Coagulation factor X (EC 3.4.21.6), or Stuart factor, is an enzyme of the coagulation cascade, encoded in humans by F10 gene. It is a serine endopeptidase (protease group S1, PA clan). Factor X is synthesized in the liver and requires vitamin K for its synthesis.
Factor X is activated, by hydrolysis, into factor Xa by both factor IX with its cofactor, factor VIII in a complex known as intrinsic pathway; and factor VII with its cofactor, tissue factor in a complex known as extrinsic pathway. It is therefore the first member of the final common pathway or thrombin pathway.
It acts by cleaving prothrombin in two places (an Arg-Thr and then an Arg-Ile bond), which yields the active thrombin. This process is optimized when factor Xa is complexed with activated co-factor V in the prothrombinase complex.
Factor Xa is inactivated by protein Z-dependent protease inhibitor (ZPI), a serine protease inhibitor (serpin). The affinity of this protein for factor Xa is increased 1000-fold by the presence of protein Z, while it does not require protein Z for inactivation of factor XI. Defects in protein Z lead to increased factor Xa activity and a propensity for thrombosis. The half life of factor X is 40–45 hours.

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