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Comparative platelet binding and kinetic studies with normal and variant factor IXa molecules.

S S Ahmad1, R Rawala-Sheikh, D M Monroe

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

The Journal of Biological Chemistry
|December 5, 1990
PubMed
Summary

Structural features of factor IXa, specifically Asp47 and cleavage at Arg145-Ala146, are crucial for high-affinity binding to activated platelets. This binding is essential for efficient factor X activation in the coagulation cascade.

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Area of Science:

  • Biochemistry
  • Hematology
  • Molecular Biology

Background:

  • Thrombin-stimulated human platelets possess specific receptors for factor IXa.
  • Platelet factor IXa receptor occupancy enhances factor X activation, a key step in blood coagulation.

Purpose of the Study:

  • To investigate the structural requirements for factor IXa binding to platelet receptors.
  • To elucidate the roles of specific amino acid residues and cleavage sites in factor IXa-platelet interactions.

Main Methods:

  • Equilibrium binding studies were performed using normal and variant factor IXa proteins (factor IXaAlabama and factor IXaChapel Hill).
  • Kinetic studies of factor X activation were conducted in the presence and absence of factors VIIIa and X.
  • Binding affinities (Kd) and catalytic efficiencies (kcat/Km) were determined for normal and variant factor IXa.

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Main Results:

  • Variant factor IXa proteins showed altered binding affinities to platelets compared to normal factor IXa.
  • Factor IXaAlabama (Asp47 substitution) and factor IXaChapel Hill (Arg145 substitution) exhibited reduced binding affinity.
  • These binding alterations correlated with reduced rates of factor X activation, indicating impaired catalytic efficiency due to lower affinity.

Conclusions:

  • Asp47 residue and cleavage at Arg145-Ala146 are critical for high-affinity factor IXa binding to activated platelets.
  • Specific structural features of factor IXa are essential for its interaction with platelet receptors and subsequent role in coagulation.