Heparin cofactor II (HCF2/HC-II, also called protease inhibitor leuserpin-2/hLS2) is a protein encoded by the SERPIND1 gene that inhibits thrombin, the final protease of the coagulation cascade. HCII is allosterically activated by binding to cell surface glycosaminoglycans (GAGs). The specificity of HCII for thrombin is conferred by a highly acidic hirudin-like N-terminal tail, which becomes available after GAG binding for interaction with the anion-binding exosite I of thrombin. HCII deficiency can lead to increased thrombin generation and a hypercoagulable state. This subgroup corresponds to clade D of the serpin superfamily. In general, SERine Proteinase INhibitors (serpins) exhibit conformational polymorphism shifting from native to cleaved, latent, delta, or polymorphic forms. Many serpins, such as antitrypsin and antichymotrypsin, function as serine protease inhibitors which regulate blood coagulation cascades. Non-inhibitory serpins perform many diverse functions such as chaperoning proteins or transporting hormones. Serpins are of medical interest because mutants have been associated with blood clotting disorders, emphysema, cirrhosis, and dementia. A classification based on evolutionary relatedness has resulted in the assignment of serpins to 16 clades designated A-P along with some orphans.
Comment:depending on the conformational state, the RC loop is surface accessible in the active form or buried and inserted as the central beta strand in the inactive form.
Structure:1JMJ_B: human heparin cofactor II native conformation RCL partially inserted into a beta-sheet and is unfavorable for complex formation with thrombin
Structure:1JMO_A: human heparin cofactor II with bound heparin results in the expulsion of the RCL to a favorable conformation for complex formation with thrombin
Jiyao W et al.(2023). "The conserved domain database in 2023.",