title: Spectroscopic and Thermodynamic Characterization of the Metal-Binding Sites in the LH1-RC Complex from Thermophilic Photosynthetic Bacterium Thermochromatium tepidum.
abstract: The light-harvesting 1 reaction center (LH1-RC) complex from thermophilic photosynthetic bacterium Thermochromatium (Tch.) tepidum exhibits enhanced thermostability and an unusual LH1 Qytransition, both induced by Ca2+binding. In this study, metal-binding sites and metal-protein interactions in the LH1-RC complexes from wild-type (B915) and biosynthetically Sr2+-substituted (B888) Tch. tepidum were investigated by isothermal titration calorimetry (ITC), atomic absorption (AA), and attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopies. The ITC measurements revealed stoichiometric ratios of approximately 1:1 for binding of Ca2+, Sr2+, or Ba2+to the LH1 αβ-subunit, indicating the presence of 16 binding sites in both B915 and B888. The AA analysis provided direct evidence for Ca2+and Sr2+binding to B915 and B888, respectively, in their purified states. Metal-binding experiments supported that Ca2+and Sr2+(or Ba2+) competitively associate with the binding sites in both species. The ATR-FTIR difference spectra upon Ca2+depletion and Sr2+substitution demonstrated that dissociation and binding of Ca2+are predominantly responsible for metal-dependent conformational changes of B915 and B888. The present results are largely compatible with the recent structural evidence that another binding site for Sr2+(or Ba2+) exists in the vicinity of the Ca2+-binding site, a part of which is shared in both metal-binding sites.
authors.affiliation: Graduate School of Agricultural Science, Kobe University , Kobe 657-8501, Japan.
authors.forename: Yukihiro
authors.identifier: 0000-0003-3747-0367
authors.initials: Y
authors.lastname: Kimura
authors.affiliation: Graduate School of Agricultural Science, Kobe University , Kobe 657-8501, Japan.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Yura
authors.affiliation: Graduate School of Agricultural Science, Kobe University , Kobe 657-8501, Japan.
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Hayashi
authors.affiliation: Graduate School of Agricultural Science, Kobe University , Kobe 657-8501, Japan.
authors.forename: Yong
authors.initials: Y
authors.lastname: Li
authors.affiliation: Graduate School of Agricultural Science, Kobe University , Kobe 657-8501, Japan.
authors.forename: Moe
authors.initials: M
authors.lastname: Onoda
authors.affiliation: Graduate School of Natural Science and Technology, Okayama University , Okayama 700-8530, Japan.
authors.forename: Long-Jiang
authors.initials: LJ
authors.lastname: Yu
authors.affiliation: Faculty of Science, Ibaraki University , Mito 310-8512, Japan.
authors.forename: Zheng-Yu
authors.initials: ZY
authors.lastname: Wang-Otomo
authors.affiliation: Graduate School of Agricultural Science, Kobe University , Kobe 657-8501, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Ohno
mesh_terms: D020816:Amino Acid Motifs; D000693:Anaerobiosis / Q000502:physiology; D001464:Barium / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry* / Q000378:metabolism; D002413:Cations, Divalent; D002841:Chromatiaceae / Q000737:chemistry* / Q000502:physiology; D006358:Hot Temperature; D007700:Kinetics; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D008958:Models, Molecular; D010788:Photosynthesis / Q000502:physiology; D011485:Protein Binding; D000072756:Protein Conformation, alpha-Helical; D000072757:Protein Conformation, beta-Strand; D054730:Protein Interaction Domains and Motifs; D021122:Protein Subunits / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D013324:Strontium / Q000737:chemistry* / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D002413:Cations, Divalent; D045342:Light-Harvesting Protein Complexes; D021122:Protein Subunits; D001464:Barium; D002118:Calcium; D013324:Strontium