Serpins are inhibitors of serine proteases that act by reacting covalently, leaving a segment attached to the protease, and then the remaining segment dissociates and rearranges itself into a new conformation.

Top left: Trypsin Right: Trypsin with its anti-trypsin bound. THe thick blue segment of the anti-trypsin (serpin) has entered the catalytic site of trypsin, and will eventually be cleaved, leaving an acyl-enzyme intermediate. Notice the 4-stranded beta-sheet colored cyan. Bottom left: The serpin anti-trypsin after being cleaved by trypsin. Notice that the blue segment has now formed as a beta-strand and inserted itself into the 4-stranded sheet.	Top left: PDB (1dpo) Earnest, T.,  Fauman, E.,  Craik, C.S.,  Stroud, R. A structure of trypsin at 120 K: comparison of low temperature and room temperature structures. Proteins v10 pp.171-187 , 1991 PubMed Right: PDB (1k9o) Ye, S.,  Cech, A.L.,  Belmares, R.,  Bergstrom, R.C.,  Tong, Y.,  Corey, D.R.,  Kanost, M.R.,  Goldsmith, E.J. The structure of a Michaelis serpin-protease complex. Nat.Struct.Biol. v8 pp.979-983 , 2001 (PubMed) Bottom left: PDB (9api) Engh, R.,  Lobermann, H.,  Schneider, M.,  Wiegand, G.,  Huber, R.,  Laurell, C.B. The S variant of human alpha 1-antitrypsin, structure and implications for function and metabolism. Protein Eng. v2 pp.407-415 , 1989 (PubMed)