Functional Coupling Between Reaction Centers and Cytochrome bc 1 Complexes

Light-induced cyclic electron transfer in purple bacteria involves two integral membrane protein complexes, the reaction center (RC) and the cytochrome bc 1 complex, and two mobile carriers that shuttle between them. The mobile carrier on the acceptor side of the RC is a quinone molecule, confined to the hydrophobic membrane/protein regions. More diversity is found for the donor side, starting with the RC, which may or may not possess a multiheme donor subunit. Depending on species, the multiheme subunit includes three or four c-type hemes with alternating high and low midpoint potentials so that the electron transfer involves a succession of uphill and downhill steps. The donor side mobile carrier, confined to the periplasmic space, is either a soluble cytochrome (c 2 or c 8), a membrane-anchored c-type cytochrome, or a high-potential iron-sulfur protein (HiPIP). The stoichiometric ratios between the components of the photosynthetic chain (generally in the order: quinones > RC > mobile cytochrome > bc 1) are variable depending on species and growth conditions. In many species where the core antenna ring surrounds the RC completely, the quinone circuit requires the crossing of this barrier. For species endowed with a low bc 1:RC ratio, the diffusion of the mobile carriers over relatively long ranges is mandatory for connecting the distant partners. In some Rhodobacter species, with a high bc 1:RC ratio, evidence has been found for the formation of specific supercomplexes associating all the components of the cyclic transfer in largely independent functional units. The mechanisms that may be responsible for the confinement of the mobile carriers in such systems are discussed.