Conformation-Activated Protonation in Reaction Centers of the Photosynthetic Bacterium Rhodobacter sphaeroides

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Abstract

Kinetics and stoichiometry of proton binding/unbinding induced by intense (1 W cm-2) and continuous illumination were measured in the isolated reaction center (RC) protein from photosynthetic purple bacterium Rhodobacter sphaeroides in the absence of an external electron donor. At high ionic strength (100 mM), large proton release (approximately 6 H+ per RC) was observed at pH 6 and substoichiometric H+-ion binding (approximately 0.3 H+ per RC) at pH 8. These observations together with optical spectroscopy on the oxidized dimer indicate that, at room temperature, two distinct conformations of the RC can be obtained depending on the pH, Eh, and illumination. Acidic pH, a large redox gap between the actual Eh of the solution and the midpoint potential of the acceptor quinone, and strong illumination favor the conversion of the RC from the dark-adapted state to the light-adapted state. These conformations differ greatly in the rates of primary photochemistry, the reoxidation of semiquinone and the rereduction of the oxidized dimer, and the protonation states of the amino acids of the protein. Whereas substoichiometric proton unbinding is observed in the P+Q redox state of the protein in the dark-adapted conformation, much larger H+-ion release is detected in the light-adapted conformation. From the pH dependence of the key processes in the conformational change and reoxidation of semiquinone, we concluded that they are controlled by protonatable groups available in the protein. A simple phenomenological model is presented that relates the rates and equilibrium constants of the electron transfer reactions and the conformational change of the RC.

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