Characteristics of iodide activation and inhibition of oxygen evolution by photosystem II

Oxygen evolution by photosystem II (PSII) is activated by chloride and other monovalent anions. In this study, the effects of iodide on oxygen evolution activity were investigated using PSII-enriched membrane fragments from spinach. In the absence of Cl-, the dependence of oxygen evolution activity on I- concentration showed activation followed by inhibition in both intact PSII and NaCl-washed PSII, which lacked the PsbP and PsbQ subunits. Using a substrate inhibition model, the range of values of the Michaelis constant KM in intact PSII (0.5-1.5 mM) was smaller than that in NaCl-washed PSII (1.5-5 mM), whereas values of the inhibition constant K-I in intact PSII (9-17 mM) were larger than those in NaCl-washed PSII (1-4 mM). Studies of I- inhibition of Cl--activated oxygen evolution in intact PSII revealed that I- was primarily an uncompetitive inhibitor, with uncompetitive constant K-i' = 37 mM and Cl--competitive constant Ki > 200 mM. This result indicated that the activating Cl- must be bound for inhibition to take place, which is consistent with the substrate inhibition model for I- activation. The S-2 state multiline and g = 4.1 EPR signals in NaCl-washed PSII were examined in the presence of 3 and 25 mM NaI, corresponding to I--activated and I--inhibited conditions, respectively. The two S2 state signals were observed at both I- concentrations, indicating that I- substitutes for Cl- in formation of the signals and that advancement to the S-2 state was not prevented by high I- concentrations. A model is presented that incorporates the results of this study, including the action of both chloride and iodide.