Binding properties and conformational change of human growth hormone upon interaction with Fe3+

Binding properties and conformational change of human growth hormone (hGH) upon interaction with Fe3+ were investigated at 27 °C in NaCl solution, 50 mM, by calorimetry and spectroscopy. UV spectroscopy indicates that thermal denaturation of hGH is an irreversible process and is accompanied by aggregation. At an optimum concentration of iron thermal denaturation of hGH becomes reversible. Results from equilibrium dialysis and isothermal titration calorimetry indicate a set of four binding sites on hGH for Fe3+. Interaction of three iron ions with hGH prevents irreversibility and aggregation. Differential scanning calorimetry confirms the UV spectroscopic finding. Domain analysis by DSC shows that in the presence of iron, there are at least two main transitions corresponding with the two groups of helices. Deconvolution of the main transitions provides two sub-transitions each, the first pair is similar, but the second pair is considerably different in the enthalpy change of unfolding. Interaction of iron ions with hGH prevents aggregation by an effect on the hydrophobicity and provides information about its structure and thermal denaturation.