The effect of stereochemistry on the thermodynamic characteristics of the binding of fenoterol stereoisomers to the β2-adrenoceptor

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.

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