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6123658 
Journal Article 
Mercury binding by methanobactin from Methylocystis strain SB2 
Baral, BS; Bandow, NL; Vorobev, A; Freemeier, BC; Bergman, BH; Herdendorf, TJ; Fuentes, N; Ellias, L; Turpin, E; Semrau, JD; Dispirito, AA 
2014 
Yes 
Journal of Inorganic Biochemistry
ISSN: 0162-0134
EISSN: 1873-3344 
141 
161-169 
English 
25265378 
WOS:000343790700019 
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.