The Effect of Stereochemistry on the Thermodynamic Characteristics of the Binding of Fenoterol Stereoisomers to the Beta(2)-Adrenoceptor

Abstract

The binding thermodynamics of the stereoisomers of fenoterol, (R,R′)-, (S,S′)-, (R,S′)-, and (S,R′)-fenoterol, to the β₂-adrenergic receptor (β₂-AR) have been determined. The experiments utilized membranes obtained from HEK cells stably transfected with cDNA encoding human β₂-AR. Competitive displacement studies using [³H]CGP-12177 as the marker ligand were conducted at 4, 15, 25, 30 and 37 °C, the binding affinities calculated and the standard enthalpic (ΔH°) and standard entropic (ΔS°) contribution to the standard free energy change (ΔG°) associated with the binding process determined through the construction of van’t Hoff plots. The results indicate that the binding of (S,S′)- and (S,R′)-fenoterol were predominately enthalpy-driven processes while the binding of (R,R′)- and (R,S′)-fenoterol were entropy-driven. All of the fenoterol stereoisomers are full agonists of the β₂-AR, and, therefore, the results of this study are inconsistent with the previously described “thermodynamic agonist–antagonist discrimination”, in which the binding of an agonist to the β-AR is entropy-driven and the binding of an antagonist is enthalpy-driven. In addition, the data demonstrate that the chirality of the carbon atom containing the β-hydroxyl group of the fenoterol molecule (the β-OH carbon) is a key factor in the determination of whether the binding process will be enthalpy-driven or entropy-driven. When the configuration at the β-OH carbon is S the binding process is enthalpy-driven while the R configuration produces an entropy-driven process.