Study on the interactional behaviour of transition metal ions with myoglobin: A detailed calorimetric, spectroscopic and light scattering analysis.

The energetics and the impact on the conformation of heme containing protein myoglobin (Mb) due to the binding of three transition metal ions (Zn2+ , Ni2+ , and Mn2+ ) have been investigated using isothermal titration calorimetry (ITC), dynamic light scattering (DLS), UV-vis, and circular dichroism (CD) spectroscopy under physiological conditions. The binding affinity of the order of 104 M-1 has been observed for all metal ions from calorimetry as well as from absorption spectroscopy. The binding of these metal ions with Mb is a spontaneous process that exposes the hydrophobic groups away from the protein core as exhibited by the negative Gibbs free energy change (ΔG) and positive heat capacity change (ΔCp ) values. Both light scattering and CD results demonstrates that the binding of Zn2+ and Mn2+ ions with Mb results in the folding whereas Ni2+ ion results in the unfolding of the protein. No direct interactions among the transition metal ions and heme moiety of Mb has been observed from absorption study. The results of these studies reveals that Mn2+ ion influences the biological functions of Mb to a larger extent in spite of its lowest affinity followed by Zn2+ and Ni2+ ions.