Structure of PZ/ZPI complex. (A) Stereo view of PZ/ZPI complex. ZPI has a typical serpin fold with 3 main β-sheets: A (light blue), B (magenta), and C (green), and a fully exposed reactive loop (red). The protease domain of PZ is colored in rainbow from N (blue) to C terminus (red), and EGF2 domain is in dark gray. The P1 residue Y387 is shown in sticks. (B) Stereo view of the PZ/ZPI-binding interface. The PZ-binding site on ZPI is centered around hG (purple) and the C terminus of hA (brown). The salt bridges are shown in magenta dashes, and hydrogen bonds in green dashes. (C) The open-up view of the PZ/ZPI interface showing the complementary electrostatic surfaces of PZ (left) and ZPI (right) with positive charges in blue and negatives in red. In addition, the hydrophobic residues (Y240 and M71 of ZPI, and L353 of PZ) dock into corresponding surface cavities (black arrows). Other residues, such as L102, P103, M211, and K245 of PZ and L289, K284, and A290 of ZPI, also make significant hydrophobic interactions in the interface. (D) Stereo view of the overlaid structures of PZ (colored in rainbow) and FXa (PDB 2UWP, in gray). Ser195, Asp194, His57, and D189 of FXa are replaced (PZ numbering in brackets) by Asp313, Met312, Lys178, and Ala307 in PZ, respectively. The S1 pocket of PZ is filled by a 3₁₀ helix formed by residues 189-192 (307-310), a bulky Trp at 193 (311), and Q216 (334), and the activation peptide of FXa is in purple. In comparison with FXa, PZ lacks the disulfide bond of 191-220 around the S1 pocket and the disulfide bond^22-27  linking the activation peptide of FXa, but has an extra disulfide bridge linking EGF2 to the protease domain (data not shown).