Fig. 6.

The highly conserved residues in the OBS. The highly conserved OBS residues (see Fig. 5) Asp^3.32, Trp^6.48, Tyr/Trp^7.43 (cyan) are shown as sticks in the inactive (PDB ID 2RH1) (A) and active (PDB ID 4LDO) (B) structures of β₂AR. Asp^3.32 forms a hydrogen bond with Tyr^7.43 in all available crystal structures of aminergic receptors, suggesting that this interaction is likely to be critical for ligand-recognition of this family. This triad is similarly conserved in the opioid receptor family, as exemplified in the structure of nociceptin opioid receptor bound to a peptide-mimetic compound (PDB ID 4EA3) (C). The ligands are shown as orange sticks; the hydrogen bonds between the nitrogen atoms of the positively charged amine groups and Asp^3.32 or between Asp^3.32 and Tyr^7.43 are indicated by dotted lines.