Orientation dependence of the Fe²⁺-NO and Fe³⁺ EPR signals associated with the non-heme iron of Photosystem II

We have examined the orientation dependence of two EPR signals associated with the iron of the ferroquinone complex of Photosystem II. The first signal with g values (4.09, 3.95 and approx. 2.0) results from the reversible interaction of the ferrous ion with NO (Petrouleas et al. (1990) Biochim. Biophys. Acta 1015, 131-140). Studies of oriented spinach BBY membranes treated with NO show that g_x lies on the membrane plane, g_y is oriented at 30° and g_z is oriented at 60° with respect to the membrane plane. The latter, most likely, indicates the approximate direction of the Fe-NO bond. Since NO displaces CO₂/HCO^-₃ upon binding (Diner et al. (1990) Biochim. Biophys. Acta 1015, 141-149), the latter physiological ligand is probably well above the membrane plane containing the iron. This places CO₂/HCO^-₃ at an approximately homologous position with the bacterial glutamate ligand (Deisenhofer and Michel (1989) EMBO J. 8, 2149-2169); it is not known, however, whether CO₂/HCO^-₃ binds as a monodentate or bidentate ligand. The second signal with characteristic g values in the region 5.6-8.1 results from the oxidized non-heme iron (Fe³⁺) [3]. The orientation results show that the g_x = 8.1 and g_z = 5.6 resonances have both maxima in the plane of the membrane. The g_y axis is, therefore, perpendicular to the plane of the membrane and runs along the homologous direction of the bacterial twofold symmetry axis. Based on the strong effect that molecules bound at the Q_B binding site have on the x-y plane anisotropy ( E D) but not on the axial field strength, D, and assuming basic similarities with the bacterial reaction center, it is suggested that, the x-y plane is defined by the two oxygen ligands and the two nitrogens from the histidine imidazoles which have contacts with the Q_A and Q_B site. The g_z-axis lies then along the N-Fe-N direction defined by the nitrogen ligands from the other two histidines. An unusual finding is that the process of membrane orientation (slow drying of the membranes) induces an axial signal, g_xy = 6, in a fraction of centers. The axial plane of the latter is perpendicular to the membrane in agreement with the orientation results of the more rhombic signals. A possible arrangement of the nitrogen and oxygen ligands in the equatorial plane (vertical to the membrane) as to impose pseudo-C₃ symmetry is discussed.