= Reactive-center loop
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.<Structure of a serpin-protease complex shows inhibition by deformation by J. A. Huntington and others>[Abstract: reactive-centre cleavage and tethered-protease displacement]{ref}
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.<The anticoagulant activation of antithrombin by heparin by Lei Jin and others>[Abstract and Figure 3: pentasaccharide-associated loop exposure]{ref}
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>.<The anticoagulant activation of antithrombin by heparin by Lei Jin and others>[Introduction and Figures 1–3: activation, cleavage and loop insertion]{ref}
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