The biogenic amine-binding pocket of ABP. (a) Stick diagram showing the side-chain structure of the binding pocket with the tryptamine ligand bound. Protein C atoms are shown in white, O atoms in red and N atoms in blue. Ligand C atoms are shown in green and N atoms in blue. Water molecules are shown as red spheres and hydrogen bonds are shown as red dashed lines. (b) Stick diagram showing the hydrogen-bonding network that acts to stabilize the aliphatic amino group of tryptamine. The color scheme is the same as in (a). (c) Left, space-filling diagram of cation–π and π–π interactions involved in ligand binding. Right, the same view as a stick diagram with electron density shown (2mFo − DFc contoured at 1.0σ). (d) Space-filling model showing tryptamine bound in a portion of the ABP central cavity. Protein atoms are colored as in (a), while tryptamine is colored yellow.