Factor IX is a vitamin K-dependent plasma protein that participates in the intrinsic pathway of blood coagulation by converting factor X to its active form in the presence of Ca(2+) ions, phospholipids, and factor VIIIa. Selective cleavage of Arg-|-Ile bond in factor X to form factor Xa. Heterodimer of a light chain and a heavy chain; disulfide-linked. Interacts with SERPINC1. Calcium binds to the gamma-carboxyglutamic acid (Gla) residues in the Gla domain. Calcium can also bind, with stronger affinity, to another site beyond the Gla domain. Under physiological ion concentrations, Ca(2+) is displaced by Mg(2+) from some of the gammaglutamate residues in the N-terminal Gla domain. This leads to a subtle conformation change that may affect the interaction with its binding protein. The iron and 2-oxoglutarate dependent 3-hydroxylation of aspartate and asparagine is (R) stereospecific within EGF domains. Activated by factor XIa, which excises the activation peptide. The propeptide can also be removed by snake venom protease. Predominantly O-glucosylated at Ser-99 by POGLUT1 in vitro. Belongs to the peptidase S1 family.