As the only known physiological activator of thrombin, factor X is a vitamin K-dependent plasma protease that plays a pivotal role in blood coagulation. Human factor X is initially synthesized in the liver as a single-chain precursor of 488 amino acid residues with a signal peptide and a pro region (residues 1 - 40). Both the intrinsic and extrinsic pathways activate factor X to Xa, which consists of light (residues 41 - 179) and heavy (residues 235 - 488) chains linked by a disulfide bond. The light chain contains a Gla and two EGF-like domains and the heavy chain corresponds to the serine protease domain. The full-length human factor X was expressed and the pro enzyme was purified and activated. The active protease was further purified and analyzed for its activity towards either peptides or proteins containing a Xa cleavage site.