Essential thrombin residues for inhibition by protein C inhibitor with the cofactors heparin and thrombomodulin.
BACKGROUND
Protein C inhibitor (PCI) and antithrombin (AT) are serine protease inhibitors (serpins) that inhibit a wide array of blood coagulation serine proteases including thrombin.


OBJECTIVE
Fifty-five Ala-scanned recombinant thrombin mutants were used to determine thrombin residues important for inhibition by PCI with and without the cofactors heparin and thrombomodulin (TM) and compared with the prototypical serpin, AT.


RESULTS
Residues around the active site (Tyr50 and Glu202) and the sodium-binding site (Glu229 and Arg233) were required for thrombin inhibition by PCI with and without cofactors. Exosite-2 residues (Arg89, Arg93, Glu94, Arg98, Arg245, Arg248, and Gln251) were critical for heparin-accelerated inhibition of thrombin by PCI. Exosite-1 residues (especially Lys65 and Tyr71) were required for enhanced PCI inhibition of thrombin-TM. Interestingly, we also found that the TM chondroitin sulfate moiety is not required for the approximately 150-fold enhanced rate of thrombin inhibition by PCI. Using the aforementioned thrombin exosite-2 mutants that were essential for heparin-catalyzed PCI-thrombin inhibition reactions we found no change in PCI inhibition rates for thrombin-TM.


CONCLUSIONS
Collectively, these results show that (i) similar thrombin exosite-2 residues are critical for the heparin-catalyzed inhibition by PCI and AT, (ii) PCI and AT are different in their thrombin-TM inhibition properties, and (iii) PCI has a distinct advantage over AT in the regulation of the activity of thrombin-TM.
Fortenberry
Y M
YM
Departments of Pathology and Laboratory Medicine, and Pharmacology and Medicine, University of North Carolina at Chapel Hill, School of Medicine, Chapel Hill, NC, USA. yforten1@jhmi.edu
Whinna
H C
HC
Cooper
S T
ST
Myles
T
T
Leung
L L K
LL
Church
F C
FC
D019943:Amino Acid Substitution; D001665:Binding Sites / Q000235:genetics; D006493:Heparin / Q000378:metabolism* / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D008958:Models, Molecular; D046912:Multiprotein Complexes; D016297:Mutagenesis, Site-Directed; D018046:Protein C Inhibitor / Q000235:genetics / Q000494:pharmacology*; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism / Q000494:pharmacology; D013917:Thrombin / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D018180:Thrombomodulin / Q000378:metabolism*
D046912:Multiprotein Complexes; D018046:Protein C Inhibitor; D011994:Recombinant Proteins; D018180:Thrombomodulin; D006493:Heparin; D013917:Thrombin