Molecular Recognition Properties of Nicotinic Ligands Determining Selectivity Between Insect and Mammalian Receptors

This investigation defines the roles of various amino acids, neighboring key conserved amino acids in loops C and D of the nicotinic acetylcholine (ACh) receptor (nAChR), in the selective molecular recognition of nicotinic ligands with diverse pharmacophores using <i>Aplysia californica</i> ACh binding protein Y55W (<i>Ac</i>-AChBP) mutants (+Q57R; + Q57R+S189 V; + Q57R+S189E; + Q57T; + Q57T+S189 V; + Q57T+S189E) and <i>Lymnaea stagnalis</i> AChBP (<i>Ls-</i>AChBP) mutants (Q55T; Q55T+S186E; Q55R) as insect and mammalian nAChR structural surrogates, respectively. <i>N</i>-nitro/cyanoimine insecticides show high affinity to four <i>Ac</i>-AChBPs containing Arg57 or Thr57 and Ser189 or Val189, except for those with Glu189. Pyrazinoyl compound selectively interacts with the three <i>Ac</i>-AChBPs containing Arg57 and Ser189, Val189, or Glu189. Cationic ligands prefer three <i>Ac</i>-AChBPs with Thr57 and Ser189, Val189, or Glu189 and two <i>Ls</i>-AChBPs providing Thr55 ± Glu186 over the four <i>Ac</i>- and <i>Ls</i>-AChBPs with Arg57/55. Accordingly, loop C contributes to <i>N</i>-nitro/cyanoimine insecticide action, and loop D controls the affinity of the pyrazinoyl or cationic ligand.