Mercury binding by methanobactin from Methylocystis strain SB2

• Methanobactin from Methylocystis strain SB2 (mb-SB2) binds Hg.
• At equimolar concentrations, mb-SB2 preferentially binds Hg2 + over Cu2.
• Hg2 + can displace Cu from mb-SB2, and the displacement is an active process.
• Mb-SB2 shows potential as a treatment option for mercury toxicity.

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 Hg2+. 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, Hg2+, Hg(CN)2 and CH3Hg+. The spectral and thermodynamic properties of binding for each form of mercury differed. UV-visible absorption spectra suggested that Hg2 + 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 Hg2 + 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 Hg2 + 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 Hg2+ could displace Cu from Cu-containing mb-SB2 and preferentially bound Hg2 + over Cu2 + at metal to mb-SB2 molar ratios above 1.0.

Chemical structures for methanobactin from Methylocystis strain SB2 and proposed coordination of Hg2 +, Hg(CN)2 and CH3Hg+.Figure optionsDownload as PowerPoint slide