Factor VIII-driven changes in activated factor IX explored by hydrogen-deuterium exchange mass spectrometry

Area of Science:

• Biochemistry
• Molecular Biology
• Hematology

Background:

• The coagulation cascade is essential for hemostasis, with activated factor IX (FIXa) and activated factor VIII (FVIIIa) playing critical roles.
• Dysfunction of FIXa or FVIIIa leads to hemophilia, a bleeding disorder.
• The mechanism by which FVIIIa enhances the inefficient FIXa enzyme has remained largely elusive.

Purpose of the Study:

• To investigate the structural and functional changes in FIXa upon assembly with FVIIIa using hydrogen-deuterium exchange-mass spectrometry (HDX-MS).
• To identify the specific regions of FIXa involved in FVIIIa interaction and allosteric regulation.
• To explore the impact of disease-associated mutations on FIXa-FVIIIa complex formation and function.

Main Methods:

• Hydrogen-deuterium exchange-mass spectrometry (HDX-MS) to monitor protein conformational changes.
• Site-directed mutagenesis to probe the function of specific interfaces.
• Functional assays to assess the activity of FIXa variants.

Main Results:

• HDX-MS revealed complex conformational changes in FIXa upon FVIIIa assembly, partially overlapping with substrate binding.
• Three helices within exosite II of FIXa were identified as crucial for FVIIIa interaction, forming an extended interactive patch.
• Disease-associated mutations in FIXa were shown to disrupt the allosteric network mediated by FVIIIa.

Conclusions:

• HDX-MS is a powerful tool for visualizing the functional consequences of enzyme-cofactor interactions in the coagulation system.
• The study elucidated key structural elements involved in FIXa-FVIIIa complex formation and allosteric modulation.
• Understanding these interactions provides insights into hemophilia pathogenesis and potential therapeutic strategies.