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) (By similarity). 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.