Effects of photoinhibition on the iron-quinone electron acceptor complex of oxygen-evolving photosystem II have been studied using low-temperature EPR and Mossbauer spectroscopy. Photoinhibition of spinach photosystem II membrane particles at 4 °C decreases the EPR signal arising from the interaction of QA- with Fe2+ to 30% in 90 min under our conditions. The free radical EPR signal from QA- induced by cyanide treatment of the iron [Sanakis, Y., et al. (1994) Biochemistry 33, 9922-9928] declines with the same kinetics as the QA-Fe2+ EPR signal. In contrast, Fe2+ is present in about 70% of the centers after 90 min of photoinhibition, as shown by its EPR-detected interaction with NO and by its Mossbauer absorption. Complete oxidation of this Fe2+ population to Fe3+ by ferricyanide is possible only in the presence of glycolate, which lowers the redox potential of the Fe3+/Fe2+ couple. In a fraction of PSII centers, which reach 30% after 90 min of photoinhibition, the iron cannot be detected. It is concluded that photoinhibition of oxygen-evolving photosystem II affects both QA and Fe2+. However, the photoinhibitory impairment of the QA redox functioning precedes the modification of the non-heme iron. In a considerable portion of the photoinhibited centers, which do not have functional QA, the non-heme iron is still present and redox active, but its redox potential is increased relative to that in the normal centers. This is probably due to a minor modification of the bicarbonate ligation site. In the rest of the photoinhibited centers, the loss of QA function is accompanied by a severe modification or release of the non-heme iron.
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