title: Multiple scattering x-ray absorption studies of Zn2+ binding sites in bacterial photosynthetic reaction centers.
abstract: Binding of transition metal ions to the reaction center (RC) protein of the photosynthetic bacterium Rhodobacter sphaeroides has been previously shown to slow light-induced electron and proton transfer to the secondary quinone acceptor molecule, Q(B). On the basis of x-ray diffraction at 2.5 angstroms resolution a site, formed by AspH124, HisH126, and HisH128, has been identified at the protein surface which binds Cd(2+) or Zn(2+). Using Zn K-edge x-ray absorption fine structure spectroscopy we report here on the local structure of Zn(2+) ions bound to purified RC complexes embedded into polyvinyl alcohol films. X-ray absorption fine structure data were analyzed by combining ab initio simulations and multiparameter fitting; structural contributions up to the fourth coordination shell and multiple scattering paths (involving three atoms) have been included. Results for complexes characterized by a Zn to RC stoichiometry close to one indicate that Zn(2+) binds two O and two N atoms in the first coordination shell. Higher shell contributions are consistent with a binding cluster formed by two His, one Asp residue, and a water molecule. Analysis of complexes characterized by approximately 2 Zn ions per RC reveals a second structurally distinct binding site, involving one O and three N atoms, not belonging to a His residue. The local structure obtained for the higher affinity site nicely fits the coordination geometry proposed on the basis of x-ray diffraction data, but detects a significant contraction of the first shell. Two possible locations of the second new binding site at the cytoplasmic surface of the RC are proposed.
authors.affiliation: Dipartimento di Fisica, Università di Bologna, Bologna, Italy.
authors.forename: Lisa
authors.initials: L
authors.lastname: Giachini
authors.forename: Francesco
authors.initials: F
authors.lastname: Francia
authors.forename: Antonia
authors.initials: A
authors.lastname: Mallardi
authors.forename: Gerardo
authors.initials: G
authors.lastname: Palazzo
authors.forename: Emilio
authors.initials: E
authors.lastname: Carpenè
authors.forename: Federico
authors.initials: F
authors.lastname: Boscherini
authors.forename: Giovanni
authors.initials: G
authors.lastname: Venturoli
mesh_terms: D015502:Absorptiometry, Photon / Q000379:methods; D001665:Binding Sites; D003198:Computer Simulation; D008956:Models, Chemical; D008958:Models, Molecular; D045322:Photosynthetic Reaction Center Complex Proteins / Q000032:analysis / Q000737:chemistry*; D011485:Protein Binding; D011487:Protein Conformation; D012242:Rhodobacter sphaeroides / Q000378:metabolism*; D014961:X-Ray Diffraction / Q000379:methods*; D015032:Zinc / Q000032:analysis / Q000737:chemistry*
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D015032:Zinc