Binding thermodynamics at the human A3 adenosine receptor

The thermodynamic parameters DeltaG , DeltaH and DeltaS of the binding equilibrium of six adenosine receptor agonists and five antagonists at adenosine A(3) receptors were determined by means of affinity measurements at six different temperatures (4, 10, 15, 20, 25 and 30) and van't Hoff plots were constructed. Affinity constants were measured on Chinese hamster ovary (CHO) cells transfected with the human A(3) receptors by inhibition assays of the binding of the selective A(3) antagonist [3H]MRE 3008F20. van't Hoff plots were linear for agonists and antagonists in the temperature range 4-30 degree. Their thermodynamic parameters fall in the ranges 21 < or = DeltaH < or = 67kJmol(-1) and 208 < or = DeltaS < or =410 J(Kmol)(-1) for agonists and -52 < or = DeltaH < or = -9 kJmol(-1) and 16 < or = DeltaS < or =81 J(K/mol)(-1) for antagonists, showing that agonist binding is always totally entropy-driven while antagonist binding is enthalpy- and entropy-driven. The results are discussed with the aim of obtaining new details on the nature of the forces driving the A(3) binding at a molecular level.