Abstract
A thermodynamic study on the interaction between magnesium ion and human growth hormone (hGH) was studied at 27Β°C in NaCl solution (50 mM) using different techniques. Two techniques of ionmetry using a Mg^2+^selective membrane electrode and isothermal titration calorimetry were applied to obtain the binding isotherm for hGHMg^2+^; results obtained by both techniques were found to be in good agreement. There is a set of three identical and noninteracting binding sites for magnesium ions. The intrinsic dissociation equilibrium constant and the molar enthalpy of binding are 46 ΞΌM and β17.7 kJ/mol, respectively. Temperature scanning UVβvisible spectroscopy was applied to elucidate the effect of Mg^2+^ binding on the protein stability, and circular dichroism (CD) spectroscopy was used to show the structural change of hGH due to the metal ion interaction. Magnesium ion binding increased the protein thermal stability by increasing the Ξ±βhelix content as well as decreasing both Ξ² and random coil structures. However, the secondary structural change of the protein returns to its native form, including a small change in the tertiary structure, in high concentrations of magnesium ion. Β© 2005 Wiley Periodicals, Inc. Biopolymers 81: 120β126, 2006
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