Figure 2.

Docking of clozapine-N-oxide (CNO) into the ligand binding pocket of the rat M₃ muscarinic receptor (M3R). (A) Chemical structures of clozapine, CNO, and tiotropium. Note that CNO differs from clozapine only in the presence of the N-oxide moiety. The structure of tiotropium, a muscarinic antagonist/inverse agonist, is shown for comparison because of the availability of a high-resolution X-ray structure of the tiotropium–rat M3R complex [17]. (B) Extracellular view of the tiotropium binding pocket [17]. Key amino acids that are of particular importance for tiotropium binding are highlighted. Both Y^3.33 and A^5.46 (red arrows) are predicted to contact the tiotropium ligand (see the text for details; note that all muscarinic receptor-based DREADDs contain the Y^3.33C and A^5.46G point mutations). The amino acids that replace Y^3.33 and A^5.46 in the DREADD constructs are indicated (C and G at the beginning of the red arrows). CNO can be docked into the M3R binding pocket in a pose very similar to that of tiotropium, without any obvious structural clashes (the two compounds are similar in size and overall shape; CNO, light blue; tiotropium, orange; Dahlia R. Weiss and Brian K. Shoichet, personal communication).