Fig. 6.

Structure of K15A trypsinogen-BPTI complex. (A) Stereo view of S195A trypsinogen (light bonds) and K15A trypsinogen (dark bonds) in their BPTI complexes. Ile16 and Asp194 have similar positions in both structures. In S195A trypsinogen, electron density is only observed for the β carbon of Lys15, whereas electron density is not observed for Ala15 in K15A trypsinogen. The autolysis loop is disordered in both structures, with electron density observed only for Trp141-Gly142 and Asp153. Nevertheless, it is clear that the autolysis loop has different conformations in the two structures as seen by the position of Gly142. The side chain of Lys15 would sterically clash with the autolysis loop conformation observed in the K15A trypsinogen structure. (B) Stereo view of wild-type trypsin (light bonds) and K15A trypsinogen (dark bonds) in their BPTI complexes. The entire autolysis loop is observed in the trypsin structure. The position of Gly142 in K15A trypsinogen is similar to that in trypsin, which suggests that the autolysis loop of K15A trypsinogen has a more trypsinlike conformation than S195A trypsinogen.