Properties of the factor Xa binding site on human platelets

Area of Science:

• Biochemistry
• Hematology
• Molecular Biology

Background:

• Coagulation Factor Xa (FXa) is a critical enzyme in the blood coagulation cascade, responsible for converting prothrombin to thrombin.
• Platelets play a pivotal role in hemostasis, not only through aggregation but also by providing a surface for coagulation factor assembly.
• The interaction between FXa and activated platelets is crucial for efficient thrombin generation.

Purpose of the Study:

• To quantify the binding affinity of human FXa to activated platelets.
• To investigate the impact of platelet binding on FXa's enzymatic activity.
• To elucidate the role of platelet-derived cofactors in FXa-platelet interactions.

Main Methods:

• Equilibrium binding studies using 125I-labeled FXa to determine binding affinity (Ka).
• Measurement of FXa enzymatic activity in the conversion of prothrombin to thrombin in the presence of platelets.
• Competition assays with related coagulation factors and functional assays using antibodies against Factor V.

Main Results:

• Human FXa exhibits a high affinity (Ka = 3-4 x 10^10 M^-1) for thrombin-activated platelets.
• Platelet-bound FXa shows a 300,000-fold increase in apparent enzymatic activity, significantly exceeding that with optimal Factor V and phospholipids.
• Calcium ions (Ca2+) are essential for FXa-platelet binding, with optimal concentration at 2.5 mM. Factor V appears to mediate this interaction.

Conclusions:

• Platelets provide a high-affinity binding site for FXa, dramatically enhancing its prothrombinase activity.
• The FXa-platelet interaction, likely mediated by platelet Factor V, is a key step in rapid thrombin generation.
• FXa bound to platelets is resistant to inactivation by antithrombin III, suggesting a role in sustained thrombin formation.