A common protein C inhibitor exosite partially controls the heparin induced activation and inhibition of serine proteases

Protein C inhibitor (PCI) maintains hemostasis by inhibiting both procoagulant and anticoagulant serine proteases, and plays important roles in coagulation, fibrinolysis, reproduction, and anti-angiogenesis. The reactive site loop of PCI traps and irreversibly inhibits the proteases like APC (activating protein C), thrombin (FIIa) and factor Xa (FXa). Previous studies on antithrombin (ATIII) had identified Tyr253 and Glu255 as functional exosites that interact and aid in the inhibition of factor IXa and FXa. Presence of exosite in PCI is not known, however a sequence comparison with the PCI from different vertebrate species and ATIII identified Glu239 to be absolutely conserved. PCI residues analogous to ATIII exosite residues were mutated to R238A and E239A. Purified variant PCI in the presence of heparin (10 μg/ml) showed a 2–4 fold decrease in the rate of inhibition of the proteases. However, the stoichiometry of inhibition of FIIa, APC, and FXa by native PCI, R238A and E239A variants were found to be close to 1.0, which also indicated the formation of stable complexes based on SDS-PAGE and western blot analysis with thrombin and APC. Our findings revealed the possible presence of an exosite in PCI that influences the protease inhibition rates.