Structural basis for sequential displacement of Ca2+ by Yb3+ in a protozoan EF-hand calcium binding protein

We have studied the displacement of Ca(2+)by the trivalent lanthanide ions (Yb3+) in a protozoan (Entamoeba histolytica) Ca2+-binding protein (EhCaBP), by NMR and thermodynamics. We have demonstrated, for the first time, how one can use in a combined fashion the utility of NMR and thermodynamics to have an insight to the relative binding specificities/affinity between Ca2+ and Yb3+. As revealed by the titration experiments, Yb3+ displaces Ca2+ from the four metal binding sites present in EhCaBP in a sequential manner. The study provides a structural origin for such a sequential Ca2+ displacement by Yb3+ in EhCaBP.