Factor Xa 
 
 Factor X is a component of the blood clotting system. It circulates in blood plasma (8 µg/mL) as an inactive precursor of a serine proteinase (1). When the blood coagulation process is initiated, factor X is converted to factor Xa (activated factor X), which forms part of a complex that converts another serine proteinase precursor, pro-thrombin, into the active enzyme thrombin. Clotting occurs when thrombin converts fibrinogen to fibrin.
 Factor X is synthesized in the liver as a 488-amino-acid residue precursor protein, pre-pro-X, the primary translation product of the factor X gene (2). Forty amino acids are cleaved from the amino terminus of pre-pro-X. The resultant 448-residue protein is modified by the attachment of sugars and, in a vitamin-K-dependent process, by the carboxylation of certain glutamic acid residues to form g-carboxy glutamic acid residues. It is cleaved by a specific proteinase that removes residues 140 to 142, so factor X circulates as a two-chain, disulfide bond-linked, enzymatically inactive protein. This heavy-chain–light-chain dimeric structure is typical of the blood coagulation proteins (3) .
 As might be expected, activation of blood coagulation is a carefully controlled process involving many clotting factors. All the details of this process are not fully understood as yet, but it is known that factor X can be converted to an active enzyme by two distinct pathways, the intrinsic and the extrinsic (4). The former pathway involves activation by factor IXa, which together with factor VIIIa forms a receptor on the phospholipid surface of blood platelets. When factor X binds to this receptor complex, a 52-residue peptide is cleaved from the amino terminus of its heavy chain to form factor Xa. The latter pathway involves activation by factor VIIa which, in an analogous fashion, forms a platelet-bound complex with tissue factor. When factor X binds to this complex, the same peptide is cleaved from the heavy chain to give factor Xa. Calcium ions, which bind to the g-carboxy glutamic acid residues of the light chain (and also to the other clotting factors), are essential for factor X activation.
Once factor X is activated, it binds along with factor Va on the surface of platelets, where it interacts with pro-thrombin to convert this zymogen into thrombin, the end product of the so-called coagulation proteinase cascade. Factor X was first recognized as an essential part of this cascade when it was shown that it could restore coagulability to the plasma of individuals who had a specific hemorrhagic disorder. It was given the name Stuart factor, which was subsequently changed to factor X in an effort to systematize the nomenclature of the clotting factors.
 
References
1. M. Hertzberg (1994) Blood Rev. 8, 56–62. 
2. C. Miao, S. P. Leytus, D. W. Chung, and E. W. Davie (1992) J. Biol. Chem. 267, 7395–7401. 
3. E. W. Davie, K. Fujikawa, M. E. Legaz, and H. Kato (1975) In Proteases and Biological Control (E. Reich, D. B. Rifkin, and E. Shaw, eds.), Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, pp. 65–77. 
4. M. P. McGee and L. C. Lee (1991) J. Biol. Chem. 266, 8079–8085.