Coagulation factor Xa–Antithrombin III complex (FXa-ATIII)

The Coagulation factor Xa–Antithrombin III complex is a critical regulatory assembly in the human blood coagulation cascade. It consists of the activated serine protease Factor Xa (FXa) and its primary endogenous inhibitor, Antithrombin III (ATIII), a member of the serpin superfamily. Factor Xa is responsible for the conversion of prothrombin to thrombin, a pivotal step in clot formation. Antithrombin III neutralizes Factor Xa by forming an initial, reversible encounter complex that transitions into a stable, inactive covalent complex, thereby preventing excessive thrombus propagation (Source: UniProt P01008, P00742). This interaction is the pharmacological basis for indirect anticoagulants like heparin and fondaparinux. These drugs bind to ATIII, inducing a conformational change that increases its affinity for Factor Xa by several thousand-fold, effectively accelerating the formation of the encounter complex (Source: PubMed PMID: 15670155). Monitoring the levels of this complex serves as a clinical biomarker for intravascular coagulation activation and the efficacy of antithrombotic therapies (Source: StatPearls, Heparin).

Other names

Factor Xa-Antithrombin complexFXa-AT complexAntithrombin-Factor Xa complexFactor Xa-Antithrombin III encounter complex

Indirect inhibition of Factor Xa through allosteric potentiation of Antithrombin III, accelerating the formation of the encounter complex and subsequent covalent inactivation.