What is the structure, production, and half-life of factor IX (FIX)?

Updated: Mar 09, 2021
  • Author: Robert A Schwartz, MD, MPH; Chief Editor: Srikanth Nagalla, MD, MS, FACP  more...
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Answer

FIX, a vitamin K–dependent single-chain glycoprotein, is synthesized first by the hepatocyte as a precursor protein (protein in vitamin K absence). It then undergoes extensive posttranslational modification to become the fully gamma-carboxylated mature zymogen that is secreted into the blood.

The precursor protein has the following parts, starting with (1) a signal peptide at the amino (NH2) terminal end (as marked in the diagram below), which directs the protein to the endoplasmic reticulum in the liver, and continuing with (2) the prepro leader sequence recognized by the gamma-glutamylcarboxylase, which is responsible for the posttranslational modification (carboxylation) of the glutamic acid residues (Gla) in the NH2 -terminal portion of the molecule. These 2 parts of the molecule are removed before the protein is secreted into the circulation.

Major components of the factor IX structure. Major components of the factor IX structure.

Single-chain plasma FIX has the Gla domain (12 gamma-carboxyglutamic acid residues) at its amino terminal end; this is a characteristic feature of all vitamin K–dependent factors. The Gla domain is responsible for Ca2+ binding, which is necessary for the binding of FIX to phospholipid membranes. The Gla region is followed by (1) two epidermal growth factor regions, (2) the activation peptide, which is removed when the single-chain zymogen FIX is converted to activated factor IX (FIXa), ie, the 2-chain active enzyme, and (3) the catalytic domain, which contains the enzymatic activity.

Before secretion from the hepatocyte, the FIX protein undergoes extensive posttranslational modifications, which include gamma-carboxylation, beta-hydroxylation, and removal of the signal peptide and propeptides, addition of carbohydrates, sulfation, and phosphorylation. Gamma-carboxylation, as demonstrated in the diagram below, is a vitamin K–dependent process in which the enzyme gamma-glutamylcarboxylase binds to specific sites on the propeptide region of the precursor protein in the liver. The process of gamma-carboxylation of the glutamic acid residues forms gamma-carboxyglutamyl (Gla) residues in the mature protein and requires reduced vitamin K, oxygen, and carbon dioxide to perform its functions.

Vitamin K–dependent carboxylation of precursor fac Vitamin K–dependent carboxylation of precursor factor IX to procoagulant factor IX. Carboxylation of glutamate (Glu) to gamma-carboxyglutamate (Gla) residues in the precursor protein of the vitamin K–dependent factors occurs in the endoplasmic reticulum of the hepatocyte. Reduced vitamin K is oxidized in this process. Warfarin prevents the reduction and recycling of oxidized vitamin K.

These Gla regions are the high affinity Ca2+ binding sites necessary for binding FIXa to lipid membranes so FIXa can express its full procoagulant activity. All of the vitamin K–dependent procoagulants and anticoagulants are biologically inactive unless the glutamic acid residues at the amino terminal end are carboxylated; the exact number of Gla regions varies with each protein.

Warfarin prevents the reduction and recycling of oxidized vitamin K (vitamin K epoxide) that is generated during this carboxylation reaction. As a result of the indirect inhibition of the carboxylation reaction resulting from a lack of available reduced vitamin K, hypocarboxylated and decarboxylated forms of the vitamin K–dependent factors are found in the circulation of patients ingesting warfarin. These abnormal forms have reduced or absent biological activity. Following these modifications, the carboxyterminal (C-terminal) region is recognized by the hepatic secretion process. Mutations that increase the charge of this region result in decreased hepatic secretion of all vitamin K–dependent proteins, including FIX, and lead to deficiencies of multiple vitamin K–dependent factors.

FIX is present in a concentration of 4-5 µg/mL with a half-life of approximately 18-24 hours. A 3-fold variation in the activity of FIX in plasma is normal. Since FIX is smaller than albumin, it distributes in both the extravascular and intravascular compartments. Following intravenous (IV) administration, recovery of FIX concentrates varies significantly, which has been ascribed to the development of nonneutralizing antibodies. In vivo binding of FIX to collagen IV has been proposed as another reason for reduced recovery of FIX following infusion of FIX concentrates in hemophilia B patients. FIX concentrates generally are replaced every 18-24 hours under steady state conditions. Lower recoveries are seen with recombinant factor IX (rFIX) compared to FIX concentrates. [6]

Extensive homology is found between FIX and the other vitamin K–dependent proteins (procoagulants factor VII [FVII], factor X [FX], factor II [FII] and anticoagulant proteins C and S), especially in the prepro sequence and the Gla regions. Despite numerous similarities, each vitamin K–dependent protein performs a different function in the hemostatic pathway, which is diagrammed in the following image.

The hemostatic pathway: role of factor IX. The hemostatic pathway: role of factor IX.

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