EFFECTS OF PHOTOINHIBITION ON THE Q(A)(-)FE(2-II STUDIED BY EPR AND MOSSBAUER-SPECTROSCOPY() COMPLEX OF PHOTOSYSTEM)

Effects of photoinhibition on the iron-quinone electron acceptor compl ex of oxygen-evolving photosystem II have been studied using low-tempe rature EPR and Mossbauer spectroscopy. Photoinhibition of spinach phot osystem II membrane particles at 4 degrees C decreases the EPR signal arising from the interaction of Q(A)(-) with Fe2+ to 30% in 90 min und er our conditions. The free radical EPR signal from Q(A)(-) induced by cyanide treatment of the iron [Sanakis, Y., et al. (1994) Biochemistr y 33, 9922-9928] declines with the same kinetics as the Q(A)(-)Fe(2+) EPR signal. In contrast, Fe2+ is present in about 70% of the centers a fter 90 min of photoinhibition, as shown by its EPR-detected interacti on with NO and by its Mossbauer absorption. Complete oxidation of this Fe2+ population to Fe3+ by ferricyanide is possible only in the prese nce of glycolate, which lowers the redox potential of the Fe3+/Fe2+ co uple. In a fraction of PSII centers, which reach 30% after 90 min of p hotoinhibition, the iron cannot be detected. It is concluded that phot oinhibition of oxygen-evolving photosystem II affects both Q(A) and Fe 2+. However, the photoinhibitory impairment of the Q(A) redox function ing precedes the modification of the non-heme iron. In a considerable portion of the photoinhibited centers, which do not have functional Q( A), the non-heme iron is still present and redox active, but its redox potential is increased relative to that in the normal centers. This i s probably due to a minor modification of the bicarbonate ligation sit e. In the rest of the photoinhibited centers, the loss of Q(A) functio n is accompanied by a severe modification or release of the non-heme i ron.