Effects of photoinhibition on the QA-Fe2+ complex of photosystem II studied by EPR and Mössbauer spectroscopy.
Effects of photoinhibition on the iron-quinone electron acceptor complex of oxygen-evolving photosystem II have been studied using low-temperature EPR and Mössbauer spectroscopy. Photoinhibition of spinach photosystem II membrane particles at 4 degrees 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 Mössbauer 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.(ABSTRACT TRUNCATED AT 250 WORDS)
Vass
I
I
Institute of Plant Biology, Biological Research Center, Szeged, Hungary.
Sanakis
Y
Y
Spetea
C
C
Petrouleas
V
V
D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D005292:Ferricyanides / Q000737:chemistry; D005296:Ferrous Compounds / Q000037:antagonists & inhibitors* / Q000737:chemistry; D005561:Formates / Q000494:pharmacology; D006016:Glycolates / Q000494:pharmacology; D007700:Kinetics; D008027:Light; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D045332:Photosystem II Protein Complex; D011809:Quinones / Q000037:antagonists & inhibitors* / Q000737:chemistry; D015204:Spectroscopy, Mossbauer; D018724:Spinacia oleracea / Q000737:chemistry
D005292:Ferricyanides; D005296:Ferrous Compounds; D005561:Formates; D006016:Glycolates; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D011809:Quinones; C031149:glycolic acid; C030544:formic acid; C007931:hexacyanoferrate III