pH-Dependent Protonation of Surface Carboxylate Groups in PsbO Enables Local Buffering and Triggers Structural Changes

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 Mn₄ CaO₅ 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 pK_a values of water-accessible aspartate and glutamate side-chain carboxylate groups by means of NMR. Their distribution is strikingly uneven, with high pK_a 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.

Keywords: NMR spectroscopy; pH titration; pKa values; photosystem II; protonation.