Thermodynamics Research Today is a free monthly online journal that collates and summarizes the latest research about Thermodynamics, including details on enthalpy, entropy, energy transitions. | ||||||||
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Spectral, kinetic, and thermodynamic properties of Cu(I) and Cu(II) binding by methanobactin from Methylosinus trichosporium OB3b.Choi DW, Zea CJ, Do YS, Semrau JD, Antholine WE, Hargrove MS, Pohl NL, Boyd ES, Geesey GG, Hartsel SC, Shafe PH, McEllistrem MT, Kisting CJ, Campbell D, Rao V, de la Mora AM, Dispirito AA Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, Ames, Iowa 50011-3211, USA. 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 DeltaG degrees of -47 kcal/mol. At Cu(II):mb ratios between 0.2 and 0.45, the K dropped to (2.6 +/- 0.46) x 10(8) with a DeltaG degrees of -11.46 kcal/mol followed by another K of (1.40 +/- 0.21) x 10(6) and a DeltaG degrees 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(I) to 4-hydroxy-5-thiocarbonyl imidazolate (HTI) resulting in the final coordination of Cu(I) by THI and HTI. The rate constant (k(obsI)) of binding of Cu(II) to THI exceeded that of the stopped flow apparatus that was used, i.e., >640 s(-)(1), whereas the coordination of copper to HTI showed a 6-8 ms lag time followed by a k(obsII) of 121 +/- 9 s(-)(1). Mb also solubilized and bound Cu(I) with a k(obsI) to THI of >640 s(-)(1), but with a slower rate constant to HTI (k(obsII) = 8.27 +/- 0.16 s(-)(1)), and appeared to initially bind Cu(I) as a monomer. Published 31 January 2006 in Biochemistry, 45(5): 1442-53.
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