Effects of calcium binding on the structure and stability of human growth hormone

Thermodynamic analysis of calcium ions binding to human growth hormone (hGH) was done at 27 degrees C in NaCl solution, 50 mM, using different techniques. The binding isotherm for hGH-Ca2+ was obtained by two techniques of ionmetry, using a Ca(2+)-selective membrane electrode, and isothermal titration calorimetry. Results obtained by two ionmetric and calorimetric methods are in good agreement. There is a set of three identical and non-interacting binding sites for calcium ions. The intrinsic dissociation equilibrium constant and the molar enthalpy of binding are 52 microM and -17.4 kJ/mol, respectively. Temperature scanning UV-vis spectroscopy was applied to elucidate the effect of Ca2+ 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. Calcium ions binding increase the protein thermal stability by increasing of the alpha helix content as well as decreasing of both beta and random coil structures.