Prothrombin (coagulation factor II) is encoded in the human by the F2-gene. It isproteolytically cleaved during theclotting process by theprothrombinase enzyme complex to form thrombin.
Thrombin (Factor IIa) (EC3.4.21.5, fibrose, thrombase, thrombofort, topical, thrombin-C, tropostasin, activated blood-coagulation factor II, E thrombin, beta-thrombin, gamma-thrombin) is aserine protease, that converts fibrinogen into strands of insolublefibrin, as well as catalyzing many other coagulation-related reactions.[5][6]
After the description of fibrinogen and fibrin,Alexander Schmidt hypothesised the existence of an enzyme that converts fibrinogen into fibrin in 1872.[7]
Prothrombin was discovered byPekelharing in 1894.[8][9][10]
Thrombin is produced by the enzymatic cleavage of two sites on prothrombin by activatedFactor X (Xa). The activity of factor Xa is greatly enhanced by binding to activatedFactor V (Va), termed theprothrombinase complex. Prothrombin is produced in the liver and is co-translationally modified in avitamin K-dependent reaction that converts 10-12glutamic acids in the N terminus of the molecule togamma-carboxyglutamic acid (Gla).[11] In the presence of calcium, the Gla residues promote the binding of prothrombin to phospholipid bilayers. Deficiency of vitamin K or administration of the anticoagulantwarfarin inhibits the production of gamma-carboxyglutamic acid residues, slowing the activation of the coagulation cascade.
In human adults, the normal blood level ofantithrombin 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.[12]
In the blood coagulation pathway, thrombin acts to convertfactor XI to XIa,VIII to VIIIa,V to Va,fibrinogen tofibrin, andXIII to XIIIa. In the conversion of fibrinogen into fibrin, thrombin catalyzes the cleavage offibrinopeptides A andB from the respectiveAα andBβ chains of fibrinogen to form fibrin monomers.[13]
Factor XIIIa is atransglutaminase that catalyzes the formation of covalent bonds between lysine and glutamine residues in fibrin. The covalent bonds increase the stability of the fibrin clot. Thrombininteracts withthrombomodulin.[14][15]
As part of its activity in the coagulation cascade, thrombin also promotesplatelet activation and aggregation via activation ofprotease-activated receptors on the cell membrane of the platelet.
Thrombin bound to thrombomodulin activatesprotein C, an inhibitor of the coagulation cascade. The activation of protein C is greatly enhanced following the binding of thrombin tothrombomodulin, an integral membraneprotein expressed byendothelial cells. Activated protein C inactivates factors Va and VIIIa. Binding of activated protein C toprotein S leads to a modest increase in its activity. Thrombin is also inactivated byantithrombin, aserine protease inhibitor.
The molecular weight of prothrombin is approximately 72,000Da. 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 largePA clan of proteases.
Prothrombin is composed of four domains; an N-terminalGla domain, twokringle domains and a C-terminaltrypsin-likeserine protease domain.Factor Xa withfactor V as a cofactor leads to cleavage of the Gla and two Kringle domains (forming together a fragment called fragment 1.2) and leave thrombin, consisting solely of the serine protease domain.[17]
As is the case for allserine 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 theβ-barrel promoting the correct conformation of the catalytic residues.[18] Contrary to crystal structures of active thrombin, hydrogen-deuterium exchange mass spectrometry studies indicate that this N-terminal Ile-NH3 does not become inserted into the β-barrel in the apo form of thrombin. However, binding of the active fragment ofthrombomodulin appears to allosterically promote the active conformation of thrombin by inserting this N-terminal region.[19]
There are an estimated 30 people in the world that have been diagnosed with the congenital form of Factor II deficiency,[20] which should not be confused with theprothrombin G20210A mutation, which is also called the factor II mutation. Prothrombin G20210A is congenital.[21]
Prothrombin G20210A is not usually accompanied by other factor mutations (i.e., the most common is factor V Leiden). The gene may be inheritedheterozygous (1 pair), or much more rarely,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 forthrombosis, such as oral contraceptives may be additive. The previously reported relationship ofinflammatory bowel disease (i.e.,Crohn's disease orulcerative colitis) and prothrombin G20210A orfactor V Leiden mutation have been contradicted by research.[22]
Activation of prothrombin is crucial in physiological and pathological coagulation. Various rare diseases involving prothrombin have been described (e.g.,hypoprothrombinemia). Anti-prothrombinantibodies inautoimmune disease may be a factor in the formation of thelupus anticoagulant (also known asantiphospholipid syndrome).Hyperprothrombinemia can be caused by the G20210A mutation.
Thrombin, a potentvasoconstrictor andmitogen, is implicated as a major factor invasospasm followingsubarachnoid hemorrhage. Blood from a rupturedcerebral aneurysm clots around a cerebralartery, releasing thrombin. This can induce an acute and prolonged narrowing of the blood vessel, potentially resulting incerebral ischemia andinfarction (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.[23][24][25]
Thrombin is implicated in the physiology ofblood 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-coatediron oxide attached to "reporter chemicals". When a peptide binds to a thrombin molecule, the report is released and appears in theurine where it can be detected. Human testing has not been conducted.[26]
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 ofrecombinant fusion protein constructs. Following purification of the fusion protein, thrombin can be used to selectively cleave between thearginine andglycine residues of the cleavage site, effectively removing thepurification tag from the protein of interest with a high degree of specificity.
Prothrombin complex concentrate andfresh 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 towarfarin.
Manipulation of prothrombin is central to the mode of action of mostanticoagulants.Warfarin and related drugs inhibitvitamin K-dependent carboxylation of several coagulation factors, including prothrombin.Heparin increases the affinity of antithrombin to thrombin (as well asfactor Xa). Thedirect 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 intoaqueous solution. It can be appliedtopically during surgery, as an aid tohemostasis. It can be useful for controlling minor bleeding from capillaries and small venules, but ineffective and not indicated for massive or brisk arterial bleeding.[27][28][29]
Thrombin, combined withfibrinogen, is sold under the brand name Fibrimex for use as a binding agent for meat. Both proteins in Fibrimex derives fromporcine orbovine blood.[30] 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.[31]
General secretary Jan Bertoft ofSwedish 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".[30]
PDB gallery | |
|---|---|
|
![1d3p: CRYSTAL STRUCTURE OF HUMAN ALPHA-THROMBIN IN COMPLEX WITH BENZO[B]THIOPHENE INHIBITOR 3](/mt/?noimg=&dark=on&url=http%3a%2f%2fwww.wikipedia.org%2fwiki%2fFile%3aPDB_1d3p_EBI.jpg)
![1d3q: CRYSTAL STRUCTURE OF HUMAN ALPHA THROMBIN IN COMPLEX WITH BENZO[B]THIOPHENE INHIBITOR 2](/mt/?noimg=&dark=on&url=http%3a%2f%2fwww.wikipedia.org%2fwiki%2fFile%3aPDB_1d3q_EBI.jpg)
![1d3t: CRYSTAL STRUCTURE OF HUMAN ALPHA THROMBIN IN COMPLEX WITH BENZO[B]THIOPHENE INHIBITOR 1](/mt/?noimg=&dark=on&url=http%3a%2f%2fwww.wikipedia.org%2fwiki%2fFile%3aPDB_1d3t_EBI.jpg)
![1tbz: HUMAN THROMBIN WITH ACTIVE SITE N-METHYL-D PHENYLALANYL-N-[5-(AMINOIMINOMETHYL)AMINO]-1-[(BENZOTHIAZOLYL)CARBONYL] BUTYL]-L-PROLINAMIDE TRIFLUROACETATE AND EXOSITE-HIRUGEN](/mt/?noimg=&dark=on&url=http%3a%2f%2fwww.wikipedia.org%2fwiki%2fFile%3aPDB_1tbz_EBI.jpg)
