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Hemostasis is a crucial process that prevents excessive blood loss from damaged blood vessels. It involves various mechanisms such as vasoconstriction, platelet adhesion and activation, and fibrin formation. The importance of each mechanism depends on the type of vessel injury. In contrast, thrombosis is the abnormal formation of a blood clot within the blood vessels, leading to potential complications if the clot obstructs blood flow. Thrombosis can be caused by increased coagulability of the...
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Blood clotting or coagulation involves extrinsic and intrinsic pathways, which ultimately merge into the common pathway, forming a fibrin clot.
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The platelet phase, the second stage of hemostasis, commences around 15-20 seconds after an injury. It follows and overlaps with the vascular phase, during which blood vessels constrict to minimize blood loss.
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After a fibrin clot is formed, the next step is clot retraction, a vital process facilitated by platelet contractile proteins, such as actin and myosin. These proteins pull the fibrin strands closer together and condense the clot. This action reduces the size of the clot, creating a smaller, denser structure that effectively seals off the damaged vessel. Clot retraction consolidates the clot and helps with wound healing by bringing the edges of the damaged blood vessel closer together.
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The coagulation phase is a critical part of the body's process to prevent blood loss following injury to blood vessels. It involves chemical reactions that form a clot to seal the injured area. The clotting process begins shortly after injury, within 15-20 seconds for severe damage and 1-2 minutes for minor injuries.
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A Microfluidic Flow Chamber Model for Platelet Transfusion and Hemostasis Measures Platelet Deposition and Fibrin Formation in Real-time
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Factor Xa dimerization competes with prothrombinase complex formation on platelet-like membrane surfaces.

Tilen Koklic1, Rima Chattopadhyay2, Rinku Majumder2

  • 1*Laboratory of Biophysics, Condensed Matter Physics Department, Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia.

The Biochemical Journal
|January 10, 2015
PubMed
Summary

Platelet membranes expose phosphatidylserine (PS), crucial for blood coagulation. Factor Va (fVa) competes with factor Xa (fXa) dimerization, releasing fXa inhibition and amplifying thrombin production.

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Area of Science:

  • Biochemistry
  • Hematology
  • Molecular Biology

Background:

  • Phosphatidylserine (PS) exposure on activated platelets is vital for blood coagulation.
  • Factor Xa (fXa) and Factor Va (fVa) form a complex enhancing prothrombin proteolysis.
  • Soluble PS and model membranes promote inactive fXa dimer formation at 5 mM Ca²⁺.

Purpose of the Study:

  • Investigate the competition between fXa dimerization and prothrombinase complex formation.
  • Determine the influence of Ca²⁺ and lipid membrane concentrations on this balance.
  • Elucidate the role of fVa in modulating fXa dimerization and thrombin generation.

Main Methods:

  • Utilized homo-FRET measurements with inactivated fXa (FEGR-fXa) to monitor dimer formation.
  • Assessed prothrombinase activity to quantify complex formation.
  • Employed quantitative global analysis of protein interaction equilibria on surfaces.
  • Conducted experiments on activated platelet-derived microparticles (MPs).

Main Results:

  • FEGR-fXa dimer formation was dissociated by the addition of fVa, indicating complex formation.
  • The dissociation constant for fXa dimer (K(fXa×fXa)(d, σ)) was approximately 10-fold lower than for the fXa-fVa complex.
  • Competition between fXa dimerization and fXa-fVa complex formation was more pronounced on MPs.
  • At physiological Ca²⁺ levels (2-5 mM), fVa effectively competed with fXa dimers.

Conclusions:

  • Factor Va actively disassembles fXa dimers, releasing inhibition.
  • This dissociation, coupled with cofactor activity, significantly amplifies thrombin production.
  • PS-exposing platelet membranes offer a novel regulatory mechanism for blood coagulation amplification and propagation.