Thrombin



Thrombin (, fibrinogenase, thrombase, thrombofort, topical, thrombin-C, tropostasin, activated blood-coagulation factor II, blood-coagulation factor IIa, factor IIa, E thrombin, beta-thrombin, gamma-thrombin) is a is_associated_with::serine protease that in humans is encoded by the F2 is_associated_with::gene. Prothrombin (coagulation factor II) is proteolytically cleaved to form thrombin in the is_associated_with::coagulation cascade, which ultimately results in the reduction of blood loss. Thrombin in turn acts as a serine protease that converts soluble is_associated_with::fibrinogen into insoluble strands of is_associated_with::fibrin, as well as catalyzing many other coagulation-related reactions.

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

Synthesis
Thrombin is produced by the enzymatic cleavage of two sites on is_associated_with::prothrombin by activated is_associated_with::Factor X (Xa). The activity of factor Xa is greatly enhanced by binding to activated is_associated_with::Factor V (Va), termed the prothrombinase complex. Prothrombin is produced in the liver and is post-translationally modified in a is_associated_with::vitamin K-dependent reaction that converts ten glutamic acids on prothrombin to is_associated_with::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 is_associated_with::warfarin inhibits the production of gamma-carboxyglutamic acid residues, slowing the activation of the coagulation cascade.

In human adults, the normal blood level of is_associated_with::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.

Mechanism of action
In the blood coagulation pathway, thrombin acts to convert is_associated_with::factor XI to XIa, VIII to VIIIa, V to Va, is_associated_with::fibrinogen to is_associated_with::fibrin, and XIII to XIIIa.

is_associated_with::Factor XIIIa is a is_associated_with::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 is_associated_with::thrombomodulin.

As part of its activity in the coagulation cascade, thrombin also promotes is_associated_with::platelet activation and aggregation via activation of is_associated_with::protease-activated receptors on the cell membrane of the platelet.

Negative feedback
Thrombin bound to thrombomodulin activates is_associated_with::protein C, an inhibitor of the coagulation cascade. The activation of protein C is greatly enhanced following the binding of thrombin to is_associated_with::thrombomodulin, an integral membrane is_associated_with::protein expressed by is_associated_with::endothelial cells. Activated protein C inactivates factors Va and VIIIa. Binding of activated protein C to is_associated_with::protein S leads to a modest increase in its activity. Thrombin is also inactivated by is_associated_with::antithrombin, a serine protease inhibitor.

Protein


The molecular weight of prothrombin is approximately 72,000 Da. 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. Structurally, it is a member of the large is_associated_with::PA clan of proteases.

Gene
The thrombin (prothrombin) gene is located on the eleventh is_associated_with::chromosome (11p11-q12).

There are an estimated 30 people in the world that have been diagnosed with the congenital form of Factor II deficiency, which should not be confused with the is_associated_with::prothrombin G20210A mutation, which is also called the factor II mutation. Prothrombin G20210A is congenital.

Prothrombin G20210A is not usually accompanied by other factor mutations (i.e., the most common is factor V Leiden). The gene may be inherited is_associated_with::heterozygous (1 pair), or much more rarely, is_associated_with::homozygous (2 pairs), and is not related to gender or blood type. Homozygous mutations increase the risk of thrombosis more than heterozygous mutations, but the relative increased risk is not well documented. Other potential risks for thrombosis, such as oral contraceptives may be additive. The previously reported relationship of inflammatory bowel disease (i.e., Crohn's disease or ulcerative colitis) and prothrombin G20210A or is_associated_with::factor V Leiden mutation have been contradicted by research.

Role in disease
Activation of prothrombin is crucial in physiological and pathological coagulation. Various rare diseases involving prothrombin have been described (e.g., is_associated_with::hypoprothrombinemia). Anti-prothrombin antibodies in is_associated_with::autoimmune disease may be a factor in the formation of the is_associated_with::lupus anticoagulant also known as (is_associated_with::antiphospholipid syndrome). is_associated_with::Hyperprothrombinemia can be caused by the G20210A mutation.

Thrombin, a potent is_associated_with::vasoconstrictor and is_associated_with::mitogen, is implicated as a major factor in is_associated_with::vasospasm following is_associated_with::subarachnoid hemorrhage. Blood from a ruptured is_associated_with::cerebral aneurysm clots around a cerebral is_associated_with::artery, releasing thrombin. This can induce an acute and prolonged narrowing of the blood vessel, potentially resulting in is_associated_with::cerebral ischemia and is_associated_with::infarction (is_associated_with::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.

Thrombin is implicated in the physiology of is_associated_with::blood clots. Its presence indicates the existence of a clot. In 2013 a system for detecting the presence of thrombin was developed in mice. It combines peptide-coated is_associated_with::iron oxide attached to "reporter chemicals". When a peptide binds to a thrombin molecule, the report is released and appears in the is_associated_with::urine where it can be detected. Human testing has not been conducted.

Research tool
Due to its high proteolytic specificity, thrombin is a valuable biochemical tool. The thrombin cleavage site (Leu-Val-Pro-Arg-Gly-Ser) is commonly included in linker regions of is_associated_with::recombinant fusion protein constructs. Following purification of the fusion protein, thrombin can be used to selectively cleave between the Arginine and Glycine residues of the cleavage site, effectively removing the is_associated_with::purification tag from the protein of interest with a high degree of specificity.

Medicine and surgery
is_associated_with::Prothrombin complex concentrate and is_associated_with::fresh frozen plasma are prothrombin-rich coagulation factor preparations that can be used to correct deficiencies (usually due to medication) of prothrombin. Indications include intractable bleeding due to is_associated_with::warfarin.

Manipulation of prothrombin is central to the mode of action of most is_associated_with::anticoagulants. is_associated_with::Warfarin and related drugs inhibit is_associated_with::vitamin K-dependent carboxylation of several coagulation factors, including prothrombin. is_associated_with::Heparin increases the affinity of antithrombin to thrombin (as well as is_associated_with::factor Xa). The is_associated_with::direct thrombin inhibitors, a newer class of medication, directly inhibit thrombin by binding to its active site.

Recombinant thrombin is available as a powder for reconstitution into is_associated_with::aqueous solution. It can be applied is_associated_with::topically during surgery, as an aid to is_associated_with::hemostasis. It can be useful for controlling minor bleeding from capillaries and small venules, but ineffective and not indicated for massive or brisk arterial bleeding.

Food production
Thrombin is sold under the brand name Fibrimex for use as a binding agent for meat. The thrombin in Fibrimex derives from is_associated_with::porcine or is_associated_with::bovine blood. According to the manufacturer it can be used to produce new kinds of mixed meats (for example combining beef and fish seamlessly). The manufacturer also states that it can be used to combine whole muscle meat, form and portion these thus cutting down on production costs without a loss in quality.

General secretary Jan Bertoft of is_associated_with::Swedish Consumers' Association has stated that "there is danger of misleading the consumers since there is no way to tell this reconstituted meat from real meat"