Factor Xa is a vitamin K-dependent glycoprotein that converts prothrombin to thrombin in the presence of factor Va, calcium and phospholipid during blood clotting. Selective cleavage of Arg-|-Thr and then Arg-|-Ile bonds in prothrombin to form thrombin. Inhibited by SERPINA5. The two chains are formed from a single-chain precursor by the excision of two Arg residues and are held together by 1 or more disulfide bonds. Forms a heterodimer with SERPINA5 (By similarity). The vitamin K-dependent, enzymatic carboxylation of some glutamate residues allows the modified protein to bind calcium. N- and O-glycosylated. Proteolytically cleaved and activated by cathepsin CTSG (By similarity). The activation peptide is cleaved by factor IXa (in the intrinsic pathway), or by factor VIIa (in the extrinsic pathway) (PubMed:1059122). The iron and 2-oxoglutarate dependent 3-hydroxylation of aspartate and asparagine is (R) stereospecific within EGF domains. Calcium also binds, with stronger affinity to another site, beyond the GLA domain. Belongs to the peptidase S1 family.