High-affinity, specific factor IXa binding to platelets is mediated in part by residues 3-11

Area of Science:

• Biochemistry
• Hematology
• Molecular Biology

Background:

• The Gla domain of factor IXa plays a role in its interaction with human platelets.
• Understanding these interactions is key to comprehending the coagulation cascade.

Purpose of the Study:

• To pinpoint the specific amino acids within the Gla domain of factor IXa responsible for mediating its binding to human platelets.
• To investigate the functional impact of these amino acids on factor IXa activity.

Main Methods:

• Utilized chimeric molecules and point mutations within the Gla domain of recombinant factor IX.
• Employed molecular modeling based on bovine prothrombin's Gla domain structure.
• Assessed factor IXa binding to thrombin-activated platelets under various conditions, including the presence of factor VIIIa and factor X.

Main Results:

• Identified specific surface structures in the Gla domain with sequence variations among factor IX, X, and VII.
• Quantified factor IXa binding to platelets, revealing approximately 550 sites per platelet with a Kd of 0.65 nM in the presence of factor VIIIa and X.
• Demonstrated that mutations at positions 4-5 or 9-11 significantly decrease binding affinity and site number.
• A chimera with the factor VII Gla domain and specific factor IX residues showed altered binding and factor X activation.

Conclusions:

• Specific amino acid residues within the Gla domain of factor IXa are essential for high-affinity binding to human platelets.
• These residues influence both the number of binding sites and the affinity of factor IXa.
• The findings provide insights into the molecular mechanisms of factor IXa-platelet interaction and potential therapeutic targets.