Factor IX
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Factor IX (plasma thromboplastin component, Christmas factor, or hemophilia B factor) is a 57-kDa vitamin K-dependent procoagulant glycoprotein. It is synthesized by the liver hepatocyte as a pre-prozymogen that requires extensive posttranslational modification. The pre-prozymogen contains a pre-peptide (hydrophobic signal peptide) at its amino terminal that transports the growing polypeptide into the lumen of the Endoplasmic Reticulum. Once inside the ER, this signal peptide is cleaved by a signal peptidase. A pro-peptide functions as a recognition element for a vitamin K-dependent carboxylase (γ-glutamyl carboxylase) which modifies 12 glutamic acid residues to gamma-carboxyglutamyl ( Gla) residues. These residues are required for the association with the anionic phospholipid surface through Ca2+-dependent binding. The Gla Domain is followed by two epidermal growth factor domains ( EGF-1 and EGF-2). The N-terminus of EGF-1 contains a Ca2+ binding site, while the C-terminus connects to a hydrophobic pocket of EGF-2 and a salt bridge through Lys122 (EGF-1 residue) and Gln74 (EGF-2). EGF-2 connects to the serine protease domain through a linker peptide and is required for a proper orientation and folding of serine proteases. To have a physiologically active factor IX, two cleaves must occur to remove a 35 amino acid region that precedes the catalytic region. The first cleave is at Arg145, generating an inactive FIXα. The second cleavage is at Arg180 results in a catalytically active molecule FIXaβ. This resulting heterodimer is held by a disulfide bridge at Cys132-Cys289. The serine protease contains a catalytic triad of His221, Asp269, and Ser365. Upon cleave at Arg180, Val181 can form a salt bridge with Asp364, which is a characteristic of active serine proteases. The active FIXa, can then interact with its cofactor, FVIIIa, to form a membrane-bound Xase complex, which activated FX to FXa.
Gene Structure and Expression:
The gene for factor IX is located on the long arm of chromosome X between positions 26.3- and 27.1 and contains eight exons and seven introns, which segregate the FIX gene into specific structural regions.
| I | II | III | IV | V | VI | VII | VIII |
Exon I encodes the hydrophobic signal peptide that targets the FIX into the lumen of the Endoplasmic Reticulum. Exon II codes for the pro-peptide and the Gla domain. Exon III encodes the hydrophobic helix, that inserts itself into the lipid membrane, anchoring FIX. Exon IV and V, code for EGF-1 and EGF-2, the serine protease region is encoded by exon VI-VIII.
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Proteopedia Page Contributors and Editors (what is this?)
Nadia Dorochko, Michal Harel, David Canner, Alexander Berchansky
