Effects of zinc binding on the structure and thermal stability of camel alpha-lactalbumin

The interaction between zinc ion and camel alpha-lactalbumin (alpha-La) has been studied using different techniques of fluorescence spectroscopy, circular dichroism, isothermal titration calorimetry, and differential scanning calorimetry (DSC). There are two sets of independent binding sites for Zn2+, two ions bind with the binding constant of 4.53 x 10(4) M-1 and four ions with the binding constant of 963 M-1. The Protein-Zn(2+)complexation is an entropy-driven process. Circular dichroism studies do not show any significant change in the secondary structure of the protein after the binding of zinc ion on the alpha-La. The interaction leads to a conformational change of the protein and exposure of hydrophobic patches on alpha-Lato the solvent as observed in ANS fluorescence spectroscopy. DSC was applied to elucidate the effect of Zn2+ binding on the protein stability. Binding of Zinc ion to alpha-La induces a partially folded structure of protein. At the Zn2+/alpha-La molar ratio of 5 and 10, a mixture of native and partially folded structures is present. When the ratio reaches 20, all molecules adopt partially folded structure. Chemometric analysis confirmed the experimental results and provided insight into the process of denaturation of alpha-La via characterizing the native, intermediate, and denatured conformations in the presence of different concentrations of Zn2+ and different thermal conditions. Thermal stability of the partially folded structure of alpha-La in the presence of Zinc ion is lower than in the native protein.