P + is completed in less than 25 μs. The reduction of the remaining P + implies two sequential reactions: (1) rereduction of cytochrome c 2 by cytochrome c 1 , ( t 1/2 ≈ 130 μs); and (2) association of this reduced cytochrome c 2 with P + ( t 1/2 ≈ 60 μs).