Prothrombinase

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The prothrombinase enzyme complex consists of factor Xa (a serine protease) and factor Va (a protein cofactor). The complex assembles on negatively charged phospholipid membranes in the presence of calcium ions. The prothrombinase complex catalyzes the conversion of prothrombin (factor II), an inactive zymogen, to thrombin (factor IIa), an active serine protease. The activation of thrombin is a critical reaction in the coagulation cascade, which functions to regulate hemostasis in the body. To produce thrombin, the prothrombinase complex cleaves two peptide bonds in prothrombin, one after Arg271 and the other after Arg320. [1] Although it has been shown that factor Xa can activate prothrombin when unassociated with the prothrombinase complex, the rate of thrombin formation is severely decreased under such circumstances. The prothrombinase complex can catalyze the activation of prothrombin at a rate 3 x 105-fold faster than can factor Xa alone. [2] Thus, the prothrombinase complex is required for the efficient production of activated thrombin and also for adequate hemostasis.

Contents

Activation of protein precursors

Both factor X and factor V circulate in the blood as inactive precursors prior to activation by the coagulation cascade. The inactive zymogen factor X consists of two chains, a light chain (136 residues) and a heavy chain (306 residues). The light chain contains an N-terminal γ-carboxyglutamic acid domain (Gla domain) and two epidermal growth factor-like domains (EGF1 and EGF2). The heavy chain consists of an N-terminal activation peptide and a serine-protease domain. [3] [4] Factor X can be activated by both the factor VIIa-tissue factor complex of the extrinsic coagulation pathway and by the tenase complex of the intrinsic pathway. The intrinsic tenase complex is composed of both factor IXa and factor VIIIa. [5] [6] The activation peptide is released when factor X is activated to factor Xa, but the heavy and light chains remain covalently linked following activation.

Factor V circulates as a single-chain procofactor which contains six domains, A1-A2-B-A3-C1-C2. [7] Thrombin activates factor V by cleaving off the B domain. Other proteases also activate factor V, but this cleavage is primarily carried out by thrombin. Following cleavage, factor Va contains a heavy chain, composed of the A1 and A2 domains and a light chain, consisting of the A3, C1, and C2 domains. The light and heavy chains of factor Va are linked via a divalent metal ion, such as calcium. [8]

Complex assembly

Prothrombinase assembly begins with the binding of factor Xa and factor Va to negatively charged phospholipids on plasma membranes. Activated factor Xa and factor Va bind to the plasma membranes of a variety of different cell types, including monocytes, platelets, and endothelial cells. [9] Both factor Xa and Va bind to the membrane independently of each other, but they both bind to mutually exclusive binding sites. [10] Both factor Xa and factor Va associate with the membrane via their light chains, with factor Xa binding via its Gla-domain in a calcium-dependent manner and factor Va via its C2 and C1 domains. [11] [12] Once bound to the plasma membrane, Factor Xa and factor Va rapidly associate in a 1:1 stoichiometric ratio to form the prothrombinase complex. [13] Assembly of the prothrombinase complex is calcium dependent. When associated with the prothrombinase complex, the catalytic efficiency of factor Xa is increased 300,000-fold compared to its efficiency alone. [2] Factor Xa and factor Va interact tightly with each other when associated on the plasma membrane. [10] Further, membrane-bound factor Va provides a strong catalytic advantage to the prothrombinase complex. Factor Va strengthens the affinity of factor Xa for the membrane and also increases the kcat of factor Xa for prothrombin. [10] [14] Factor Va also decreases the Km of the reaction by enhancing the binding of prothrombin to the prothrombinase complex. [15]

Activity

The fully assembled prothrombinase complex catalyzes the conversion of the zymogen prothrombin to the serine protease thrombin. Specifically, Factor Xa cleaves prothrombin in two locations, following Arg271 and Arg320 in human prothrombin. [1] Because there are two cleavage events, prothrombin activation can proceed by two pathways. In one pathway, prothrombin is first cleaved at Arg271. This cleavage produces Fragment 1•2, comprising the first 271 residues, and the intermediate prethrombin 2, which is made up of residues 272-579. Fragment 1•2 is released as an activation peptide, and prethrombin 2 is cleaved at Arg320, yielding active thrombin. The two chains formed after the cleavage at Arg320, termed the A and B chains, are linked by a disulfide bond in active thrombin. In the alternate pathway for thrombin activation, prothrombin is first cleaved at Arg320, producing a catalytically active intermediate called meizothrombin. [16] Meizothrombin contains fragment 1•2 A chain linked to the B chain by a disulfide bond. Subsequent cleavage of meizothrombin by factor Xa at Arg271 gives fragment 1•2 and active thrombin, consisting of the A and B chains linked by a disulfide bond. When thrombin is generated by factor Xa alone, the first pathway predominates and prothrombin is first cleaved after Arg271, producing prethrombin 2, which is subsequently cleaved after Arg320. [17] If factor Xa acts as a component of the prothrombinase complex, however, the second pathway is favored, and prothrombin is first cleaved after Arg320, producing meizothrombin, which is cleaved after Arg271 to produce active thrombin. [17] [18] Thus, the formation of the prothrombinase complex alters the sequence of prothrombin bond cleavage.

Inactivation

Factor Va is inactivated following cleavage by activated protein C. Activated protein C cleaves factor Va in both its light and heavy chains. Cleavage in the heavy chain reduces the ability of factor V to bind to factor Xa. [19] Activated protein C interacts tightly and exclusively with the light chain of factor Va, and this interaction is calcium independent. [20] Factor Xa can help to prevent the inactivation of factor Va by protecting factor Va from activated protein C. [21] It is likely that factor Xa and activated protein C compete for similar sites on factor Va. [9] Factor Xa is inhibited by the antithrombin III/heparin system, which also acts to inhibit thrombin. [9]

Role in disease

Deficiencies of either protein components of the prothrombinase complex are very rare. Factor V deficiency, also called parahemophilia, is a rare autosomal recessive bleeding disorder with an approximate incidence of 1 in 1,000,000. [22] Congenital factor X deficiency is also extremely rare, affecting an estimated 1 in 1,000,000. [23]

A point mutation in the gene encoding factor V can lead to a hypercoagulability disorder called factor V Leiden. In factor V Leiden, a G1691A nucleotide replacement results in an R506Q amino acid mutation. Factor V Leiden increases the risk of venous thrombosis by two known mechanisms. First, activated protein C normally inactivates factor Va by cleaving the cofactor at Arg306, Arg506, and Arg679. [24] The factor V Leiden mutation at Arg506 renders factor Va resistant to inactivation by activated protein C. As a result of this resistance, the half-life of factor Va in plasma is increased, resulting in increased thrombin production and increased risk of thrombosis. [25] Secondly, under normal conditions, if factor V is cleaved by activated protein C instead of thrombin, it can serve as a cofactor for activated protein C. [25] Once bound to factor V, activated protein C cleaves and inactivates factor VIIIa. The mutated form of factor V present in factor V Leiden, however, serves as a less efficient cofactor of activated protein C. Thus, Factor VIIIa is less efficiently inactivated in factor V Leiden, further increasing the risk of thrombosis. [25] In fact, Factor V Leiden is the most common cause of inherited thrombosis. [26]

Heterozygous factor V Leiden is present in approximately 5% of the white population in the United States and homozygous factor V Leiden is found less than 1% of this population. [27] Factor V Leiden is much more common in individuals of Northern European descent and in some Middle Eastern populations. It is less common in Hispanic populations, and rare in African, Asian, and Native American populations. [27] Factor V Leiden is an important risk factor for venous thromboembolism, that is, deep vein thrombosis or pulmonary embolism. [28] In fact, heterozygous factor V Leiden increases one's risk of recurrent venous thromboembolism by 40%. [29]

Anticoagulant drugs

Inhibition of factor Xa prevents thrombin activation, thereby preventing clot formation. Thus, Factor Xa is used as both a direct and indirect target of several anticoagulant drugs. For example, the drug Fondaparinux is an indirect inhibitor of factor Xa. Fondaparinux binds to antithrombin III and activates the molecule for factor Xa inhibition. In fact, Fondaparinux imparts an increased affinity of antithrombin III to factor Xa, and this increased affinity results in a 300-fold increase in the antithrombin III inhibitory effect on factor Xa. [30] After the antithrombin III binds to factor Xa, the Fondaparinux is released and can activate another antithrombin. [31] Another drug that indirectly inhibits factor Xa is Idraparinux. Idraparinux also binds antithrombin III, however with a 30-fold increase in affinity as compared to Fondaparinux. [32] Idraparinux has an increased half-life as compared to Fondaparinux and can be administered on a weekly basis, whereas Fondaparinux must be subcutaneously injected daily. [33]

Rivaroxaban, Apixaban and Edoxaban are direct factor Xa inhibitors. [34] [35] [36]

Rivaroxaban, Apixaban and Edoxaban bind to the active site of factor Xa, regardless of whether factor Xa is bound in the prothrombinase complex or is in its free form. [35] [37] These direct factor Xa inhibitors can be administered orally, as can dabigatran etexilate, which is a direct thrombin inhibitor.

Fondaparinux, Rivaroxaban, Apixaban, Dabigatran Etexilate and Edoxaban are currently used as FDA-approved anticoagulant drugs. Development of Idraparinux was discontinued. [38]

Related Research Articles

<span class="mw-page-title-main">Coagulation</span> Process of formation of blood clots

Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechanism of coagulation involves activation, adhesion and aggregation of platelets, as well as deposition and maturation of fibrin.

Factor V Leiden is a variant of human factor V, which causes an increase in blood clotting (hypercoagulability). Due to this mutation, protein C, an anticoagulant protein that normally inhibits the pro-clotting activity of factor V, is not able to bind normally to factor V, leading to a hypercoagulable state, i.e., an increased tendency for the patient to form abnormal and potentially harmful blood clots. Factor V Leiden is the most common hereditary hypercoagulability disorder amongst ethnic Europeans. It is named after the Dutch city of Leiden, where it was first identified in 1994 by Rogier Maria Bertina under the direction of Pieter Hendrick Reitsma. Despite the increased risk of venous thromboembolisms, people with one copy of this gene have not been found to have shorter lives than the general population. It is an autosomal dominant genetic disorder with incomplete penetrance.

<span class="mw-page-title-main">Thrombin</span> Enzyme involved in blood coagulation in humans

Thrombin is a serine protease, an enzyme that, in humans, is encoded by the F2 gene. During the clotting process, prothrombin is proteolytically cleaved by the prothrombinase enzyme complex to form thrombin. Thrombin in turn acts as a serine protease that converts soluble fibrinogen into insoluble strands of fibrin, as well as catalyzing many other coagulation-related reactions.

<span class="mw-page-title-main">Antithrombin</span> Mammalian protein found in Homo sapiens

Antithrombin (AT) is a small glycoprotein that inactivates several enzymes of the coagulation system. It is a 464-amino-acid protein produced by the liver. It contains three disulfide bonds and a total of four possible glycosylation sites. α-Antithrombin is the dominant form of antithrombin found in blood plasma and has an oligosaccharide occupying each of its four glycosylation sites. A single glycosylation site remains consistently un-occupied in the minor form of antithrombin, β-antithrombin. Its activity is increased manyfold by the anticoagulant drug heparin, which enhances the binding of antithrombin to factor IIa (thrombin) and factor Xa.

Low-molecular-weight heparin (LMWH) is a class of anticoagulant medications. They are used in the prevention of blood clots and treatment of venous thromboembolism and in the treatment of myocardial infarction.

<span class="mw-page-title-main">Protein S</span>

Protein S is a vitamin K-dependent plasma glycoprotein synthesized in the liver. In the circulation, Protein S exists in two forms: a free form and a complex form bound to complement protein C4b-binding protein (C4BP). In humans, protein S is encoded by the PROS1 gene. Protein S plays a role in coagulation.

<span class="mw-page-title-main">Protein C</span> Mammalian protein found in Homo sapiens

Protein C, also known as autoprothrombin IIA and blood coagulation factor XIV, is a zymogen, that is, an inactive enzyme. The activated form plays an important role in regulating anticoagulation, inflammation, and cell death and maintaining the permeability of blood vessel walls in humans and other animals. Activated protein C (APC) performs these operations primarily by proteolytically inactivating proteins Factor Va and Factor VIIIa. APC is classified as a serine protease since it contains a residue of serine in its active site. In humans, protein C is encoded by the PROC gene, which is found on chromosome 2.

<span class="mw-page-title-main">Thrombophilia</span> Abnormality of blood coagulation

Thrombophilia is an abnormality of blood coagulation that increases the risk of thrombosis. Such abnormalities can be identified in 50% of people who have an episode of thrombosis that was not provoked by other causes. A significant proportion of the population has a detectable thrombophilic abnormality, but most of these develop thrombosis only in the presence of an additional risk factor.

<span class="mw-page-title-main">Factor X</span> Mammalian protein found in Homo sapiens

Factor X, also known by the eponym Stuart–Prower factor, is an enzyme of the coagulation cascade. It is a serine endopeptidase. Factor X is synthesized in the liver and requires vitamin K for its synthesis.

<span class="mw-page-title-main">Factor V</span> Mammalian protein found in humans

Factor V is a protein of the coagulation system, rarely referred to as proaccelerin or labile factor. In contrast to most other coagulation factors, it is not enzymatically active but functions as a cofactor. Deficiency leads to predisposition for hemorrhage, while some mutations predispose for thrombosis.

<span class="mw-page-title-main">Hirudin</span>

Hirudin is a naturally occurring peptide in the salivary glands of blood-sucking leeches that has a blood anticoagulant property. This is essential for the leeches' habit of feeding on blood, since it keeps a host's blood flowing after the worm's initial puncture of the skin.

<span class="mw-page-title-main">Enteropeptidase</span> Class of enzymes

Enteropeptidase is an enzyme produced by cells of the duodenum and is involved in digestion in humans and other animals. Enteropeptidase converts trypsinogen into its active form trypsin, resulting in the subsequent activation of pancreatic digestive enzymes. Absence of enteropeptidase results in intestinal digestion impairment.

<span class="mw-page-title-main">Fondaparinux</span> Chemical compound

Fondaparinux is an anticoagulant medication chemically related to low molecular weight heparins. It is marketed by Viatris. A generic version developed by Alchemia is marketed within the US by Dr. Reddy's Laboratories.

<span class="mw-page-title-main">Tissue factor pathway inhibitor</span> Single-chain polypeptide capable of inhibiting blood clotting Factor Xa

Tissue factor pathway inhibitor is a single-chain polypeptide which can reversibly inhibit factor Xa (Xa). While Xa is inhibited, the Xa-TFPI complex can subsequently also inhibit the FVIIa-tissue factor complex. TFPI contributes significantly to the inhibition of Xa in vivo, despite being present at concentrations of only 2.5 nM.

<span class="mw-page-title-main">Activated protein C resistance</span> Medical condition

Activated protein C resistance (APCR) is a hypercoagulability characterized by a lack of a response to activated protein C (APC), which normally helps prevent blood from clotting excessively. This results in an increased risk of venous thrombosis, which resulting in medical conditions such as deep vein thrombosis and pulmonary embolism. The most common cause of hereditary APC resistance is factor V Leiden mutation.

<span class="mw-page-title-main">Heparin cofactor II</span> Protein-coding gene in the species Homo sapiens

Heparin cofactor II (HCII), a protein encoded by the SERPIND1 gene, is a coagulation factor that inhibits IIa, and is a cofactor for heparin and dermatan sulfate.

<span class="mw-page-title-main">Carboxypeptidase B2</span>

Carboxypeptidase B2 (CPB2), also known as carboxypeptidase U (CPU), plasma carboxypeptidase B (pCPB) or thrombin-activatable fibrinolysis inhibitor (TAFI), is an enzyme that, in humans, is encoded by the gene CPB2.

Prothrombin fragment 1+2 (F1+2), also written as prothrombin fragment 1.2 (F1.2), is a polypeptide fragment of prothrombin generated by the in vivo cleavage of prothrombin into thrombin by the enzyme prothrombinase. It is released from the N-terminus of prothrombin. F1+2 is a marker of thrombin generation and hence of coagulation activation. It is considered the best marker of in vivo thrombin generation.

The activated protein C resistance (APCR) test is a coagulation test used in the evaluation and diagnosis of activated protein C (APC) resistance, a form of hypercoagulability. Hereditary APC resistance is usually caused by the factor V Leiden mutation, whereas acquired APC resistance has been linked to antiphospholipid antibodies, pregnancy, and estrogen therapy. APC resistance can be measured using either an activated partial thromboplastin time (aPTT)-based test or an endogenous thrombin potential (ETP)-based test.

Coagulation activation markers are biomarkers of net activation of coagulation and fibrinolysis. Examples include prothrombin fragment 1+2 (F1+2), thrombin–antithrombin complex (TAT), fibrinopeptide A (FpA), fibrin monomers (FMs), plasmin-α2-antiplasmin complex (PAP), activated protein C–protein C inhibitor (APC-PCI), and D-dimer (DD). These compounds are markers of thrombin generation, fibrin generation, and fibrinolysis. Coagulation activation markers, particularly D-dimer, are useful in the diagnosis of acute venous thromboembolism. They may also be useful in the assessment of hypercoagulability and venous thromboembolism risk.

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  38. ↑ Gross PL, Weitz JI (March 2008). "New anticoagulants for treatment of venous thromboembolism". Arterioscler. Thromb. Vasc. Biol. 28 (3): 380–6. doi: 10.1161/ATVBAHA.108.162677 . PMID   18296593. S2CID   3074344.

See also

coagulation cascade hemostasis

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