Tropisetron adopts two poses in the orthosteric site of variant II. (a) The chemical structure of tropisetron. (b) The interacting residues of variant II are shown with C positions colored white for the principal side and cyan for the complementary side, with one tropisetron pose (yellow C positions). Two water molecules discussed in the text are depicted as blue spheres; O and N positions are red and blue, respectively. Selected hydrogen-bonding interactions are shown as blue dashed lines. The second pose, which is common with that adopted in WT AcAChBP (PDB entry 2wnc), is shown with black C atoms.