pH Dependence of the Donor Side Reactions in Ca2+-Depleted Photosystem II

• We have studied how low pH affects the water-oxidizing complex in Photosystem II when depleted of the essential Ca2+ ion cofactor. For these samples, it was found that the EPR signal from the YZ radical decays faster at low pH than at high pH. At 20 C, YZ decays with biphasic kinetics. At pH 6.5, the fast phase encompasses about 65% of the amplitude and has a lifetime of ~0.8 s, while the slow phase has a lifetime of ~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 ~0.3 s. The kinetic changes occurred with an approximate pKa of 4.5. Low pH also affected the induction of the so-called split radical EPR signal from the S2YZ state that is induced in Ca2+-depleted PSII membranes because of an inability of YZ to oxidize the S2 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 pKa of 4.5. In the same samples, the stable multiline signal from the S2 state, which is modified by the removal of Ca2+, was decreased by the illumination to the same extent at all pHs. It is proposed that decreased induction of the S2YZ state at lower pH was not due to inability to oxidize the modified S2 state induced by the Ca2+ depletion. Instead, we propose that the low pH makes YZ able to oxidize the S2 state, making the S2 S3 transition available in Ca2+-depleted PSII. Implications of these results for the catalytic role of Ca2+ and the role of proton transfer between the Mn cluster and YZ during oxygen evolution is discussed.

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NATURVETENSKAP  -- Biologi -- Biokemi och molekylärbiologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Biochemistry and Molecular Biology (hsv//eng)

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