Mercury binding by methanobactin from Methylocystis strain SB2

J Inorg Biochem. 2014 Dec;141:161-169. doi: 10.1016/j.jinorgbio.2014.09.004. Epub 2014 Sep 18.

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.

Keywords: Chalkophore; Mercury binding; Mercury mobilization; Methanobactin; Methanotroph.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / isolation & purification
  • Bacterial Proteins / metabolism
  • Binding Sites
  • Cations, Divalent
  • Copper / chemistry*
  • Copper / metabolism
  • Gene Expression
  • Imidazoles / chemistry*
  • Imidazoles / isolation & purification
  • Imidazoles / metabolism
  • Kinetics
  • Mercury / chemistry*
  • Mercury / metabolism
  • Methylocystaceae / chemistry*
  • Methylocystaceae / growth & development
  • Methylocystaceae / metabolism
  • Methylosinus trichosporium / chemistry
  • Oligopeptides / chemistry*
  • Oligopeptides / isolation & purification
  • Oligopeptides / metabolism
  • Protein Binding
  • Thermodynamics

Substances

  • Bacterial Proteins
  • Cations, Divalent
  • Imidazoles
  • Oligopeptides
  • methanobactin
  • Copper
  • Mercury