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 (By similarity). Interacts with the heterodimeric snake venom coagulation factor IX-binding protein (PubMed:12695512). Detected in blood plasma (at protein level) (PubMed:12695512). Calcium binds to the gamma-carboxyglutamic acid (Gla) residues in the Gla domain (PubMed:12695512). Calcium can also bind, with stronger affinity, to another site beyond the Gla domain (By similarity). 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 (PubMed:12695512). Activated by factor XIa, which excises the activation peptide. The propeptide can also be removed by snake venom protease. The iron and 2-oxoglutarate dependent 3-hydroxylation of aspartate and asparagine is (R) stereospecific within EGF domains. Predominantly O-glucosylated at Ser-99 by POGLUT1 in vitro. Belongs to the peptidase S1 family.