Interaction of trypsin with its inhibitor. Structure of a complex of trypsin (yellow) and pancreatic trypsin inhibitor (red). Notice that lysine 15 of the inhibitor penetrates into the active site of the enzyme. There it forms an ionic bond with aspartate 189 in the active site. Also notice that bound inhibitor and the free inhibitor are almost identical in structure.
[Drawn from 1BPI.pdb.]