Binding pockets of the β₁- and β₁-adrenergic receptors for subtype- selective agonists

We examined the subtype-selective binding site of the β-adrenergic receptors (βARs). The β₁/β₂-chimeric receptors showed the importance of the second and seventh transmembrane domains (TM2 and TM7) of the β₂AR for the binding of the β₂-selective agonists such as formoterol and procaterol. Alanine-substituted mutants of TM7 of the β₂AR showed that Tyr³⁰⁸ located at the top of TM7, mainly contributed to β₂ selectivity. However, Tyr³⁰⁸ interacted with formoterol and procaterol in two different ways. The results of Ala- and Phe-substituted mutants indicated that the phenyl group of Tyr³⁰⁸ interacted with the phenyl group in the N-substituent of formoterol (hydrophobic interaction), and the hydroxyl group of Tyr³⁰⁸ interacted with the protonated amine of procaterol (hydrophilic interaction). In contrast to β₂AR, TM2 is a major determinant that β₁-selective agonists such as denopamine and T-0509 bound the β₁AR with high affinity. Three amino acids (Leu¹¹⁰, Thr¹¹⁷, and Val¹²⁰) in TM2 of the β₁AR were identified as major determinants for β₁-selective binding of these agonists. Three-dimensional models built on the basis of the predicted structure of rhodopsin showed that Tyr³⁰⁸ of the β₂AR covered the binding pocket formed by TM2 and TM7 from the upper side, and Thr¹¹⁷ of the β₁AR located in the middle of the binding pocket to provide a hydrogen bonding for the β₁-selective agonists. These data indicate that TM2 and TM7 of the βAR formed the binding pocket that binds the βAR subtype-selective agonists with high affinity.