Kallistatin (KST, also called proteinase inhibitor 4/PI4, or kallikrein inhibitor/KAL) is a protein that in humans is encoded by the SERPINA4 gene. Kallistatin inhibits human amidolytic and kininogenase activities of tissue kallikrein. Heparin blocks kallistatin's complex formation with tissue kallikrein and abolishes its inhibitory effect on tissue kallikrein's activity. Kallistatin was found to be expressed in human liver, stomach, pancreas, kidney, aorta, testes, prostate, artery, atrium, ventricle, lung, renal proximal tubular cell, and a colonic carcinoma cell line T84. This family belongs to the clade A of the serpin superfamily, which includes the classical serine proteinase inhibitors, alpha-1-antitrypsin and alpha-1-antichymotrypsin, protein C inhibitor, kallistatin, and non-inhibitory serpins, like corticosteroid and thyroxin binding globulins. 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.
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