Photosystem II (PSII), the light-absorbing complex of photosynthesis that evolves oxygen, requires chloride for activation of the oxygen evolving complex (OEC). In this study, fluoride was characterized as an inhibitor of Cl−-activated oxygen evolution in higher plant PSII. It was confirmed to be primarily a competitive inhibitor in intact PSII, with Cl−-competitive inhibition constant Ki = 2 mM and uncompetitive inhibition constant \documentclass[12pt]{minimal}
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\begin{document}$$ {\text{K}}_{\text{i}}^{\prime } $$\end{document} = 79 mM. A pH dependence study showed that fluoride inhibition was more pronounced at lower pH values. In order to determine the location of the fluoride effect, PSII preparations lacking various amounts of the PsbQ subunit were prepared. The competitive F− inhibition constant and the Michaelis constant for Cl− activation increased with loss of the PsbQ subunit, while the uncompetitive F− inhibition constant was relatively insensitive to loss of PsbQ. The S2 state EPR signals from PSII lacking PsbQ responded to Ca2+ and Cl− removal and to F− treatment similar to intact PSII, with enhancement of the g = 4.1 signal and suppression of the multiline signal, but the effects were more pronounced in PSII lacking PsbQ. Together, these results support the interpretation that the PsbQ subunit has a role in retaining anions within the OEC.
