Antithrombin III–Factor Xa complex (AT-FXa)

The Antithrombin III–Factor Xa complex is a stable, covalent molecular assembly that represents the inactivated state of the coagulation protease Factor Xa (UniProt P00742). Antithrombin III, a member of the serine protease inhibitor (serpin) superfamily, functions as the primary endogenous regulator of the blood-clotting cascade by neutralizing Factor Xa (StatPearls: Antithrombin III Deficiency). The formation of this complex is a critical physiological process to prevent excessive thrombus formation and maintain vascular patency (PubMed: PMID 15607132). Pharmacologically, the rate of complex formation is dramatically enhanced by anticoagulant drugs such as heparin and fondaparinux, which bind to antithrombin and facilitate its interaction with Factor Xa (DrugBank: DB00105). In clinical settings, the presence of the Antithrombin III–Factor Xa complex in plasma serves as a specific biomarker for the activation of the coagulation system and the efficacy of anticoagulant therapy (PubMed: PMID 2361464). Dysregulation or deficiency in the components of this complex is directly linked to hypercoagulable states and an increased risk of venous thromboembolism (NIH: MedlinePlus). Therapeutic strategies targeting the formation of this complex are foundational in treating conditions like deep vein thrombosis, pulmonary embolism, and acute coronary syndromes. While the complex itself is an end-product of inhibition, understanding its kinetics and structural biology is essential for the development of next-generation anticoagulants with improved safety profiles.

Other names

ATIII-FXa complexAntithrombin-Factor Xa complexSERPINC1-F10 complexAntithrombin-Xa complexAT-FXa complex

Heparinoids and pentasaccharides bind to Antithrombin III, inducing an allosteric conformational change in its reactive center loop, which increases its affinity for Factor Xa and facilitates the formation of the inactive Antithrombin III–Factor Xa complex (PubMed: PMID 15607132).