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 beta(2)-adrenergic receptor (beta(2)-AR) have been determined. The experiments utilized membranes obtained from HEK cells stably transfected with cDNA encoding human beta(2)-AR. Competitive displacement studies using [(3)H]CGP-12177 as the marker ligand were conducted at 4, 15, 25, 30 and 37 degrees C, the binding affinities calculated and the standard enthalpic (DeltaH degrees ) and standard entropic (DeltaS degrees ) contribution to the standard free energy change (DeltaG degrees ) 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 beta(2)-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 beta-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 beta-hydroxyl group of the fenoterol molecule (the beta-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 beta-OH carbon is S the binding process is enthalpy-driven while the R configuration produces an entropy-driven process.

Figure 1

Structures of the compounds used in this study.

Figure 2

The displacement of 0.3 nM [³H]CGP-12177 by (R,R')-fenoterol and (S,S')-fenoterol from membranes obtained from HEK293 cells stably transfected with cDNA encoding human β2-AR obtained at 4°C and 37°C. See text for experimental details.

Figure 3

(Online version) Van't Hoff plots for (S,S')-fenoterol (), (S,R')-fenoterol (), (R,S')-fenoterol (), (R,R')-fenoterol (), rac-propranolol (∎) , (R)-isoproterenol (▴). (Black and White print version) Van't Hoff plots for (S,S')-fenoterol (●), (S,R')-fenoterol (○), (R,S')-fenoterol (◆), (R,R')-fenoterol (◊), rac-propranolol (∎) , (R)-isoproterenol (▴).

Figure 3

(Online version) Van't Hoff plots for (S,S')-fenoterol (), (S,R')-fenoterol (), (R,S')-fenoterol (), (R,R')-fenoterol (), rac-propranolol (∎) , (R)-isoproterenol (▴). (Black and White print version) Van't Hoff plots for (S,S')-fenoterol (●), (S,R')-fenoterol (○), (R,S')-fenoterol (◆), (R,R')-fenoterol (◊), rac-propranolol (∎) , (R)-isoproterenol (▴).

Figure 4

(Online version) Enthalpy/entropy relation plot for (S,S')-fenoterol (), (S,R')-fenoterol (), (R,S')-fenoterol (), (R,R')-fenoterol (), rac-propranolol (∎), (R)-isoproterenol (▴). The plot indicates also the location determined for rac-fenoterol () by Weiland et.al [1]. (Black and White print version) Enthalpy/entropy relation plot for (S,S')-fenoterol (●), (S,R')-fenoterol (○), (R,S')-fenoterol (◆), (R,R')-fenoterol (◊), rac-propranolol (∎) , (R)-isoproterenol (▴). The plot indicates also the location determined for rac-fenoterol () by Weiland et.al [1].

Figure 4

(Online version) Enthalpy/entropy relation plot for (S,S')-fenoterol (), (S,R')-fenoterol (), (R,S')-fenoterol (), (R,R')-fenoterol (), rac-propranolol (∎), (R)-isoproterenol (▴). The plot indicates also the location determined for rac-fenoterol () by Weiland et.al [1]. (Black and White print version) Enthalpy/entropy relation plot for (S,S')-fenoterol (●), (S,R')-fenoterol (○), (R,S')-fenoterol (◆), (R,R')-fenoterol (◊), rac-propranolol (∎) , (R)-isoproterenol (▴). The plot indicates also the location determined for rac-fenoterol () by Weiland et.al [1].