Table 1 Thermodynamic parameters of the duck Na,K-ATPase binding to AMP, ADP and ATP determined by isothermal titration calorimetry^a

Ligand	T, °C	K_a^b, M⁻¹	K_d^c, μM	ΔH^d, kcal/mole	TΔS^e, kcal/mole	ΔG^f, kcal/mole
AMP	25	1.1 × 10⁶	0.91	−1.40	6.83	−8.23
AMP	30	1.8 × 10⁶	0.56	−1.81	6.86	−8.67
AMP	37	1.7 × 10⁶	0.59	−2.19	6.65	−8.84
ADP	25	1.4 × 10⁷	0.07	−5.68	4.06	−9.74
ADP	30	2.4 × 10⁷	0.04	−6.00	4.23	−10.23
ADP	37	2.6 × 10⁷	0.04	−6.21	4.31	−10.52
ATP	10	1.2 × 10⁷	0.08	7.70	16.87	−9.17
ATP	25	1.6 × 10⁷	0.06	−4.00	5.82	−9.82
ATP	37	2.0 × 10⁷	0.05	−12.80	−2.44	−10.36

^aAll measurements were performed three to four times in imidazole buffer (25 mM imidazole, 1 mM DTT, 1 mM EDTA, 3 mM NaCl and 250 mM sucrose, pH 7.5).
^bK_a – affinity constant; standard deviation did not exceed ±20%.
^cK_d – dissociation constant; calculated as 1/K_a.
^dΔH – enthalpy variation; standard deviation did not exceed ±10%.
^eTΔS– entropy variation; calculated from the equation ΔG = ΔH − TΔS.
^fΔG – Gibbs energy; calculated from the equation ΔG = −RTlnK_a.

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