Occupancy of anion binding exosite 2 on thrombin determines Ca2+ dependence of protein C activation.

摘要: Abstract Thrombomodulin (TM) binds thrombin to form a complex that activates the plasma anticoagulant zymogen protein C. TM is an integral membrane glycoprotein contains chondroitin sulfate moiety. Interaction with involves both component of TM, specifically growth factor-like repeats 4-6 (TM 4-6), and sulfate. Removal decreases affinity for approximately 10-fold shifts Ca2+ dependence C activation from simple saturation at > or = 500 microM distinct optimum 100 Ca2+. Thrombin possesses two regions high positive charge, anion binding exosites 1 2. Anion exosite interacts factor region while 2 involved in prothrombin fragment heparin. We demonstrate recombinant truncated membrane-spanning domain, can bind when present either covalently attached (meizothrombin des-fragment 1) reversible association. With meizothrombin 1, independent presence on TM. At 0.27 mM Ca2+, containing (Kd(app) 0.3 nM) 45 times tighter than 14 nM). However, chondroitin-free 2.4 only 4 9.4 These studies suggest occupancy by alters conformation resulting altered activation.