Structure of the core complex of Blastochloris viridis and of Rhodobacter sphaeroides

In purple photosynthetic bacteria, two light harvesting complexes (LH), LH2 and LH1, ensure the collection of light. Then, the excitation energy is funneled towards the reaction center (RC), where after two photoreactions and proton captures, ubiquinol (QH2) formed at the QB site of the RC dissociates into the membrane. The cytochrome (cyt) bc1 complex utilizes QH2 and oxidized cytochrome c2 as reductant and oxidant, respectively. The net result is a cyclic electron transfer that promotes the formation of a proton gradient across the membrane, which is utilized for ATP synthesis by F1F0ATPsynthase (for review see Hu 2002, #1264). The description of the bacterial photosynthetic apparatus at atomic level is nearly complete with the structures of two RC, two LH2, and the cytochrome bc1 complex (see http://blanco.biomol.uci.edu/Membrane_ Proteins_xtal.html). The last component not yet solved is the core complex formed by the LH1 and the RC, in which the transformation of light energy into charge separation occurs. A central question is the coupling between the RC and the cytochrome bc1 complex, or how the quinones produced by the RC are transferred through the LH1 fence to reach the cytochrome bc1 complex. Here we present the structural analysis of two different core complexes, from Blastochloris (Blc.) viridis and from Rhodobacter (Rb) sphaeroides.