title: Hydrogen bonds between nitrogen donors and the semiquinone in the Qi-site of the bc1 complex.
abstract: The ubisemiquinone stabilized at the Qi-site of the bc1 complex of Rhodobacter sphaeroides forms a hydrogen bond with a nitrogen from the local protein environment, tentatively identified as ring N from His-217. The interactions of 14N and 15N have been studied by X-band (approximately 9.7 GHz) and S-band (3.4 GHz) pulsed EPR spectroscopy. The application of S-band spectroscopy has allowed us to determine the complete nuclear quadrupole tensor of the 14N involved in H-bond formation and to assign it unambiguously to the Nepsilon of His-217. This tensor has distinct characteristics in comparison with H-bonds between semiquinones and Ndelta in other quinone-processing sites. The experiments with 15N showed that the Nepsilon of His-217 was the only nitrogen carrying any considerable unpaired spin density in the ubiquinone environment, and allowed calculation of the isotropic and anisotropic couplings with the Nepsilon of His-217. From these data, we could estimate the unpaired spin density transferred onto 2s and 2p orbitals of nitrogen and the distance from the nitrogen to the carbonyl oxygen of 2.38+/-0.13A. The hyperfine coupling of other protein nitrogens with semiquinone is <0.1 MHz. This did not exclude the nitrogen of the Asn-221 as a possible hydrogen bond donor to the methoxy oxygen of the semiquinone. A mechanistic role for this residue is supported by kinetic experiments with mutant strains N221T, N221H, N221I, N221S, N221P, and N221D, all of which showed some inhibition but retained partial turnover.
authors.affiliation: Department of Veterinary Clinical Medicine, University of Illinois, Urbana, Illinois 61801, USA. dikanov@uiuc.edu
authors.forename: Sergei A
authors.initials: SA
authors.lastname: Dikanov
authors.forename: J Todd
authors.initials: JT
authors.lastname: Holland
authors.forename: Burkhard
authors.initials: B
authors.lastname: Endeward
authors.forename: Derrick R J
authors.initials: DR
authors.lastname: Kolling
authors.forename: Rimma I
authors.initials: RI
authors.lastname: Samoilova
authors.forename: Thomas F
authors.initials: TF
authors.lastname: Prisner
authors.forename: Antony R
authors.initials: AR
authors.lastname: Crofts
mesh_terms: D019943:Amino Acid Substitution; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics; D016227:Benzoquinones / Q000737:chemistry*; D001665:Binding Sites; D014450:Electron Transport Complex III / Q000737:chemistry* / Q000235:genetics; D006860:Hydrogen Bonding; D008958:Models, Molecular; D020125:Mutation, Missense; D020836:Protein Structure, Quaternary; D012242:Rhodobacter sphaeroides / Q000201:enzymology* / Q000235:genetics; D013329:Structure-Activity Relationship; D014961:X-Ray Diffraction
chemical_list: D001426:Bacterial Proteins; D016227:Benzoquinones; C025232:semiquinone radicals; D014450:Electron Transport Complex III