Structure comparison of AtHPPD-HPPA complex with holo-AtHPPD. (a) Superposition of the active sites of holo-AtHPPD (yellow) and AtHPPD-HPPA (light blue). (b) The H-bond network in the active site of AtHPPD-HPPA complex. The key residues are shown as sticks and the H-bonds are indicated with black dash lines. Gray electron densities (2Fo–Fc map) correspond to Gln293 contoured at 1.0 σ.