Factor X (F.X) is a vitamin K-dependent glycoprotein produced in the liver. The concentration of F.X in plasma is ~10ug/ml (~170nM). Factor X is expressed as a two-chain molecule with a molecular weight of 59kD. The light chain (17kD) of F.X contains a calcium-binding domain consisting of one hydroxyaspartic acid and 11 gamma carboxyglutamic acid (gla) residues. These residues allow F.X to bind to membranes that contain acidic phospholipids in a calcium dependent manner. This is followed by two EGF-like domains. The heavy chain of F.X (42kD) consists of the catalytic domain, carbohydrate and the activation peptide. Activation of F.X to the active enzyme (F.Xa) results from cleavage at residue Arg52 in the heavy chain of F.X by a complex of F.IXa, cofactor VIIIa, calcium and negatively charged phospholipid surface (the tenase complex), or by the F.VIIa-tissue factor complex. Both activation pathways result in the release of the activation peptide from the N-terminal of the heavy chain. The F.Xa generated is a serine protease responsible for the activation of prothrombin to thrombin in the presence of a phospholipid membrane, calcium and cofactor Va. The activity of F.Xa in plasma is inhibited by antithrombin (ATIII), alpha1-antitrypsin, alpha2-macroglobulin and tissue factor pathway inhibitor (TFPI). The inhibitory activity of ATIII is stimulated ~1000-fold by heparin.