pH-dependence of the four steps in the S-cycle in Photosystem II

Abstract

We have determined the pH dependence individually for all four redox transitions in the oxygen evolving complex of Photosystem II by EPR spectroscopy. In these experiments, OEC is advanced to the appropriate S-state at normal pH. Then the pH is rapidly changed and a new flash is given. The samples are analyzed by EPR spectroscopy before and after the flash. The S-state advancement at different pH¿s is determined by measurements of the decrease or increase the EPR signals from the different S-state populations after the last flash. In most cases we followed the changes at S0 (S3 ® S0, S0 ® S1) or S2 populations (S1 ® S2, S2 ® S3) determined by the S0 and S2 multiline EPR signal intensities. Data evaluation of the individual S-transitions showed that all, except the S1 ® S2 transition, are blocked at low pH. The pKs for the inhibition is 4.3-4.7. We have also observed that high pH blocks the S2 ® S3 and S3 ® S0 transitions with pKs of 9.2 and 8.0, respectively. This suggests that the decrease of the steady state oxygen evolution at high pH, which occurs with a pH 7.7-8.0, is dominated by the inhibition of the S3 ® S0 transition. The results are in good agreement with a model which suggests that the redox potentials of YZ/YZ· and the Si/Si+1 states are directly affected by the pH in the bulk medium.