Involvement of the protein-protein interactions in the thermodynamics of the electron-transfer process in the reaction centers from Rhodopseudomonas viridis
Reaction centers from Rhodopseudomonas viridis were reconstituted into dimyristoylphosphatidylcholine (DMPC) and dielaidoylphosphatidylcholine (DEPC) liposomes. Freeze-fracture electron micrographs were performed on the samples frozen from temperatures above and below the phase transition temperatures of those lipids (Tc = 23 and 9.5 degrees C, in DMPC and DEPC, respectively). Above Tc, in the fluid conformation of the lipids, the reaction centers are randomly distributed in the vesicle membranes. Below Tc, aggregation of the proteins occurs. The Arrhenius plots of the rate constants of the charge recombination between P+ and QA- display a break at about 24 degrees C in DMPC vesicles and about 10 degrees C in DEPC vesicles (P represents the primary electron donor, a dimer of bacteriochlorophyll, and QA the primary quinone electron acceptor). This is in contrast to what was previously observed for the proteoliposomes of egg yolk phosphatidylcholine and for chromatophores [Baciou, L., Rivas, E., & Sebban, P. (1990) Biochemistry 29, 2966-2976], for which Arrhenius plots were linear. In DMPC and DEPC proteoliposomes, the activation parameters were very different on the two sides of Tc (delta H degrees for T less than Tc = 2.5 times delta H degrees for T greater than Tc), leading however, to the same delta G degrees values. Taking into account the structural and thermodynamic data, we suggest that, in vivo, protein-protein interactions play a role in the thermodynamic parameters associated with the energy stabilization process within the reaction centers.