A glycoprotein synthesized in the liver, secreted as a zymogen, and circulating in blood complexed with high molecular weight kininogen (HK). Unlike other coagulation serine proteases, FXIa has two active catalytic sites. FXI participates in the intrinsic pathway of coagulation by converting FIX to FIXa. FXI can be activated either by Thrombin in a feedback mechanism to amplify further Thrombin generation or by autoactivation in the presence of negatively charged materials. In vitro, FXI can be activated by FXIIa in the presence of HK and a negatively charged surface. The slide show entitled Coagulation Pathways (All Components) shows how FXI relates to the other coagulation factors. See also the slide show entitled Intrinsic Pathway-APTT.
