TY - JOUR
T1 - Semiquinone-iron complex of Photosystem II
T2 - EPR signals assigned to the low-field edge of the ground state doublet of QA •-Fe2+ and QB•-Fe 2+
AU - Sedoud, Arezki
AU - Cox, Nicholas
AU - Sugiura, Miwa
AU - Lubitz, Wolfgang
AU - Boussac, Alain
AU - Rutherford, A. William
PY - 2011/7/12
Y1 - 2011/7/12
N2 - The quinone-iron complex of the electron acceptor complex of Photosystem II was studied by EPR spectroscopy in Thermosynechococcus elongatus. New g ∼ 2 features belonging to the EPR signal of the semiquinone forms of the primary and secondary quinone, i.e., QA•-Fe2+ and QB•-Fe2+, respectively, are reported. In previous studies, these signals were missed because they were obscured by the EPR signal arising from the stable tyrosyl radical, TyrD•. When the TyrD• signal was removed, either by chemical reduction or by the use of a mutant lacking TyrD, the new signals dominated the spectrum. For QA•-Fe2+, the signal was formed by illumination at 77 K or by sodium dithionite reduction in the dark. For Q B•-Fe2+, the signal showed the characteristic period-of-two variations in its intensity when generated by a series of laser flashes. The new features showed relaxation characteristics comparable to those of the well-known features of the semiquinone-iron complexes and showed a temperature dependence consistent with an assignment to the low-field edge of the ground state doublet of the spin system. Spectral simulations are consistent with this assignment and with the current model of the spin system. The signal was also present in QB •-Fe2+ in plant Photosystem II, but in plants, the signal was not detected in the QA•-Fe2+ state.
AB - The quinone-iron complex of the electron acceptor complex of Photosystem II was studied by EPR spectroscopy in Thermosynechococcus elongatus. New g ∼ 2 features belonging to the EPR signal of the semiquinone forms of the primary and secondary quinone, i.e., QA•-Fe2+ and QB•-Fe2+, respectively, are reported. In previous studies, these signals were missed because they were obscured by the EPR signal arising from the stable tyrosyl radical, TyrD•. When the TyrD• signal was removed, either by chemical reduction or by the use of a mutant lacking TyrD, the new signals dominated the spectrum. For QA•-Fe2+, the signal was formed by illumination at 77 K or by sodium dithionite reduction in the dark. For Q B•-Fe2+, the signal showed the characteristic period-of-two variations in its intensity when generated by a series of laser flashes. The new features showed relaxation characteristics comparable to those of the well-known features of the semiquinone-iron complexes and showed a temperature dependence consistent with an assignment to the low-field edge of the ground state doublet of the spin system. Spectral simulations are consistent with this assignment and with the current model of the spin system. The signal was also present in QB •-Fe2+ in plant Photosystem II, but in plants, the signal was not detected in the QA•-Fe2+ state.
UR - http://www.scopus.com/inward/record.url?scp=79959990120&partnerID=8YFLogxK
U2 - 10.1021/bi200313p
DO - 10.1021/bi200313p
M3 - Article
SN - 0006-2960
VL - 50
SP - 6012
EP - 6021
JO - Biochemistry
JF - Biochemistry
IS - 27
ER -