Comparison of the coordination of SOTI-III with those of other peptide trypsin inhibitors. (a) Superposition of SOTI-III with other canonical inhibitors. Trypsin is shown in surface representation. SOTI-III (chain A) is colored purple, STI lime (PDB entry 1avw
; Song & Suh, 1998), SFTI-I black (PDB entry 1sfi
; Luckett et al., 1999), CPTI-II blue (PDB entry 2btc
; Helland et al., 1999) and BPTI orange (PDB entry 1f7z
; Pasternak et al., 2001). Inhibitors were superposed based on trypsin main-chain atoms. (b) Close-up of the active-site residues and the inhibitor loop with arginine or lysine P1 residues. The coloring of the inhibitors is as in (a). Active-site residues are shown as sticks, with C, N and O atoms in white, blue and red, respectively. (c) Superposition with the SOTI-III complex of 99 deposited trypsin inhibitor structures that are close to residues 12–15 of SOTI-III in chain D (<3 Å; see Supplementary Material). SOTI-III is shown in purple and trypsin is shown in white. Superposed trypsin molecules (blue lines) align well, while the superposed inhibitors (gray lines) do not align or address the Phe14 binding site of SOTI-III. Tyr45 of trypsin in the SOTI-III complex has a different rotamer conformation. (d) Close-up of the inhibitor loop region in (c) showing the P1 residue Arg32 (red box). The inhibitors superpose well with the SOTI-III inhibitor in this region.