Factor IX (), also known as Christmas factor, is one of the

serine protease Serine proteases (or serine endopeptidases) are enzymes that cleave peptide bonds in proteins. Serine serves as the nucleophilic amino acid at the (enzyme's) active site. They are found ubiquitously in both eukaryotes and prokaryotes. Serin ...

s involved in

coagulation Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a thrombus, blood clot. It results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The process of co ...

; it belongs to peptidase family S1. Deficiency of this protein causes haemophilia B. It was discovered in 1952 after a young boy named Stephen Christmas was found to be lacking this exact factor, leading to haemophilia. Coagulation factor IX is on the

World Health Organization's List of Essential Medicines The WHO Model List of Essential Medicines (aka Essential Medicines List or EML), published by the World Health Organization (WHO), contains the medications considered to be most effective and safe to meet the most important needs in a health s ...

Physiology

Factor IX is produced as a

zymogen In biochemistry, a zymogen (), also called a proenzyme (), is an inactive precursor of an enzyme. A zymogen requires a biochemical change (such as a hydrolysis reaction revealing the active site, or changing the configuration to reveal the activ ...

, an inactive precursor. It is processed to remove the

signal peptide A signal peptide (sometimes referred to as signal sequence, targeting signal, localization signal, localization sequence, transit peptide, leader sequence or leader peptide) is a short peptide (usually 16–30 amino acids long) present at the ...

, glycosylated and then cleaved by factor XIa (of the contact pathway) or

factor VII Coagulation factor VII (, formerly known as proconvertin) is a protein involved in coagulation and, in humans, is encoded by gene ''F7''. It is an enzyme of the serine protease class. Once bound to tissue factor released from damaged tissues, ...

a (of the tissue factor pathway) to produce a two-chain form, where the chains are linked by a disulfide bridge. When activated into factor IXa, in the presence of Ca²⁺, membrane phospholipids, and a Factor VIII cofactor, it hydrolyses one

arginine Arginine is the amino acid with the formula (H2N)(HN)CN(H)(CH2)3CH(NH2)CO2H. The molecule features a guanidinium, guanidino group appended to a standard amino acid framework. At physiological pH, the carboxylic acid is deprotonated (−CO2−) a ...

isoleucine Isoleucine (symbol Ile or I) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the depro ...

bond in

factor X Coagulation factor X (), or Stuart factor, is an enzyme of the coagulation cascade, encoded in humans by ''F10'' gene. It is a serine endopeptidase (protease group S1, PA clan). Factor X is synthesized in the liver and requires vitamin K for i ...

to form factor Xa. Factor IX is inhibited by antithrombin. Factor IX expression increases with age in humans and mice. In mouse models, mutations within the promoter region of factor IX have an age-dependent phenotype.

Domain architecture

Factors VII, IX, and X all play key roles in

blood coagulation Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The process of coagulatio ...

and also share a common domain architecture. The factor IX protein is composed of four

protein domain In molecular biology, a protein domain is a region of a protein's Peptide, polypeptide chain that is self-stabilizing and that Protein folding, folds independently from the rest. Each domain forms a compact folded Protein tertiary structure, thre ...

s: the Gla domain, two tandem copies of the EGF domain and a C-terminal

trypsin Trypsin is an enzyme in the first section of the small intestine that starts the digestion of protein molecules by cutting long chains of amino acids into smaller pieces. It is a serine protease from the PA clan superfamily, found in the dig ...

-like peptidase domain which carries out the catalytic cleavage. The N-terminal EGF domain has been shown to at least in part be responsible for binding tissue factor. Wilkinson ''et al''. conclude that residues 88 to 109 of the second EGF domain mediate binding to platelets and assembly of the factor X activating complex. The structures of all four domains have been solved. A structure of the two EGF domains and the trypsin-like domain was determined for the pig protein. The structure of the Gla domain, which is responsible for Ca(II)-dependent phospholipid binding, was also determined by

NMR Nuclear magnetic resonance (NMR) is a physical phenomenon in which atomic nucleus, nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near and far field, near field) and respond by producing ...

. Several structures of 'super active' mutants have been solved, which reveal the nature of factor IX activation by other proteins in the clotting cascade.

Genetics

Because the

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

for factor IX is located on the

X chromosome The X chromosome is one of the two sex chromosomes in many organisms, including mammals, and is found in both males and females. It is a part of the XY sex-determination system and XO sex-determination system. The X chromosome was named for its u ...

(Xq27.1-q27.2), loss-of-function mutations thereof are

X-linked recessive ''Main Article'': Sex linkage X-linked recessive inheritance is a mode of Mendelian inheritance, inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be always expressed in males (who are necessarily hemizygous for ...

: males experience the disease phenotype much more frequently than females. At least 534 disease-causing mutations in this gene have been discovered. The F9 gene was first cloned in 1982 by Kotoku Kurachi and Earl Davie. Polly, a transgenic cloned Poll Dorset sheep carrying the gene for factor IX, was produced by Dr Ian Wilmut at the Roslin Institute in 1997.

Role in disease

Deficiency of factor IX causes Christmas disease ( hemophilia B). Over 3000 variants of factor IX have been described, affecting 73% of the 461 residues; some cause no symptoms, but many lead to a significant bleeding disorder. The original Christmas disease mutation was identified by sequencing of Christmas' DNA, revealing a mutation which changed a cysteine to a serine. Recombinant factor IX is used to treat Christmas disease. Formulations include: * nonacog alfa (brand name Benefix) * nonacog gamma (brand name Rixubis) * albutrepenonacog alfa (brand name Idelvion) * eftrenonacog alfa (brand name Alprolix) * nonacog beta pegol (brand name Refixia) * coagulation factor IX ecombinant(Benefix) * coagulation factor IX ecombinant(Idelvion) * coagulation factor IX (recombinant), Fc fusion protein (Alprolix) * coagulation factor IX ecombinant(Ixinity) * coagulation factor IX ecombinant(Rebinyn) * coagulation factor IX ecombinant(Rixubis) * coagulation factor IX (human) (Alphanine SD) Some rare mutations of factor IX result in elevated clotting activity, and can result in clotting diseases, such as

deep vein thrombosis Deep vein thrombosis (DVT) is a type of venous thrombosis involving the formation of a blood clot in a deep vein, most commonly in the legs or pelvis. A minority of DVTs occur in the arms. Symptoms can include pain, swelling, redness, and enl ...

. This gain of function mutation renders the protein hyperfunctional and is associated with familial early-onset thrombophilia. Factor IX deficiency is treated by injection of purified factor IX produced through cloning in various animal or animal cell vectors. Tranexamic acid may be of value in patients undergoing surgery who have inherited factor IX deficiency in order to reduce the perioperative risk of bleeding. A list of all the mutations in Factor IX is compiled and maintained by EAHAD. Coagulation factor IX is on the

References

External links