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Ixolaris: a factor Xa heparin-binding exosite inhibitor
Robson Q Monteiro1, Alireza R Rezaie, José M C Ribeiro
1Instituto de Bioquímica Médica, Programa de Biologia Estrutural, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, 21941-590, Brazil.
The Biochemical Journal
|December 25, 2004
Summary
Ixolaris, a tick inhibitor, specifically targets the Factor Xa heparin-binding exosite. This interaction is crucial for inhibiting thrombin generation and offers a novel therapeutic target.
Area of Science:
- Biochemistry
- Hematology
- Molecular Biology
Background:
- Ixolaris is a novel two-Kunitz tissue factor pathway inhibitor (TFPI) derived from tick salivary glands.
- Unlike human TFPI, Ixolaris exhibits unique binding characteristics to Factor Xa (FXa), involving exosites.
Purpose of the Study:
- To elucidate the specific binding site and mechanism of action of Ixolaris on FXa.
- To investigate the role of the FXa heparin-binding exosite (HBE) in the Ixolaris-FXa interaction.
Main Methods:
- Enzyme inhibition assays using antithrombin-heparin and antithrombin-pentasaccharide complexes.
- FXa binding studies with immobilized heparin.
- Allosteric modulation analysis of FXa catalytic activity.
- Site-directed mutagenesis of the FXa HBE.
Main Results:
- Ixolaris specifically binds to the FXa HBE, distinct from its interaction with the active site.
- Ixolaris binding reduces FXa inhibition by antithrombin-heparin and impairs heparin binding.
- Mutagenesis studies identified key amino acids (Arg-93, Arg-165, Lys-169) critical for Ixolaris interaction.
- Ixolaris effectively inhibits thrombin generation by the prothrombinase complex.
Conclusions:
- Ixolaris is the first characterized inhibitor that selectively targets the FXa heparin-binding exosite.
- The FXa HBE is essential for Ixolaris binding and inhibitory activity.
- Ixolaris represents a potential therapeutic agent for modulating coagulation.