Role of calcium ion in the generation of factor XIII activity

Peptidic Inhibitors and a Fluorescent Probe for the Selective Inhibition and Labelling of Factor XIIIa Transglutaminase

Factor XIIIa (FXIIIa) is a transglutaminase of major therapeutic interest for the development of anticoagulants due to its essential role in the blood coagulation cascade. While numerous FXIIIa inhibitors have been reported, they failed to reach clinical evaluation due to their lack of metabolic stability and low selectivity over transglutaminase 2 (TG2). Furthermore, the chemical tools available for the study of FXIIIa activity and localization are extremely limited. To combat these shortcomings, we designed, synthesised, and evaluated a library of 21 novel FXIIIa inhibitors. Electrophilic warheads, linker lengths, and hydrophobic units were varied on small molecule and peptidic scaffolds to optimize isozyme selectivity and potency. A previously reported FXIIIa inhibitor was then adapted for the design of a probe bearing a rhodamine B moiety, producing the innovative KM93 as the first known fluorescent probe designed to selectively label active FXIIIa with high efficiency (k_inact/K_I = 127,300 M^-1 min^-1) and 6.5-fold selectivity over TG2. The probe KM93 facilitated fluorescent microscopy studies within bone marrow macrophages, labelling FXIIIa with high efficiency and selectivity in cell culture. The structure-activity trends with these novel inhibitors and probes will help in the future study of the activity, inhibition, and localization of FXIIIa.

Hemostasis components in cerebral amyloid angiopathy and Alzheimer's disease

Increased cerebrovascular amyloid-β (Aβ) deposition represents the main pathogenic mechanisms characterizing Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA). Whereas an increasing number of studies define the contribution of fibrin(ogen) to neurodegeneration, how other hemostasis factors might be pleiotropically involved in the AD and CAA remains overlooked. Although traditionally regarded as pertaining to hemostasis, these proteins are also modulators of inflammation and angiogenesis, and exert cytoprotective functions. This review discusses the contribution of hemostasis components to Aβ cerebrovascular deposition, which settle the way to endothelial and blood-brain barrier dysfunction, vessel fragility, cerebral bleeding, and the associated cognitive changes. From the primary hemostasis, the process that refers to platelet aggregation, we discuss evidence regarding the von Willebrand factor (vWF) and its regulator ADAMTS13. Then, from the secondary hemostasis, we focus on tissue factor, which triggers the extrinsic coagulation cascade, and on the main inhibitors of coagulation, i.e., tissue factor pathway inhibitor (TFPI), and the components of protein C pathway. Last, from the tertiary hemostasis, we discuss evidence on FXIII, involved in fibrin cross-linking, and on components of fibrinolysis, including tissue-type plasminogen activator (tPA), urokinase-type plasminogen activator (uPA) and its receptor uPA(R), and plasminogen activator inhibitor-1 (PAI-1). Increased knowledge on contributors of Aβ-related disease progression may favor new therapeutic approaches for early modifiable risk factors.

Proteolytic and nonproteolytic activation mechanisms result in conformationally and functionally different forms of coagulation factor XIII A

Factor XIIIA (FXIIIA) is a transglutaminase that cross-links intra- and extracellular protein substrates. FXIIIA is expressed as an inactive zymogen, and during blood coagulation, it is activated by removal of an activation peptide by the protease thrombin. No such proteolytic FXIIIA activation is known to occur in other tissues or the intracellular form of FXIIIA. For those locations, FXIIIA is assumed instead to undergo activation by Ca²⁺ ions. Previously, we demonstrated a monomeric state for active FXIIIA. Current analytical ultracentrifugation and kinetic experiments revealed that thrombin-activated FXIIIA has a higher conformational flexibility and a stronger affinity toward glutamine substrate than does nonproteolytically activated FXIIIA. The proteolytic activation of FXIIIA was further investigated in a context of fibrin clotting. In a series of fibrin cross-linking assays and scanning electron microscopy studies of plasma clots, the activation rates of FXIIIA V34X variants were correlated with the extent of fibrin cross-linking and incorporation of nonfibrous protein into the clot. Overall, the results suggest conformational and functional differences between active FXIIIA forms, thus expanding the understanding of FXIIIA function. Those differences may serve as a basis for developing therapeutic strategies to target FXIIIA in different physiological environments. ENZYMES: Factor XIIIA ( EC 2.3.2.13).

Factor XIII topology: organization of B subunits and changes with activation studied with single-molecule atomic force microscopy

Essentials Factor XIII is a heterotetramer with 2 catalytic A subunits and 2 non-catalytic B subunits. Structure of active and inactive factor XIII was studied with atomic force microscopy. Inactive factor XIII is made of an A2 globule and 2 flexible B subunits extending from it. Activated factor XIII separates into a B2 homodimer and 2 monomeric active A subunits. SUMMARY: Background Factor XIII (FXIII) is a precursor of the blood plasma transglutaminase (FXIIIa) that is generated by thrombin and Ca2+ and covalently crosslinks fibrin to strengthen blood clots. Inactive plasma FXIII is a heterotetramer with two catalytic A subunits and two non-catalytic B subunits. Inactive A subunits have been characterized crystallographically, whereas the atomic structure of the entire FXIII and B subunits is unknown and the oligomerization state of activated A subunits remains controversial. Objectives Our goal was to characterize the (sub)molecular structure of inactive FXIII and changes upon activation. Methods Plasma FXIII, non-activated or activated with thrombin and Ca2+ , was studied by single-molecule atomic force microscopy. Additionally, recombinant separate A and B subunits were visualized and compared with their conformations and dimensions in FXIII and FXIIIa. Results and Conclusions We showed that heterotetrameric FXIII forms a globule composed of two catalytic A subunits with two flexible strands comprising individual non-catalytic B subunits that protrude on one side of the globule. Each strand corresponds to seven to eight out of 10 tandem repeats building each B subunit, called sushi domains. The remainder were not seen, presumably because they were tightly bound to the globular A2 dimer. Some FXIII molecules had one or no visible strands, suggesting dissociation of the B subunits from the globular core. After activation of FXIII with thrombin and Ca2+ , B subunits dissociated and formed B2 homodimers, whereas the activated globular A subunits dissociated into monomers. These results characterize the molecular organization of FXIII and changes with activation.

Proline 36 of the Factor XIII Activation Peptide Plays a Crucial Role in Substrate Recognition and Zymogen Activation

The activation peptide of blood coagulation factor XIII (AP-FXIII) has important functions in stabilizing the FXIII-A₂ dimer and regulating FXIII activation. Contributions of many of its 37 amino acids to these functions have been described. However, the role of proline 36, which is adjacent to the thrombin cleavage site at Arg37, has not yet been studied in detail. We approached this question when we came across a patient with congenital FXIII deficiency in whom we detected a novel Pro36Ser mutation. We expressed the mutant FXIII-A Pro36Ser protein in Chinese hamster ovary cells and found that this mutation does not influence FXIII-A expression but significantly inhibits proteolytic activation by thrombin. The enzymatic transglutaminase activity is not affected as it can be induced in the presence of high Ca²⁺ concentrations. We performed nuclear magnetic resonance analysis to investigate AP-FXIII-thrombin interactions, which showed that the mutant Ser36 peptide binds less well to the thrombin surface than the native Pro36 peptide. The Arg37 at the P₁ position still makes strong interactions with the active site cleft but the P₄-P₂ residues (³⁴VVS³⁶) appear to be less well positioned to contact the neighbouring thrombin active site region. In conclusion, we have characterized a novel mutation in AP-FXIII representing only the fourth case of the rare FXIII-A type II deficiency. This case served as a perfect in vivo model to shed light on the crucial role of Pro36 in the proteolytic activation of FXIII-A. Our results contribute to the understanding of structure-function relationship in FXIII.

The role of β-barrels 1 and 2 in the enzymatic activity of factor XIII A-subunit

Essentials The roles of β-barrels 1 and 2 in factor XIII (FXIII) are currently unknown. FXIII truncations lacking β-barrel 2, both β-barrels, or full length FXIII, were made. Removing β-barrel 2 caused total loss of activity, removing both β-barrels returned 30% activity. β-barrel 2 is necessary for exposure of the active site cysteine during activation.

SUMMARY

Background Factor XIII is composed of an activation peptide segment, a β-sandwich domain, a catalytic core, and, finally, β-barrels 1 and 2. FXIII is activated following cleavage of its A-subunits by thrombin. The resultant transglutaminase activity leads to increased resistance of fibrin clots to fibrinolysis. Objectives To assess the functional roles of β-barrels 1 and 2 in FXIII, we expressed and characterized the full-length FXIII A-subunit (FXIII-A) and variants truncated to residue 628 (truncated to β-barrel 1 [TB1]), residue 515 (truncated to catalytic core [TCC]), and residue 184 (truncated to β-sandwich). Methods Proteins were analyzed by gel electrophoresis, circular dichroism, fluorometric assays, and colorimetric activity assays, clot structure was analyzed by turbidity measurements and confocal microscopy, and clot formation was analyzed with a Chandler loop system. Results and Conclusions Circular dichroism spectroscopy and tryptophan fluorometry indicated that full-length FXIII-A and the truncation variants TCC and TB1 retain their secondary and tertiary structure. Removal of β-barrel 2 (TB1) resulted in total loss of transglutaminase activity, whereas the additional removal of β-barrel 1 (TCC) restored enzymatic activity to ~ 30% of that of full-length FXIII-A. These activity trends were observed with physiological substrates and smaller model substrates. Our data suggest that the β-barrel 1 domain protects the active site cysteine in the FXIII protransglutaminase, whereas the β-barrel 2 domain is necessary for exposure of the active site cysteine during activation. This study demonstrates the importance of individual β-barrel domains in modulating access to the FXIII active site region.

Activation of factor XIII is accompanied by a change in oligomerization state

Factor XIII A (FXIIIA) is a member of the transglutaminase enzyme family that cross-links both intra- and extracellular protein substrates. To prevent undesired cross-linking, FXIIIA is expressed as an inactive zymogen and exists intracellularly as an A₂ homodimer. In plasma, FXIII A₂ is complexed with two protective factor XIII B subunits (A₂ B₂ ) that dissociate upon activation of the zymogen. Based on limited experimental data, activated FXIII was considered a dimer of two catalytically active A subunits. However, accumulating but indirect evidence has suggested activation may lead to a monomeric state instead. In the present study, we employed analytical ultracentrifugation (AUC) to directly explore the oligomerization state of zymogen as well as active FXIIIA in solution. We first confirmed that the zymogen was a FXIIIA₂ dimer. When we activated FXIIIA nonproteolytically (by high mm Ca²⁺ ), the protein dissociated to monomers. More importantly, FXIIIA incubation with its physiological partner, the protease thrombin, led to a monomeric state as well. AUC studies of partially cleaved FXIIIA further suggested that thrombin cleavage of a single activation peptide in a zymogen dimer is sufficient to weaken intersubunit interactions, initiating the transition to monomer. The enzymatic activity of the thrombin-cleaved species was higher than nonproteolytically activated enzyme, suggesting that displacement of the activation peptide renders the FXIIIA more accessible to substrates. Thus, results provide evidence that FXIII undergoes a change in oligomerization state as part of the activation process, and emphasize the role of the activation peptide in preventing FXIIIA catalytic activity.

ENZYMES

Factor XIIIA (EC2.3.2.13).

Biochemical and structural analysis of the interaction between β-amyloid and fibrinogen

The majority of patients with Alzheimer disease (AD) suffer from impaired cerebral circulation. Accumulating evidence suggests that fibrinogen, the main protein component of blood clots, plays an important role in this circulatory dysfunction in AD. Fibrinogen interacts with β-amyloid (Aβ), forming plasmin-resistant abnormal blood clots, and increased fibrin deposition is found in the brains of AD patients and mouse models. In this study, we investigated the biochemical and structural details of the Aβ-fibrinogen interaction. We identified the central region of Aβ42 as the most critical region for the interaction, which can be inhibited by specific antibodies against the central region of Aβ and by naturally occurring p3 peptides, Aβ17-40 and Aβ17-42. X-ray crystallographic analysis revealed that Aβ42 binding to fragment D of fibrinogen induced a structural change in the C-terminal region of the fibrinogen β-chain (β384-393). Furthermore, we identified an additional Aβ-binding site within the αC region of fibrinogen. Aβ binding to this αC region blocked plasmin-mediated fibrin cleavage at this site, resulting in the generation of increased levels of a plasmin-resistant fibrin degradation fragment. Overall, our study elucidates the Aβ-fibrinogen interaction and clarifies the mechanism by which Aβ-fibrinogen binding delays fibrinolysis by plasmin. These results may facilitate the development of effective therapeutics against the Aβ-fibrinogen interaction to treat cerebrovascular abnormalities in AD.

Blood coagulation factor XIII and factor XIII deficiency

Factor XIII (FXIII) is a multifunctional pro-γ-transglutaminase that, in addition to its well-known role in hemostasis, has a crucial role in angiogenesis, maintenance of pregnancy, wound healing, bone metabolism, and even cardio protection. FXIII deficiency (FXIIID) is a rare bleeding disorder (RBD) with an estimated incidence of one per two million that is accompanied by life-threatening bleeding such as umbilical cord bleeding, recurrent spontaneous miscarriage, and intracranial hemorrhage (ICH). Today, the disease is successfully managed by FXIII concentrate and recombinant FXIII for prophylaxis, management of minor and major bleeding, treatment of ICH, and successful delivery in women with recurrent pregnancy loss. Molecular analysis of patients with FXIIID revealed a wide spectrum of mutations, most frequently missense mutations in the FXIII-A subunit, with a few recurrent mutations observed worldwide. In vitro expression studies revealed that most of the missense mutations cause intracellular instability of the FXIII protein and, subsequently, FXIIID.

Newly-Recognized Roles of Factor XIII in Thrombosis

Arterial and venous thromboses are major contributors to coagulation-associated morbidity and mortality. Greater understanding of mechanisms leading to thrombus formation and stability is expected to lead to improved treatment strategies. Factor XIII (FXIII) is a transglutaminase found in plasma and platelets. During thrombosis, activated FXIII cross-links fibrin and promotes thrombus stability. Recent studies have provided new information about FXIII activity during coagulation and its effects on clot composition and function. These findings reveal newly-recognized roles for FXIII in thrombosis. Herein, we review published literature on FXIII biology and effects on fibrin structure and stability, epidemiologic data associating FXIII with thrombosis, and evidence from animal models indicating FXIII has an essential role in determining thrombus stability, composition, and size.

Viscoelasticity and Ultrastructure in Coagulation and Inflammation: Two Diverse Techniques, One Conclusion

The process of blood clotting has been studied for centuries. A synopsis of current knowledge pertaining to haemostasis and the blood components, including platelets and fibrin networks which are closely involved in coagulation, are discussed. Special emphasis is placed on tissue factor (TF), calcium and thrombin since these components have been implicated in both the coagulation process and inflammation. Analysis of platelets and fibrin morphology indicate that calcium, tissue factor and thrombin at concentrations used during viscoelastic analysis (with thromboelastography or TEG) bring about alterations in platelet and fibrin network ultrastructure, which is similar to that seen in inflammation. Scanning electron microscopy indicated that, when investigating platelet structure in disease, addition of TF, calcium or thrombin will mask disease-induced alterations associated with platelet activation. Therefore, washed platelets without any additives is preferred for morphological analysis. Furthermore, morphological and viscoelastic analysis confirmed that thrombin activation is the preferred method of fibrin activation when investigating fibrin network ultrastructure.

The activation peptide of coagulation factor XIII is vital for its expression and stability

BACKGROUND

The human activation peptide of factor XIII (AP-FXIII) comprises the first 37 amino acids of the N-terminus and holds the FXIII in an inactive state. FXIII is activated either proteolytically by cleavage of AP-FXIII by thrombin, or non-proteolytically by high calcium concentrations.

OBJECTIVE

To investigate the role of AP-FXIII in the expression and stability of FXIII.

METHODS

We cloned 13 FXIII variants with progressive truncations of AP-FXIII from the N-terminus (delN-FXIII-A), expressed them in mammalian cells, and measured their thermostability, activation, and transglutaminase activity. We also used in silico calculations to analyze the stability of hypothetical delN-FXIII dimers and to identify crucial motifs within AP-FXIII.

RESULTS

Variants with deletions longer than the first 10 amino acids and an R11Q point mutant were not expressed as proteins. In silico calculations indicated that the sequence (8) FGGR(12) R plays a substantial role in intersubunit interactions in FXIII-A2 homodimers. In agreement with this prediction, the temperature stability of delN-FXIII variants decreased with increasing length of deletion. These results may suggest a role of the N-terminus of AP-FXIII in dimer stability. Substantial sequence homology was found among activation peptides of vertebrate and even invertebrate (crustacean) FXIII-A orthologs, which further supports our conclusion.

CONCLUSIONS

We conclude that deletion of 11 or more N-terminal amino acids disrupts intersubunit interactions, which may prevent FXIII-A2 homodimer formation. Therefore, AP-FXIII plays an important role in the stability of the FXIII-A2 dimer.

Factor XIII activity mediates red blood cell retention in venous thrombi

Venous thrombi, fibrin- and rbc-rich clots triggered by inflammation and blood stasis, underlie devastating, and sometimes fatal, occlusive events. During intravascular fibrin deposition, rbc are thought to become passively trapped in thrombi and therefore have not been considered a modifiable thrombus component. In the present study, we determined that activity of the transglutaminase factor XIII (FXIII) is critical for rbc retention within clots and directly affects thrombus size. Compared with WT mice, mice carrying a homozygous mutation in the fibrinogen γ chain (Fibγ390-396A) had a striking 50% reduction in thrombus weight due to reduced rbc content. Fibrinogen from mice harboring the Fibγ390-396A mutation exhibited reduced binding to FXIII, and plasma from these mice exhibited delayed FXIII activation and fibrin crosslinking, indicating these residues mediate FXIII binding and activation. FXIII-deficient mice phenocopied mice carrying Fibγ390-396A and produced smaller thrombi with fewer rbc than WT mice. Importantly, FXIII-deficient human clots also exhibited reduced rbc retention. The addition of FXIII to FXIII-deficient clots increased rbc retention, while inhibition of FXIII activity in normal blood reduced rbc retention and produced smaller clots. These findings establish the FXIII-fibrinogen axis as a central determinant in venous thrombogenesis and identify FXIII as a potential therapeutic target for limiting venous thrombosis.

Tissue-regenerating functions of coagulation factor XIII

The protransglutaminase factor XIII (FXIII) has recently attracted attention within the field of tissue regeneration, as it has been found that FXIII significantly influences wound healing by exerting a multitude of functions. It supports hemostasis by enhancing platelet adhesion to damaged endothelium, and by its cross-linking activity it stabilizes the formed fibrin clot. Furthermore, FXIII limits bacterial dissemination from the wound and incorporates macromolecules of importance for cellular infiltration, supporting cell migration and survival. FXIII-mediated complex formation of the vascular endothelial growth factor receptor 2 and αV β3 integrin is important for angiogenesis, supporting the formation of granulation tissue. Chronic inflammatory conditions involving bleeding and activation of the coagulation cascade have been shown to lead to reduced FXIII levels in plasma. Of particular importance for this review is the fact that patients suffering from inflammatory bowel disease (IBD) have reduced FXIII antigen levels and activity. Furthermore, these patients show impaired mucosal healing, which supports the inflammatory state of the disease. This review summarizes the role of FXIII in the healing of wounds, and briefly summarizes the previous use of FXIII in clinical settings. Moreover, it addresses the potential role for FXIII as a therapeutic agent in the healing of persistent wounds during chronic conditions, with an emphasis on IBD.

The activation peptide cleft exposed by thrombin cleavage of FXIII-A(2) contains a recognition site for the fibrinogen α chain

Formation of a stable fibrin clot is dependent on interactions between factor XIII and fibrin. We have previously identified a key residue on the αC of fibrin(ogen) (Glu396) involved in binding activated factor XIII-A(2) (FXIII-A(2)*); however, the functional role of this interaction and binding site(s) on FXIII-A(2)* remains unknown. Here we (1) characterized the functional implications of this interaction; (2) identified by liquid-chromatography-tandem mass spectrometry the interacting residues on FXIII-A(2)* following chemical cross-linking of fibrin(ogen) αC389-402 peptides to FXIII-A(2)*; and (3) carried out molecular modeling of the FXIII-A(2)*/peptide complex to identify contact site(s) involved. Results demonstrated that inhibition of the FXIII-A(2)*/αC interaction using αC389-402 peptide (Pep1) significantly decreased incorporation of biotinamido-pentylamine and α2-antiplasmin to fibrin, and fibrin cross-linking, in contrast to Pep1-E396A and scrambled peptide controls. Pep1 did not inhibit transglutaminase-2 activity, and incorporation of biotinyl-TVQQEL to fibrin was only weakly inhibited. Molecular modeling predicted that Pep1 binds the activation peptide cleft (AP-cleft) within the β-sandwich domain of FXIII-A(2)* localizing αC cross-linking Q366 to the FXIII-A(2)* active site. Our findings demonstrate that binding of fibrin αC389-402 to the AP-cleft is fundamental to clot stabilization and presents this region of FXIII-A(2)* as a potential site involved in glutamine-donor substrate recognition.

Regulation of the activities of the mammalian transglutaminase family of enzymes

Mammalian transglutaminases catalyze post-translational modifications of glutamine residues on proteins and peptides through transamidation or deamidation reactions. Their catalytic mechanism resembles that of cysteine proteases. In virtually every case, their enzymatic activity is modulated by elaborate strategies including controlled gene expression, allostery, covalent modification, and proteolysis. In this review, we focus on our current knowledge of post-translational regulation of transglutaminase activity by physiological as well as synthetic allosteric agents. Our discussion will primarily focus on transglutaminase 2, but will also compare and contrast its regulation with Factor XIIIa as well as transglutaminases 1 and 3. Potential structure-function relationships of known mutations in human transglutaminases are analyzed.

Factor XIII: a coagulation factor with multiple plasmatic and cellular functions

Factor XIII (FXIII) is unique among clotting factors for a number of reasons: 1) it is a protransglutaminase, which becomes activated in the last stage of coagulation; 2) it works on an insoluble substrate; 3) its potentially active subunit is also present in the cytoplasm of platelets, monocytes, monocyte-derived macrophages, dendritic cells, chondrocytes, osteoblasts, and osteocytes; and 4) in addition to its contribution to hemostasis, it has multiple extra- and intracellular functions. This review gives a general overview on the structure and activation of FXIII as well as on the biochemical function and downregulation of activated FXIII with emphasis on new developments in the last decade. New aspects of the traditional functions of FXIII, stabilization of fibrin clot, and protection of fibrin against fibrinolysis are summarized. The role of FXIII in maintaining pregnancy, its contribution to the wound healing process, and its proangiogenic function are reviewed in details. Special attention is given to new, less explored, but promising fields of FXIII research that include inhibition of vascular permeability, cardioprotection, and its role in cartilage and bone development. FXIII is also considered as an intracellular enzyme; a separate section is devoted to its intracellular activation, intracellular action, and involvement in platelet, monocyte/macrophage, and dendritic cell functions.

Platelet factor XIII gene expression and embolic propensity in atrial fibrillation

Nearly 15% of patients with non-valvular atrial fibrillation (NVAF) have left atrial appendage thrombus (LAAT) by transesophageal echocardiography (TEE) and yet the annual stroke rate averages 5%. The aim of this study was to identify variables influencing embolic propensity of LAAT. Platelet RNA was extracted from platelet-rich regions within formalin-fixed, paraffin-embedded specimens obtained from NVAF patients during cardiac surgery (26 LAAT from 23 patients) or peripheral embolectomy (51 thrombi from 41 patients). Platelet RNA was also assessed from whole blood from 40 NVAF patients. Expression of six platelet-predominate genes: H2A histone family, A1 domain of factor XIII, integrin α₂bβ₃; glycoprotein IX, platelet factor 4, glycoprotein Ib, was performed using TaqMan MGB-probe based quantitative real-time polymerase chain reaction. Platelet factor XIII subunit A gene expression was significantly lower in embolised compared to non-embolised thrombi as determined by normalised cycle threshold values (4.0 ± 1.2 v 2.8 ± 1.8, p=0.02). Expression of other genes did not differ by embolic status. In conclusion, RNA extracted from formalin-fixed, paraffin-embedded platelet-rich tissues can be used for analysis of platelet-predominate gene expression. Variable factor XIII gene expression in thrombi generated during NVAF may in part explain the propensity to embolisation.

Role of calcium in the conformational dynamics of factor XIII activation examined by hydrogen-deuterium exchange coupled with MALDI-TOF MS

Factor XIII catalyzes formation of γ-glutamyl-ε-lysyl crosslinks within fibrin clots. FXIII A(2) can be activated proteolytically with thrombin and low mM Ca(2+) or nonproteolytically with high monovalent/divalent cations along with low mM Ca(2+). Physiologically, FXIII A(2) is poised to respond to transient influxes of Ca(2+) in a Na(+) containing environment. A successful strategy to monitor FXIII conformational events is hydrogen-deuterium exchange (HDX) coupled with mass spectrometry. FXIII A(2) was examined in the presence of different cations (Ca(2+), Mg(2+), Ba(2+), Cu(2+), Na(+), TMAC(+), and EDA(2+)) ranging from 1 to 2mM, physiological Ca(2+) concentration, to 50-500mM for nonproteolytic activation. Increases in FXIII solvent exposure could already be observed at 1mM Ca(2+) for the dimer interface, the catalytic site, and glutamine substrate regions. By contrast, solvent protection was observed at the secondary cleavage site. These events occurred even though 1mM Ca(2+) is insufficient for FXIII activation. The metals 1mM Mg(2+), 1mM Ba(2+), and 1mM Cu(2+) each led to conformational changes, many in the same FXIII regions as Ca(2+). FXIII could also be activated nonproteolytically with 500mM tetramethylammonium chloride (TMAC(+)) and 500mM ethylenediamine (EDA(2+)), both with 2mM Ca(2+). These different HDX studies help reveal the first FXIII segments that respond to physiological Ca(2+) levels.

Interactions between factor XIII and the αC region of fibrinogen

Fibrinogen αC residues 242-424 have been shown to have a major regulatory role in the activation of factor XIII-A2B2 (FXIII-A2B2); however, the interactions underpinning this enhancing effect have not been determined. Here, we have characterized the binding of recombinant (r)FXIII-A subunit and FXIII-A2B2 with fibrin(ogen) and fibrin αC residues 233-425. Using recombinant truncations of the fibrin αC region 233-425 and surface plasmon resonance, we found that activated rFXIII-A bound αC 233-425 (Kd of 2.35 ± 0.09μM) which was further localized to αC 389-403. Site-directed mutagenesis of this region highlighted Glu396 as a key residue for binding of activated rFXIII-A. The interaction was specific for activated rFXIII-A and depended on the calcium-induced conformational change known to occur in rFXIII-A during activation. Furthermore, nonactivated FXIII-A2B2, thrombin-cleaved FXIII-A2B2, and activated FXIII-A2B2 each bound fibrin(ogen) and specifically αC region 371-425 with high affinity (Kd < 35nM and Kd < 31nM, respectively), showing for the first time the potential involvement of the αC region in binding to FXIII-A2B2. These results suggest that in addition to fibrinogen γ′ chain binding, the fibrin αC region also provides a platform for the binding of FXIII-A2B2 and FXIII-A subunit.

Factor XIII: novel structural and functional aspects

Factor (F)XIII is a protransglutaminase that, in addition to maintaining hemostasis, has multiple plasmatic and intracellular functions. Its plasmatic form (pFXIII) is a tetramer of two potentially active A (FXIII-A) and two inhibitory/carrier B (FXIII-B) subunits, whereas its cellular form (cFXIII) is a dimer of FXIII-A. FXIII-A belongs to the family of transglutaminases (TGs), which show modest similarity in the primary structure, but a high degree of conservatism in their domain and sub-domain secondary structure. FXIII-A consists of an activation peptide, a β-sandwich, a catalytic and two β-barrel domains. FXIII-B is a glycoprotein consisting of 10 repetitive sushi domains each held together by two internal disulfide bonds. The structural elements of FXIII-A involved in the interaction with FXIII-B have not been elucidated; in FXIII-B the first sushi domain seems important for complex formation. In the circulation pFXIII is bound to the fibrinogen γ'-chain through its B subunit. In the process of pFXIII activation first thrombin cleaves off the activation peptide from FXIII-A, then in the presence of Ca(2+) FXIII-B dissociates and FXIII-A becomes transformed into an active transglutaminase (FXIIIa). The activation is highly accelerated by the presence of fibrin(ogen). cFXIII does not require proteolysis for intracellular activation. The three-dimensional structure of FXIIIa has not been resolved. Based on analogies with transglutaminase-2, a three-dimensional structure of FXIIIa was developed by molecular modeling, which shows good agreement with the drastic structural changes demonstrated by biochemical studies. The structural requirements for enzyme-substrate interaction and for transglutaminase activity are also reviewed.

Substrate specificity of microbial transglutaminase as revealed by three-dimensional docking simulation and mutagenesis

Transglutaminases (TGases) are used in fields such as food and pharmaceuticals. Unlike other TGases, microbial transglutaminase (MTG) activity is Ca²⁺-independent, broadening its application. Here, a three-dimensional docking model of MTG binding to a peptide substrate, CBZ-Gln-Gly, was simulated. The data reveal CBZ-Gln-Gly to be stretched along the MTG active site cleft with hydrophobic and/or aromatic residues interacting directly with the substrate. Moreover, an oxyanion binding site for TGase activity may be constructed from the amide groups of Cys64 and/or Val65. Alanine mutagenesis verified the simulated binding region and indicated that large molecules can be widely recognized on the MTG cleft.

Thrombin-activated thrombelastography for evaluation of thrombin interaction with thrombin inhibitors

For intravenous anticoagulation, heparin has been the mainstay drug, but its use may be contraindicated in heparin-induced thrombocytopenia and thrombosis. Heparin alternatives including direct thrombin inhibitors are available, but clotting assays (e.g. partial thromboplastin time) measure the time required to form fibrin gel when only a small amount of thrombin is generated. It was hypothesized that the extent of thrombin inhibition varies among inhibitors, and thrombin-activated thrombelastography would provide useful data on therapeutic responses to thrombin inhibitors. Thrombin was added (0-100 nmol/l final concentration) to nonrecalcified whole blood to evaluate clot formation on thrombelastography. Effects of direct thrombin inhibitors (argatroban 3.75 microg/ml, bivalirudin 15 microg/ml, and lepirudin 3.0 microg/ml), and heparin cofactor II activator and dermatan disulfate (20 microg/ml) were evaluated in the presence of 100 nmol/l thrombin. The interactions of thrombin and respective inhibitors were also compared by fluorogenic thrombin substrate cleavage. Increasing concentrations of thrombin progressively shortened the lag time and increased viscoelasticity on thrombelastography. Only 20 nmol/l thrombin caused instantaneous clotting, but maximal viscoelastic force was obtained at 50-100 nmol/l thrombin. All thrombin inhibitors prolonged the lag time (lepirudin > bivalirudin > argatroban = dermatan disulfate), but full recovery of thrombelastography viscoelasticity was observed with argatroban and bivalirudin. Lepirudin abrogated clotting, and dermatan disulfate suppressed clot development on thrombelastography. Thrombin substrate cleavage was observed only for bivalirudin, and heparin cofactor II without dermatan disulfate. The modified thrombelastography technique using nonrecalcified whole blood may be useful in evaluating the extent and reversibility of thrombin blockade with direct or indirect thrombin inhibitors.

Interaction of a blood coagulation factor on electrically polarized hydroxyapatite surfaces

Although the polarization treatment of hydroxyapatite (HA) remarkably enhances the osteoconductivity, the mechanisms have not yet been completely understood. The interaction of proteins in blood and tissue fluids with biomaterials are reportedly triggers for later cellular responses and played a major role in osteoconductive processes. Considering this, we disclosed the interaction of polarized HA surface with a coagulation factor, fibrin stabilizing factor XIII (FXIII). The HA activated FXIII even in Ca2+ free buffer, based on the SDS-PAGE detections of alpha-polymer and gamma-dimer bands assigned to stabilized fibrin. The Ca2+ ions, possibly released from the HA surfaces, were examined whether they initiate the activation of the FXIII. It was experimentally proved by ICP analysis that the induced large negative charges on the electrically polarized HA significantly increased the released Ca2+ concentration for the short pre-incubation time of 3 min. The more Ca2+ ions released from the negatively charged HA (N-HA) surfaces were more effective in the activation of the FXIII, resulting in the rapider disappearance of the gamma-chain bands in fibrin. The slightly lower Ca2+ concentration in the positively charged HA, compared to the nonpolarized HA activated the FXIII at an almost equal rate. The accelerated activation contributed to the stabilization of fibrin scaffold. Therefore, the polarity difference of the induced charges of the polarized HA surface altered the rate of the FXIII activation. The early stage interaction of the HA surfaces with blood proteins was considered to be an essential process of the accelerated new bone formation near implanted N-HA surface.

Factor XIII (FXIII) and angiogenesis

Factor XIII is a plasma transglutaminase that participates in the final stage of the coagulation cascade. Thrombin-activated FXIII (FXIIIa) catalyzes the formation of covalent cross-links between gamma-glutamyl and epsilon-lysyl residues on adjacent fibrin chains in polymerized fibrin to yield the mature clot. In addition to its role in hemostasis, FXIII is known to participate in wound healing and embryo implantation, which are processes involving angiogenesis. In this review, we discuss the role of FXIII in angiogenesis and the molecular mechanisms underlying its proangiogenic effects. The FXIII role in tissue repair and remodeling may at least in part be attributed to its pro-angiogenic activity.

Crystal structure of microbial transglutaminase from Streptoverticillium mobaraense

The crystal structure of a microbial transglutaminase from Streptoverticillium mobaraense has been determined at 2.4 A resolution. The protein folds into a plate-like shape, and has one deep cleft at the edge of the molecule. Its overall structure is completely different from that of the factor XIII-like transglutaminase, which possesses a cysteine protease-like catalytic triad. The catalytic residue, Cys(64), exists at the bottom of the cleft. Asp(255) resides at the position nearest to Cys(64) and is also adjacent to His(274). Interestingly, Cys(64), Asp(255), and His(274) superimpose well on the catalytic triad "Cys-His-Asp" of the factor XIII-like transglutaminase, in this order. The secondary structure frameworks around these residues are also similar to each other. These results imply that both transglutaminases are related by convergent evolution; however, the microbial transglutaminase has developed a novel catalytic mechanism specialized for the cross-linking reaction. The structure accounts well for the catalytic mechanism, in which Asp(255) is considered to be enzymatically essential, as well as for the causes of the higher reaction rate, the broader substrate specificity, and the lower deamidation activity of this enzyme.

Role of factor XIII in fibrin clot formation and effects of genetic polymorphisms

Factor XIII and fibrinogen are unusual among clotting factors in that neither is a serine protease. Fibrin is the main protein constituent of the blood clot, which is stabilized by factor XIIIa through an amide or isopeptide bond that ligates adjacent fibrin monomers. Many of the structural and functional features of factor XIII and fibrin(ogen) have been elucidated by protein and gene analysis, site-directed mutagenesis, and x-ray crystallography. However, some of the molecular aspects involved in the complex processes of insoluble fibrin formation in vivo and in vitro remain unresolved. The findings of a relationship between fibrinogen, factor XIII, and cardiovascular or other thrombotic disorders have focused much attention on these 2 proteins. Of particular interest are associations between common variations in the genes of factor XIII and altered risk profiles for thrombosis. Although there is much debate regarding these observations, the implications for our understanding of clot formation and therapeutic intervention may be of major importance. In this review, we have summarized recent findings on the structure and function of factor XIII. This is followed by a review of the effects of genetic polymorphisms on protein structure/function and their relationship to disease.

Crystal structure of red sea bream transglutaminase

The crystal structure of the tissue-type transglutaminase from red sea bream liver (fish-derived transglutaminase, FTG) has been determined at 2.5-A resolution using the molecular replacement method, based on the crystal structure of human blood coagulation factor XIII, which is a transglutaminase zymogen. The model contains 666 residues of a total of 695 residues, 382 water molecules, and 1 sulfate ion. FTG consists of four domains, and its overall and active site structures are similar to those of human factor XIII. However, significant structural differences are observed in both the acyl donor and acyl acceptor binding sites, which account for the difference in substrate preferences. The active site of the enzyme is inaccessible to the solvent, because the catalytic Cys-272 hydrogen-bonds to Tyr-515, which is thought to be displaced upon acyl donor binding to FTG. It is postulated that the binding of an inappropriate substrate to FTG would lead to inactivation of the enzyme because of the formation of a new disulfide bridge between Cys-272 and the adjacent Cys-333 immediately after the displacement of Tyr-515. Considering the mutational studies previously reported on the tissue-type transglutaminases, we propose that Cys-333 and Tyr-515 are important in strictly controlling the enzymatic activity of FTG.

Identification of the calcium binding site and a novel ytterbium site in blood coagulation factor XIII by x-ray crystallography

The presence or absence of calcium determines the activation, activity, oligomerization, and stability of blood coagulation factor XIII. To explore these observed effects, we have determined the x-ray crystal structure of recombinant factor XIII A2 in the presence of calcium, strontium, and ytterbium. The main calcium binding site within each monomer involves the main chain oxygen atom of Ala-457, and also the side chains from residues Asn-436, Asp-438, Glu-485, and Glu-490. Calcium and strontium bind in the same location, while ytterbium binds several angstroms removed. A novel ytterbium binding site is also found at the dimer two-fold axis, near residues Asp-270 and Glu-272, and this site may be related to the reported inhibition by lanthanide metals (Achyuthan, K. E., Mary, A., and Greenberg, C. S. (1989) Biochem. J. 257, 331-338). The overall structure of ion-bound factor XIII is very similar to the previously determined crystal structures of factor XIII zymogen, likely due to the constraints of this monoclinic crystal form. We have merged the three independent sets of water molecules in the structures to determine which water molecules are conserved and possibly structurally significant.

Novel aspects of blood coagulation factor XIII. I. Structure, distribution, activation, and function

Blood coagulation factor XIII (FXIII) is a protransglutaminase that becomes activated by the concerted action of thrombin and Ca2+ in the final stage of the clotting cascade. In addition to plasma, FXIII also occurs in platelets, monocytes, and monocyte-derived macrophages. While the plasma factor is a heterotetramer consisting of paired A and B subunits (A2B2), its cellular counterpart lacks the B subunits and is a homodimer of potentially active A subunits (A2). The gene coding for the A and B subunits has been localized to chromosomes 6p24-25 and 1q31-32.1, respectively. The genomic as well as the primary protein structure of both subunits has been established, and most recently the three-dimensional structure of recombinant cellular FXIII has also been revealed. Monocytes/macrophages synthesize their own FXIII, and very likely FXIII in platelets is synthesized by the megakaryocytes. Cells of bone marrow origin seem to be the primary site for the synthesis of subunit A in plasma FXIII, but hepatocytes might also contribute. The B subunit of plasma FXIII is synthesized in the liver. Plasma FXIII circulates in association with its substrate precursor, fibrinogen. Fibrin(ogen) has an important regulatory role in the activation of plasma FXIII. The most important steps of the activation of plasma FXIII are the proteolytic removal of activation peptide by thrombin, the dissociation of subunits A and B, and the exposure of the originally buried active site on the free A subunits. The end result of this process is the formation of an active transglutaminase, which cross-links peptide chains through epsilon(gamma-glutamyl)lysyl isopeptide bonds. Cellular FXIII in platelets becomes activated through a nonproteolytic process. When intracytoplasmic Ca2+ is raised during platelet activation, the zymogen--in the absence of subunit B--assumes an active configuration. The protein substrates of activated FXIII include components of the clotting-fibrinolytic system, adhesive and contractile proteins. The main physiological function of plasma FXIII is to cross-link fibrin and protect it from the fibrinolytic plasmin. The latter effect is achieved mainly by covalently linking alpha 2 antiplasmin, the most potent physiological inhibitor of plasmin, to fibrin. Plasma FXIII seems to be involved in wound healing and tissue repair, and it is essential to maintaining pregnancy. Cellular FXIII, if exposed to the surface of the cells, might support or perhaps take over the hemostatic functions of plasma FXIII; however, its intracellular role has remained mostly unexplored.

Transglutaminase inhibition by 2-[(2-oxopropyl)thio]imidazolium derivatives: mechanism of factor XIIIa inactivation

The physiologic role of several transglutaminases could be more precisely defined with the development of specific inhibitors for these enzymes. In addition, specific plasma transglutaminase (fXIIIa) inhibitors may have therapeutic utility in the treatment of thrombosis. For these purposes, the inactivation of fXIIIa and human erythrocyte transglutaminase (HET) by 2-[(2-oxopropyl)thio]imidazolium derivatives, which comprise a novel class of transglutaminase inactivators, was studied. As a specific example, 1,3,4,5-tetramethyl-2-[(2-oxopropyl)thio]imidazolium chloride (III) inactivated fXIIIa with an apparent second-order rate constant (specificity constant of inactivation) of 6.3 x 10(4) M-1 s-1, corresponding to a rate 4 x 10(7) times greater than its reaction rate with glutathione (GSH). The mechanism of fXIIIa inactivation by this class of compounds was investigated utilizing two [14C]-isotopic regioisomers of 1,3-dimethyl-2-[(2-oxopropyl)thio]imidazolium iodide (II). Structural analyses demonstrated that acetonylation of the active site cysteinyl residue of fXIIIa occurred along with the stoichiometric release of the complementary fragment of the inactivator as the corresponding thione. Kinetic analysis of the inactivation of fXIIIa by nonquarternary analogs of II and III indicated the formation of a reversible complex between the inactivator and fXIIIa prior to irreversible modification of the enzyme. At 1 mM, III displayed no detectable levels of inhibition or inactivation with several serine proteases and thiol reagent-sensitive enzymes. 2-[(2-Oxopropyl)thio]imidazolium derivatives and the related molecule 2-(1-acetonylthio)-5-methylthiazolo-[2,3]-1,3,4-thiadiazo lium perchlorate (I), when present at the time of clot formation at 1-10 microM, enhanced the rates of tissue plasminogen activator catalyzed clot lysis in vitro. These inactivators prevented the fXIIIa-catalyzed covalent incorporation of alpha 2-antiplasmin into the alpha chain of fibrin and the formation of high molecular weight fibrin alpha chain polymers, providing the basis for the observed enhancements in clot lysis rates.

Factor XIII: inherited and acquired deficiency

Factor XIII (XIII), an enzyme found in plasma (present as a pro-enzyme), platelets and monocytes, is essential for normal haemostasis. It may also have a role to play in the processes of wound healing and tissue repair. Inherited XIII deficiency results in a life-long, severe bleeding diathesis which, if untreated, carries a very high risk of death in early life from intracranial bleeding. XIII is a zymogen requiring thrombin and calcium for activation. In plasma, XIII has two subunits: the 'a' subunit, which is the active enzyme, and the 'b' subunit which is a carrier protein. Activated XIII modifies the structure of clot by covalently crosslinking fibrin through an epsilon (gamma-glutamyl)lysine link. It also crosslinks other proteins, including fibronectin and alpha-2-plasmin inhibitor (alpha-2PI), into the clot through the same link. Clot modified by XIII is physically stronger, relatively more resistant to fibrinolysis and may be a more suitable medium for the ingrowth of fibroblasts. Inheritance of factor XIII is autosomal recessive. The majority of patients with the inherited defect show no XIII activity and absence of 'a' subunit protein in plasma, platelets and monocytes. At the molecular level, the defect is not a major gene rearrangement or deletion, but most likely a single point mutation which may be different in each family. Because of the severity of the bleeding diathesis, prophylaxis is desirable and has been shown to be very effective as the in vivo half-life of plasma XIII is long, and low plasma levels are sufficient for haemostasis. Acquired inhibitors have been reported in only two cases with inherited XIII deficiency. Acquired XIII deficiency has been described in a variety of diseases and bleeding has been controlled by therapy with large doses of XIII in such conditions as Henoch-Schönlein purpura, various forms of colitis, erosive gastritis and some forms of leukaemia. Large dose XIII therapy has also been used in an endeavour to promote wound healing after surgery and bone union in non-healing fractures. The use of XIII in these conditions remains controversial. Very rarely a bleeding diathesis results from the development of a specific inhibitor to XIII arising de novo, often as a complication in the course of a disease or in association with long-term drug therapy. The bleeding diathesis in these patients is difficult to treat.

Fibrin--recombinant human factor XIII a-subunit association

The association of factor XIII a-subunits with fibrin was characterized using recombinant human placental factor XIII (rFXIII) and native fully hydrated fibrin clots formed from purified fibrinogen and thrombin. Binding was assessed using small columns containing fibrin and perfusing them with radioiodinated rFXIII. Results show that thrombin activation of rFXIII led to fibrin binding. The association was partially reversible since much of the bound enzyme could be removed by percolating clots with more buffer. Binding was blocked by antibody directed against the COOH-terminal part of fibrinogen A alpha-chain (A alpha 389-402) and also by the COOH-terminal A alpha-chain peptide fragment A alpha 241-476 (Hi2-DSK).

Interactions of factor XIII with fibrin as substrate and cofactor

Factor XIIIa (a2') is a homodimeric transglutaminase that is formed via limited alpha-thrombin-catalyzed proteolysis of the platelet (a2) or plasma (a2b2) factor XIII zymogen in a reaction that results in proteolytic removal of a 37-aminoacyl residue peptide from the N-terminus of the a chains and exposure of the active-site thiol group in the resulting a' chains of factor XIIIa. In this study, we characterized interactions of factor XIII and factor XIIIa with fibrin, a natural substrate for factor XIIIa and a cofactor for the alpha-thrombin-catalyzed activation of plasma factor XIII. The carbamylmethyl derivatives of the active-site thiol group of platelet factor XIII (CMa2) and factor XIIIa (CMa2') were prepared, and their interactions with fibrin were measured. The enzyme-like derivative (CMa2') which contained nicked a' chains bound more tightly to fibrin (Kd = 2.1 microM) than did CMa2 (Kd = 14 microM), the platelet zymogen-like derivative with intact a chains, but the binding of each was weaker than the binding of plasma factor XIII zymogen (a2b2) to fibrin (Kd = 0.20 microM) under the same conditions. Saturation of fibrin with plasma factor XIII zymogen (a2b2) did not affect the binding of CMa2' to fibrin, suggesting that the plasma factor XIII zymogen (a2b2) and the active-site-modified form of factor XIIIa (CMa2') bind to separate, noninteracting sites of fibrin.(ABSTRACT TRUNCATED AT 250 WORDS)