title: pH dependence of the donor side reactions in Ca2+-depleted photosystem II.
abstract: We have studied how low pH affects the water-oxidizing complex in Photosystem II when depleted of the essential Ca(2+) ion cofactor. For these samples, it was found that the EPR signal from the Y(Z)(*) radical decays faster at low pH than at high pH. At 20 degrees C, Y(Z)(*) decays with biphasic kinetics. At pH 6.5, the fast phase encompasses about 65% of the amplitude and has a lifetime of approximately 0.8 s, while the slow phase has a lifetime of approximately 22 s. At pH 3.9, the kinetics become totally dominated by the fast phase, with more than 90% of the signal intensity operating with a lifetime of approximately 0.3 s. The kinetic changes occurred with an approximate pK(a) of 4.5. Low pH also affected the induction of the so-called split radical EPR signal from the S(2)Y(Z)(*) state that is induced in Ca(2+)-depleted PSII membranes because of an inability of Y(Z)(*) to oxidize the S(2) state. At pH 4.5, about 50% of the split signal was induced, as compared to the amplitude of the signal that was induced at pH 6.5-7, using similar illumination conditions. Thus, the split-signal induction decreased with an apparent pK(a) of 4.5. In the same samples, the stable multiline signal from the S(2) state, which is modified by the removal of Ca(2+), was decreased by the illumination to the same extent at all pHs. It is proposed that decreased induction of the S(2)Y(Z)(*) state at lower pH was not due to inability to oxidize the modified S(2) state induced by the Ca(2+) depletion. Instead, we propose that the low pH makes Y(Z)(*) able to oxidize the S(2) state, making the S(2) --> S(3) transition available in Ca(2+)-depleted PSII. Implications of these results for the catalytic role of Ca(2+) and the role of proton transfer between the Mn cluster and Y(Z) during oxygen evolution is discussed.
authors.affiliation: Department of Biochemistry, Centre for Chemistry and Chemical Engineering P.O. Box 124, Lund University, Sweden.
authors.forename: Stenbjörn
authors.initials: S
authors.lastname: Styring
authors.forename: Yashar
authors.initials: Y
authors.lastname: Feyziyev
authors.forename: Fikret
authors.initials: F
authors.lastname: Mamedov
authors.forename: Warwick
authors.initials: W
authors.lastname: Hillier
authors.forename: Gerald T
authors.initials: GT
authors.lastname: Babcock
mesh_terms: D002118:Calcium / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals; D006863:Hydrogen-Ion Concentration; D008954:Models, Biological; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism*; D045332:Photosystem II Protein Complex; D011522:Protons; D018724:Spinacia oleracea / Q000378:metabolism; D014867:Water / Q000737:chemistry
chemical_list: D005609:Free Radicals; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D011522:Protons; D014867:Water; D002118:Calcium