Figure 2.

Ligand recognition of histamine receptors. A) Detailed interaction of amthamine with H₂R. Dashed lines represent hydrogen bonds. B) Amthamine induced cAMP accumulation in HEK293 cells expressing H₂R mutants of the residues in orthosteric pocket (n = 3, ordinary one‐way ANOVA, ****P < 0.0001, ND, not determinable, which refers to cannot be established over the tested concentration range, ns refers to no significance between the WT and mutant). C) Structural comparisons of ligand recognition between histamine‐H₁R and amthamine‐H₂R structures. D) Sequence alignment of the orthosteric pocket of histamine receptors. Residues that interact with ligands are highlighted with colored circles. E) Dose‐dependent curves for amthamine induced cAMP accumulation in HEK293 cells expressing the H₂R mutants (V99^3.33Y, D186^5.42T, T190^5.46N) that are not conserved in H₁R (n  = 3). F‐G) Detailed interactions of histamine F) and immepip G) with H₃R. H) Agonists induced G_i dissociation in HEK293 cells expressing H₃R mutants of the residues in orthosteric pocket by NanoBiT assays (n = 3, ordinary one‐way ANOVA, *P = 0.0456, ****P < 0.0001, NR refers to no response to the ligand, ND, not determinable, which refers to cannot be established over the tested concentration range, ns refers to no significance between the WT and mutant).