Structure of the ΔI16V17/Q156K trypsinogen-BPTI (green) and S195A trypsinogen-BPTI complexes (orange). The dashed lines indicate hydrogen bonds. The hydrogen bonding distances are Lys156-Thr21, 3.0 Å; Lys156-water 633, 3.0 Å; and water 633-Asp153, 2.6Å. As in ΔI16V17 trypsinogen, Lys15 forms a salt bridge with Asp194 and can be seen overlaying Ile16 of S195A trypsinogen. The autolysis loop of ΔI16V17/Q156K trypsinogen assumes a trypsinlike conformation and is more ordered than in S195A trypsinogen, with electron density visible for Trp141-Gly142-Asn143 and Pro152-Asp153. Asp153 has two conformations (only one is shown for clarity) in the ΔI16V17/Q156K trypsinogen structure. Although only the β carbon of Lys15 is visible in S195A trypsinogen, it is clear that the side chain of Lys15 would clash with Asn153 in the trypsinlike conformation of ΔI16V17/Q156K trypsinogen.