Substitution of cu2+ in the reaction center diquinone electron acceptor complex of rhodobacter sphaeroides: Determination of the metal-ligand coordination

S. K. Buchanan, G Charles Dismukes

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Abstract

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(∥) = 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, 58 5a] 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 [A = 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.

Original languageEnglish
Pages (from-to)5049-5055
Number of pages7
JournalBiochemistry
Volume26
Issue number16
Publication statusPublished - 1987

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Rhodobacter sphaeroides
Copper
Substitution reactions
Metals
Electrons
Ligands
Nitrogen
Electron Spin Resonance Spectroscopy
Histidine
Paramagnetic resonance
Rhodopseudomonas
Imidazoles
Hamiltonians
Quinones
Hydrogen
Iron
Crystal structure
Binding Sites
Atoms
Geometry

ASJC Scopus subject areas

  • Biochemistry

Cite this

@article{e047eed14e854197ba8cb6e881e2afd2,
title = "Substitution of cu2+ in the reaction center diquinone electron acceptor complex of rhodobacter sphaeroides: Determination of the metal-ligand coordination",
abstract = "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(∥) = 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, 58 5a] 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 [A∥ = 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.",
author = "Buchanan, {S. K.} and Dismukes, {G Charles}",
year = "1987",
language = "English",
volume = "26",
pages = "5049--5055",
journal = "Biochemistry",
issn = "0006-2960",
publisher = "American Chemical Society",
number = "16",

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TY - JOUR

T1 - Substitution of cu2+ in the reaction center diquinone electron acceptor complex of rhodobacter sphaeroides

T2 - Determination of the metal-ligand coordination

AU - Buchanan, S. K.

AU - Dismukes, G Charles

PY - 1987

Y1 - 1987

N2 - 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(∥) = 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, 58 5a] 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 [A∥ = 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.

AB - 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(∥) = 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, 58 5a] 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 [A∥ = 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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