Thrombin

Overview
Thrombin (activated Factor II [IIa]) is a coagulation protein that has many effects in the coagulation cascade. It is a serine protease that converts soluble fibrinogen into insoluble strands of fibrin, as well as catalyzing many other coagulation-related reactions.

Genetics
The prothrombin gene is located on the eleventh chromosome (11p11-q12). The molecular weight of prothrombin is approximately 72000mol-1; in contrast, the molecular weight of thrombin is 36000mol-1. Once activated, the catalytic domain of prothrombin is released from prothrombin fragment 1.2. There are an estimated 30 people in the world that have been diagnosed with the congenital form of Factor II deficiency (Degen, 1995), which should not be confused with a mutation of prothrombin. The prothrombin gene mutation is called Factor II mutation. Factor II mutation is congenital. The Factor II mutated gene is not usually accompanied by other factor mutations (i.e. the most common is Factor V Leiden). The gene may be inherited heterozygous, or much more rarely, homozygous, 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 mutation or Factor V Leiden mutation have been contradicted by research.

Generation
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 post-translationally modified in a vitamin K-dependent reaction that converts ten glutamic acids on prothrombin to gamma-carboxyglutamic acid (Gla). In the presence of calcium, the Gla residues promote the binding of thrombin to phospholipid bilayers (see the picture). 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 beings the level prothrombin in the blood stream increases after birth and typically peaks on the 8th day after which the prothrombin level lowers to normal levels.

Action
Thrombin converts fibrinogen to an active form that assembles into fibrin. Thrombin also activates factor XI, factor V, and factor VIII. This positive feedback accelerates the production of thrombin.

Factor XIII is also activated by thrombin. 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.

Platelets
In addition to its activity in the coagulation cascades, thrombin also promotes platelet activation, via activation of protease-activated receptors on the platelet.

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

Role in disease
Activation of prothrombin is crucial in physiological and pathological coagulation. Various rare diseases involving prothrombin have been described (e.g., hypoprothrombinemia). Anti-thrombin antibodies in autoimmune disease may be a factor in the formation of the lupus anticoagulant.

Prothrombin 20210a mutation
Substitution of adenine for guanine at position 20210 of the prothrombin gene, known as the prothrombin 20210a mutation, prothrombin 20210 mutation and, less precise, just factor II mutation, leads to hypercoagulability and may be a factor that contributes to infertility. Its prevalence is thought to be approximately 2% in caucasians and 0.5% in blacks.

Position 20210 falls outside of the reading frame for the protein and is thought to play a role in promoting prothrombin production; the substitution leads to high levels of prothrombin.

Biotechnology
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 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 purification tag from the protein of interest with a high degree of specificity.

Pharmacology
Prothrombin complex concentrate and 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 warfarin.

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

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