Characteristics of Iodide Activation and Inhibition of Oxygen Evolution by Photosystem II

Abstract

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 K(M) 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 K(i) > 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 S(2) 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.

Fluoride inhibition of photosystem II and the effect of removal of the PsbQ subunit

“…We have in the past observed complete loss of the g = 4.1 signal, as well as the multiline signal, in PSII-PsbP/PsbQ that had been washed to remove Ca 2? and Cl - (Bryson et al 2005). We have also observed complete loss of the g = 4.1 signal in PSII-PsbP/PsbQ in which Cl -had been replaced with F -by successive washes (unpublished observation).…”

EPR spectroscopy of the manganese cluster of photosystem II

“…Cl -is required for formation of the multiline signal and the signal height has been found to be correlated with oxygen evolution activity (Yachandra et al 1986). Other activating anions (Br -, NO 3 -, I -) have also been found to support multiline signal formation (Damoder et al 1986;Yachandra et al 1986;Ono et al 1987;Lindberg and Andréasson 1996;Olesen and Andréasson 2003;Bryson et al 2005). On the other hand, Cl -depletion or substitution of Cl -with anions such as F -or N 3 -, which are competitors of Cl -activation, prevent multiline signal formation (Fig.…”

Current status of the role of Cl− ion in the oxygen-evolving complex

“…In addition, NO 3 -was found to slow the S 3 -S 0 transition (Wincencjusz et al 1998). In the absence of Cl -, I -acts as an activator and then as an inhibitor of O 2 evolution activity in thylakoids, intact PSII and PsbP-and PsbQ-depleted PSII from spinach (Kelly and Izawa 1978;Olesen and André asson 2003;Bryson et al 2005). In the presence of Cl -, I -was found to act as an uncompetitive inhibitor of Cl -activation (Bryson et al 2005).…”