Electron Transfer between Primary and Secondary Donors in Photosynthetic Bacteria: Evidence for “Supercomplexes”
Most of the carriers of the photosynthetic electron transfer chain are included in large transmembrane protein complexes, two in the case of bacterial photosynthesis and three for oxygenic photosynthesis. On a time-scale of several seconds, these complexes can be considered as immobilized in the membrane. Two types of soluble electron carriers establish a link between the membrane complexes: ubiquinone or plastoquinone diffuse in the lipid phase of the membrane, while cyt c2 or plastocyanin which are hydrosoluble, diffuse in the periplasmic space or the internal aquous phase of the thylakoid. In a classical view of the photosynthetic apparatus, the soluble carriers are supposed to diffuse rapidly over long distances; therefore, we can expect that in the dark or under weak illumination, the carriers of the electron transfer chain are close to thermodynamic equilibrium. In such circumstances, the localization of the different electron carriers within the membrane should be of little functional importance.
KeywordsElectron Transfer Reaction Continuous Illumination Electron Transfer Chain Midpoint Potential Secondary Donor
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