Factor IX (FIX) is a vitamin K–dependent single-chain glycoprotein produced in the liver, that undergoes extensive posttranslational modification, to become a γ-carboxylated zymogen that is secreted into the blood. This carboxylation, that all of the vitamin K–dependent pro- and anticoagulants undergo, make these zymogens biologically active. In blood plasma the concentration FIX present is about 4-5 µg/mL with a half-life of approximately 18-24 hours.
Factor IX is a serine protease that, circulates in the blood as an inactive zymogen. The activation of FIX to activated FIX (FIXa) occurs on the surface of phospholipid membranes. It can be activated via both the intrinsic and extrinsic pathway. Regarding the extrinsic way the tissue factor/factor FVIIIa complex (TF/FVIIa complex) is responsible for the activation of FIX. In the intrinsic pathway FIX is activated by Factor XIa (FXIa). Both activations are in need of the presence of calcium ions.
Once activated, FIXa forms a complex with Factor X (FX), activated Factor VIII (FVIIIa), phospholipids and Ca2+, called the tenase-complex. This tenase-complex has the potency to convert the FX to activated Factor X (FXa), which then converts prothrombin into thrombin that results in the formation of fibrin, and hence in blood coagulation.
Inhibition or deactivation of FIXa is performed by binding to antithrombin III (AT III, which is greatly accelerated by the presence of heparin), nexin-2, protein Z-dependent protease inhibitor or endocytic hepatocyte receptors or it can be degraded by neutrophil elastase.
FIX, also known as the Christmas Factor, was discovered in 1952 after a young boy named Stephen Christmas. He suffered from haemophilia, or ‘Christmas disease’ that was caused by the deficiency of this protein.
The deficiency of FIX in human plasma results in the disease known as haemophilia B.
Current treatment of these haemophilia patients is restricted to protein-replacement therapy.
Patients used to be periodically treated by plasma transfusions, later by transfusions of prothrombin complex concentrates (PCC) or with plasma derived FIX preparations and even a recombinant FIX product.
All those therapies are very expensive for patients and the healthcare system.
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