Characterization of fluoride inhibition in photosystem II lacking extrinsic PsbP and PsbQ subunits

Photosynthetic oxygen evolution occurs through the oxidation of water at a catalytic Mn₄CaO₅ cluster in photosystem II and is promoted by chloride, which binds at two sites near the Mn₄CaO₅ cluster. Fluoride is a competitive inhibitor of chloride activation, but study of its effects is complicated by the possibility that it may form an insoluble CaF₂ complex. In this study, the effects of fluoride were studied using PSII lacking the PsbP and PsbQ subunits, which help to regulate the requirements for the inorganic cofactors Ca²⁺ and Cl^-. In this preparation, which allows easy exchange of ions, it was found that F^- does not directly remove Ca²⁺ even when catalytic turnovers take place, suggesting that fluoride is not able to access the inner coordination sphere of Ca²⁺. By monitoring the loss in O₂ evolution activity, the dissociation constant of F^- was estimated to be about 1 mM in intact PSII, consistent with previous studies, and about 77 mM in PSII lacking the extrinsic subunits. The significantly higher value for PSII lacking PsbP and PsbQ is consistent with results for other ions. The effects of F^- on electron transfer to Tyr Z was also studied and found to show similar trends in PSII with and without the two extrinsic subunits, but with a more pronounced effect in PSII lacking the extrinsic subunits. These results indicate that in PSII lacking PsbP and PsbQ, fluoride does not directly interact with or remove Ca²⁺ and inhibits O₂ evolution in a manner comparable to PSII with the extrinsic subunits intact.