The formation of a fibrin clot at the site of an injury to the wall of a normal blood vessel is an essential part of the process to stop blood loss after vascular injury. The reactions that lead to fibrin clot formation are commonly described as a cascade, in which the product of each step is an enzyme or cofactor needed for following reactions to proceed efficiently. The entire clotting cascade can be divided into three portions, the extrinsic pathway, the intrinsic pathway, and the common pathway. The extrinsic pathway begins with the release of tissue factor at the site of vascular injury and leads to the activation of factor X. The intrinsic pathway provides an alternative mechanism for activation of factor X, starting from the activation of factor XII. The common pathway consists of the steps linking the activation of factor X to the formation of a multimeric, cross-linked fibrin clot. Each of these pathways includes not only a cascade of events that generate the catalytic activities needed for clot formation, but also numerous positive and negative regulatory events.
View original pathway at:Reactome.
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Fibrinogen is a hexamer, containing two fibrinogen alpha chains, two fibrinogen beta chains, and two fibrinogen gamma chains, held together by disulfide bonds.
Fibrin is a hexamer of two fibrinogen alpha chains, two fibrinogen beta chains, and two fibrinogen gamma chains, held together by disulfide bonds. It is formed in vivo by the thrombin-catalyzed removal of amino terminal fibinopeptides from the A alpha and B beta chains of fibrinogen. This fibrin hexamer ("fibrin monomer") is the subunit that multimerizes to form a fibrin clot ("fibrin multimer").
The fibrin "monomers" formed by the action of thrombin on fibrinogen associate spontaneously into multimers. This association can follow several distinct pathways and may be able to form several types of higher-order structures. All of these possibilities are represented in Reactome as a fibrin trimer.
Activated thrombin cleaves the A chains of factor XIII tetramers in a reaction stimulated by the presence of fibrin multimers. The amino terminal portions of the A chains are released as activation peptides, which have no known function. The resulting factor XIII tetramer remains catalytically inactive.
In the blood coagulation process, prothrombin is proteolytically cleaved to form thrombin (factor IIa) which in turn, acts as a serine protease that converts soluble fibrinogen into insoluble strands of fibrin. Specifically, thrombin converts factor XI to XIa, factor VIII to VIIIa, factor V to Va, fibrinogen to fibrin, and factor XIII to XIIIa. The direct oral anticoagulant (DOAC) drug dabigatran is a potent, competitive direct thrombin inhibitor (DTI) that reversibly and specifically binds both clot-bound and free thrombin, as well as inhibiting thrombin-induced platelet aggregation (Wienen et al. 2007, Stangier et al. 2007). Commercially it is formulated as a lipophilic prodrug, dabigatran etexilate, to promote gastrointestinal absorption before it is metabolised to the active drug. The kidneys excrete the majority (80%) of the unchanged drug (Stangier et al. 2007).
The membrane-bound Va:Xa (prothrombinase) complex rapidly activates large amounts of thrombin.
Factor Xa (aka Factor X heavy chain), a cleavage product of coagulation factor X (F10), is a vitamin K-dependent glycoprotein able to convert prothrombin to thrombin during the blood clotting process. Factor Xa is a target for direct oral anticoagulant (DOAC) drugs that are direct factor Xa inhibitors (the so-called 'xabans') and used in the treatment and prevention of thromboembolic disorders (Galanis et al. 2014). Rivaroxaban (brand name Xarelto) was the first medically approved drug of this class (Abrams & Emerson 2009, Misselwitz et al. 2011). In patients with non-valvular atrial fibrillation, 'xabans' appears to be as effective as warfarin in preventing nonhemorrhagic strokes and embolic events (Patel et al. 2011, Gomez-Outes et al. 2013). The most serious side-effect of rivaroxaban is GI bleeding, and with there being no antidote for rivaroxaban, bleeding events can be difficult to manage (Siegal et al. 2014). Rivaroxaban binds to and inhibits both free factor Xa and factor Xa bound in the prothrombinase complex (Roehrig et al. 2005). Unlike warfarin, the 'xabans' exhibit a predictable dose response and do not require routine coagulation monitoring.
Factors Va and Xa associate on a membrane surface to form a complex in which the activity of factor Xa on prothrombin is greatly increased (Mann et al. 1988). The presence of negatively charged phospholipid in the membrane greatly facilitates this process, a feature that may contribute to its localization, as such phospholipids are normally on the cytosolic face of the plasma membrane (Devaux 1992), but could be exposed to the extracellular space following platelet activation or mechanical injury to endothelial cells.
Activated thrombin (factor IIa) catalyzes the conversion of factor V to factor Va (activated factor V). The activation peptide released in this reaction has no known function.
Membrane-bound factor Xa catalyzes the activation of small amounts of thrombin. The amino terminal portion of prothrombin is released as an activation peptide, which can be cleaved further by activated thrombin. Neither the full-length activation peptide nor its cleavage products have known functions.
Factor Xa (aka Factor X heavy chain), a cleavage product of coagulation factor X (F10), is a vitamin K-dependent glycoprotein able to convert prothrombin to thrombin during the blood clotting process. Factor Xa is a target for direct oral anticoagulant (DOAC) drugs that are direct factor Xa inhibitors (the so-called 'xabans') and used in the treatment and prevention of thromboembolic disorders (Galanis et al. 2014). Rivaroxaban (brand name Xarelto) was the first medically approved drug of this class (Abrams & Emerson 2009, Misselwitz et al. 2011). In patients with non-valvular atrial fibrillation, 'xabans' appears to be as effective as warfarin in preventing nonhemorrhagic strokes and embolic events (Patel et al. 2011, Gomez-Outes et al. 2013). The most serious side-effect of rivaroxaban is GI bleeding, and with there being no antidote for rivaroxaban, bleeding events can be difficult to manage (Siegal et al. 2014). Rivaroxaban binds to and inhibits both free factor Xa and factor Xa bound in the prothrombinase complex (Roehrig et al. 2005). Unlike warfarin, the 'xabans' exhibit a predictable dose response and do not require routine coagulation monitoring.
Factor VIIa, bound to tissue factor at the endothelial cell surface (the "extrinsic tenase complex"), catalyzes the formation of activated factor X with high efficiency. The amino terminal part of the heavy chain of factor X, the factor X activation peptide, is released. (This peptide has no known function.)
Factor VII, bound to tissue factor at the endothelial cell surface, catalyzes the activation of factor X from plasma with moderate efficiency. The amino terminal part of the heavy chain of factor X, the factor X activation peptide, is released. (This peptide has no known function.)
Antithrombin III binds to membrane-associated heparin, e.g., on the surface of a normal endothelial cell. This binding event increases the affinity of antithrombin III for thrombin approximately 1000-fold.
Factor VIIa, bound to tissue factor at the endothelial cell surface, catalyzes the formation of activated factor IX with high efficiency. The amino terminal part of the heavy chain of factor IX, the factor IX activation peptide, is released. (This peptide has no known function.)
TFPI binds to the factor VIIa:TF complex and to factor Xa at the endothelial surface, forming a stable heterotetrameric complex in which factor VIIa is catalytically inactive.
The alpha and beta chains of fibrinogen hexamer are cleaved by thrombin to generate fibrin monomer (Ni et al. 1989). The amino terminal regions of the cleaved alpha and beta chains are released (fibrinopeptides A and B respectively).
In the blood coagulation process, prothrombin is proteolytically cleaved to form thrombin (factor IIa) which in turn, acts as a serine protease that converts soluble fibrinogen into insoluble strands of fibrin. Specifically, thrombin converts factor XI to XIa, factor VIII to VIIIa, factor V to Va, fibrinogen to fibrin, and factor XIII to XIIIa. The direct oral anticoagulant (DOAC) drug dabigatran is a potent, competitive direct thrombin inhibitor (DTI) that reversibly and specifically binds both clot-bound and free thrombin, as well as inhibiting thrombin-induced platelet aggregation (Wienen et al. 2007, Stangier et al. 2007). Commercially it is formulated as a lipophilic prodrug, dabigatran etexilate, to promote gastrointestinal absorption before it is metabolised to the active drug. The kidneys excrete the majority (80%) of the unchanged drug (Stangier et al. 2007).
Fibrin monomers rapidly and spontaneously associate into large multimers, binding to one another via sites created by fibrinopeptide release (Laudano and Doolittelle 1980). The process of multimerization, and the range of multimer structures that can form in vivo and in vitro, have been studied in detail (Doolittle 1984). Here, multimer size has arbitrarily been set to three fibrin monomers.
Once the A chains of the Factor XIII tetramer have been cleaved by thrombin, the complex dissociates and the resulting A chain dimer binds Ca++ (one per peptide monomer) to form activated factor XIII (factor XIIIa).
Fibrin multimers are stabilized by the formation of multiple covalent crosslinks between the side chains of specific lysine and glutamine residues in fibrinogen alpha and gamma chains, catalyzed by factor XIIIa.
Antithrombin III in the complex is cleaved by thrombin, thereupon undergoing a conformational change that stabilizes the thrombin:antithrombin III complex, trapping and inactivating the thrombin moiety.
The same conformational change that traps thrombin in its complex with cleaved antithrombin III also decreases the affinity of the latter for heparin, and the complex of cleaved antithrombin III and thrombin dissociates from the cell-bound heparin molecule.
Activated protein C cleaves peptide bonds in activated factor V (factor Va), converting it to an inactive form (factor Vi). APC proteolysis involves cleavage of the factor Va heavy chain at Arg-334 (306 if signal peptide is not included) and Arg-534 (506 with no signal peptide) (Nicolaes et al. 1985). Most factor Va molecules are initially cleaved at Arg-534, yielding a partially active intermediate, followed by complete inactivation through cleavage at Arg-334 (Kalafatis et al. 1994). Factor Xa inhibits Arg-534 cleavage but this effect is mitigated by Protein S (Norstrom et al. 2006). A mutation of the APC cleavage sites in Fv at Arg-534Gln a.k.a. FVLeiden is the most common identifiable hereditary risk factor for venous thrombosis among Caucasians (Camire 2011).
Thrombin complexed with thrombomodulin at the endothelial cell surface cleaves the heavy chain of protein C, generating activated protein C and an activation peptide. The activation peptide has no known function.
Activated thrombin (factor IIa) binds to thrombomodulin at the external face of the plasma membrane, forming a thrombin:thrombomodulin complex. In this complexed form, the activity of thrombin towards protein C is greatly increased, and as thrombomodulin is particularly abundant on the surfaces of endothelial cells, this association plays a major role in restricting clot formation.
Factor VIII binds to von Willebrand factor to form a complex. This complex stabilizes factor VIII, which otherwise has a very short half-life in the blood.
Factor VIII (Vehar et al. 1984) is a heterodimer containing a heavy and a light polypeptide chain, generated by the proteolytic cleavage of a single large precursor polypeptide. Several forms of the heavy chain are found in vivo, all functionally the same but differing in the amount of the B domain removed by proteolysis. The single form annotated here is the shortest one (Eaton et al. 1986; Hill-Eubanks et al. 1989).
In vitro, von Willebrand factor (Titani et al. 1986) can form complexes with factor VIII with a 1:1 stoichiometry. The complexes that form in vivo, however, involve large multimers of von Willebrand factor and varied, but always low, proportions of factor VIII (Vlot et al. 1995). A stoichiometry of one molecule of factor VIII associated with 50 of von Willebrand factor is typical in vivo, and is used here to annotate the factor VIII:von Willebrand factor complex.
Factor VIII complexed to von Willibrand factor in the blood is cleaved into several smaller polypeptides that remain associated. The acidic polypeptide on the aminoterminal side of the A3 domain of the light chain is released, however, and as this polypeptide mediates the association of factor VIII with von Willibrand factor, the activated factor VIII is released. While several proteases are capable of catalyzing these cleavages in vitro, only thrombin is active on factor VIII:von Willibrand factor complexes under physiological conditions (Eaton et al. 1986; Hill-Eubanks et al. 1989; Lollar et al. 1988; Pieters et al. 1989).
In the blood coagulation process, prothrombin is proteolytically cleaved to form thrombin (factor IIa) which in turn, acts as a serine protease that converts soluble fibrinogen into insoluble strands of fibrin. Specifically, thrombin converts factor XI to XIa, factor VIII to VIIIa, factor V to Va, fibrinogen to fibrin, and factor XIII to XIIIa. The direct oral anticoagulant (DOAC) drug dabigatran is a potent, competitive direct thrombin inhibitor (DTI) that reversibly and specifically binds both clot-bound and free thrombin, as well as inhibiting thrombin-induced platelet aggregation (Wienen et al. 2007, Stangier et al. 2007). Commercially it is formulated as a lipophilic prodrug, dabigatran etexilate, to promote gastrointestinal absorption before it is metabolised to the active drug. The kidneys excrete the majority (80%) of the unchanged drug (Stangier et al. 2007).
Plasma factor XI binds to the platelet glycoprotein Ib:IX:V complex (Baglia et al. 2002; Greengard et al. 1986). In the body, this reaction occurs specifically on the surfaces of activated platelets, but not on endothelial cells (Baird and Walsh 2002). The stoichiometry of the platelet glycoprotein Ib:IX:V complex has not been established directly, but is inferred from the relative abundances of its components in platelet membranes (Modderman et al. 1992; Shrimpton et al. 2002).
Factor IXa, in a complex with factor VIIIa on the surfaces of activated platelets (the "intrinsic tenase complex"), catalyzes the formation of activated factor X with high efficiency. The amino terminal part of the heavy chain of factor X, the factor X activation peptide, is released. (This peptide has no known function.)
Prekallikrein (PK) associates specifically with kininogen (HK) on cell surfaces. In vivo, this reaction may occur primarily on the surfaces of endothelial cells in response to platelet activation (Lin et al. 1997; Motta et al. 1998; Mahdi et al. 2003).
Prekallikrein in a complex with kininogen and C1q binding protein on the plasma membrane is cleaved to generate active kallikrein, which remains bound to the complex. In the body, this reaction appears to occur on the surfaces of endothelial cells and may require the presence of activated platelets. Recent work indicates that the protease that cleaves prekallikrein under these conditions is prolylcarboxypeptidase. Although this enzyme was originally isolated from lysosomes (Odya et al. 1978; Tan et al. 1993), it is associated with plasma membranes of cultured human endothelial cells in vitro (Moreira et al. 2002; Shariat-Madar et al. 2002), and the purified recombinant enzyme efficiently cleaves prekallikrein (Shariat-Madar et al. 2004). In contrast factor XII, despite its activity on prekallikrein in vitro, appears not to be responsible for prekallikrein activation on the cell surface (Rojkjaer et al. 1998).
Factors VIIIa and IXa associate on cell surfaces to form a complex that very efficiently catalyzes the activation of factor X, the so-called "intrinsic tenase complex". In vitro, negatively charged phospholipids can provide an appropriate surface. In the body, the surface is provided by the plasma membranes of activated platelets (Gilbert and Arena 1996).
Factor XI, bound to the cell surface, is converted to activated factor XI (factor XIa). Chemically, this reaction involves the cleavage of a single peptide bond in each subunit of the factor XI homodimer; intra- and inter-chain disulfide bonds hold the resulting four polypeptides together (Bouma and Griffin 1977; Kurachi and Davie 1977; McMullen et al. 1991). In the body, this reaction occurs on the surfaces of activated platelets (Greengard et al. 1986; Baglia et al. 2002; Baird and Walsh 2002); when this reaction occurs as a step in the intrinsic ("contact") pathway of blood coagulation, it is catalyzed by activated factor XIIa (Kurachi and Davie 1977, Baglia and Walsh 2000) which in turn is generated through the interactions of factor XII, kallikrein, and kininogen on endothelial cell surfaces (Schmaier 2004).
The cleavage of kininogen (HK, high molecular weight kininogen) yields activated kininogen and the vasoactive peptide bradykinin (Kerbirou and Griffin 1979; Lottspeich et al. 1985; Kellerman et al. 1986). In vivo, this reaction is catalyzed by activated kallikrein, takes places within the kallikrein:kininogen:C1q binding protein tetramer complex on the endothelial cell surface, and results in the release of kallikrein and bradykinin (Motta et al. 1998). The hormonal functions of bradykinin will be annotated in a future version of Reactome.
Cleavage of a single peptide bond converts factor XII to activated factor XII (factor XIIa) (Fujikawa and McMullen 1983; McMullen and Fujikawa 1985). Identification of the catalytic activity or activities responsible for this cleavage has not been straightforward. Studies in vitro have demonstrated the autoactivation of factor XII as well as activation by kallikrein. Both reactions require the presence of negatively charged surfaces and are accelerated in the presence of kininogen (high molecular weight kininogen, HK) (Griffin and Cochrane 1976; Meier et al. 1977; Silverberg et al. 1980). Recent work suggests that factor XII activation in vivo may occur primarily on endothelial cell surfaces and that, as in vitro, association with kininogen may accelerate the reaction (Mahdi et al. 2002; Schmaier 2004), although alternative pathways and alternative mechanisms for associating factor XII with the cell surface have not been excluded (Joseph et al. 2001).
Factor XIa, bound to platelet glycoprotein (GP) Ib:IX:V on the platelet cell surface, catalyzes the formation of activated factor IX with high efficiency in a reaction that requires Ca++. The amino terminal part of the heavy chain of factor IX, the factor IX activation peptide, is released. (This peptide has no known function.)
Activated kallikrein binds to alpha2-macroglobulin (Sottrup-Jensen et al. 1984), forming a stable and enzymatically inactive complex. Under normal conditions in vivo, this reaction appears to be responsible for the inactivation of about 1/6 of activated kallikrein (with C1Inh responsible for the inactivation of about 5/6) (Harpel et al. 1985).
Kininogen (high molecular weight kininogen; HK) associates with C1q binding protein on the cell surface in a reaction dependent on Zn++ (Joseph et al. 1996). In the body, the Zn++ needed to drive this reaction may be provided locally by Zn++ release from activated platelets (Mahdi et al. 2002). The C1q binding protein is inferred to form tetramers based on the properties of purified recombinant protein in vitro (Ghebrehiwet et al. 1994); the stoichiometry of the cell surface complex has not been determined directly.
Activated factor XII (factor XIIa) binds to C1Inh (C1 inhibitor - Bock et al. 1986) to form a stable, inactive complex (Schneider et al. 1973). While several protease inhibitors can form stable complexes with XIIa in vitro, only C1Inh does so to a significant extent under normal conditions in vivo (Pixley et al. 1985).
Activated kallikrein binds to C1Inh (plasma protease C1 inhibitor) (Bock et al. 1986), forming a stable and enzymatically inactive complex. This reaction appears to be the major means by which kallikrein is inactivated (kallikrein can also be inactivated by binding to alpha2-macroglobulin) (Harpel et al. 1985; Ratnoff et al. 1969).
Factor XI, bound to the cell surface, is converted to activated factor XI (factor XIa). In the body, this reaction occurs on the surfaces of activated platelets (Baglia et al. 2002). Small quantities of factor XI can be activated in a reaction catalyzed by factor XIIa, to initiate formation of a fibrin clot. However, the efficient activation of larger quantities of factor XI, needed to propagate the blood clotting process, appears to be mediated by thrombin (Baglia and Walsh 2000; Gailani and Broze 1993; Naito and Fujikawa 1991; Oliver et al. 1999; Monroe et al. 2002).
In the blood coagulation process, prothrombin is proteolytically cleaved to form thrombin (factor IIa) which in turn, acts as a serine protease that converts soluble fibrinogen into insoluble strands of fibrin. Specifically, thrombin converts factor XI to XIa, factor VIII to VIIIa, factor V to Va, fibrinogen to fibrin, and factor XIII to XIIIa. The direct oral anticoagulant (DOAC) drug dabigatran is a potent, competitive direct thrombin inhibitor (DTI) that reversibly and specifically binds both clot-bound and free thrombin, as well as inhibiting thrombin-induced platelet aggregation (Wienen et al. 2007, Stangier et al. 2007). Commercially it is formulated as a lipophilic prodrug, dabigatran etexilate, to promote gastrointestinal absorption before it is metabolised to the active drug. The kidneys excrete the majority (80%) of the unchanged drug (Stangier et al. 2007).
SERPIND1 (Heparin cofactor 2) is a serine endopeptidase inhibitor (SERPIN) that acts as a pseudosubstrate for activated thrombin, forming a stable complex which has the effect of inactivating thrombin protease activity (Church et al. 1985), although with slower kinetics than SERPINC1 (antithrombin-III). The presence of the glycosaminoglycans heparin or dermatan sulphate increases thrombin inactivation 1000-fold (Van Deerlin & Tollefsen 199) by facilitating the interaction between the active site of thrombin and the reactive site of SERPIND1. Thrombin specificity is conferred by a 90-residue N-terminal extension that contains two acidic motifs containing sulphated Tyr residues, resembling the C-terminus of hirudin (Tollefsen et al. 1997). SERPIND1 also inhibits chymotrypsin and neutrophil cathepsin G, but in a glycosaminoglycan independent manner (Church et al. 1985). In contrast to SERPINC1 deficiency, SERPIND1 deficiency is not associated with venous thrombosis (Corral et al. 2004).
Protein C is best known for its anticoagulant activity, the proteolytic inactivation of FVa and FVIIIa on negatively charged phospholipid membranes. This is enhanced by cofactors protein S and FV (Rosing et al. 1995, Norstrom et al. 2006). Inactivation of FVa involves APC-mediated cleavages at Arg306 and Arg506. The rapid cleavage at Arg506 is kinetically favored over cleavage at Arg306, but results only in partial inactivation of FVa, whereas the slower cleavage at Arg306 results in a complete loss of FVa function (Kalafatis et al. 1994, Nicolaes et al. 1995). Protein S accelerates factor Va inactivation by selectively promoting the slow cleavage at Arg306 (Rosing et al. 1995). A mutation of the APC cleavage sites in FV Arg506Gln a.k.a. FVLeiden is the most common identifiable hereditary risk factor for venous thrombosis among Caucasians (Camire 2011). APC also has a role in the inactivation of FVIIIa (Regan et al. 1994). Similar to FVa inactivation, FVIIIa is cleaved by APC at Arg336 in the A1 subunit and at Arg562 in the A2 subunit, with either resulting in a complete loss of cofactor activity (O'Brien et al. 2000, Manithody et al. 2003). Both protein S and FV but not FVa enhance inactivation of FVIIIa by APC (O'Brien et al. 2000,57). By acting on FVa and FVIIIa Protein C down-regulates both primary and secondary thrombin formation, delaying clot formation and diminishing activation of TAFI, enhanced susceptibility of the clot to fibrinolysis, respectively. The latter effects of APC on secondary thrombin formation is sometimes referred to as APC’s profibrinolytic effect (Bajzar et al. 1996).
Physiological activation of protein C on the endothelial cell surface requires the binding of protein C to the endothelial protein C receptor PROCR (EPCR) as well as binding of thrombin to thrombomodulin (TM) (Stavenuiter et al. 2013). PROCR binding to protein C (Fukudome & Esmon 1994) augments by at least 5-fold the effect of thrombin-thrombomodulin on the rate of protein C activation (Stearns-Kurosawa et al. 1996, Taylor et al. 2001).
SERPINA5, also called Plasma serine protease inhibitor or Protein C inhibitor, inactivates serine proteases by binding irreversibly to their serine activation site. It is involved in the regulation of intravascular and extravascular proteolytic activities, promoting coagulation by inhibiting the anticoagulant complex Activated protein C (APC), but also acts as an anticoagulant factor by inhibiting blood coagulation factors such as prothrombin, factor XI, factor Xa, plasma kallikrein and fibrinolytic enzymes such as tissue- and urinary-type plasminogen activators. Its inhibitory activity is greatly enhanced in the presence of glycosaminoglycans (GAGs), heparin, thrombomodulin and phospholipids vesicles (Suzuki et al. 1985).
SERPINA5 inhibits activated protein C In the blood plasma and inhibits thromibin as part of the thrombin:thrombomodulin complex (Rezaie et al. 1995). On the other hand, PCI can also inhibit coagulation factors (Radtke et al. 2007). The SERPINA5:APC complex is a marker of thrombotic events (Kolbel et al. 2006), which suggests that despite low circulating SERPINA5 concentrations and rates of APC inhibition, its predominant role is procoagulatory (Li & Huntington 2008). This is due to the enhancing effect of GAGs, which line the vascular endothelium. Both SERPINA5 and APC bind to GAGs. The presence of heparin in vitro accelerates the maximal rate of inhibition by over 2000-fold (when accounting for dissociation constants) (Yang et al. 2002).
SERPINE2 (Protease nexin-1, PN1) is a specific and extremely efficient inhibitor of thrombin. Unlike other thrombin inhibitors belonging to the serpin family, SERPINE2 does not circulate in the blood (Bouton et al. 2012). Rather, it is bound to glycosaminoglycans on the surface of cell types including macrophages, smooth muscle cells and platelets, where it inhibits the signaling functions of thrombin. SERPINE2 sets the threshold for thrombin-induced platelet activation (Gronke et al. 1987, Boulaftali et al. 2010) and has been implicated in atherosclerosis (Bouton et al. 2012). Recent studies have demonstrated an important antithrombotic effect of platelet SERPINE2 in vitro and in vivo (Boulaftali et al. 2010).
Activated protein C (APC) can either dissociate from PROCR to exert its anticoagulant activity, or remain bound to PROCR where it influences multiple direct cellular activities. Dissociation of APC from PROCR allows APC to associate with other cell membrane surface molecules, various microparticles, or lipoproteins (e.g., high-density lipoprotein). As an anticoagulant, APC cleaves the activated cofactors Va (fVa) and VIIIa (fVIIIa), yielding inactivated cofactors, fVi and fVIIIi. This proteolytic inactivation is enhanced by protein cofactors (e.g., protein S, factor V) and lipids cofactors (e.g., phosphatidylserine, cardiolipin, glucosylceramide, or HDL).
Activated protein C binds to Protein S on appropriate cell surfaces where it inactivates factors Va and VIIIa. Protein S is best known as a cofactor for the Activated protein C (APC)-catalyzed inactivation of factor Va (Walker 1980). Protein S must be membrane-bound to display this cofactor activity (Hackeng et al. 1993). Protein S binding brings the active site of APC closer to the phospholipid cell surface (Yegneswaran et al. 1999).
APC proteolysis involves cleavage of the factor Va heavy chain at Arg-306 and Arg-506 (Nicolaes et al. 1985). Most factor Va molecules are initially cleaved at Arg506, yielding a partially active intermediate, followed by complete inactivation through cleavage at Arg306 (Kalafatis et al. 1994). Protein S stimulates the cleavage at Arg306 ~20-fold (Rosing et al. 1995) and also counteracts the protective effect of factor Xa on Arg506 cleavage (Norstrom et al. 2006).
Protein S also enhances the APC-mediated inactivation of factor VIIIa (van de Poel et al. 2001). Protein S and factor V act as synergistic cofactors in the APC-mediated inactivation of factor VIIIa (Shen & Dahlback 1994, Somajo et al. 2014).
A soluble form of PROCR (sEPCR) fully retains the ability to bind Protein C and Activated protein C (Kurosawa et al. 1997). This form increases up to 5-fold in patients with sepsis or systemic lupus erythematosus (Kurosawa et al. 1998), either from vascular injury or through a regulated proteolytic release of soluble receptor (Gu et al. 2000). sEPCR inhibits protein C activation over large vessel endothelium in culture, reflecting competition between the soluble and cell surface forms of PROCR (Liaw et al. 2000).
Activated Protein C (APC) is best known for its anticoagulant activity, the proteolytic inactivation of FVa and FVIIIa on negatively charged phospholipid membranes. This is enhanced by cofactors protein S and factor V (Rosing et al. 1995, Norstrom et al. 2006).
APC inactivates FVIIIa (Regan et al. 1994) with a mechanism similar to its inactivation of FVa. FVIIIa is cleaved by APC at Arg355 (336 if numbering excludes signal peptide) in the A1 subunit and at Arg581 (562 if numbering excludes signal peptide) in the A2 subunit (O'Brien et al. 2000, Manithody et al. 2003). The Arg355 cleavage is 6-fold faster than the Arg581 cleavage but does not fully inactivate factor VIIIa if dissociation of the A2 subunit is blocked (Gale et al. 2008). Protein S and Factor V (but not FVa) enhance the inactivation of FVIIIa by APC (O'Brien et al. 2000). Protein S and factor V both enhance cleavage at both sites, more so at Arg581 (Gale et al. 2008).
The A2 subunit of FVIIIa spontaneously dissociates, inactivating FVIIIa with a half-life of about 2 min (Fay et al. 1991).
By acting on FVa and FVIIIa Protein C down-regulates both primary and secondary thrombin formation, delaying clot formation and diminishing activation of TAFI, enhanced susceptibility of the clot to fibrinolysis, respectively. The latter effects of APC on secondary thrombin formation is sometimes referred to as APC’s profibrinolytic effect (Bajzar et al. 1996).
Soluble PROCR binds to activated neutrophils via PRTN3, also cknown as myeloblastin and (Leukocyte) proteinase-3 (Kurosawa et al. 2000). PRTN3 is the most abundant serine protease in neutrophils (Campbell et al. 2000). After neutrophil activation, PRTN3 is secreted from azurophil granules, rebinding to the neutrophil surface through an association with CD177 (NB1) a 60-kDa glycosyl-phosphatidylinositol (GPI)-linked cell surface glycoprotein, which is expressed on a subpopulation of neutrophils in 97% of healthy individuals (Knuckleburg et al. 2012). PRTN3 is partially protected from inactivation when associated with CD177 (Campbell et al. 2000) which may increase its efficacy. CD177 is a heterophilic binding partner for endothelial cell platelet-endothelial cell adhesion molecule (PECAM)-1, which is expressed at endothelial cell junctions where transmigration occurs (Sun et al. 2000) suggesting that CD177 directs at least a subpopulation of PRTN3 molecules to these areas to aid neutrophil diapedesis, perhaps through PRTN3 degradation of cell junction proteins or the extracellular matrix.
Membrane-bound thrombin-activated factor VIII (fVIIIa) functions as a cofactor for factor IXa in the factor Xase complex. Factors VIIIa and IXa associate with anionic phospholipid surfaces with high affinity (Respective Kd values ?1 nM and ~15nM, Gilbert et al. 1990, Mertens & Bertina 1984, Greengard et al. 1986). Studies using physiologic surfaces provide evidence for coordinated binding interactions of the enzyme, cofactor and substrate to discrete surface structures. For example, the presence of both (active site-modified) factor IXa and factor X increased both the number and the affinity of binding sites on activated platelets for factor VIIIa (Ahmad et al. 2000). However classical receptors for the constituents of factor Xase have not been identified (Fay 2004).
Cleavage of factor VIII light chain promotes a change in the conformation of the C2 domain that facilitates dissociation from VWF and enhances the affinity of factor VIIIa for anionic phospholipid surfaces (Saenko et al. 1998).
Membrane-bound thrombin-activated factor VIII (fVIIIa) functions as a cofactor for factor IXa in the factor Xase complex. Factors VIIIa and IXa associate with anionic phospholipid surfaces with high affinity (Respective Kd values ?1 nM and ~15nM, Gilbert et al. 1990, Mertens & Bertina 1984, Greengard et al. 1986). Studies using physiologic surfaces provide evidence for coordinated binding interactions of the enzyme, cofactor and substrate to discrete surface structures. For example, the presence of both (active site-modified) factor IXa and factor X increased both the number and the affinity of binding sites on activated platelets for factor VIIIa (Ahmad et al. 2000). However classical receptors for the constituents of factor Xase have not been identified (Fay 2004).
F2R (PAR1) mediates multiple cytoprotective effects of Activated proein C (APC) (Riewald et al. 2002, Griffin et al. 2007). In most, but not all, reported studies of APC’s beneficial effects on endothelial cells, the cellular receptors EPCR and F2R are required. These cytoprotective effects include anti-apoptotic activities, anti-inflammatory activities, protection of endothelial barrier functions, and favorable alteration of gene expression profiles. This paradigm in which EPCR-bound APC activates F2R to initiate signaling is consistent with many in vitro and in vivo data. Localization of APC signaling to caveolin-1-rich microdomains (caveolae) may help differentiate mechanisms for cytoprotective APC signaling versus proinflammatory thrombin signaling. Additional mechanisms for APC effects on cells may involve other receptors. These effects include APC anti-inflammatory effects on leukocytes or cytoprotective effects on dendritic cells and neurons. Other receptors may include F2RL2 (PAR3), various integrins e.g., Mac-1 (CD11b/CD18), Beta-1 integrins, Beta-3 integrins, S1P1, or the apolipoprotein E receptor 2 (LRP8) (Mosnier et al. 2007).
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thrombin inactivating
complexeskininogen:C1q binding protein
tetramer(factor
IIa):SERPIND1XIa:GPIb:GPIX:GPV
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complexesthrombin inactivating
complexesthrombin inactivating
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complexesIn the blood coagulation process, prothrombin is proteolytically cleaved to form thrombin (factor IIa) which in turn, acts as a serine protease that converts soluble fibrinogen into insoluble strands of fibrin. Specifically, thrombin converts factor XI to XIa, factor VIII to VIIIa, factor V to Va, fibrinogen to fibrin, and factor XIII to XIIIa. The direct oral anticoagulant (DOAC) drug dabigatran is a potent, competitive direct thrombin inhibitor (DTI) that reversibly and specifically binds both clot-bound and free thrombin, as well as inhibiting thrombin-induced platelet aggregation (Wienen et al. 2007, Stangier et al. 2007). Commercially it is formulated as a lipophilic prodrug, dabigatran etexilate, to promote gastrointestinal absorption before it is metabolised to the active drug. The kidneys excrete the majority (80%) of the unchanged drug (Stangier et al. 2007).
Factor Xa (aka Factor X heavy chain), a cleavage product of coagulation factor X (F10), is a vitamin K-dependent glycoprotein able to convert prothrombin to thrombin during the blood clotting process. Factor Xa is a target for direct oral anticoagulant (DOAC) drugs that are direct factor Xa inhibitors (the so-called 'xabans') and used in the treatment and prevention of thromboembolic disorders (Galanis et al. 2014). Rivaroxaban (brand name Xarelto) was the first medically approved drug of this class (Abrams & Emerson 2009, Misselwitz et al. 2011). In patients with non-valvular atrial fibrillation, 'xabans' appears to be as effective as warfarin in preventing nonhemorrhagic strokes and embolic events (Patel et al. 2011, Gomez-Outes et al. 2013). The most serious side-effect of rivaroxaban is GI bleeding, and with there being no antidote for rivaroxaban, bleeding events can be difficult to manage (Siegal et al. 2014). Rivaroxaban binds to and inhibits both free factor Xa and factor Xa bound in the prothrombinase complex (Roehrig et al. 2005). Unlike warfarin, the 'xabans' exhibit a predictable dose response and do not require routine coagulation monitoring.
Factor Xa (aka Factor X heavy chain), a cleavage product of coagulation factor X (F10), is a vitamin K-dependent glycoprotein able to convert prothrombin to thrombin during the blood clotting process. Factor Xa is a target for direct oral anticoagulant (DOAC) drugs that are direct factor Xa inhibitors (the so-called 'xabans') and used in the treatment and prevention of thromboembolic disorders (Galanis et al. 2014). Rivaroxaban (brand name Xarelto) was the first medically approved drug of this class (Abrams & Emerson 2009, Misselwitz et al. 2011). In patients with non-valvular atrial fibrillation, 'xabans' appears to be as effective as warfarin in preventing nonhemorrhagic strokes and embolic events (Patel et al. 2011, Gomez-Outes et al. 2013). The most serious side-effect of rivaroxaban is GI bleeding, and with there being no antidote for rivaroxaban, bleeding events can be difficult to manage (Siegal et al. 2014). Rivaroxaban binds to and inhibits both free factor Xa and factor Xa bound in the prothrombinase complex (Roehrig et al. 2005). Unlike warfarin, the 'xabans' exhibit a predictable dose response and do not require routine coagulation monitoring.
In the blood coagulation process, prothrombin is proteolytically cleaved to form thrombin (factor IIa) which in turn, acts as a serine protease that converts soluble fibrinogen into insoluble strands of fibrin. Specifically, thrombin converts factor XI to XIa, factor VIII to VIIIa, factor V to Va, fibrinogen to fibrin, and factor XIII to XIIIa. The direct oral anticoagulant (DOAC) drug dabigatran is a potent, competitive direct thrombin inhibitor (DTI) that reversibly and specifically binds both clot-bound and free thrombin, as well as inhibiting thrombin-induced platelet aggregation (Wienen et al. 2007, Stangier et al. 2007). Commercially it is formulated as a lipophilic prodrug, dabigatran etexilate, to promote gastrointestinal absorption before it is metabolised to the active drug. The kidneys excrete the majority (80%) of the unchanged drug (Stangier et al. 2007).
Factor VIII (Vehar et al. 1984) is a heterodimer containing a heavy and a light polypeptide chain, generated by the proteolytic cleavage of a single large precursor polypeptide. Several forms of the heavy chain are found in vivo, all functionally the same but differing in the amount of the B domain removed by proteolysis. The single form annotated here is the shortest one (Eaton et al. 1986; Hill-Eubanks et al. 1989).
In vitro, von Willebrand factor (Titani et al. 1986) can form complexes with factor VIII with a 1:1 stoichiometry. The complexes that form in vivo, however, involve large multimers of von Willebrand factor and varied, but always low, proportions of factor VIII (Vlot et al. 1995). A stoichiometry of one molecule of factor VIII associated with 50 of von Willebrand factor is typical in vivo, and is used here to annotate the factor VIII:von Willebrand factor complex.
In the blood coagulation process, prothrombin is proteolytically cleaved to form thrombin (factor IIa) which in turn, acts as a serine protease that converts soluble fibrinogen into insoluble strands of fibrin. Specifically, thrombin converts factor XI to XIa, factor VIII to VIIIa, factor V to Va, fibrinogen to fibrin, and factor XIII to XIIIa. The direct oral anticoagulant (DOAC) drug dabigatran is a potent, competitive direct thrombin inhibitor (DTI) that reversibly and specifically binds both clot-bound and free thrombin, as well as inhibiting thrombin-induced platelet aggregation (Wienen et al. 2007, Stangier et al. 2007). Commercially it is formulated as a lipophilic prodrug, dabigatran etexilate, to promote gastrointestinal absorption before it is metabolised to the active drug. The kidneys excrete the majority (80%) of the unchanged drug (Stangier et al. 2007).
In the blood coagulation process, prothrombin is proteolytically cleaved to form thrombin (factor IIa) which in turn, acts as a serine protease that converts soluble fibrinogen into insoluble strands of fibrin. Specifically, thrombin converts factor XI to XIa, factor VIII to VIIIa, factor V to Va, fibrinogen to fibrin, and factor XIII to XIIIa. The direct oral anticoagulant (DOAC) drug dabigatran is a potent, competitive direct thrombin inhibitor (DTI) that reversibly and specifically binds both clot-bound and free thrombin, as well as inhibiting thrombin-induced platelet aggregation (Wienen et al. 2007, Stangier et al. 2007). Commercially it is formulated as a lipophilic prodrug, dabigatran etexilate, to promote gastrointestinal absorption before it is metabolised to the active drug. The kidneys excrete the majority (80%) of the unchanged drug (Stangier et al. 2007).
SERPINA5 inhibits activated protein C In the blood plasma and inhibits thromibin as part of the thrombin:thrombomodulin complex (Rezaie et al. 1995). On the other hand, PCI can also inhibit coagulation factors (Radtke et al. 2007). The SERPINA5:APC complex is a marker of thrombotic events (Kolbel et al. 2006), which suggests that despite low circulating SERPINA5 concentrations and rates of APC inhibition, its predominant role is procoagulatory (Li & Huntington 2008). This is due to the enhancing effect of GAGs, which line the vascular endothelium. Both SERPINA5 and APC bind to GAGs. The presence of heparin in vitro accelerates the maximal rate of inhibition by over 2000-fold (when accounting for dissociation constants) (Yang et al. 2002).
Protein S is best known as a cofactor for the Activated protein C (APC)-catalyzed inactivation of factor Va (Walker 1980). Protein S must be membrane-bound to display this cofactor activity (Hackeng et al. 1993). Protein S binding brings the active site of APC closer to the phospholipid cell surface (Yegneswaran et al. 1999).
APC proteolysis involves cleavage of the factor Va heavy chain at Arg-306 and Arg-506 (Nicolaes et al. 1985). Most factor Va molecules are initially cleaved at Arg506, yielding a partially active intermediate, followed by complete inactivation through cleavage at Arg306 (Kalafatis et al. 1994). Protein S stimulates the cleavage at Arg306 ~20-fold (Rosing et al. 1995) and also counteracts the protective effect of factor Xa on Arg506 cleavage (Norstrom et al. 2006).
Protein S also enhances the APC-mediated inactivation of factor VIIIa (van de Poel et al. 2001). Protein S and factor V act as synergistic cofactors in the APC-mediated inactivation of factor VIIIa (Shen & Dahlback 1994, Somajo et al. 2014).
APC inactivates FVIIIa (Regan et al. 1994) with a mechanism similar to its inactivation of FVa. FVIIIa is cleaved by APC at Arg355 (336 if numbering excludes signal peptide) in the A1 subunit and at Arg581 (562 if numbering excludes signal peptide) in the A2 subunit (O'Brien et al. 2000, Manithody et al. 2003). The Arg355 cleavage is 6-fold faster than the Arg581 cleavage but does not fully inactivate factor VIIIa if dissociation of the A2 subunit is blocked (Gale et al. 2008). Protein S and Factor V (but not FVa) enhance the inactivation of FVIIIa by APC (O'Brien et al. 2000). Protein S and factor V both enhance cleavage at both sites, more so at Arg581 (Gale et al. 2008).
The A2 subunit of FVIIIa spontaneously dissociates, inactivating FVIIIa with a half-life of about 2 min (Fay et al. 1991).
By acting on FVa and FVIIIa Protein C down-regulates both primary and secondary thrombin formation, delaying clot formation and diminishing activation of TAFI, enhanced susceptibility of the clot to fibrinolysis, respectively. The latter effects of APC on secondary thrombin formation is sometimes referred to as APC’s profibrinolytic effect (Bajzar et al. 1996).
Membrane-bound thrombin-activated factor VIII (fVIIIa) functions as a cofactor for factor IXa in the factor Xase complex. Factors VIIIa and IXa associate with anionic phospholipid surfaces with high affinity (Respective Kd values ?1 nM and ~15nM, Gilbert et al. 1990, Mertens & Bertina 1984, Greengard et al. 1986). Studies using physiologic surfaces provide evidence for coordinated binding interactions of the enzyme, cofactor and substrate to discrete surface structures. For example, the presence of both (active site-modified) factor IXa and factor X increased both the number and the affinity of binding sites on activated platelets for factor VIIIa (Ahmad et al. 2000). However classical receptors for the constituents of factor Xase have not been identified (Fay 2004).
kininogen:C1q binding protein
tetramer(factor
IIa):SERPIND1XIa:GPIb:GPIX:GPV
complexXIa:GPIb:GPIX:GPV
complexXIa:GPIb:GPIX:GPV
complexWillebrand factor
multimerWillebrand factor
multimerantithrombin
III:heparinantithrombin
III:heparin