Thermodynamics of metal ion binding and denaturation of a calcium binding protein from entamoeba histolytica

Abstract

The thermodynamics of the binding of calcium and magnesium ions to a calcium binding protein from Entamoeba histolytica was investigated by isothermal titration calorimetry (ITC) in 20 mM MOPS buffer (pH 7.0) at 20 °C. Enthalpy titration curves of calcium show the presence of four Ca²⁺ binding sites. There exist two low-affinity sites for Ca²⁺, both of which are exothermic in nature and with positive cooperative interaction between them. Two other high affinity sites for Ca²⁺ exist of which one is endothermic and the other exothermic, again with positive cooperative interaction. The binding constants for Ca²⁺ at the four sites have been verified by a competitive binding assay, where CaBP competes with a chromophoric chelator 5, 5'-Br₂ BAPTA to bind Ca²⁺ and a Ca²⁺ titration employing intrinsic tyrosine fluorescence of the protein. The enthalpy of titration of magnesium in the absence of calcium is single site and endothermic in nature. In the case of the titrations performed using protein presaturated with magnesium, the amount of heat produced is altered. Further, the interaction between the high-affinity sites changes to negative cooperativity. No exchange of heat was observed throughout the addition of magnesium in the presence of 1 mM calcium. Titrations performed on a cleaved peptide comprising the N-terminus and the central linker show the existence of two Ca²⁺ specific sites. These results indicate that this CaBP has one high-affinity Ca-Mg site, one high-affinity Ca-specific site, and two low-affinity Ca-specific sites. The thermodynamic parameters of the binding of these metal ions were used to elucidate the energetics at the individual site(s) and the interactions involved therein at various concentrations of the denaturant, guanidine hydrochloride, ranging from 0.05 to 6.5 M. Unfolding of the protein was also monitored by titration calorimetry as a function of the concentration of the denaturant. These data show that at a GdnHCl concentration of 0.25 M the binding affinity for the Mg²⁺ ion is lost and there are only two sites which can bind to Ca²⁺, with substantial loss of cooperativity. At concentrations beyond 2.5 M GdnHCl, at which the unfolding of the tertiary structure of this protein is observed by near UV CD spectroscopy, the binding of Ca²⁺ ions is lost. We thus show that the domain containing the two low-affinity sites is the first to unfold in the presence of GdnHCl. Control experiments with change in ionic strength by addition of KCl in the range 0.25-1 M show the existence of four sites with altered ion binding parameters.