1 research outputs found
pHâdependent protonation of surface carboxylate groups in PsbO enables local buffering and triggers structural changes
Photosystemâ
II (PSII) catalyzes the splitting of water, releasing protons and dioxygen. Its highly conserved subunit PsbO extends from the oxygenâevolving 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 waterâaccessible aspartate and glutamate sideâchain 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. pHâdependent changes in backbone chemical shifts in the area of the lumenâexposed loops are observed, indicating conformational changes. In conclusion, we present a siteâspecific analysis of carboxylate group proton affinities in PsbO, providing a basis for further understanding of proton transport in photosynthesis