Prothrombin: A coagulation (clotting) factor that is needed for the normal clotting of blood. A cascade of biochemical events leads to the formation of the final clot. In this cascade, prothrombin is a precursor to thrombin.
A prothrombin time test measures how quickly your blood clots. Sometimes called a PT or pro time test, a prothrombin time test uses a sample of your blood.
Prothrombin is a protein produced by your liver. It is one of many factors in your blood that help it to clot appropriately.
If you take a blood-thinning medication such as warfarin (Coumadin), your prothrombin time test results will be expressed as a ratio called the international normalized ratio (INR).
T (also known as factor II) is a 72-kDa vitamin K-dependent glycoprotein synthesized in the liver. It circulates at a concentration of about 100 μg/mL in normal plasma as a single-chain glycoprotein of 579 amino acids. During its hepatic biosynthesis, PT undergoes γ-carboxylation leading to 10 γ-carboxyglutamic residues, named GLA domain. This domain, located on fragment 1 of the PT molecule, is essential for the calcium-dependent binding of PL to PT, which is, in turn, necessary for the conversion of PT into the biologically active form, α-thrombin. The physiologic activation of PT is mediated by the prothrombinase complex, which enlists activated factor X, factor V, calcium, and PL. The prothrombinase complex converts PT into thrombin only when negatively charged PL bind PT. Once activated, thrombin triggers fibrinogen polymerization into fibrin. Thrombin binds to fibrin via its kringle domain, constituted by two kringle structures and a C-terminal serine protease. Moreover, thrombin binds to thrombomodulin on the surface of endothelial cells and activates protein C, which exerts its anticoagulant activity by digesting factor V then depriving the prothrombinase complex of its most important cofactor