Two Plastoquinone A Molecules Are Required for Photosystem II Activity: Analysis in Hexane-Extracted Photosystem II Particles

Prenylquinones were extracted with hexane from lyophilized oxygen-evolving photosystem II particles prepared from spinach chloroplasts. Determination by high performance liquid chromatography showed that two molecules of plastoquinone A remained per reaction center after the extraction, in contrast to the presence of three to four plastoquinone A molecules before the extraction. Electron transfer from water to phenyl-p-quinone was not inhibited by the extraction. Measurement of EPR signal II and microsecond chlorophyll fluorescence kinetics showed that hexane did not extract quinones which were acting as the secondary electron donor (Z) and the primary electron acceptor (QA) in photosystem II. These results, as well as the effect of quinone extraction on oxygen evolution, indicate that two molecules of plastoquinone A acting as Z and QA are essential for the activity of photosystem II. An artificial donor phenyl-p-quinone probably accepts electron from QA at the same site as the intrinsic secondary electron acceptor (QB). QA and Z seem to be surrounded by special microenvironments which differ from that of bulk quinones, and are resistant to hexane treatment.