Substrate and product binding to AidH. (a) Active site of the AidHS102G–C6-HSL (enzyme–substrate) complex. The substrate C6-HSL binds to AidHS102G by hydrogen-bonding interactions with the residues in the active site. The water molecule W1 close to the substrate is indicated as a red sphere. (b) Active site of the wild-type AidH–C4-HS (enzyme–product) complex. The electron density of some H atoms (shown as a green mesh) can be observed in panel 1. In panel 2, the product C4-HS binds in the active site of AidH. In (a) and (b), the 2Fo − Fc electron density for the bound C6-HSL and C4-HS contoured at the 1.5σ level is shown as a grey mesh. C6-HSL and C4-HS are indicated in yellow stick representation. (c) Wall-eyed stereo presentation of the superimposed active sites of the AidHS102G–C6-HSL complex and the AidHE219G–C6-HS complex, revealing the positional divergence in His248 and the loop where Glu219 is located. Residues of the AidHS102G–C6-HSL complex and AidHE219G–C6-HS complex are shown in marine and pink, respectively.