title: Crystallographic and Computational Analysis of the Barrel Part of the PsbO Protein of Photosystem II: Carboxylate-Water Clusters as Putative Proton Transfer Relays and Structural Switches.
abstract: In all organisms that employ oxygenic photosynthesis, the membrane-extrinsic PsbO protein is a functionally important component of photosystem II. To study the previously proposed proton antenna function of carboxylate clusters at the protein-water interface, we combined crystallography and simulations of a truncated cyanobacterial (Thermosynechococcus elongatus) PsbO without peripheral loops. We expressed the PsbO β-barrel heterologously and determined crystal structures at resolutions of 1.15-1.5 Å at 100 K at various pH values and at 297 K and pH 6. (1) Approximately half of the 177 surface waters identified at 100 K are resolved at 297 K, suggesting significant occupancy of specific water sites at room temperature, and loss of resolvable occupancy for other sites. (2) Within a loop region specific to cyanobacterial PsbO, three residues and four waters coordinating a calcium ion are well ordered even at 297 K; the ligation differs for manganese. (3) The crystal structures show water-carboxylate clusters that could facilitate fast Grotthus-type proton transfer along the protein surface and/or store protons. (4) Two carboxylate side chains, which are part of a structural motif interrupting two β-strands and connecting PsbO to photosystem II, are within hydrogen bonding distance at pH 6 (100 K). Simulations indicate coupling between protein structure and carboxylate protonation. The crystal structure determined at 100 K and pH 10 indicates broken hydrogen bonding between the carboxylates and local structural change. At pH 6 and 297 K, both conformations were present in the crystal, suggesting conformational dynamics in the functionally relevant pH regime. Taken together, crystallography and molecular dynamics underline a possible mechanism for pH-dependent structural switching.
authors.affiliation: Institut für Biologie, Strukturbiologie/Biochemie, Humboldt-Universität zu Berlin , Unter den Linden 6, 10099 Berlin, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Bommer
authors.affiliation: Fachbereich Physik, Theoretical Molecular Biophysics, Freie Universität Berlin , Arnimallee 14, 14195 Berlin, Germany.
authors.forename: Ana-Nicoleta
authors.initials: AN
authors.lastname: Bondar
authors.affiliation: Institut für Biologie, Biophysik der Photosynthese, Humboldt-Universität zu Berlin , Unter den Linden 6, 10099 Berlin, Germany.
authors.forename: Athina
authors.initials: A
authors.lastname: Zouni
authors.affiliation: Institut für Biologie, Strukturbiologie/Biochemie, Humboldt-Universität zu Berlin , Unter den Linden 6, 10099 Berlin, Germany.
authors.forename: Holger
authors.initials: H
authors.lastname: Dobbek
authors.affiliation: Fachbereich Physik, Biophysics and Photosynthesis, Freie Universität Berlin , Arnimallee 14, 14195 Berlin, Germany.
authors.forename: Holger
authors.initials: H
authors.lastname: Dau
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D002264:Carboxylic Acids / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D056004:Molecular Dynamics Simulation; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D011522:Protons; D014867:Water / Q000737:chemistry
chemical_list: D001426:Bacterial Proteins; D002264:Carboxylic Acids; D045332:Photosystem II Protein Complex; D011522:Protons; D014867:Water