Description of Factor IX (FIX)
Factor IX (FIX, Christmas Factor) is a vitamin K-dependent glycoprotein produced in the liver. Plasma concentration of FIX is normally around 5 μg/ml (87 nM) in plasma. The biological importance of FIX is demonstrated in Haemophilia B (Christmas disease), an X-linked congenital bleeding disease resulting from a quantitative (low activity and low antigen) or qualitative (low activity, normal antigen) defect in FIX function.
In its proenzyme or zymogen form FIX is a single chain molecule of 55,000 daltons. It contains two EGF-like domains and an amino-terminal domain containing 12 γ-carboxy-glutamic acid (Gla) residues. These Gla residues allow FIX to bind divalent metal ions and participate in calcium-dependent binding interactions. The activation of F.IX occurs by limited proteolysis in the presence of calcium by activated factor XI (FXIa) and/or by a complex of VIIa/tissue factor/phospholipid and activated Factor X between residues Arg146-Ala147 and between Arg180-Val181. The terminal activated product in either case is FIXaβ, a two-chain enzyme consisting of a heavy chain (28,000 daltons), a light chain (18,000 daltons) and an activation peptide product of 11,000 daltons. FIX can also be cleaved into inactive products by thrombin and by elastase.
The activity of FIXaβ in plasma is inhibited by antithrombin and this inhibition is accelerated 1000-fold in the presence of optimal concentrations of heparin 1-3.