Phospholipid-enriched bacterial chromatophores. A system suited to investigate the ubiquinone-mediated interactions of protein complexes in photosynthetic oxidoreduction processes.

Fusion of phospholipid vesicles with photosynthetic chromatophores from Rhodopseudomonas sphaeroides was induced by freezing and thawing. After sucrose density gradient sedimentation, bands containing closed vesicles characterized by different phospholipid to reaction center molar ratios could be isolated and analyzed morphologically and functionally by means of electron microscopy and fast spectroscopy, respectively. Analogously to data reported for phospholipid-enriched mitochondrial inner membranes (Schneider, H., Lemasters, J. J., and Hackenbrock, C. R. (1982) J. Biol. Chem. 257, 10793), the rate of photosynthetic electron transfer in phospholipid-enriched chromatophores decreased with increasing distance between integral membrane complexes. A fast cyclic electron transfer could be restored when the concentration of the ubiquinone pool within the lipid bilayer was reconstituted by additions of exogenous ubiquinone. These results suggest that cyclic electron transfer between reaction center and ubiquinol-cytochrome c2 oxidoreductase complexes in phospholipid-enriched chromatophores is limited by the lateral diffusion of the quinone molecules in the membrane plane. The observation that dilution of the quinone pool in the lipid bilayer affects the rate of photosynthetic electron transport contrasts with previously reported data which indicated that up to 80% of the quinone pool can be removed without altering the kinetic parameters of the overall process. These conflicting results can be reconciled by a model which assumes that the relative orientation of the protein complexes, possibly controlled by protein-protein interactions within the lipid bilayer, plays a key role in the effectiveness of the molecular collisions. According to a diffusion-limited mechanism, this would lead to a fast electron transfer during the photosynthetic reactions.