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. VAP cleaves the fusion proteins of Sendai virus, NDV, and influenza virus a at a specific single arginine-containing site, and plays a key role in the viral spreading in the allantoic sac. Selective cleavage of Arg-|-Thr and then Arg-|-Ile bonds in prothrombin to form thrombin. 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. Liver and chorioallantoic membrane. The vitamin K-dependent, enzymatic carboxylation of some glutamate residues allows the modified protein to bind calcium. The activation peptide is cleaved by factor IXa (in the intrinsic pathway), or by factor VIIa (in the extrinsic pathway). The iron and 2-oxoglutarate dependent 3-hydroxylation of aspartate and asparagine is (R) stereospecific within EGF domains. Belongs to the peptidase S1 family.