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Thromb Haemost 2005; 94(01): 60-68
DOI: 10.1160/TH05-02-0097
Blood Coagulation, Fibrinolysis and Cellular Haemostasis
Schattauer GmbH

The functional significance of the autolysis loop in protein C and activated protein C

Likui Yang
1   Edward A. Doisy Department of Biochemistry and Molecular Biology, Saint Louis University School of Medicine, Saint Louis, Missouri, USA
,
Chandrashekhara Manithody
1   Edward A. Doisy Department of Biochemistry and Molecular Biology, Saint Louis University School of Medicine, Saint Louis, Missouri, USA
,
Alireza R. Rezaie
1   Edward A. Doisy Department of Biochemistry and Molecular Biology, Saint Louis University School of Medicine, Saint Louis, Missouri, USA
› Author AffiliationsFinancial support: The research discussed herein was supported by grants awarded by the National Heart, Lung, and Blood Institute of the National Institutes of Health (HL 62565 and HL 68571 to ARR).
Further Information

Publication History

Received 07 February 2005

Accepted after resubmission 28 March 2005

Publication Date:
05 December 2017 (online)

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Summary

The autolysis loop of activated protein C (APC) is five residues longer than the autolysis loop of other vitamin K-dependent coagulation proteases. To investigate the role of this loop in the zymogenic and anticoagulant properties of the molecule, a protein C mutant was constructed in which the autolysis loop of the protein was replaced with the corresponding loop of factor X. The protein C mutant was activated by thrombin with ~5-fold higher rate in the presence of Ca2+. Both kinetics and direct binding studies revealed that the Ca2+ affinity of the mutant has been impaired ∼3-fold. The result of a factorVa degradation assay revealed that the anticoagulant function of the mutant has been improved 4–5-fold in the absence but not in the presence of protein S. The improvement was due to a better recognition of both the P1-Arg506 and P1-Arg306 cleavage sites by the mutant protease. However, the plasma half-life of the mutant was markedly shortened due to faster inactivation by plasma serpins. These results suggest that the autolysis loop of protein C is critical for the Ca2+-dependence of activation by thrombin. Moreover, a longer autolysis loop in APC is not optimal for interaction with factor Va in the absence of protein S, but it contributes to the lack of serpin reactivity and longer half-life of the protease in plasma.

Keywords

Activated protein C - thrombin - thrombomodulin - factorVa - antithrombin
 

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