Factor X (the eponym Stuart–Prower factor) is an enzyme of the coagulation cascade. 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 and factor VII with its cofactor, tissue factor . 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.