Substrate-binding tunnel of AidH and the conformational change of the tunnel entrance. (a) Sliced-surface view of the substrate-binding tunnel. The tunnel lying between the core and cap domains is lined by hydrophobic residues and has an overall positive charge. (b) The entrance of the tunnel is located on the cap domain and surrounded by hydrophobic residues. In (a) and (b), the molecular surface is coloured according to the electrostatic potential. Positive and negative potentials are shown in blue and red, respectively. (c) The distance between Phe189 and Phe192 changes upon substrate/product binding. In panel 1, free AidH, C4-bonded AidH and C6-bonded AidHS102G (and AidHE219G) are shown in purple, cyan and yellow (and green), respectively. Panel 2 shows that there is no distance change in free-form AidHS102G (orange) and AidHE219G (grey).