title: Activating anions that replace Cl- in the O2-evolving complex of photosystem II slow the kinetics of the terminal step in water oxidation and destabilize the S2 and S3 states.
abstract: Photosystem II, the multisubunit protein complex that oxidizes water to O2, requires the inorganic cofactors Ca2+ and Cl- to exhibit optimal activity. Chloride can be replaced functionally by a small number of anionic cofactors (Br-, NO3-, NO2-, I-), but among these anions, only Br- is capable of restoring rates of oxygen evolution comparable to those observed with Cl-. UV absorption difference spectroscopy was utilized in the experiments described here as a probe to monitor donor side reactions in photosystem II in the presence of Cl- or surrogate anions. The rate of the final step of the water oxidation cycle was found to depend on the activating anion bound at the Cl- site, but the kinetics of this step did not limit the light-saturated rate of oxygen evolution. Instead, the lower oxygen evolution rates supported by surrogate anions appeared to be correlated with an instability of the higher oxidation states of the oxygen-evolving complex that was induced by addition of these anions. Reduction of these states takes place not only with I- but also with NO2- and to a lesser extent even with NO3- and Br- and is not related to the ability of these anions to bind at the Cl- binding site. Rather, it appears that these anions can attack higher oxidation states of the oxygen evolving complex from a second site that is not shielded by the extrinsic 17 and 23 kDa polypeptides and cause a one-electron reduction. The decrease of the oxygen evolution rate may result from accumulated damage to the reaction center protein by the one-electron oxidation product of the anion.
authors: Biophysics Department, Huygens Laboratory, Leiden University, P.O. Box 9504, NL-2300 RA Leiden, The Netherlands.
authors: H
authors: H
authors: Wincencjusz
authors: C F
authors: CF
authors: Yocum
authors: H J
authors: HJ
authors: van Gorkom
mesh_terms: D000838:Anions / Q000737:chemistry*; D001665:Binding Sites; D002712:Chlorides / Q000737:chemistry; D004579:Electron Transport; D007700:Kinetics; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism; D045332:Photosystem II Protein Complex; D013056:Spectrophotometry, Ultraviolet; D018724:Spinacia oleracea; D014867:Water / Q000737:chemistry
chemical_list: D000838:Anions; D002712:Chlorides; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D014867:Water; D010100:Oxygen