| Affiliation: | 1. Leibniz-Forschungsinstitut für Molekulare Pharmakologie, Department of NMR-Supported Structural Biology, Robert-Rössle-Strasse 10, 13125 Berlin, Germany;2. Leibniz-Forschungsinstitut für Molekulare Pharmakologie, Department of NMR-Supported Structural Biology, Robert-Rössle-Strasse 10, 13125 Berlin, Germany Freie Universität Berlin, Department of Biology, Chemistry and Pharmacy, Thielallee 63, 14195 Berlin, Germany;3. Max-Delbrück-Centrum für Molekulare Medizin, Robert-Rössle-Strasse 10, 13125 Berlin, Germany;4. Freie Universität Berlin, Department of Physics, Theoretical Molecular Biophysics, Arnimallee 14, 14195 Berlin, Germany;5. Humboldt-Universität zu Berlin, Institute of Biology, Philippstrasse 13, 10099 Berlin, Germany;6. Freie Universität Berlin, Department of Physics, Biophysics and Photosynthesis, Arnimallee 14, 14195 Berlin, Germany |
| Abstract: | Photosystem II (PSII) catalyzes the splitting of water, releasing protons and dioxygen. Its highly conserved subunit PsbO extends from the oxygen-evolving center (OEC) into the thylakoid lumen and stabilizes the catalytic Mn4CaO5 cluster. The high degree of conservation of accessible negatively charged surface residues in PsbO suggests additional functions, as local pH buffer or by affecting the flow of protons. For this discussion, we provide an experimental basis, through the determination of pKa values of water-accessible aspartate and glutamate side-chain carboxylate groups by means of NMR. Their distribution is strikingly uneven, with high pKa values around 4.9 clustered on the luminal PsbO side and values below 3.5 on the side facing PSII. pH-dependent changes in backbone chemical shifts in the area of the lumen-exposed loops are observed, indicating conformational changes. In conclusion, we present a site-specific analysis of carboxylate group proton affinities in PsbO, providing a basis for further understanding of proton transport in photosynthesis. |
