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.

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.

[12] 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.

[13] Factor XIIIa is a transglutaminase that catalyzes the formation of covalent bonds between lysine and glutamine residues in fibrin.

The catalytic domain is released from prothrombin fragment 1.2 to create the active enzyme thrombin, which has a molecular weight of 36,000 Da.

[17] As is the case for all serine proteases, prothrombin is converted to active thrombin by proteolysis of an internal peptide bond, exposing a new N-terminal Ile-NH3.

The historic model of activation of serine proteases involves insertion of this newly formed N-terminus of the heavy chain into the β-barrel promoting the correct conformation of the catalytic residues.

Anti-prothrombin antibodies in autoimmune disease may be a factor in the formation of the lupus anticoagulant (also known as antiphospholipid syndrome).

This can induce an acute and prolonged narrowing of the blood vessel, potentially resulting in cerebral ischemia and infarction (stroke).

Beyond its key role in the dynamic process of thrombus formation, thrombin has a pronounced pro-inflammatory character, which may influence the onset and progression of atherosclerosis.

Acting via its specific cell membrane receptors (protease activated receptors: PAR-1, PAR-3 and PAR-4), which are abundantly expressed in all arterial vessel wall constituents, thrombin has the potential to exert pro-atherogenic actions such as inflammation, leukocyte recruitment into the atherosclerotic plaque, enhanced oxidative stress, migration and proliferation of vascular smooth muscle cells, apoptosis and angiogenesis.

The thrombin cleavage site (Leu-Val-Pro-Arg-Gly-Ser) is commonly included in linker regions of recombinant fusion protein constructs.

Warfarin and related drugs inhibit vitamin K-dependent carboxylation of several coagulation factors, including prothrombin.

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