Molecular modeling of a putative antagonist binding site on helix III of the β-adrenoceptor

Summary

In recent biochemical studies it was demonstrated that residue Asp¹¹³ of theβ-adrenoceptor (β-AR) is an indispensable amino acid for the binding ofβ-AR antagonists. Earlier fluorescence studies showed that a tryptophan-rich region of theβ-AR is involved in the binding of propranolol, the prototypeβ-AR antagonist. Bearing these two biochemical findings in mind, we explored theβ-AR part containing Asp¹¹³, for an energetically favorable antagonist binding site. This was done by performing molecular docking studies with the antagonist propranolol and a specificβ-AR peptide which included, besides Asp¹¹³, two possibly relevant tryptophan residues. In the docking calculations, the propranolol molecule was allowed to vary all its internal torsional angles. The receptor peptide was kept in anα-helix conformation, while side chains relevant to ligand binding were flexible to enable optimal adaptations to the ligand's binding conformation. By means of force-field calculations the total energy was minimized, consisting of the intramolecular energies of both ligand and receptor peptide, and the intermolecular energy. We found an antagonist binding site, consisting of amino acids Asp¹¹³ and Trp¹⁰⁹, which enabled energetically favorable interactions with the receptor-binding groups of propranolol. According to these results, binding involves three main interaction points: (i) a reinforced ionic bond; (ii) a hydrogen bond; and (iii) a hydrophobic/charge transfer interaction. The deduced binding site shows a difference in affinity between the levo- and dextrorotatory isomers of propranolol caused by a difference in ability to form a hydrogen bond, which is in conformity with the experimentally observed stereoselectivity. Moreover, it also provides an explanation for theβ ₁-selectivity ofp-phenyl substituted phenoxypropanolamines like betaxolol. Thep-phenyl substituent of betaxolol was shown to be sterically hindered upon binding to theβ ₂-AR peptide, whereas this hindrance is very likely to be much less with theβ ₁-AR peptide. Finally, the proposed antagonist binding site is discussed in the light of some recent biochemical findings and theories.