Mechanism of inhibition and decoupling of oxygen evolution from electron transfer in photosystem II by fluoride, ammonia and acetate

• Treatment of PSII by F−, NH4Cl, acetate results in appearance of split EPR signal and decoupling effect.
• We imply that these inhibitors disturb the hydrogen bond network around Ca2 +.
• All investigated inhibitors have other inhibition sites at higher concentrations.

Ca2 + extraction from oxygen-evolving complex (OEC) of photosystem II (PSII) is accompanied by decoupling of oxygen evolution/electron transfer processes [Semin et al. Photosynth. Res. 98 (2008) 235] and appearance of a broad EPR signal at g = 2 (split “S3” signal) what can imply the relationship between these effects. Split signal have been observed not only in Ca-depleted PSII but also in PSII membranes treated by fluoride anions, sodium acetate, and NH4Cl. Here we investigated the question: can such compounds induce the decoupling effect during treatment of PSII like Ca2 + extraction does? We found that F−, sodium acetate, and NH4Cl inhibit O2 evolution in PSII membranes more effectively than the reduction of artificial electron acceptor 2,6-dichlorophenolindophenol, i.e. the action of these compounds is accompanied by decoupling of these processes in OEC. Similarity of effects observed after Ca2 + extraction and F−, CH3COO− or NH4Cl treatments suggests that these compounds can inactivate function of Ca2 +. Such inactivation could originate from disturbance of the network of functionally active hydrogen bonds around OEC formed with participation of Ca2 +. This inhibition effect is observed in the region of low concentration of inhibitors. Increasing of inhibitor concentration is accompanied by appearance of other sites of inhibition.