Spectral, kinetic, and thermodynamic properties of Cu(I) and Cu(II) binding by methanobactin from Methylosinus trichosporium OB3b

To examine the potential role of methanobactin (mb) as the extracellular component of a copper acquisition system in Methylosinus trichosporium OB3b, the metal binding properties of mb were examined. Spectral (UV−visible, fluorescence, and circular dichroism), kinetic, and thermodynamic data suggested copper coordination changes at different Cu(II):mb ratios. Mb appeared to initially bind Cu(II) as a homodimer with a comparatively high copper affinity at Cu(II):mb ratios below 0.2, with a binding constant (K) greater than that of EDTA (log K = 18.8) and an approximate ΔG° of −47 kcal/mol. At Cu(II):mb ratios between 0.2 and 0.45, the K dropped to (2.6 ± 0.46) × 108 with a ΔG° of −11.46 kcal/mol followed by another K of (1.40 ± 0.21) × 106 and a ΔG° of −8.38 kcal/mol at Cu(II):mb ratios of 0.45−0.85. The kinetic and spectral changes also suggested Cu(II) was initially coordinated to the 4-thiocarbonyl-5-hydroxy imidazolate (THI) and possibly Tyr, followed by reduction to Cu(I), and then coordination of Cu(...