- Molecular Weight: 56,000
- Normal plasma concentration 0.5 μg/ml
- 10-40% of normal concentration required for haemostasis
- Vitamin K dependent blood clotting factor
- Activated FIXa is cofactor of Factor VIII
The factor IX gene on the long arm of the X chromosome (Xq27.1) encodes a precursor protein consisting of a signal peptide, propeptide followed by a glutamic acid–rich domain, 2 epidermal growth factor (EGF) domains, an activation peptide, and a catalytic domain. Activation of factor IX occurs by cleavage by factor VIIa-tissue factor complex or activated factor XI. This releases the activation peptide resulting in a circulating light chain and a heavy chain connected by the disulfide bond. Numerous posttranslational modifications are necessary for its normal function, including tyrosine sulfation, serine phosphorylation, and O- and N-linked glycosylation. Haemophilia B is caused by various mutations of the factor IX gene. A rare type of warfarin sensitivity results from another mutation (Ala-10) which causes bleeding with a therapeutic range INR but the APPT is disproportionately prolonged.