The reactive-center loop is a mobile segment of a serpin that presents a target peptide bond to a protease. For inhibitory serpins, attack on this segment initiates structural rearrangement that can trap the attacking enzyme. It is therefore both a recognition surface and part of a mechanical inhibitory mechanism.[ref]
Structure of a serpin-protease complex shows inhibition by deformation by J. A. Huntington and others. Abstract: reactive-centre cleavage and tethered-protease displacement
Its exposure can itself be regulated. In antithrombin, a 1997 comparison of structures with a high-affinity heparin fragment linked activation to expulsion of partly inserted loop residues from the main beta sheet, making the loop more accessible.[ref]
The anticoagulant activation of antithrombin by heparin by Lei Jin and others. Abstract and Figure 3: pentasaccharide-associated loop exposure
Two uses of loop movement should be distinguished. Exposing the loop prepares an inhibitor for reaction; inserting the cleaved loop into the beta sheet participates in trapping the protease. Comparing these states explains why the same protein can differ greatly in inhibitory activity without changing its protein primary structure.[ref]
The anticoagulant activation of antithrombin by heparin by Lei Jin and others. Introduction and Figures 1–3: activation, cleavage and loop insertion
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