Structural basis for sequential displacement of Ca 2+ by Yb 3+ in a protozoan EF‐hand calcium binding protein

We have studied the displacement of Ca 2+ by the trivalent lanthanide ions (Yb 3+ ) in a protozoan ( Entamoeba histolytica ) Ca 2+ ‐binding protein ( Eh CaBP), 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 Ca 2+ and Yb 3+ . As revealed by the titration experiments, Yb 3+ displaces Ca 2+ from the four metal binding sites present in Eh CaBP in a sequential manner. The study provides a structural origin for such a sequential Ca 2+ displacement by Yb 3+ in Eh CaBP.