Exosite-dependent regulation of the protein C anticoagulant pathway

Activated protein C (APC) is a vitamin K-dependent anticoagulant serine protease in plasma that downregulates the coagulation cascade by degrading cofactors Va and VIIIa by limited proteolysis. In addition to its anticoagulant function, APC also exhibits potent profibrinolytic and anti-inflammatory properties. The proteolytic activity of APC in plasma is slowly inhibited by three serpins: protein C inhibitor, plasminogen activator inhibitor-1, and alpha(1)-antitrypsin. Recent structural and mutagenesis data have indicated that basic residues of three exposed surface loops known as the 39-loop (Lys(37)-Lys(39)), 60-loop (Lys(62), Lys(63)), and 70-80-loop (Arg(74), Arg(75), and Lys(78)) (chymotrypsin numbering) constitute an anion-binding exosite in APC that interacts with these macromolecular substrates and inhibitors. Moreover, this exosite plays a critical role in the thrombomodulin-dependent activation of the zymogen protein C by thrombin. This article briefly reviews how the binding of physiological protein and polysaccharide cofactors on this exosite modulates the protein C anticoagulant pathway in plasma.