Thrombin
Thrombin
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Thrombin

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Thrombin

Prothrombin (coagulation factor II) is encoded in the human by the F2-gene. It is proteolytically cleaved during the clotting process by the prothrombinase enzyme complex to form thrombin.

Thrombin (Factor IIa) (EC 3.4.21.5, fibrose, thrombase, thrombofort, topical, thrombin-C, tropostasin, activated blood-coagulation factor II, E thrombin, beta-thrombin, gamma-thrombin) is a serine protease, that converts fibrinogen into strands of insoluble fibrin, as well as catalyzing many other coagulation-related reactions.

After the description of fibrinogen and fibrin, Alexander Schmidt hypothesised the existence of an enzyme that converts fibrinogen into fibrin in 1872.

Prothrombin was discovered by Pekelharing in 1894.

Thrombin is produced by the enzymatic cleavage of two sites on prothrombin by activated Factor X (Xa). The activity of factor Xa is greatly enhanced by binding to activated Factor V (Va), termed the prothrombinase complex. Prothrombin is produced in the liver and is co-translationally modified in a vitamin K-dependent reaction that converts 10-12 glutamic acids in the N terminus of the molecule to gamma-carboxyglutamic acid (Gla). In the presence of calcium, the Gla residues promote the binding of prothrombin to phospholipid bilayers. Deficiency of vitamin K or administration of the anticoagulant warfarin inhibits the production of gamma-carboxyglutamic acid residues, slowing the activation of the coagulation cascade.

In human adults, the normal blood level of antithrombin activity has been measured to be around 1.1 units/mL. Newborn levels of thrombin steadily increase after birth to reach normal adult levels, from a level of around 0.5 units/mL 1 day after birth, to a level of around 0.9 units/mL after 6 months of life.

In the blood coagulation pathway, thrombin acts to convert factor XI to XIa, VIII to VIIIa, V to Va, fibrinogen to fibrin, and XIII to XIIIa. In the conversion of fibrinogen into fibrin, thrombin catalyzes the cleavage of fibrinopeptides A and B from the respective and chains of fibrinogen to form fibrin monomers.

Factor XIIIa is a transglutaminase that catalyzes the formation of covalent bonds between lysine and glutamine residues in fibrin. The covalent bonds increase the stability of the fibrin clot. Thrombin interacts with thrombomodulin.

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