The structure of the Fe(II) site in the “ferroquinone” electron acceptor complex of bacterial reaction centers has been studied by using electron paramagnetic resonance (EPR) of reaction centers prepared from Rhodobacter sphaeroides cells in which the metal is biosynthetically replaced by Cu(II) during growth. In the dark, a typical Cu(II) spectrum is observable having axial symmetry with g = 2.19 and g = 2.05 and with resolved copper hyperfine peaks (A = 0.0203 cm-1 (199 G); A = 0.0019 cm-1). Comparison of the g values and copper hyperfine splittings with those of structurally characterized copper complexes indicates that the ligand geometry in the reaction center is primarily tetragonal with little distortion away from a coplanar set of four nitrogen ligands. All of the peaks of the Cu(II) spectrum show additional ligand hyperfine splitting, arising from coupling to four nitrogen atoms that are indistinguishable an(ii) = 0.00145 cm-1 (14 G); AN(±) = 0.0017 cm-1(18 G)]. This structure is observed at all pH values between 8.0 and 10.0. These nitrogens are likely to be due to four histidine (imidazole) ligands to Cu in the Fe binding site, analogous to the histidine ligands to Fe found in the Rhodopseudomonas viridis crystal structure [Deisenhofer, J., Epp, O., Miki, K., Huber, R., & Michel, H. (1985) Nature (London) 318, 618]. All of the spectral features were satisfactorily simulated by using an appropriate spin Hamiltonian to extract the spectroscopic factors reported here. This result is in contrast to a recent report [Feher, G., Isaacson, R.A., Debus, R.J., & Okamura, M.Y. (1986) Biophys. J. 49, 585a] in which reaction centers were extracted of iron and reconstituted with copper. The EPR spectrum of the copper in this case exhibits greatly reduced hyperfine structure from only three indistinguishable nitrogens, indicating loss or distortion of one imidazole ligand. Furthermore, the substantially reduced copper hyperfine splitting [A1 = 143 G (0.0154 cm-1); A = unresolved] indicates that the local symmetry about Cu(II) is significantly reduced from the native tetragonal symmetry for the biosynthetically incorporated Cu(II) site. These changes appear to have little influence on the electron-transfer rate between the primary and secondary quinones which are directly hydrogen bonded to two of the coordinated imidazoles.
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