Binding pockets of the beta(1)- and beta(2)-adrenergic receptors for subtype-selective agonists

We examined the subtype-selective binding site of the beta-adrenergic recep tors (beta ARs). The beta(1)/beta(2)-chimeric receptors showed the importan ce of the second and seventh transmembrane domains (TM2 and TM7) of the bet a(2)AR for the binding of the beta(2)-selective agonists such as formoterol and procaterol. Alanine-substituted mutants of TM7 of the beta(2)AR showed that Tyr(308), located at the top of TM7, mainly contributed to beta(2) se lectivity. However, Tyr(308) interacted with formoterol and procaterol in t wo different ways. The results of Ala- and Phe-substituted mutants indicate d that the phenyl group of Tyr(308) interacted with the phenyl group in the N-substituent of formoterol (hydrophobic interaction), and the hydroxyl gr oup of Tyr(308) interacted with the protonated amine of procaterol (hydroph ilic interaction). In contrast to beta(2)AR, TM2 is a major determinant tha t beta(1)-selective agonists such as denopamine and T-0509 bound the beta(1 )AR with high affinity. Three amino acids (Leu(110), Thr(117), and Val(120) ) in TM2 of the beta(1)AR were identified as major determinants for beta(1) -selective binding of these agonists. Three-dimensional models built on the basis of the predicted structure of rhodopsin showed that Tyr(308) of the beta(2)AR covered the binding pocket formed by TM2 and TM7 from the upper s ide, and Thr(117) of the beta(1)AR located in the middle of the binding poc ket to provide a hydrogen bonding for the beta(1)-selective agonists. These data indicate that TM2 and TM7 of the beta AR formed the binding pocket th at binds the beta AR subtype-selective agonists with high affinity.