Factor X is a vitamin K dependent, two-chain glycoprotein zymogen (Mr = 59,000) synthesized in the liver that circulates in plasma at a concentration of approximately 10 μg/mL. The NH2-terminal light chain (Mr = 17,000) possesses the gamma-carboxyglutamic (gla) residues that enable factor X/Xa to bind to phospholipids/membranes in a calcium dependent manner. The COOH-terminal heavy chain (Mr = 42,000) possesses the latent catalytic domain.

Activation to factor Xa occurs by interaction with the intrinsic factor Xase complex (Factor VIIa/IXa/Ca+2/phospholipid) or the extrinsic factor Xase complex (Factor VIIa/tissue factor/ Ca+2/phospholipid). Both complexes cleave the molecule at Arg52-lle53, releasing an activation peptide from the heavy chain, resulting in factor Xa as a two-chain molecule where the light chain remains with a Mr of 17,000 and the heavy chain has been reduced to a Mr of 29,000.

Factor Xa provides the enzymatic activity of the Prothrombinase complex (Factor Xa/FactorVa/ Ca+2/phospholipid) which converts prothrombin to thrombin. While factor Xa can convert prothrombin to thrombin alone, its activity is greatly enhanced when a part of the complex. Its activity may be inhibited by inactivation of the factor Va cofactor or directly by a natural inhibitor such as antithrombin III.