FIGURE 1.

Structure of the rShPI-1/K13L·PPE complex (PDB code 3UOU). a, schematic model of the overall structure of the complex between PPE (green, surface representation) and rShPI-1/K13L (blue), showing the changed P1 residue Leu¹³ of the inhibitor in stick representation. The primary (P6-P5′ sites) and secondary binding loops are highlighted in red and orange, respectively, and the linking disulfide bridge, Cys¹²–Cys³⁶, is shown in yellow. b, close view of the entire complex interface centered on the S1 pocket of PPE, illustrating the formation of an antiparallel β-sheet by the inhibitor loops within the concave binding pocket of the enzyme that is typical for canonical inhibitors. The binding loops of rShPI-1/K13L (stick representation) are well defined by the 2F_o − F_c map (blue) countered at 1σ. The side chains of the catalytic triad residues His⁵⁷, Asp¹⁰², and Ser¹⁹⁵ as well as Ser¹⁸⁹ at the bottom of the S1 pocket of PPE are highlighted in stick representation. c, buried surface area (BSA) of rShPI-1/K13L residues (black) involved in the PPE interface compared with that of wild-type rShPI-1A (gray) bound to trypsin. The buried surface area represents a percentage of the total surface area that is buried after complex formation. The amino acid sequence of rShPI-1/K13L and the corresponding Pn sites are shown.