English: 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​). ^{[1] [2]}