Cationic screening of charged surface groups (carboxylates) affects electron transfer steps in photosystem-II water oxidation and quinone reduction

The functional or regulatory role of long-distance interactions between protein surface and interior represents an insufficiently understood aspect of protein function. Cationic screening of surface charges determines the morphology of thylakoid membrane stacks. We show that it also influences directly the light-driven reactions in the interior of photosystem II (PSII). After laser-flash excitation of PSII membrane particles from spinach, time courses of the delayed recombination fluorescence (10 μs–10 ms) and the variable chlorophyll-fluorescence yield (100 μs–1 s) were recorded in the presence of chloride salts. At low salt-concentrations, a stimulating effect was observed for the S-state transition efficiency, the time constant of O₂-formation at the Mn₄Ca-complex of PSII, and the halftime of re-oxidation of the primary quinone acceptor (Q_a) by the secondary quinone acceptor (Q_b). The cation valence determined the half-effect concentrations of the stimulating salt effect, which were around 6 μM, 200 μM and 10 mM for trivalent (LaCl₃), bivalent (MgCl₂, CaCl₂), and monovalent cations (NaCl, KCl), respectively. A depressing high-salt effect also depended strongly on the cation valence (onset concentrations around 2 mM, 50 mM, and 500 mM). These salt effects are proposed to originate from electrostatic screening of negatively charged carboxylate sidechains, which are found in the form of carboxylate clusters at the solvent-exposed protein surface. We conclude that the influence of electrostatic screening by solvent cations manifests a functionally relevant long-distance interaction between protein surface and electron-transfer reactions in the protein interior. A relation to regulation and adaptation in response to environmental changes is conceivable.