Factor XI or plasma thromboplastin antecedent is the zymogen form of factor XIa, one of the enzymes of the coagulation cascade.
It is a serine protease.
Factor XI is encoded by the F11 gene in chromosome 4.
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.
Zymogen factor XI 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 including high molecular weight ,
kininogen, prekallikrein, factor XII, factor XI, and factor IX.
Factor XIa activates factor IX by selectively cleaving peptide bonds.
Factor IXa, in turn, forms a complex with Factor VIIIa (FIXa-FVIIIa) and activates factor X.
Physiological inhibitors of factor XIa include protein Z-dependent protease inhibitor .
Although synthesized as a single polypeptide chain, FXI circulates as a homodimer.
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 found on chromosome 4q32-35.
Deficiency of factor XI causes the rare hemophilia C, which 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 occur in many other disease states, including Noonan syndrome.
High levels of factor XI have been implicated in thrombosis.
Pharmacological inhibitors of factor XI that are under clinical development.