A new approach to docking in the β₂-adrenergic receptor that exploits the domain structure of G-protein-coupled receptors

A novel technique for docking ligands to the β₂-adrenergic receptor is described which exploits the domain structure of this class of receptors. The ligands (norepinephrine an agonist pindolol, a partial agonist; and propranolol, an antagonist) were docked into the receptor using the key conserved aspartate on helix 3 (D(113)) as an initial guide to the placement of the amino group and GRID maps (Goodford, P. J. J. Med. Chem. 1985, 28, 849) to identify the likely binding regions of the hydrophobic (and hydroxyl) moieties on the A domain (comprising of helices 1-5). The essence of the new approach involved pulling the B domain, which includes helices 6 and 7, away from the other domain by 5-7 Å. During the subsequent minimization and molecular dynamics, the receptor ligand complex reformed to yield structures which were very well supported by site-directed mutagenesis data. In particular, the model predicted a number of important interactions between the antagonist and key residues on helix 7 (notably Leu³¹¹ and Asn³¹²) which have not been described in many previous computer simulation studies. The justification for this new approach is discussed in terms of (a) phase space sampling and (b) mimicking the natural domain dynamics which may include domain swapping and dimerization to farm a 5,6-domain-swapped dimer. The observed structural changes in the receptor when pindolol, the partial agonist, was docked were midway between those observed for propranolol and norepinephrine. These structural changes, particularly the changes in helix- helix interactions at the dimer interface, support the idea that the receptors have a very dynamic structure and may shed some light on the activation process. The receptor model used in these studies is well supported by experiment, including site-directed mutagenesis (helices 1-7), zinc binding studies (helices 2, 3, 5, and 6), the substituted cysteine accessibility method (helices 3, 5, and 7), and site-directed spin-labeling studies (helices 3-6).