The interaction of human factor VIIa with tissue factor

Area of Science:

• Biochemistry
• Molecular Biology
• Hematology

Background:

• The extrinsic pathway of blood coagulation is initiated by the complex of factor VIIa and tissue factor (TF).
• This complex, known as the extrinsic tenase complex (extrinsic Xase), is essential for activating factor X.
• Understanding the precise interactions and dependencies of this complex is vital for comprehending hemostasis and developing therapeutic strategies.

Purpose of the Study:

• To investigate the binding kinetics and thermodynamics of human recombinant factor VIIa (rFVIIa) with recombinant human tissue factor (rTF) incorporated into lipid vesicles.
• To determine the role of membrane phospholipids in stabilizing the rFVIIa-rTF complex and its catalytic activity.
• To elucidate the membrane dependence of the extrinsic Xase complex function, particularly concerning factor X activation.

Main Methods:

• Binding studies using synthetic peptidyl p-nitroanilide substrates (S2238) to monitor factor VIIa catalytic activity upon interaction with TF.
• Equilibrium binding analysis of rFVIIa to TF reconstituted into phosphatidylcholine (TF/PC) or phosphatidylcholine/phosphatidylserine (TF/PCPS) vesicles.
• Measurements of factor X activation rates using both membrane-bound and solution-phase TF to assess catalytic efficiency.

Main Results:

• The interaction between factor VIIa and TF showed saturable binding kinetics without cooperativity, with a dissociation constant (Kd) of 259 +/- 60 pM.
• Incorporation of TF into either TF/PC or TF/PCPS vesicles, or using TF in solution without phospholipids, yielded identical binding parameters, indicating membrane independence for enzyme-cofactor interaction.
• While the enzyme-cofactor binding was membrane-independent, the rate of factor X activation decreased approximately 50-fold when using solution-phase TF compared to membrane-bound TF.

Conclusions:

• The binding interaction between factor VIIa and tissue factor is not significantly stabilized by membrane phospholipids.
• The observed membrane dependence of the extrinsic Xase complex function is primarily attributed to the influence of the membrane surface on the utilization or presentation of factor X.
• These findings highlight the critical role of the membrane surface in modulating the catalytic efficiency of the extrinsic pathway of coagulation.