The Factor IX γ-Carboxyglutamic Acid (Gla) Domain Is Involved in Interactions between Factor IX and Factor XIa

During hemostasis, factor IX is activated to factor IXaβ by factor VIIa and factor XIa. The glutamic acid-rich γ-carboxyglutamic acid (Gla) domain of factor IX is involved in phospholipid binding and is required for activation by factor VIIa. In contrast, activation by factor XIa is not phospholipid-dependent, raising questions about the importance of the Gla for this reaction. We examined binding of factors IX and IXaβ to factor XIa by surface plasmon resonance. Plasma factors IX and IXaβ bind to factor XIa with K d values of 120 ± 11 n m and 110 ± 8 n m , respectively. Recombinant factor IX bound to factor XIa with a K d of 107 n m , whereas factor IX with a factor VII Gla domain (rFIX/VII-Gla) and factor IX expressed in the presence of warfarin (rFIX-desγ) did not bind. An anti-factor IX Gla monoclonal antibody was a potent inhibitor of factor IX binding to factor XIa ( K i 34 n m ) and activation by factor XIa ( K i 33 n m ). In activated partial thromboplastin time clotting assays, the specific activities of plasma and recombinant factor IX were comparable (200 and 150 units/mg), whereas rFIX/VII-Gla activity was low (