title: Mercury binding by methanobactin from Methylocystis strain SB2.
abstract: Methanobactin (mb) is a post-translationally modified copper-binding compound, or chalkophore, secreted by many methane-oxidizing bacteria or methanotrophs in response to copper limitation. In addition to copper, methanobactin from Methylosinus trichosporium OB3b (mb-OB3b) has been shown to bind a variety of metals including Hg(2+). In this report, Hg binding by the structurally unique methanobactin from Methylocystis strain SB2 (mb-SB2) was examined and compared to mb-OB3b. Mb-SB2 is shown to bind the common forms of Hg found in aqueous environments, Hg(2+), Hg(CN)2 and CH3Hg(+). The spectral and thermodynamic properties of binding for each form of mercury differed. UV-visible absorption spectra suggested that Hg(2+) binds to both the oxazolone and imidazolone rings of mb-SB2, whereas CH3Hg(+) appeared to only bind to the oxazolone ring. Hg(CN)2 showed spectral properties between Hg(2+) and CH3Hg(+). Isothermal titration calorimetry (ITC) showed both Hg(CN)2 and CH3Hg(+) fit into two-site binding models. For Hg(CN)2 the first site was exothermic and the second endothermic. Both binding sites in CH3Hg(+) were exothermic, but at equilibrium the reaction never moved back to the baseline, suggesting a slow residual reaction. ITC results for Hg(2+) were more complex and suggested a 3- or 4-site model. The spectral, kinetic and thermodynamic changes following Hg binding by mb-SB2 also differed from the changes associated with mb-OB3b. Like mb-OB3b, copper did not displace Hg bound to mb-SB2. In contrast to mb-OB3b Hg(2+) could displace Cu from Cu-containing mb-SB2 and preferentially bound Hg(2+) over Cu(2+) at metal to mb-SB2 molar ratios above 1.0.
authors.affiliation: Roy J. Carver Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, Ames, IA, 50011, USA.
authors.forename: Bipin S
authors.initials: BS
authors.lastname: Baral
authors.affiliation: Roy J. Carver Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, Ames, IA, 50011, USA.
authors.forename: Nathan L
authors.initials: NL
authors.lastname: Bandow
authors.affiliation: Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, MI, 48109-2125, USA.
authors.forename: Alexy
authors.initials: A
authors.lastname: Vorobev
authors.affiliation: Roy J. Carver Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, Ames, IA, 50011, USA.
authors.forename: Brittani C
authors.initials: BC
authors.lastname: Freemeier
authors.affiliation: Roy J. Carver Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, Ames, IA, 50011, USA.
authors.forename: Brandt H
authors.initials: BH
authors.lastname: Bergman
authors.affiliation: Roy J. Carver Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, Ames, IA, 50011, USA.
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Herdendorf
authors.affiliation: Roy J. Carver Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, Ames, IA, 50011, USA.
authors.forename: Nathalie
authors.initials: N
authors.lastname: Fuentes
authors.affiliation: Roy J. Carver Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, Ames, IA, 50011, USA.
authors.forename: Luke
authors.initials: L
authors.lastname: Ellias
authors.affiliation: Roy J. Carver Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, Ames, IA, 50011, USA.
authors.forename: Erick
authors.initials: E
authors.lastname: Turpin
authors.affiliation: Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, MI, 48109-2125, USA.
authors.forename: Jeremy D
authors.initials: JD
authors.lastname: Semrau
authors.affiliation: Roy J. Carver Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, Ames, IA, 50011, USA. Electronic address: aland@iastate.edu.
authors.forename: Alan A
authors.initials: AA
authors.lastname: DiSpirito
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D001665:Binding Sites; D002413:Cations, Divalent; D003300:Copper / Q000737:chemistry* / Q000378:metabolism; D015870:Gene Expression; D007093:Imidazoles / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D007700:Kinetics; D008628:Mercury / Q000737:chemistry* / Q000378:metabolism; D042083:Methylocystaceae / Q000737:chemistry* / Q000254:growth & development / Q000378:metabolism; D020602:Methylosinus trichosporium / Q000737:chemistry; D009842:Oligopeptides / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D011485:Protein Binding; D013816:Thermodynamics
chemical_list: D001426:Bacterial Proteins; D002413:Cations, Divalent; D007093:Imidazoles; D009842:Oligopeptides; C491147:methanobactin; D003300:Copper; D008628:Mercury
keywords: Chalkophore; Mercury binding; Mercury mobilization; Methanobactin; Methanotroph