Serpin
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![A protease (grey) bound to a serpin reactive centre loop (RCL, blue). When the protease's catalytic triad (red) cleaves the RCL, it becomes trapped in an inactive conformation. (PDB: 1K9O)](/Images/godic/202502/10/Serpin_and_protease3738.png")
![The native state of serpins is an equilibrium between a fully stressed state (left) and a partially relaxed state (right). The five-stranded A-sheet (light blue) contains two functionally important regions for the serpin's mechanism, the breach and the shutter. The reactive centre loop (RCL, blue) exists in a dynamic equilibrium between the fully exposed conformation (left) and a conformation where it is partially inserted into the breach of the A-sheet (right).(PDB: 1QLP, 1YXA)[42][43]](/Images/godic/202502/10/Serpin_equilibrium3738.png")
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Serpins are a superfamily of proteins with similar structures that were first identified for their protease inhibition activity and are found in all kingdoms of life. The acronym serpin was originally coined because the first serpins to be identified act on chymotrypsin-like serine proteases (serine protease inhibitors). They are notable for their unusual mechanism of action, in which they irreversibly inhibit their target protease by undergoing a large conformational change to disrupt its active site. This contrasts with the more common competitive mechanism for protease inhibitors that bind to and block access to the protease active site.