Factor Va-factor Xa interactions: molecular sites involved in enzyme:cofactor assembly

Area of Science:

• Biochemistry
• Molecular Biology
• Hematology

Background:

• Thrombin generation via the prothrombinase complex is central to hemostasis.
• Activated factor V (FVa) is an essential cofactor for factor Xa (FXa) in prothrombin activation.
• FXa's catalytic efficiency dramatically increases upon assembly with FVa on phospholipid membranes.

Purpose of the Study:

• To elucidate the molecular interactions between factor V (FV) and factor Xa (FXa).
• To understand the molecular events governing the assembly of the FXa:FVa complex.
• To investigate the role of FV activation, specifically B-domain release, in enhancing cofactor activity.

Main Methods:

• Utilized recombinant FV mutants to study FVa and FXa assembly.
• Employed a functional prothrombin activation assay to investigate FXa:FVa interaction on phospholipids.
• Developed a novel direct binding assay to assess FXa:FVa interaction independently of prothrombin.

Main Results:

• All three thrombin cleavage sites in FV contribute to increased FXa affinity.
• The B-domain of intact FV exerts an inhibitory effect on the FV-FXa interaction.
• FV activation, involving B-domain release, is critical for optimal FXa binding and procoagulant activity.

Conclusions:

• The B-domain of FV plays a crucial inhibitory role in the unbound state, preventing premature coagulation.
• FV activation exposes binding sites for FXa, facilitating efficient prothrombinase complex formation.
• Understanding these molecular mechanisms provides insight into regulating coagulation and preventing thrombotic disorders.