| Article ID | Journal | Published Year | Pages | File Type |
|---|---|---|---|---|
| 1942464 | Biochimica et Biophysica Acta (BBA) - Bioenergetics | 2012 | 9 Pages |
The active site for water oxidation in Photosystem II (PSII) goes through five sequential oxidation states (S0 to S4) before O2 is evolved. It consists of a Mn4CaO5 cluster and TyrZ, 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 Mn4CaO5 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 Ca2 + or Sr2 + biosynthetically assembled into the Mn4 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 S3TyrZ• → (S3TyrZ•)′ → S0 reactions in which the fast phase, S3TyrZ• → (S3TyrZ•)′, reflects the electrostatically triggered expulsion of one proton from the catalytic center caused by the positive charge near/on TyrZ• and the slow phase corresponds to the S0 and O2 formations and to a second proton release. The t1/2 for S0 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 TyrZ• reduction was the limiting step. The kinetic effects are interpreted by a model in which the Ca2 + 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 Ca2 + binding sites by which proton release may be channelled.
Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (74 K)Download as PowerPoint slideHighlights► Chloride is substituted for iodide in Photosystem II from T. elongatus. ► Ca/I-PSII and Sr/I-PSII have a high O2 evolving activity. ► The t1/2 of S3TyrZ•′ to S0 is 1.1 ms in Ca/Cl-PSII and 6 ms in Ca/I-PSII. ► The t1/2 of S3TyrZ•′ to S0 is 4.8 ms in Sr/Cl-PSII and 45 ms in Sr/I-PSII. ► Ca2+ and Cl– interact via the H-bond and/or H2O network(s) connecting the 2 sites.
