title: 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.
authors.affiliation: Department of Chemistry and Biochemistry, University of North Carolina-Greensboro, Greensboro, North Carolina 27402, USA.
authors.forename: David I
authors.initials: DI
authors.lastname: Bryson
authors.forename: Ninad
authors.initials: N
authors.lastname: Doctor
authors.forename: Rachelle
authors.initials: R
authors.lastname: Johnson
authors.forename: Sergei
authors.initials: S
authors.lastname: Baranov
authors.forename: Alice
authors.initials: A
authors.lastname: Haddy
mesh_terms: D001665:Binding Sites; D001667:Binding, Competitive; D002712:Chlorides / Q000737:chemistry / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D007454:Iodides / Q000737:chemistry* / Q000378:metabolism; D007700:Kinetics; D008956:Models, Chemical; D010100:Oxygen / Q000037:antagonists & inhibitors* / Q000737:chemistry* / Q000378:metabolism; D045332:Photosystem II Protein Complex / Q000037:antagonists & inhibitors* / Q000737:chemistry* / Q000378:metabolism; D012965:Sodium Chloride / Q000737:chemistry; D012974:Sodium Iodide / Q000737:chemistry; D018724:Spinacia oleracea
chemical_list: D002712:Chlorides; D007454:Iodides; D045332:Photosystem II Protein Complex; D012965:Sodium Chloride; D012974:Sodium Iodide; D010100:Oxygen