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 Ca²⁺ 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 °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 ∼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 pK_a of 4.5. Low pH also affected the induction of the so-called split radical EPR signal from the S₂Y_Z^• state that is induced in Ca²⁺-depleted PSII membranes because of an inability of Y_Z^• 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 pK_a 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 S₂Y_Z^• 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 Y_Z^• 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 Y_Z during oxygen evolution is discussed.