Flexibility of EF-hand motifs: structural and thermodynamic studies of Calcium Binding Protein-1 from Entamoeba histolytica with Pb2+, Ba2+, and Sr2+

Table 2 Thermodynamic parameters of metal binding to EhCaBP1in 20 mM MOPS buffer pH 7.4 at 25°C obtained and calculated from ITC results

Metal	Site*	K_a(M^-1)	ΔH(kcal.mol^-1)	ΔS(Cal.mol^-1.K^-1)	ΔG(kcal.mol^-1)	Dominating forces (inferred)
Ca²⁺	1	2.56E5 ± 9.1E4	−3.33 ± 0.30	13.6	−7.38	H-bonding/hydrophobic interactions
	2	1.60E3 ± 9.0E2	−2.42 ± 0.28	6.54	−4.37	H-bonding/hydrophobic interactions
	3	2.42E5 ± 8.3E4	8.80 ± 0.36	53.2	−7.34	Hydrophobic interactions
	4	1.97E5 ± 8.6E4	−4.85 ± 0.94	7.94	−7.22	H-bonding/hydrophobic interactions
Sr²⁺	1	5.57E4 ± 9.6E3	−6.74 ± 0.51	−0.90	−6.47	H-bonding /conformational change
	2	6.44E4 ± 7.0E3	−10.35 ± 1.98	−12.7	−6.56	H-bonding /conformational change
	3	1.06E5 ± 1.5E4	19.42 ± 2.71	88.1	−6.83	Hydrophobic interactions
	4	1.49E5 ± 2.1E4	−7.44 ± 1.31	−1.28	−7.06	H-bonding /conformational change
Ba²⁺	1	1.02E5 ± 1.5E4	−3.84 ± 0.17	10.0	−6.82	H-bonding/hydrophobic interactions
	2	1.88E5 ± 2.0E4	−2.15 ± 0.39	16.9	−7.18	H-bonding/hydrophobic interactions
	3	7.15E4 ± 6.1E3	3.23 ± 1.02	33.1	−6.62	Hydrophobic interactions
	4	6.25E4 ± 9.3E3	−1.09 ± 2.14	18.3	−6.55	H-bonding/hydrophobic interactions
	5	3.16E4 ± 1.9E3	−0.69 ± 0.23	18.3	−6.14	H-bonding/hydrophobic interactions

*Sites 1, 2, 3, 4 and 5 only correspond to the order of occupied sites during the non-linear fitting of ITC data by sequential mode. These are not exchange digits for the loop of EF-I, EF-II etc.

ISSN: 2046-1682