Domain Architecture
Factors VII, IX, and X all play key roles in blood coagulation and also share a common domain architecture. The factor IX protein is composed of four protein domains. These are the Gla domain, two tandem copies of the EGF domain and a C-terminal trypsin-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 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.
Several structures of 'super active' mutants have been solved which reveal the nature of Factor IX activation by other proteins in the clotting cascade.
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