Effect of different methods of Ca extraction from PSII oxygen-evolving complex on the Q oxidation kinetics.

Lumenal extrinsic proteins PsbO, PsbP, and PsbQ of photosystem II (PSII) protect the catalytic cluster MnCaO of oxygen-evolving complex (OEC) from the bulk solution and from soluble compounds in the surrounding medium. Extraction of PsbP and PsbQ proteins by NaCl-washing together with chelator EGTA is followed also by the depletion of Ca cation from OEC. In this study, the effects of PsbP and PsbQ proteins, as well as Ca extraction from OEC on the kinetics of the reduced primary electron acceptor (Q) oxidation, have been studied by fluorescence decay kinetics measurements in PSII membrane fragments. We found that in addition to the impairment of OEC, removal of PsbP and PsbQ significantly slows the rate of electron transfer from Q to the secondary quinone acceptor Q. Electron transfer from Q to Q in photosystem II membranes with an occupied Q site was slowed down by a factor of 8. However, addition of EGTA or CaCl to NaCl-washed PSII did not change the kinetics of fluorescence decay. Moreover, the kinetics of Q oxidation by Q in Ca-depleted PSII membranes obtained by treatment with citrate buffer at pH 3.0 (such treatment keeps all extrinsic proteins in PSII but extracts Ca from OEC) was not changed. The results obtained indicate that the effect of NaCl-washing on the Q to Q electron transport is due to PsbP and PsbQ extrinsic proteins extraction, but not due to Ca depletion.