Studies on the water-oxidizing system by the effects of different treatments in chloroplasts |
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Authors: | Marie-José Delrieu |
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Affiliation: | Laboratoire de Photosynthèse, CNRS, BP 1, 91190 Gif-sur-Yvette France |
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Abstract: | Treatments such as trypsinization (50 μg/ml per mg Chl for 1 h), osmotic shock of the chloroplasts or mild heating altered the oxygen evolution in such a way that the properties of the Photosystem II were simplified. After these treatments, the damping of the oscillation pattern of O2 yields induced by a flash series remained the same, irrespective of the level of inhibition induced by the treatment. This damping did not decrease with increasing flash energy, as observed in untreated chloroplasts. The light saturation curve of the S2 → S3 transition of the O2 evolving system no more exhibited the slow-increasing phase at high flash energy observed under normal conditions. The kinetic properties of the O2-evolving system were also simplified. After the treatments cited above, deactivation of S2 and S3 were identical and accelerated with respect to untreated chloroplasts. Turnover kinetics of the transitions S1 → S2 and S2 → S3 were also similar and simpler without a lag for S2 → S3. These results indicate that the treatments mentioned above disconnect one donor from the O2-evolving complex. This donor, under normal conditions, contributes to the increase of the quantum yield of the transition S2 → S3 at high flash energy. This donor is here denoted by D. Our results are in agreement with the following working hypothesis: the large miss, observed on the S2 → S3 transition without any contribution of the donor D, may be due to the fact that the system needs a conformation change of the O2-evolving complex in the S2 state, so that the main donor Y can oxidize the second H2O molecule in the water-splitting complex. In the inactive state corresponding to the absence of a conformation change, the donor D, being different in configuration, is likely to oxidize the S2 state into an S3 state at high light intensity. |
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Keywords: | Oxygen evolution Photosystem II Chlorophyll fluorescence Water splitting (Lettuce chloroplast) Chl chlorophyll DCMU 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea PS II Photosystem II Tricine Hepes 4-(2-hydroxyethyl)-1-piperazineethanesulphonic acid cyt cytochrome |
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