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| Content Provider | World Health Organization (WHO)-Global Index Medicus |
|---|---|
| Author | Ishida, Naoko Boussac, Alain Sugiura, Miwa Rappaport, Fabrice |
| Description | Author Affiliation: Boussac A ( iBiTec-S, CNRS UMR 8221, CEA Saclay, 91191 Gif-sur-Yvette, France. alain.boussac@cea.fr) |
| Abstract | The active site for water oxidation in Photosystem II (PSII) goes through five sequential oxidation states (S $_{0}$ to S $_{4}$ ) before O $_{2}$ is evolved. It consists of a Mn $_{4}$ CaO $_{5}$ cluster and Tyr $_{Z}$ , a redox-active tyrosine residue. Chloride ions have been known for long time to be required for the function of the enzyme. However, X-ray data have shown that they are located about 7 Å away from the Mn $_{4}$ CaO $_{5}$ cluster, a distance that seems too large to be compatible with a direct involvement of chloride in the water splitting chemistry. We have investigated the role of this anion by substituting I $^{−}$ for Cl $^{−}$ in the cyanobacterium Thermosynechococcus elongatus with either Ca $^{2$ + or Sr $^{2$ + biosynthetically assembled into the Mn $_{4}$ cluster. The electron transfer steps affected by the exchanges were investigated by time-resolved UV–visible absorption spectroscopy, time-resolved EPR at room temperature and low temperature cw-EPR spectroscopy. In both Ca-PSII and Sr-PSII, the Cl $^{−}$ /I $^{−}$ exchange considerably slowed down the two S $_{3}$ Tyr $_{Z}^{•}$ → (S $_{3}$ Tyr $_{Z}^{•}$ )′ → S $_{0}$ reactions in which the fast phase, S $_{3}$ Tyr $_{Z}^{•}$ → (S $_{3}$ Tyr $_{Z}^{•}$ )′, reflects the electrostatically triggered expulsion of one proton from the catalytic center caused by the positive charge near/on Tyr $_{Z}^{•}$ and the slow phase corresponds to the S $_{0}$ and O $_{2}$ formations and to a second proton release. The $t_{1/2}$ for S $_{0}$ formation increased from 1.1 ms in Ca/Cl-PSII to ≈ 6 ms in Ca/I-PSII and from 4.8 ms in Sr/Cl-PSII to ≈ 45 ms in Sr/I-PSII. In all cases the Tyr $_{Z}^{•}$ reduction was the limiting step. The kinetic effects are interpreted by a model in which the Ca $^{2$ + binding site and the Cl $^{−}$ binding site, although spatially distant, interact. This interaction is likely mediated by the H-bond and/or water molecules network(s) connecting the Cl $^{−}$ and Ca $^{2$ + binding sites by which proton release may be channelled. |
| ISSN | 00063002 |
| Journal | Biochimica et Biophysica Acta (BBA) - Reviews on Cancer |
| Issue Number | 5 |
| Volume Number | 1817 |
| Language | English |
| Publisher | Elsevier |
| Publisher Date | 2012-05-01 |
| Publisher Place | Netherlands |
| Access Restriction | Open |
| Subject Keyword | Chlorides Metabolism Cyanobacteria Iodides Photosystem II Protein Complex Absorption Calcium Electron Spin Resonance Spectroscopy Kinetics Lasers Tyrosine Research Support, Non-U.S. Gov't Biochemistry |
| Content Type | Text |
| Resource Type | Article |
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