Activation of factor IX by factor XIa.

Blood coagulation factor IX is activated during hemostasis by two distinct mechanisms. Activation through factor VIIa/tissue factor occurs early in the course of fibrin clot formation. Activation by factor XIa appears to be important for maintaining the integrity of the clot over time. In general, coagulation proteases are activated on a phospholipid surface in the presence of a protein cofactor. Until recently, activation of factor IX by factor XIa was thought to be the exception to this rule, as phospholipid has no effect on the reaction and no cofactor had been identified. These curious observations suggest that factor IX is activated by factor XIa in the fluid phase. A large amount of new evidence now indicates that factor IX activation by factor XIa occurs on the surface of activated platelets. The data suggest, however, that this reaction differs significantly from other protease-substrate interactions on the platelet surface. This is likely to be due, in part, to the unusual structure of the factor XI molecule.