The structure of a Michaelis serpin-protease complex

Serine protease inhibitors (serpins) regulate the activities of circulating proteases. Serpins inhibit proteases by acylating the serine hydroxyl at t heir active sites. Before deacylation and complete proteolysis of the serpi n can occur, massive conformational changes are triggered in the serpin whi le maintaining the covalent linkage between the protease and serpin. Here w e report the structure of a serpin-trypsin Michaelis complex, which we visu alized by using the S195A trypsin mutant to prevent covalent complex format ion. This encounter complex reveals a more extensive interaction surface th an that present in small inhibitor-protease complexes and is a template for modeling other serpin-protease pairs. Mutations of several serpin residues at the interface reduced the inhibitory activity of the serpin. The serine residue C-terminal to the scissile peptide bond is found in a closer than usual interaction with His 57 at the active site of trypsin.