Coagulation factor Xa (within the prothrombinase complex) (FXa)

Coagulation factor Xa (FXa) is a vitamin K-dependent serine protease that plays a central role in the coagulation cascade by serving as the primary enzyme responsible for converting prothrombin into thrombin. While FXa can exist in a free form, its catalytic efficiency is increased by approximately 300,000-fold when it is incorporated into the prothrombinase complex, which consists of FXa, its cofactor factor Va, calcium ions, and a phospholipid surface (typically provided by activated platelets). This complex represents the 'common pathway' of coagulation, where the intrinsic and extrinsic pathways converge to generate the burst of thrombin necessary for fibrin clot formation. Because of its rate-limiting role in thrombin generation, FXa is a major therapeutic target for anticoagulants used to prevent and treat thromboembolic disorders such as deep vein thrombosis, pulmonary embolism, and stroke in patients with atrial fibrillation. Modern direct factor Xa inhibitors are designed to inhibit both free and prothrombinase-bound FXa, providing effective anticoagulation with a more predictable pharmacokinetic profile compared to traditional vitamin K antagonists.

Other names

Activated factor XStuart-Prower factorThrombokinaseProthrombinase-bound FXaFactor Xa

Direct oral anticoagulants (DOACs) like rivaroxaban and apixaban act as competitive inhibitors that bind directly to the active site of factor Xa, whether it is free in plasma or assembled within the prothrombinase complex. Indirect inhibitors like heparin and fondaparinux act by accelerating the activity of antithrombin III, which then neutralizes factor Xa.