title: Regulation of the primary quinone binding conformation by the H subunit in reaction centers from Rhodobacter sphaeroides.
abstract: Unlike photosystem II (PSII) in higher plants, bacterial photosynthetic reaction centers (bRCs) from Proteobacteria have an additional peripheral membrane subunit "H". The H subunit is necessary for photosynthetic growth, but can be removed chemically in vitro. The remaining LM dimer retains its activity to perform light-induced charge separation. Here we investigate the influence of the H subunit on interactions between the primary semiquinone and the protein matrix, using a combination of site-specific isotope labeling, pulsed electron paramagnetic resonance (EPR), and density functional theory (DFT) calculations. The data reveal substantially weaker binding interactions between the primary semiquinone and the LM dimer than observed for the intact bRC; the amount of electron spin transferred to the nitrogen hydrogen bond donors is significantly reduced, the methoxy groups are more free to rotate, and the spectra indicate a heterogeneous mixture of bound semiquinone states. These results are consistent with a loosening of the primary quinone binding pocket in the absence of the H subunit.
authors.affiliation: Department of Biochemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
authors.forename: Chang
authors.initials: C
authors.lastname: Sun
authors.affiliation: Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.|Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
authors.forename: Alexander T
authors.initials: AT
authors.lastname: Taguchi
authors.affiliation: School of Chemistry, University of Manchester , Manchester M13 9PL, U.K.
authors.forename: Nathan J
authors.initials: NJ
authors.lastname: Beal
authors.affiliation: School of Chemistry, University of Manchester , Manchester M13 9PL, U.K.
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: O'Malley
authors.affiliation: Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
authors.forename: Sergei A
authors.initials: SA
authors.lastname: Dikanov
authors.affiliation: Department of Biochemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.|Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
authors.forename: Colin A
authors.initials: CA
authors.lastname: Wraight
mesh_terms: D016227:Benzoquinones / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D006860:Hydrogen Bonding; D008958:Models, Molecular; D045332:Photosystem II Protein Complex / Q000737:chemistry; D011487:Protein Conformation; D012242:Rhodobacter sphaeroides / Q000737:chemistry*
chemical_list: D016227:Benzoquinones; D045332:Photosystem II Protein Complex; C004532:quinone
keywords: DFT; hyperfine coupling; isotope labeling; pulsed EPR; site-directed mutagenesis; ubisemiquinone