Energetics and Mechanism of Ca2+ displacement by lanthanides in a calcium binding protein

The displacement of Ca2+ by trivalent lanthanide ions (Tm3+) in a protozoan (Entamoeba histolytica) Ca2+ binding protein has been studied by NMR and isothermal calorimetry (ITC). The study provides a basis for understanding the behavior of lanthanides when used as a substitute for Ca2+, the pattern of sequential binding, the structural changes involved, the range and magnitude of paramagnetic interaction, and the associated energetics and mechanism. The progressive Ca2+ displacement from site III first, followed by displacement from site II, I, and IV, as observed during the NMR titration experiments, is interpreted in the light of ITC data to provide a deeper insight into the intradomain and, for the first time, interdomain cooperativity and information about the statistical phenomenon involved in it. A theoretical model governing Ca2+ displacement is provided. The small structural changes involved in Ca2+ displacement by a diamagnetic lanthanide (La3+) has also been monitored.