Factor XI or plasma thromboplastin antecent is one of the enzymes (EC 3.4.21.27) of the coagulation cascade. Like many other coagulation factors, it is a serine protease.

Genetics

The gene for factor XI is located on the fourth chromosome (4q35).

Physiology

Factor XI is produced by the liver and circulates in its inactive form. It is activated into factor XIa by factor XII, and is therefore a member of the "contact pathway" (with includes HMWK, prekallikrein , factor XII, factor XI and factor IX).

Factor XIa activates factor IX by selectively cleaving arg-ala and arg-val peptide bonds. Factor IXa, in turn, activates factor X.

Inhibitors of factor XIa include protein Z-dependent protease inhibitor (ZPI, a member of the serine protease inhibitor/serpin class of proteins), which is independent of protein Z (its action on factor X, however, is protein Z-dependent, hence its name).

Role in disease

Deficiency of factor XI causes the rare hemophilia C; this mainly occurs in Ashkenazi Jews, although it has been described in other populations. It is an autosomal recessive disorder. There is little spontaneous bleeding, but surgical procedures may cause excessive blood loss, and prophylaxis is required.

Low levels of factor XI also occur in many other disease states, including Noonan syndrome.

High levels of factor XI have been implicated in thrombosis, although it is uncertain what determines these levels and how serious the procoagulant state is.

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