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Related Experiment Videos

Platelet receptor occupancy with factor IXa promotes factor X activation.

S S Ahmad1, R Rawala-Sheikh, P N Walsh

  • 1Department of Medicine, Temple University School of Medicine, Philadelphia, Pennsylvania 19140.

The Journal of Biological Chemistry
|November 25, 1989
PubMed
Summary

Activated platelets facilitate factor X activation, with factor IXa binding tightly to a platelet receptor in the presence of factor VIIIa. This interaction is crucial for the enzyme

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Area of Science:

  • Biochemistry
  • Hematology
  • Cell Biology

Background:

  • Platelets play a critical role in hemostasis and thrombosis.
  • Factor X activation is a key step in the coagulation cascade, leading to thrombin generation.
  • The precise mechanism and location of factor X activation on activated platelets are not fully elucidated.

Purpose of the Study:

  • To investigate the role of activated platelets as a platform for factor X activation.
  • To determine the functional consequences of factor IXa binding to platelets.
  • To elucidate the relationship between factor IXa binding and factor X activation rates.

Main Methods:

  • Equilibrium binding studies to determine the dissociation constant (Kd) of factor IXa binding to thrombin-activated platelets.

Related Experiment Videos

  • Kinetic experiments measuring factor X activation rates in the presence of factor IXa, factor VIIIa, and activated platelets.
  • Polyacrylamide gel electrophoresis (PAGE) to analyze the structural integrity of bound factor IXa.
  • Enzyme inhibition studies using a site-inhibited factor IXa mutant.

    • Main Results:

    • Factor IXa binds to activated platelets with high affinity (Kd = 0.56 nM), closely matching the concentration required for half-maximal factor X activation (0.53 nM).
    • Factor IXa bound to platelets remains structurally intact and does not form covalent complexes with platelet proteins.
    • Inhibition studies indicate that factor Xa generation is not required for factor IXa binding, and bound factor IXa is kinetically coupled to factor X activation.

    Conclusions:

    • Activated platelets serve as a specific binding site for factor IXa.
    • Factor IXa, when bound to a platelet receptor in the presence of factor VIIIa, is the primary enzyme responsible for factor X activation.
    • This finding highlights the critical role of the platelet surface in amplifying the coagulation cascade.