Fig. 3.

Molecular model of ligand-free β₂-AR. (A) Helices H2–H4 are proposed to form the packing core of the receptor. The positions of Ala-78 (2.49), Asp-113 (3.32), Val-114 (3.33), Ser-161 (4.53), and Ser-165 (4.57) are highlighted. Asp-113 provides the counter ion for the positively charged amine in the ligand. The electrostatic interaction between the ligand and Asp-113 parallels the interaction between the protonated retinal Schiff base in rhodopsin and its Glu-113 counter ion. (B) Helices H5–H7 are shown with approximately the same receptor orientation as in A. Ser-120 interacts with Asn-318 on H7 and is in close proximity to Phe-282 on H6. Ala-128 is packed against Val-214 and Val-218 on H5. Both models are viewed from the extracellular surface of the receptor.