Factor XI or plasma thromboplastin antecedent is the zymogen form of factor XIa, one of the enzymes of the coagulation cascade. Like many other coagulation factors, it is a serine protease. In humans, Factor XI is encoded by the F11 gene.
Factor XI (FXI) is produced by the liver and circulates as a homo-dimer in its inactive form. The plasma half-life of FXI is approximately 52 hours. The zymogen factor is activated into factor XIa by factor XIIa (FXIIa), thrombin, and FXIa itself; due to its activation by FXIIa, FXI is a member of the "contact pathway" (which 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).
Although synthesized as a single polypeptide chain, FXI circulates as a homodimer. Every chain has a relative molecular mass of approximately 80000. Typical plasma concentrations of FXI are 5 μg/mL, corresponding to a plasma concentration (of FXI dimers) of approximately 30 nM.
The FXI gene is 23kb in length, has 15 exons, and is found on chromosome 4q32-35.
Role in disease
Deficiency of factor XI causes the rare hemophilia C; this mainly occurs in Ashkenazi Jews and is believed to affect approximately 8% of that population. Less commonly, hemophilia C can be found in Jews of Iraqi ancestry and in Israeli Arabs. The condition has been described in other populations at around 1% of cases. 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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- The MEROPS online database for peptidases and their inhibitors: S01.213
1zlr: Factor XI catalytic domain complexed with 2-guanidino-1-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)ethyl nicotinate