Evidence for two Qa-like quinone binding sites in the reaction centre of Rhodopseudomonas viridis |
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Authors: | Michael C W Evans |
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Institution: | Department of Botany and Microbiology, University College London, London, U.K. |
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Abstract: | The oxidation-reduction potential of the iron-quinone electron acceptors in the reaction centre of Rhodopseudomonas viridis has been reinvestigated. In chromatophores treated with o-phenanthroline to remove the secondary acceptor Qb, two steps were observed in the reduction of the primary electron acceptor Qa with Em ≈ − 100 and ≈ − 330 mV. In isolated reaction centres only one step was observed in the reduction of Qa with E ≈ −150 mV. Reconstitution of the reaction centres with additional menaquinone resulted in an increase in the Qa EPR signal and reconstitution of the low-potential step in the oxidation-reduction titration. Reconstitution with ubiquinone resulted in the recovery of the secondary quinone Qb. The addition of ubiquinone did not reconstitute the low-potential step of Qa reduction, or affect the reconstitution of this step by menaquinone. It is concluded that menaquinone can bind to two sites on the reaction centre. Both have properties of the Qa site but with different pK values. It is unlikely that either is the same as the Qb site. |
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Keywords: | Photosynthesis Photosynthetic reaction centre Quinone Iron-quinone (Rps viridis) |
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