The subunit structure of thrombin-activated factor V. Isolation of activated factor V, separation of subunits, and reconstitution of biological activity

Activated protein C, protein S, and tissue factor pathway inhibitor cooperate to inhibit thrombin activation

INTRODUCTION

Thrombin, the enzyme which converts fibrinogen into a fibrin clot, is produced by the prothrombinase complex, composed of factor Xa (FXa) and factor Va (FVa). Down-regulation of this process is critical, as excess thrombin can lead to life-threatening thrombotic events. FXa and FVa are inhibited by the anticoagulants tissue factor pathway inhibitor alpha (TFPIα) and activated protein C (APC), respectively, and their common cofactor protein S (PS). However, prothrombinase is resistant to either of these inhibitory systems in isolation.

MATERIALS AND METHODS

We hypothesized that these anticoagulants function best together, and tested this hypothesis using purified proteins and plasma-based systems.

RESULTS

In plasma, TFPIα had greater anticoagulant activity in the presence of APC and PS, maximum PS activity required both TFPIα and APC, and antibodies against TFPI and APC had an additive procoagulant effect, which was mimicked by an antibody against PS alone. In purified protein systems, TFPIα dose-dependently inhibited thrombin activation by prothrombinase, but only in the presence of APC, and this activity was enhanced by PS. Conversely, FXa protected FVa from cleavage by APC, even in the presence of PS, and TFPIα reversed this protection. However, prothrombinase assembled on platelets was still protected from inhibition, even in the presence of TFPIα, APC, and PS.

CONCLUSIONS

We propose a model of prothrombinase inhibition through combined targeting of both FXa and FVa, and that this mechanism enables down-regulation of thrombin activation outside of a platelet clot. Platelets protect prothrombinase from inhibition, however, supporting a procoagulant environment within the clot.

Regulation of factor V and factor V-short by TFPIα: Relationship between B-domain proteolysis and binding

Coagulation factor V (FV) plays an anticoagulant role but serves as a procoagulant cofactor in the prothrombinase complex once activated to FVa. At the heart of these opposing effects is the proteolytic removal of its central B-domain, including conserved functional landmarks (basic region, BR; 963–1008 and acidic region 2, AR2; 1493–1537) that enforce the inactive FV procofactor state. Tissue factor pathway inhibitor α (TFPIα) has been associated with FV as well as FV-short, a physiologically relevant isoform with a shortened B-domain missing the BR. However, it is unclear which forms of FV are physiologic ligands for TFPIα. Here, we characterize the binding and regulation of FV and FV-short by TFPIα via its positively charged C-terminus (TFPIα-BR) and examine how bond cleavage in the B-domain influences these interactions. We show that FV-short is constitutively active and functions in prothrombinase like FVa. Unlike FVa, FV-short binds with high affinity (K_d ∼1 nM) to TFPIα-BR, which blocks procoagulant function unless FV-short is cleaved at Arg¹⁵⁴⁵, removing AR2. Importantly, we do not observe FV binding (μM detection limit) to TFPIα. However, cleavage at Arg⁷⁰⁹ and Arg¹⁰¹⁸ displaces the FV BR, exposing AR2 and allowing TFPIα to bind via its BR. We conclude that for full-length FV, the detachment of FV BR from AR2 is necessary and sufficient for TFPIα binding and regulation. Our findings pinpoint key forms of FV, including FV-short, that act as physiologic ligands for TFPIα and establish a mechanistic framework for assessing the functional connection between these proteins.

Factor VIII and Factor V Membrane Bound Complexes

The formation of membrane-bound complexes between specific coagulation factors at different cell surfaces is required for effective blood clotting. The most important of these complexes, the intrinsic Tenase and Prothrombinase complexes, are formed on the activated platelet surface during the propagation phase of coagulation. These two complexes are highly specific in their assembly mechanism and function modulated by anionic membranes, thus offering desirable targets for pharmaceutical interventions. Factor V (FV) and factor VIII (FVIII) are highly homologous non-enzymatic proteins. In their active state, FVa and FVIIIa serve as cofactors for the respective serine proteases factor Xa (FXa) and factor IXa (FIXa), significantly increasing their catalytic activity. This is achieved by forming well organized membrane-bound complexes at the phosphatidylserine rich activated platelet membrane in the presence of Ca²⁺ ions. The tenase (FVIIIa/FIXa) complex, catalyzes the proteolytic conversion of FX to FXa. Subsequently the prothrombinase (FVa/FXa) complex catalyzes the conversion of prothrombin to thrombin, required for efficient blood clotting. Although significant knowledge of FV and FVIII biochemistry and regulation has been achieved, the molecular mechanisms of their function are yet to be defined. Understanding the geometric assembly of the tenase and prothrombinase complexes is paramount in defining the structural basis of bleeding and thrombotic disorders. Such knowledge will enable the design of efficient pro- and anticoagulant therapies critical for regulating abnormal hemostasis. In this chapter, we will summarize the findings to date, showing our achievement in the field and outlining the future findings required to grasp the complexity of these proteins.

Hemodilution and Endothelial Cell Regulation of Whole Blood Coagulation

Background

Beyond localized damage to the circulatory system and surrounding tissue, trauma stresses endothelial cells throughout the vasculature, potentially leading to hemorrhagic or thrombotic complications away from the injury site.

Objective

Use a whole blood endothelial cell model to define the effects of crystalloid fluid therapy on protein C pathway regulation of tissue factor-initiated coagulation.

Methods

Tissue factor-initiated coagulation was studied in the presence of EA.hy926 cells. Blood was diluted to 70% or 40% using normal saline or lactated ringers. Analyses of coagulation dynamics included clot times, thrombin formation (thrombin-antithrombin complex), FV activation/inactivation, fibrinogen consumption, FXIII activation, and platelet activation.

Results

In all donors, the onset of thrombin generation was not altered in 70% blood using either diluent; with the blood component reduced to 40%, clot time was prolonged two-fold when normal saline was utilized but was unchanged with lactated ringers. The timing of the activations of FV, fibrinogen, and platelets paralleled the effects of dilution on clot times. Extensive inactivation of FVa was observed in undiluted blood and where lactated ringers was the diluent but not in trials with 40% blood/60% normal saline.

Conclusion

Feedback inhibition of tissue factor-initiated coagulation by the protein C pathway is not compromised by hemodilution with crystalloids.

Spellbinding Effects of the Acidic COOH-Terminus of Factor Va Heavy Chain on Prothrombinase Activity and Function

Human factor Va (hfVa) is the important regulatory subunit of prothrombinase. Recent modeling data have suggested a critical role for amino acid Arg⁷⁰¹ of hfVa for human prothrombin (hPro) activation by prothrombinase. Furthermore, it has also been demonstrated that hfVa has a different effect than that of bovine fVa on prethrombin-1 activation by prothrombinase. The difference between the two cofactor molecules was also found within the Asn⁷⁰⁰-Arg⁷⁰¹ dipeptide in the human factor V (hfV) molecule, which is replaced by the Asp-Glu sequence in bfV. As a consequence, we produced a recombinant hfV (rhfV) molecule with the substitution ⁷⁰⁰NR⁷⁰¹→DE. rhfV^NR→DE together with the wild-type molecule (rhfV^WT) were expressed in COS7 cells, purified, and tested for their capability to function within prothrombinase. Kinetic studies showed that the K_d of rhfVa^NR→DE for human fXa as well as the k_cat and K_m of prothrombinase made with rhfVa^NR→DE for hPro activation were similar to the values obtained following hPro activation by prothrombinase made with rhfVa^WT. Remarkably, sodium dodecyl sulfate polyacrylamide gel electrophoresis analyses of hPro activation time courses demonstrated that the rate of cleavage of hPro by prothrombinase reconstituted with rhfVa^NR→DE was significantly delayed with substantial accumulation of meizothrombin, and delayed thrombin generation, when compared to activation of hPro by prothrombinase made with rhfVa^WT. These unanticipated results provide significant insights on the role of the carboxyl-terminal end of the heavy chain of hfVa for hPro cleavage and activation by prothrombinase and show that residues ⁷⁰⁰NR⁷⁰¹ regulate at least in part the enzyme-substrate/product interaction during fibrin clot formation.

Rethinking events in the haemostatic process: role of factor V and TFPI

Regulatory mechanisms responsible for limiting blood clot formation are critical for maintaining normal haemostasis. Dysregulation can lead to bleeding (e.g. haemophilia) or thrombosis. New findings showing that tissue factor pathway inhibitor-alpha (TFPIα) binds coagulation factor V(a) and inhibits prothrombinase assembly highlights that our understanding of the initiation of coagulation is evolving. Work over the past decade on the biochemistry of FV activation has laid the groundwork for deciphering the mechanistic bases that may underpin how TFPIα mediates these anticoagulant effects. Collectively, these new findings are re-shaping our thinking about how coagulation is initiated at the site of injury. These ideas could have important clinical implications and help identify new ways to bias the coagulation response for the treatment of haemophilia and other disorders of the haemostatic process.

The Importance of Exosite Interactions for Substrate Cleavage by Human Thrombin

Thrombin is a serine protease of the chymotrypsin family that acts both as a procoagulant and as an anticoagulant by cleaving either factor VIII, factor V and fibrinogen or protein C, respectively. Numerous previous studies have shown that electropositive regions at a distance from the active site, so called exosites, are of major importance for the cleavage by human thrombin. Upstream of all the known major cleavage sites for thrombin in factor VIII, factor V and fibrinogen are clusters of negatively charged amino acids. To study the importance of these sites for the interaction with the exosites and thereby the cleavage by thrombin, we have developed a new type of recombinant substrate. We have compared the cleavage rate of the minimal cleavage site, involving only 8-9 amino acids (typically the P4-P4' positions) surrounding the cleavage site, with the substrates also containing the negatively charged regions upstream of the cleavage sites. The results showed that addition of these regions enhanced the cleavage rate by more than fifty fold. However, the enhancement was highly dependent on the sequence of the actual cleavage site. A minimal site that showed poor activity by itself could be cleaved as efficiently as an optimal cleavage site when presented together with these negatively charged regions. Whereas sites conforming closely to the optimal site were only minimally enhanced by the addition of these regions. The possibility to mimic this interaction for the sites in factor V and factor VIII by recombinant substrates, which do not have the same folding as the full size target, indicates that the enhancement was primarily dependent on a relatively simple electrostatic interaction. However, the situation was very different for fibrinogen and protein C where other factors than only charge is of major importance.

Factor Xa dimerization competes with prothrombinase complex formation on platelet-like membrane surfaces

Exposure of phosphatidylserine (PS) molecules on activated platelet membrane surface is a crucial event in blood coagulation. Binding of PS to specific sites on factor Xa (fXa) and factor Va (fVa) promotes their assembly into a complex that enhances proteolysis of prothrombin by approximately 10⁵. Recent studies demonstrate that both soluble PS and PS-containing model membranes promote formation of inactive fXa dimers at 5 mM Ca²⁺. In the present study, we show how competition between fXa dimerization and prothrombinase formation depends on Ca²⁺ and lipid membrane concentrations. We used homo-FRET measurements between fluorescein-E-G-R-chloromethylketone (CK)-Xa [fXa irreversibly inactivated by alkylation of the active site histidine residue with FEGR (FEGR-fXa)] and prothrombinase activity measurements to reveal the balance between fXa dimer formation and fXa-fVa complex formation. Changes in FEGR-fXa dimer homo-FRET with addition of fVa to model-membrane-bound FEGR-fXa unambiguously demonstrated that formation of the FEGR-fXa-fVa complex dissociated the dimer. Quantitative global analysis according to a model for protein interaction equilibria on a surface provided an estimate of a surface constant for fXa dimer dissociation (K(fXa×fXa)(d, σ)) approximately 10-fold lower than K(fXa×fVa)(d,σ) for fXa-fVa complex. Experiments performed using activated platelet-derived microparticles (MPs) showed that competition between fXa dimerization and fXa-fVa complex formation was even more prominent on MPs. In summary, at Ca²⁺ concentrations found in the maturing platelet plug (2-5 mM), fVa can compete fXa off of inactive fXa dimers to significantly amplify thrombin production, both because it releases dimer inhibition and because of its well-known cofactor activity. This suggests a hitherto unanticipated mechanism by which PS-exposing platelet membranes can regulate amplification and propagation of blood coagulation.

2013 scientific sessions Sol Sherry distinguished lecture in thrombosis: polyphosphate: a novel modulator of hemostasis and thrombosis

Polyphosphate is a highly anionic, linear polymer of inorganic phosphates that is found throughout biology, including in many infectious microorganisms. Recently, polyphosphate was discovered to be stored in a subset of the secretory granules of human platelets and mast cells, and to be secreted on activation of these cells. Work from our laboratory and others has now shown that polyphosphate is a novel, potent modulator of the blood clotting and complement systems that likely plays roles in hemostasis, thrombosis, inflammation, and host responses to pathogens. Therapeutics targeting polyphosphate may have the potential to limit thrombosis with fewer hemorrhagic complications than conventional anticoagulant drugs that target essential proteases of the blood clotting cascade.

Polyphosphate accelerates factor V activation by factor XIa

Factor Va enhances the rate of prothrombin activation by factor Xa by four to five orders of magnitude. Production of initiating levels of factor Va from its precursor, factor V, is a critical event early in haemostasis, as factor V exhibits negligible cofactor activity. While thrombin is the most potent physiological back-activator of factor V, the first prothrombinase complexes require a source of factor Va prior to thrombin generation. A recent study by Whelihan et al. (J Thromb Haemost 2010; 8:1532-1539) identified factor XIa as a candidate for the initial thrombin-independent activation of factor V, although this reaction was slow and required relatively high concentrations of factors V and XIa. Activated platelets secrete polyphosphate, which we previously showed to be potently procoagulant. We now report that polyphosphate greatly accelerates factor V activation by factor XIa, and that this is supported by polyphosphate polymers of the size secreted by activated human platelets. This finding provides additional evidence that factor XIa-mediated generation of factor Va may contribute to the initiation of haemostasis.

Biology of tissue factor pathway inhibitor

Recent studies of the anticoagulant activities of the tissue factor (TF) pathway inhibitor (TFPI) isoforms, TFPIα and TFPIβ, have provided new insight into the biochemical and physiological mechanisms that underlie bleeding and clotting disorders. TFPIα and TFPIβ have tissue-specific expression patterns and anticoagulant activities. An alternative splicing event in the 5' untranslated region allows for translational regulation of TFPIβ expression. TFPIα has 3 Kunitz-type inhibitor domains (K1, K2, K3) and a basic C terminus, whereas TFPIβ has the K1 and K2 domains attached to a glycosylphosphatidyl inositol-anchored C terminus. TFPIα is the only isoform present in platelets, whereas endothelial cells produce both isoforms, secreting TFPIα and expressing TFPIβ on the cell surface. TFPIα and TFPIβ inhibit both TF-factor VIIa-dependent factor Xa (FXa) generation and free FXa. Protein S enhances FXa inhibition by TFPIα. TFPIα produces isoform-specific inhibition of prothrombinase during the initiation of coagulation, an anticoagulant activity that requires an exosite interaction between its basic C terminus and an acidic region in the factor Va B domain. Platelet TFPIα may be optimally localized to dampen initial thrombin generation. Similarly, endothelial TFPIβ may be optimally localized to inhibit processes that occur when endothelial TF is present, such as during the inflammatory response.

Improved hemostasis in hemophilia mice by means of an engineered factor Va mutant

BACKGROUND

Factor (F)VIIa-based bypassing not always provides sufficient hemostasis in hemophilia.

OBJECTIVES

To investigate the potential of engineered activated factor V (FVa) variants as bypassing agents in hemophilia A.

METHODS

Activity of FVa variants was studied in vitro using prothrombinase assays with purified components and in FV- and FVIII-deficient plasma using clotting and thrombin generation assays. In vivo bleed reduction after the tail clip was studied in hemophilia A mice.

RESULTS AND CONCLUSIONS

FVa mutations included a disulfide bond connecting the A2 and A3 domains and ones that rendered FVa resistant to inactivation by activated protein C (APC). '(super) FVa,' a combination of the A2-A3 disulfide (A2-SS-A3) to stabilize FVa and of APC-cleavage site mutations (Arg506/306/679Gln), had enhanced specific activity and complete APC resistance compared with wild-type FVa, FVL eiden (Arg506Gln), or FVaL eiden (A2-SS-A3). Furthermore, (super) FVa potently increased thrombin generation in vitro in FVIII-deficient plasma. In vivo, (super) FVa reduced bleeding in FVIII-deficient mice more effectively than wild-type FVa. Low-dose (super) FVa, but not wild-type FVa, decreased early blood loss during the first 10 min by more than two-fold compared with saline and provided bleed protection for the majority of mice, similar to treatments with FVIII. During the second 10 min after tail cut, (super) FVa at high dose, but not wild-type FVa, effectively reduced bleeding. These findings suggest that (super) FVa enhances not only clot formation but also clot stabilization. Thus, (super) FVa efficiently improved hemostasis in hemophilia in vitro and in vivo and may have potential therapeutic benefits as a novel bypassing agent in hemophilia.

Amino acid region 1000-1008 of factor V is a dynamic regulator for the emergence of procoagulant activity

Single chain factor V (fV) circulates as an M_r 330,000 quiescent pro-cofactor. Removal of the B domain and generation of factor Va (fVa) are vital for procoagulant activity. We investigated the role of the basic amino acid region 1000–1008 within the B domain of fV by constructing a recombinant mutant fV molecule with all activation cleavage sites (Arg⁷⁰⁹/Arg¹⁰¹⁸/Arg¹⁵⁴⁵) mutated to glutamine (fV^Q3), a mutant fV molecule with region 1000–1008 deleted (fV^ΔB9), and a mutant fV molecule containing the same deletion with activation cleavage sites changed to glutamine (fV^ΔB9/Q3). The recombinant molecules along with wild type fV (fV^WT) were transiently expressed in COS-7L cells, purified, and assessed for their ability to bind factor Xa (fXa) prior to and following incubation with thrombin. The data showed that fV^Q3 was severely impaired in its interaction with fXa before and after incubation with thrombin. In contrast, K_D(app) values for fV^ΔB9 (0.9 nm), fVa^ΔB9 (0.4 nm), and fV^ΔB9/Q3 (0.7 nm) were similar to the affinity of fVa^WT for fXa (0.3 nm). Two-stage clotting assays revealed that although fV^Q3 was deficient in its clotting activity, fV^ΔB9/Q3 had clotting activity comparable with fVa^WT. The k_cat value of prothrombinase assembled with fV^ΔB9/Q3 was minimally affected, whereas the K_m value of the reaction was increased 57-fold compared with the K_m value obtained with prothrombinase assembled with fVa^WT. These findings strongly suggest that amino acid region 1000–1008 of fV is a regulatory sequence protecting the organisms from spontaneous binding to fXa and unnecessary prothrombinase complex formation, which in turn results in catastrophic physiological consequences.

Restoring the procofactor state of factor Va-like variants by complementation with B-domain peptides

Coagulation factor V (FV) circulates as an inactive procofactor and is activated to FVa by proteolytic removal of a large inhibitory B-domain. Conserved basic and acidic sequences within the B-domain appear to play an important role in keeping FV as an inactive procofactor. Here, we utilized recombinant B-domain fragments to elucidate the mechanism of this FV autoinhibition. We show that a fragment encoding the basic region (BR) of the B-domain binds with high affinity to cofactor-like FV(a) variants that harbor an intact acidic region. Furthermore, the BR inhibits procoagulant function of the variants, thereby restoring the procofactor state. The BR competes with FXa for binding to FV(a), and limited proteolysis of the B-domain, specifically at Arg(1545), ablates BR binding to promote high affinity association between FVa and FXa. These results provide new insight into the mechanism by which the B-domain stabilizes FV as an inactive procofactor and reveal how limited proteolysis of FV progressively destabilizes key regulatory regions of the B-domain to produce an active form of the molecule.

A new regulatory function of activated factor V: inhibition of the activation by tissue factor/factor VII(a) of factor X

BACKGROUND

We observed that minute amounts of thrombin or the enzyme Russell's viper venom activating factor V (RVV-V) added to plasma strongly diminish the potential of that plasma to generate thrombin after being triggered by tissue factor.

OBJECTIVE

To find the mechanism behind this phenomenon.

METHODS AND RESULTS

Thrombin generation (TG) initiated by tissue factor (TF) is strongly and dose-dependently inhibited by addition of activated factor V (FVa) or by addition of a factor V activator (thrombin or RVV-V). No inhibition is seen when TG is triggered via the intrinsic pathway or by direct activation of factor X. The effect is independent of proteins C and S and tissue factor pathway inhibitor (TFPI). In factor VII-deficient plasma the effect is seen when it is spiked with recombinant factor VII (FVII) and to a much lesser extent when spiked with recombinant FVIIa. In a purified system, FVa also dose-dependently inhibits the activation of FX by TF/FVII(a). The inhibitory effect is neutralized by antibodies against the light chain of FVa but not by antibodies against the heavy chain.

CONCLUSIONS

Our observations can be explained by assuming that FVa, via its light chain, binds to the complex TF/FVII(a) and prevents it from activating FX. We assume that this mechanism reduces the possibility that thrombin and factor Xa escaping from a wound area into the circulation, together with blood-borne tissue factor, would trigger intravascular coagulation.

Phosphatidylserine-induced factor Xa dimerization and binding to factor Va are competing processes in solution

A soluble, short chain phosphatidylserine, 1,2-dicaproyl-sn-glycero-3-phospho-l-serine (C6PS), binds to discrete sites on FXa, FVa, and prothrombin to alter their conformations, to promote FXa dimerization (K(d) ~ 14 nM), and to enhance both the catalytic activity of FXa and the cofactor activity of FVa. In the presence of calcium, C6PS binds to two sites on FXa, one in the epidermal growth factor-like (EGF) domain and one in the catalytic domain; the latter interaction is sensitive to Na(+) binding and probably represents a protein recognition site. Here we ask whether dimerization of FXa and its binding to FVa in the presence of C6PS are competitive processes. We monitored FXa activity at 5, 20, and 50 nM FXa while titrating with FVa in the presence of 400 μM C6PS and 3 or 5 mM Ca(2+) to show that the apparent K(d) of FVa-FXa interaction increased with an increase in FXa concentration at 5 mM Ca(2+), but the K(d) was only slightly affected at 3 mM Ca(2+). A mixture of 50 nM FXa and 50 nM FVa in the presence of 400 μM C6PS yielded both Xa homodimers and Xa·Va heterodimers, but no FXa dimers bound to FVa. A mutant FXa (R165A) that has reduced prothrombinase activity showed both weakened dimerization (K(d) ~ 147 nM) and weakened FVa binding (apparent K(d) values of 58, 92, and 128 nM for 5, 20, and 50 nM R165A FXa, respectively). Native gel electrophoresis showed that the GLA-EGF(NC) fragment of FXa (lacking the catalytic domain) neither dimerized nor formed a complex with FVa in the presence of 400 μM C6PS and 5 mM Ca(2+). Our results demonstrate that the dimerization site and FVa-binding site are both located in the catalytic domain of FXa and that these sites are linked thermodynamically.

A bipartite autoinhibitory region within the B-domain suppresses function in factor V

Activation of blood coagulation factor V (FV) is a key reaction of hemostasis. FV circulates in plasma as an inactive procofactor, and proteolytic removal of a large central B-domain converts it to an active cofactor (FVa) for factor Xa (FXa). Here we show that two short evolutionary conserved segments of the B-domain, together termed the procofactor regulatory region, serve an essential autoinhibitory function. This newly identified motif consists of a basic (963-1008) and an acidic (1493-1537) region and defines the minimal sequence requirements to maintain FV as a procofactor. Our data suggest that dismantling this autoinhibitory region via deletion or proteolysis is the driving force to unveil a high affinity binding site(s) for FXa. These findings document an unexpected sequence-specific role for the B-domain by negatively regulating FV function and preventing activity of the procofactor. These new mechanistic insights point to new ways in which the FV procofactor to cofactor transition could be modulated to alter hemostasis.

Modeling of human factor Va inactivation by activated protein C

BACKGROUND

Because understanding of the inventory, connectivity and dynamics of the components characterizing the process of coagulation is relatively mature, it has become an attractive target for physiochemical modeling. Such models can potentially improve the design of therapeutics. The prothrombinase complex (composed of the protease factor (F)Xa and its cofactor FVa) plays a central role in this network as the main producer of thrombin, which catalyses both the activation of platelets and the conversion of fibrinogen to fibrin, the main substances of a clot. A key negative feedback loop that prevents clot propagation beyond the site of injury is the thrombin-dependent generation of activated protein C (APC), an enzyme that inactivates FVa, thus neutralizing the prothrombinase complex. APC inactivation of FVa is complex, involving the production of partially active intermediates and "protection" of FVa from APC by both FXa and prothrombin. An empirically validated mathematical model of this process would be useful in advancing the predictive capacity of comprehensive models of coagulation.

RESULTS

A model of human APC inactivation of prothrombinase was constructed in a stepwise fashion by analyzing time courses of FVa inactivation in empirical reaction systems with increasing number of interacting components and generating corresponding model constructs of each reaction system. Reaction mechanisms, rate constants and equilibrium constants informing these model constructs were initially derived from various research groups reporting on APC inactivation of FVa in isolation, or in the presence of FXa or prothrombin. Model predictions were assessed against empirical data measuring the appearance and disappearance of multiple FVa degradation intermediates as well as prothrombinase activity changes, with plasma proteins derived from multiple preparations. Our work integrates previously published findings and through the cooperative analysis of in vitro experiments and mathematical constructs we are able to produce a final validated model that includes 24 chemical reactions and interactions with 14 unique rate constants which describe the flux in concentrations of 24 species.

CONCLUSION

This study highlights the complexity of the inactivation process and provides a module of equations describing the Protein C pathway that can be integrated into existing comprehensive mathematical models describing tissue factor initiated coagulation.

Evolving methods for single nucleotide polymorphism detection: Factor V Leiden mutation detection

BACKGROUND

The many techniques used to diagnose the Factor V Leiden (FVL) mutation, the most common hereditary hypercoagulation disorder in Eurasians, and the most frequently requested genetic test reflect the evolving strategies in protein and DNA diagnosis.

METHODS

Here, molecular methods to diagnose the FVL mutation are discussed.

RESULTS

Protein-based detection assays include the conventional functional activated protein C resistance coagulation test and the recently reported antibody-mediated sensor detection; and DNA-based assays include approaches that use electrophoretic fractionation e.g., restriction fragment length polymorphism, denaturing gradient gel electrophoresis, and single-stranded conformational PCR analysis, DNA hybridization (e.g., microarrays), DNA polymerase-based assays, e.g., extension reactions, fluorescence polarization template-directed dye-terminator incorporation, PCR assays (e.g., amplification-refractory mutation system, melting curve analysis using real-time quantitative PCR, and helicase-dependent amplification), DNA sequencing (e.g., direct sequencing, pyrosequencing), cleavase-based Invader assay and ligase-based assays (e.g., oligonucleotide ligation assay and ligase-mediated rolling circle amplification).

CONCLUSION

The method chosen by a laboratory to diagnose FVL not only depends on the available technical expertise and equipment, but also the type, variety, and extent of other genetic disorders being diagnosed.

Thrombin and vascular inflammation

Vascular endothelium is a key regulator of homeostasis. In physiological conditions it mediates vascular dilatation, prevents platelet adhesion, and inhibits thrombin generation. However, endothelial dysfunction caused by physical injury of the vascular wall, for example during balloon angioplasty, acute or chronic inflammation, such as in atherothrombosis, creates a proinflammatory environment which supports leukocyte transmigration toward inflammatory sites. At the same time, the dysfunction promotes thrombin generation, fibrin deposition, and coagulation. The serine protease thrombin plays a pivotal role in the coagulation cascade. However, thrombin is not only the key effector of coagulation cascade; it also plays a significant role in inflammatory diseases. It shows an array of effects on endothelial cells, vascular smooth muscle cells, monocytes, and platelets, all of which participate in the vascular pathophysiology such as atherothrombosis. Therefore, thrombin can be considered as an important modulatory molecule of vascular homeostasis. This review summarizes the existing evidence on the role of thrombin in vascular inflammation.

Cross-reactivity of rabbit anti-bovine prothrombin/thrombin IgGs with bovine factor V/Va-related antigens

The purpose of this study was to determine whether rabbit anti-bovine prothrombin/thrombin immunoglobulin Gs (IgGs) would cross-react with bovine factor V/Va-related antigens. Bovine prothrombin, crude thrombin, as well as 2 purified versions of thrombin, that is, thrombin 4A (the previous version of Thrombin-JMI marketed prior to 2008) and 4B (the currently marketed version of Thrombin-JMI), were administrated to individual groups of rabbits on days 0, 21, 42, 91, 123, and 151 using standard immunologic methods. Blood was drawn from each rabbit on days 30, 50, 105, 137, and 165 and the pooled antisera from individual groups were purified to obtain the IgGs using protein G affinity columns. By probing bovine factor V/Va samples, the possible cross-reactivity of each IgG collected at different time points (from day 30 to day 165) was explored using Western blotting techniques. The results indicated that rabbit anti-bovine prothrombin and crude thrombin IgGs could cross-react strongly with bovine factor V/Va in an immunization time-dependent manner. However, antibodies generated in thrombin 4A-treated rabbits presented much weaker cross-reactivity with bovine factor V/Va. Furthermore, no cross-reactivity with bovine factor V/Va-related antigens was observed when the anti-bovine thrombin 4B IgG collected at any time point was used. The results suggest that thrombin 4B preparation contains the least bovine factor V/Va contaminants among the bovine prothrombin/thrombin preparations studied and the amount of bovine factor V/Va contaminants in bovine thrombin 4B is too small to elicit the generation of antibodies against bovine factor V/Va in rabbits.

Taking the Thrombin “Fork”

The proverb that probably best exemplifies my career in research is attributable to Yogi Berra (http://www.yogiberra.com/), ie, “when you come to a fork in the road … take it.” My career is a consequence of chance interactions with great mentors and talented students and the opportunities provided by a succession of ground-breaking improvements in technology.

APCR, factor V gene known and novel SNPs and adverse pregnancy outcomes in an Irish cohort of pregnant women

Background

Activated Protein C Resistance (APCR), a poor anticoagulant response of APC in haemostasis, is the commonest heritable thrombophilia. Adverse outcomes during pregnancy have been linked to APCR. This study determined the frequency of APCR, factor V gene known and novel SNPs and adverse outcomes in a group of pregnant women.

Methods

Blood samples collected from 907 pregnant women were tested using the Coatest^® Classic and Modified functional haematological tests to establish the frequency of APCR. PCR-Restriction Enzyme Analysis (PCR-REA), PCR-DNA probe hybridisation analysis and DNA sequencing were used for molecular screening of known mutations in the factor V gene in subjects determined to have APCR based on the Coatest^® Classic and/or Modified functional haematological tests. Glycosylase Mediated Polymorphism Detection (GMPD), a SNP screening technique and DNA sequencing, were used to identify SNPs in the factor V gene of 5 APCR subjects.

Results

Sixteen percent of the study group had an APCR phenotype. Factor V Leiden (FVL), FV Cambridge, and haplotype (H) R2 alleles were identified in this group. Thirty-three SNPs; 9 silent SNPs and 24 missense SNPs, of which 20 SNPs were novel, were identified in the 5 APCR subjects. Adverse pregnancy outcomes were found at a frequency of 35% in the group with APCR based on Classic Coatest^® test only and at 45% in the group with APCR based on the Modified Coatest^® test. Forty-eight percent of subjects with FVL had adverse outcomes while in the group of subjects with no FVL, adverse outcomes occurred at a frequency of 37%.

Conclusions

Known mutations and novel SNPs in the factor V gene were identified in the study cohort determined to have APCR in pregnancy. Further studies are required to investigate the contribution of these novel SNPs to the APCR phenotype. Adverse outcomes including early pregnancy loss (EPL), preeclampsia (PET) and intrauterine growth restriction (IGUR) were not significantly more frequent in subjects with APCR compared to normal pregnant women however Pregnancy induced hypertension (PIH) was found to be associated with FVL in our study group.

The molecular basis of factor V and VIII procofactor activation

Activation of precursor proteins by specific and limited proteolysis is a hallmark of the hemostatic process. The homologous coagulation factors (F)V and FVIII circulate in an inactive, quiescent state in blood. In this so-called procofactor state, these proteins have little, if any procoagulant activity and do not participate to any significant degree in their respective macromolecular enzymatic complexes. Thrombin is considered a key physiological activator, cleaving select peptide bonds in FV and FVIII which ultimately leads to appropriate structural changes that impart cofactor function. As the active cofactors (FVa and FVIIIa) have an enormous impact on thrombin and FXa generation, maintaining FV and FVIII as inactive procofactors undoubtedly plays an important regulatory role that has likely evolved to maintain normal hemostasis. Over the past three decades there has been widespread interest in studying the proteolytic events that lead to the activation of these proteins. While a great deal has been learned, mechanistic explanations as to how bond cleavage facilitates conversion to the active cofactor species remain incompletely understood. However, recent advances have been made detailing how thrombin recognizes FV and FVIII and also how the FV B-domain plays a dominant role in maintaining the procofactor state. Here we review our current understanding of the molecular process of procofactor activation with a particular emphasis on FV.

Differential contributions of Glu96, Asp102 and Asp111 to coagulation Factor V/Va metal ion binding and subunit stability

Blood coagulation FV (Factor V) is activated by thrombin-mediated excision of the B domain, resulting in a non-covalent heterodimer, FVa (activated FV). Previous studies implicated Glu96, Asp102 and Asp111 in the essential Ca2+-dependent FVa subunit interaction. In the present study, FV E96A, D102A and D111A were purified and evaluated for function, subunit dissociation and metal ion binding. Chromogenic and clotting assays in the presence of procoagulant vesicles showed that each variant was inhibited (∼20–40%). D111A was further inhibited (>90%) after cleavage by thrombin. Comparable function was observed on activated platelets. D111A inhibition correlated to spontaneous subunit dissociation and severely impaired Ca2+ binding. The Cu2+ interaction was also inhibited, suggesting interdependent Ca2+ and Cu2+ binding to FV. The parental FV (FV-810; wild-type human FV missing residues 811–1491) used here is fully active without proteolysis because the B domain is truncated. Therefore, a FVa-like functional configuration exists for intact D111A independent of normal metal ion interactions. Unlike D111A, the thrombin-mediated FVa derived from E96A and D102A had only moderately enhanced subunit dissociation upon chelation and had normal metal ion binding. For FV-810-, E96A- and D102A-derived FVa, loss of function after chelation significantly preceded subunit dissociation. This study defines the highly conserved segment spanning Glu96–Asp111 in FV as multifunctional. Of the three amino acids evaluated, Asp111 is essential and probably functions through direct and indirect effects on Ca2+ and Cu2+ interactions. Glu96 and Asp102 individually influence FV/FVa by more subtle effects, possibly at the metal ion-dependent subunit interface.

Conformational change path between closed and open forms of C2 domain of coagulation factor V on a two-dimensional free-energy surface

We test a hypothesis that the closed form of the C2 domain of coagulation factor V is more stable than the open form in an aqueous environment using a two-dimensional free-energy calculation with a simple dielectric solvent model. Our result shows that while the free-energy difference between two forms is small, favoring the closed form, a two-dimensional free-energy surface (FES) reveals that a transition state (1.53 kcal/mol) exists between the two conformations. By mapping the one-dimensional order parameter DeltaQ onto the two-dimensional FES, we search the conformational change path with the highest Boltzmann weighting factor between the closed and open form of the factor V C2 domain. The predicted transition path from the closed to open form is not that of simple side chain movements, but instead concerted movements of several loops. We also present a one-dimensional free-energy profile using a collective order parameter, which in a coarse manner locates the energy barriers found on the two-dimensional FES.

Comparison of immunogenic potentials of bovine thrombin preparations

Using a membrane filtration step, bovine crude thrombin was purified into thrombin 4A and 4B preparations. The purpose of this study was to determine whether the improved purity of a bovine thrombin preparation can reduce its overall immunogenic potential and lower the risk of development of factor V antibodies. Bovine crude thrombin and its purified versions, thrombin 4A and 4B, were administered to individual groups of rabbits on days 0, 21, 42, 91, 123, and 151 using standard immunologic methods. Blood was drawn from each rabbit on days 30, 50, 105, 137, and 165, and the pooled antisera from individual groups were purified to obtain the Ig Gs using protein G affinity columns. Using Western blotting, the specificity of each immunoglobulin G collected at the first time point (day 30) and last time point (day 165) was determined. The results of Western blotting using the Ig Gs collected on days 30 and 165 were consistent; both demonstrating that thrombin 4B has the least immunogenic potential among the 3 thrombin preparations tested. Compared with the immunoglobulin Gs collected on day 30, the Ig Gs from day 165 did not show obvious difference regarding their ability to detect antigens in bovine thrombin samples. Neither showed cross-reactivity with human coagulation factors nor the recognition of bovine factor Va antigens. These results suggest that despite the presence of a trace amount of bovine factor Va antigen in bovine thrombin preparations, these contaminants failed to elicit the generation of antibodies against factor Va light chain in rabbit.

Cooperative regulation of the activity of factor Xa within prothrombinase by discrete amino acid regions from factor Va heavy chain

The prothrombinase complex catalyzes the activation of prothrombin to alpha-thrombin. We have repetitively shown that amino acid region (695)DYDY(698) from the COOH terminus of the heavy chain of factor Va regulates the rate of cleavage of prothrombin at Arg(271) by prothrombinase. We have also recently demonstrated that amino acid region (334)DY(335) is required for the optimal activity of prothrombinase. To assess the effect of these six amino acid residues on cofactor activity, we created recombinant factor Va molecules combining mutations at amino acid regions 334-335 and 695-698 as follows: factor V(3K) ((334)DY(335) --> KF and (695)DYDY(698) --> KFKF), factor V(KF/4A) ((334)DY(335) --> KF and (695)DYDY(698) --> AAAA), and factor V(6A) ((334)DY(335) --> AA and (695)DYDY(698) --> AAAA). The recombinant factor V molecules were expressed and purified to homogeneity. Factor Va(3K), factor Va(K4/4A), and factor Va(6A) had reduced affinity for factor Xa, when compared to the affinity of the wild-type molecule (factor Va(Wt)) for the enzyme. Prothrombinase assembled with saturating concentrations of factor Va(3K) had a 6-fold reduced second-order rate constant for prothrombin activation compared to the value obtained with prothrombinase assembled with factor Va(Wt), while prothrombinase assembled with saturating concentrations of factor Va(KF/4A) and factor Va(6A) had approximately 1.5-fold reduced second-order rate constants. Overall, the data demonstrate that amino acid region 334-335 together with amino acid region 695-698 from factor Va heavy chain are part of a cooperative mechanism within prothrombinase regulating cleavage and activation of prothrombin by factor Xa.

Anticoagulant mechanism of factor IX/factor X-binding protein isolated from the venom of Trimeresurus flavoviridis

Anticoagulant mechanism of the coagulation factor IX/factor X-binding protein (IX/X-bp) isolated from the venom of Trimeresurus flavoviridis was investigated. IX/X-bp had no effect on the amidase activity of factor Xa measured with a synthetic peptide substrate Boc-Leu-Gly-Arg-pNA. Prothrombin activation by factor Xa without cofactors, such as factor Va and phospholipids, was only slightly influenced by IX/X-bp. However, prothrombin activation by factor Xa in the presence of factor Va resulted in IX/X-bp inhibiting the increase of k(cat) of thrombin formation through inhibition of interaction between factor Xa and factor Va. IX/X-bp also inhibited the decrease of K(m) for thrombin formation through interaction with phospholipids. Thus, IX/X-bp appears to act as an anticoagulant protein by inhibiting the interaction between factor Xa and its cofactors in the prothrombinase complex by binding to the Gla domain of factor Xa.

Role of the acidic hirudin-like COOH-terminal amino acid region of factor Va heavy chain in the enhanced function of prothrombinase

Prothrombinase activates prothrombin through initial cleavage at Arg(320) followed by cleavage at Arg(271). This pathway is characterized by the generation of an enzymatically active, transient intermediate, meizothrombin, that has increased chromogenic substrate activity but poor clotting activity. The heavy chain of factor Va contains an acidic region at the COOH terminus (residues 680-709). We have shown that a pentapeptide from this region (DYDYQ) inhibits prothrombin activation by prothrombinase by inhibiting meizothrombin generation. To ascertain the function of these regions, we have created a mutant recombinant factor V molecule that is missing the last 30 amino acids from the heavy chain (factor V(Delta680-709)) and a mutant molecule with the (695)DYDY (698) --> AAAA substitutions (factor V(4A)). The clotting activities of both recombinant mutant factor Va molecules were impaired compared to the clotting activity of wild-type factor Va (factor Va (Wt)). Using an assay employing purified reagents, we found that prothrombinase assembled with factor Va(Delta680-709) displayed an approximately 39% increase in k cat, while prothrombinase assembled with factor Va(4A) exhibited an approximately 20% increase in k cat for the activation of prothrombin as compared to prothrombinase assembled with factor Va(Wt). Gel electrophoresis analyzing prothrombin activation by prothrombinase assembled with the mutant molecules revealed a delay in prothrombin activation with persistence of meizothrombin. Our data demonstrate that the COOH-terminal region of factor Va heavy chain is indeed crucial for coordinated prothrombin activation by prothrombinase because it regulates meizothrombin cleavage at Arg(271) and suggest that this portion of factor Va is partially responsible for the enhanced procoagulant function of prothrombinase.

A phosphatidylserine binding site in factor Va C1 domain regulates both assembly and activity of the prothrombinase complex

Tightly associated factor V(a) (FVa) and factor X(a) (FXa) serve as the essential prothrombin-activating complex that assembles on phosphatidylserine (PS)-containing platelet membranes during blood coagulation. We have previously shown that (1) a soluble form of PS (C6PS) triggers assembly of a fully active FVa-FXa complex in solution and (2) that 2 molecules of C6PS bind to FVa light chain with one occupying a site in the C2 domain. We expressed human factor V(a) (rFVa) with mutations in either the C1 domain (Y1956,L1957)A, the C2 domain (W2063,W2064)A, or both C domains (Y1956,L1957,W2063,W2064)A. Mutations in the C1 and C1-C2 domains of rFVa reduced the rate of activation of prothrombin to thrombin by FXa in the presence of 400 muM C6PS by 14 000- to 15 000-fold relative to either wild-type or C2 mutant factor rFVa. The K(d')s of FXa binding with rFVa (wild-type, C2 mutant, C1 mutant, and C1-C2 mutant) were 3, 4, 564, and 624 nM, respectively. Equilibrium dialysis experiments detected binding of 4, 3, and 2 molecules of C6PS to wild-type rFVa, C1-mutated, and C1,C2-mutated rFVa, respectively. Because FVa heavy chain binds 2 molecules of C6PS, we conclude that both C2 and C1 domains bind one C6PS, with binding to the C1 domain regulating prothrombinase complex assembly.

Citrate anticoagulation and the dynamics of thrombin generation

BACKGROUND

Sodium citrate has been used as an anticoagulant to stabilize blood and blood products for over 100 years, presumably by sequestering Ca(++) ions in vitro. Anticoagulation of blood without chelation can be achieved by inhibition of the contact pathway by corn trypsin inhibitor (CTI).

OBJECTIVE

To evaluate the influence of citrate anticoagulation on the performance of blood, platelet-rich and platelet-poor plasma assays.

METHODS

Blood was anticoagulated in three ways: by collection into citrate, CTI and citrate with CTI. Plasma was prepared using each anticoagulation regimen. Functional analyses included calibrated automated thrombography, thromboelastography, plasma clotting, the synthetic coagulation proteome and platelet aggregation. Coagulation reactions were initiated with tissue factor-phospholipid and Ca(++) (when indicated).

RESULTS

In all cases, citrate anticoagulation resulted in reaction dynamics significantly altered relative to blood or plasma stabilized with CTI alone. Subsequent experiments showed that calcium citrate itself impairs coagulation dynamics.

CONCLUSION

Coagulation analyses using blood that has been exposed to citrate and recalcified do not yield reliable depictions of the natural dynamics of blood coagulation processes.

The role of thrombin exosites I and II in the activation of human coagulation factor V

Human blood coagulation Factor V (FV) is a plasma protein with little procoagulant activity. Limited proteolysis at Arg(709), Arg(1018), and Arg(1545) by thrombin or Factor Xa (FXa) results in the generation of activated FV, which serves as a cofactor of FXa in prothrombin activation. Both thrombin exosites I and II have been reported to be involved in FV activation, but the relative importance of these regions in the individual cleavages remains unclear. To investigate the role of each exosite in FV activation, we have used recombinant FV molecules with only one of the three activation cleavage sites available, in combination with exosite I- or II-specific aptamers. In addition, structural requirements for exosite interactions located in the B-domain of FV were probed using FV B-domain deletion mutants and comparison with FV activating enzymes from the venom of Russell's viper (RVV-V) and of Levant's viper (LVV-V) known to activate FV by specific cleavage at Arg(1545). Our results indicate that thrombin exosite II is not involved in cleavage at Arg(709) and that both thrombin exosites are important for recognition and cleavage at Arg(1545). Efficient thrombin-catalyzed FV activation requires both the N- and C-terminal regions of the B-domain, whereas only the latter is required by RVV-V and LVV-V. This indicates that proteolysis of FV by thrombin at Arg(709), Arg(1018), and Arg(1545) show different cleavage requirements with respect to interactions mediated by thrombin exosites and areas that surround the respective cleavage sites. In addition, interactions between exosite I of thrombin and FV are primarily responsible for the different cleavage site specificity as compared with activation by RVV-V or LVV-V.

Structural similarities and functional diversity of eukaryotic discoidin-like domains

The discoidin domain is a approximately 150 amino acid motif common in both eukaryotic and prokaryotic proteins. It is found in a variety of extracellular, intracellular and transmembrane multidomain proteins characterized by a considerable functional diversity, mostly involved in developmental processes. The biological role of the domain depends on its interactions with different molecules, including growth factors, phospholipids and lipids, galactose or its derivatives, and collagen. The conservation of the motif, as well as the serious physiological consequences of discoidin domain disorders underscore the importance of the fold, while the ability to accommodate such an extraordinarily broad range of ligand molecules makes it a fascinating research target. In present review we characterize the distinctive features of discoidin domains and briefly outline the biological role of this module in various eukaryotic proteins.

Inhibitory sequences within the B-domain stabilize circulating factor V in an inactive state

Blood coagulation factor V circulates as a procofactor with little or no procoagulant activity. It is activated to factor Va by thrombin following proteolytic removal of a large central B-domain. Although this reaction is well studied, the mechanism by which bond cleavage and B-domain release facilitate the transition to the active cofactor state has not been defined. Here we show that deletion or substitution of specific B-domain sequences drives the expression of procoagulant function without the need for proteolytic processing. Conversion to the constitutively active cofactor state is related, at least in part, to a cluster of amino acids that is highly basic and well conserved across the vertebrate lineage. Our findings demonstrate that discrete sequences in the B-domain serve to stabilize the inactive procofactor state, with proteolysis primarily functioning to remove these inhibitory constraints. These unexpected results provide new insight into the mechanism of factor V activation.

From gamma-carboxy-glutamate to protein C

The story I shall recount started in 1969, when I was given the opportunity to join the Department of Clinical Chemistry at the University Hospital in Malmö. I had just finished medical school at the university in the neighboring town of Lund. Parallel to pursuing my medical studies I had spent some time in the Department of Biochemistry. I did not know much about biochemistry, but it was enough for me to realize that I wanted to do laboratory research rather than developing a clinical career. I was happy to accept an offer to start working in the laboratory, particularly as the head of the department, Professor Carl-Bertil Laurell, had an excellent reputation. As it turned out, I came to spend almost all of my professional life in the laboratory.

Factor V C2 domain contains a major thrombin-binding site responsible for thrombin-catalyzed factor V activation

Factor (F)V is converted into its active form, FVa, by limited proteolysis. Thrombin-catalyzed activation of FV is essential for its full cofactor activation. Previously, we reported that thrombin was bound to the C2 domain in the light chain of FVIII. As FV has a similar domain structure to FVIII, we focused on the FV C2 domain as a possible binding region for thrombin. Kinetic parameters, measured by surface plasmon resonance, revealed that the K(d) values of anhydro-thrombin for FV, FVa, and the FV C2 domain were 66, 240, and 670 nmol L(-1), respectively. FV activation was increased by approximately 9-fold by the addition of thrombin. In the presence of the FV C2 domain, this increase of the FV activation was inhibited. However, FV activation was not inhibited by the addition of the FVIII C2 domain. FV was cleaved into a 105-kDa heavy chain and a 71/74-kDa light chain by thrombin-catalyzed proteolysis at Arg709, Arg1018 and Arg1545. In the presence of the FV C2 domain, the cleavage was inhibited at all sites. Proteolysis was not affected by the addition of the FVIII C2 domain. These results indicated that the FV C2 domain contains a major binding site for thrombin and that this domain is necessary for the proteolysis at all cleavage sites. Furthermore, the present results also suggested that thrombin has an independent binding site for FV different from that for FVIII.

Interactions of the C2 domain of human factor V with a model membrane

Activated coagulation Factor V is an important cofactor of the coagulation cascade that catalyzes the formation of the prothrombinase complex on the surface of membranes rich in phosphatidyl-L-serine (PS). Here we report molecular dynamics simulations of the two crystallographic structures (the open and closed conformations) of domain C2 of coagulation Factor V (FaVC2). The calculations were performed in water (1.5 ns for each conformation) and in the presence of a neutral phospholipid bilayer model (POPE; 10 ns for each conformation) in order to describe the dynamics of the free (plasma circulating) and membrane bound forms of FaVC2. Water simulations confirmed the hypothesis that the plasma circulating form is in the closed conformation. In contrast, the membrane simulations showed that both conformations are energetically compatible with membrane binding. We have investigated the mechanism, the dynamics, and the energetics of the binding process. Our data are consistent with published estimates of the immersion depth of the aromatic residues (W26 and W27), and with mutagenesis studies involving specific residues located on the spikes at the bottom of the FaVC2 structure. Electrostatic interactions between the phospholipid head groups and hydrophilic residues at the bottom of the structure play a key role in the binding process by creating a large number of hydrogen bonds that anchor the protein to the membrane. The simulations identified a stable phospholipid binding pocket reminiscent of a previously suggested PS interaction site. Our structural data could contribute to the design of potential inhibitors able to disrupt membrane association.

pH-dependent association of factor VIII chains: enhancement of affinity at physiological pH by Cu2+

Reconstitution of factor VIII from isolated heavy chain (HC) and light chain (LC) shows pH-dependence. In the presence of Ca2+, up to 80% of native factor VIII activity was recovered over a wide range of pH. In contrast, affinity of HC and LC was maximal at pH 6.5-6.75 (Kd approximately 4 nM), whereas a Kd approximately 20 nM was observed at physiological pH (7.25). The effect of Cu2+ (0.5 microM total Cu2+) on maximal activity regenerated was negligible at pH 6.25-8.0. However, this level of Cu2+ increased the inter-chain affinity by approximately 5-fold at pH 7.25. This effect resulted from an approximately 1.5-fold increased association rate constant (k(on)) and an approximately 3-fold reduced dissociation rate constant (k(off)). High affinity (Kd=5.3 fM) of the factor VIII heterodimer for Cu2+ was estimated by increases in cofactor activity. No significant increase in inter-chain affinity was observed when either isolated chain was reacted with Cu2+ followed by addition of the complementary chain. Together, these results suggest that the protonation state of specific residues modulates inter-chain affinity. Furthermore, copper ion contributes to the maintenance of the heterodimer at physiologic pH by a mechanism consistent with bridging the two chains.

A1 subunit-mediated regulation of thrombin-activated factor VIII A2 subunit dissociation

Factor VIII (fVIII) is the plasma protein that is missing or deficient in hemophilia A. In contrast, elevated levels of fVIII are associated with an increased risk of arterial and venous thrombosis. fVIII is activated by thrombin to form a non-covalently linked A1/A2/A3-C1-C2 heterotrimer. At physiological concentrations, fVIIIa decays as a result of A2 subunit dissociation, which may help regulate the balance between hemostasis and thrombosis. A2 subunit dissociation is faster in human fVIIIa than in porcine fVIIIa, which may represent an evolutionary adaptation associated with the development of the upright posture and venous stasis in the lower extremities. To investigate the basis for the different decay kinetics of human and porcine fVIIIa, hybrid fVIII molecules representing all possible combinations of human and porcine A domains were isolated. The kinetics of fVIIIa decay were measured and fit to a model describing a reversible bimolecular reaction in which the dissociation rate constant, k, and dissociation constant, Kd, were the fitted parameters. Substitution of the porcine A1 domain into human fVIIIa produced a dissociation rate constant indistinguishable from porcine fVIIIa. Subsequently, substitution of the second cupredoxin-like A1 subdomain resulted in a dissociation rate constant similar to porcine fVIIIa, whereas substitution of the first cupredoxin-like A1 subdomain resulted in a dissociation rate constant intermediate between human and porcine fVIIIa. We propose that cupredoxin-like A1 subdomains in fVIII contain inter-species differences that are a result of selective pressure on the dissociation rate constant.

Coagulation factor V: a plethora of anticoagulant molecules

PURPOSE OF REVIEW

Thrombin is necessary for survival and is produced after activation of prothrombin by prothrombinase at the site of a vascular injury. While the enzyme component of prothrombinase alone, factor Xa, bound to a membrane surface can activate prothrombin, incorporation of the cofactor molecule, factor Va, into prothrombinase results in a five orders of magnitude increase in the catalytic efficiency of factor Xa that provides the physiologic pathway for thrombin generation. While the kinetic constants and the identity of peptide bonds cleaved in prothrombin to generate alpha-thrombin have been long established, the peptidyl portions of the factor Va molecule responsible for its interactions with factor Xa, prothrombin, and the lipid surface are still the subject of intense investigation. In this review, we summarize the current state of knowledge with respect to the interactions of the factor Va molecule with the various components of prothrombinase.

RECENT FINDINGS

Binding sites for factor Xa have been identified on both the heavy and light chains of factor Va. Two amino acid regions that interact with factor Xa have been delineated on the heavy chain of the cofactor. It has also been demonstrated that the carboxyl-terminal portion of the heavy chain of factor Va contains hirudin-like motifs and appears to be responsible for the interaction of factor Va with prothrombin. This region of the molecule is important for procofactor activation by thrombin as well as cofactor function. Finally, the membrane-binding site of factor Va is contributed by several elements of the light chain and involves both electrostatic and hydrophobic interactions.

SUMMARY

The absence or dysfunction of factor Va leads to hemorrhagic diseases while prolonged existence of the active cofactor species is associated with thrombosis. Thus, modulation of the incorporation of factor Va into prothrombinase in vivo by using synthetic peptides that have the potential to impair factor Va binding to any of the components of prothrombinase, will allow for control of the rate of thrombin generation at the site of vascular damage. As a consequence, a systematic definition of the regions of factor Va governing its incorporation within prothrombinase will provide the scaffold for the synthesis of potent anticoagulant molecules that could modulate thrombin formation and suppress excessive clotting in thrombotic individuals.

Mutation of hydrophobic residues in the factor Va C1 and C2 domains blocks membrane-dependent prothrombin activation

The binding of factor (FVa) to phosphatidylserine (PS) membranes regulates assembly of the prothrombinase complex. Two pairs of solvent-exposed amino acids, Tyr(1956)/Leu(1957) in the C1 domain and Trp(2063)/Trp(2064) in the C2 domain, each make significant contributions to the affinity of FVa for PS membranes, but individually neither pair of amino acids is required for prothrombinase assembly on 25% PS membranes. In this study we characterize a FVa mutant with alanine substitutions in both the C1 and C2 domains: (Y1956,L1957,W2063,W2064)A. We conclude that: (i) prothrombinase assembly on PS membranes requires Trp(2063)/Trp(2064) and/or Tyr(1956)/Leu(1957); (ii) combined mutation of Trp(2063)/Trp(2064) and Tyr(1956)/Leu(1957) results in only a modest 4-fold decrease in the rate of thrombin generation in the absence of membranes; (iii) the present data provide experimental support for the joint participation of the C1 and C2 domains in the binding of FVa to phospholipid membranes as suggested by the recently solved structure for FVai (A1/A3-C1-C2).

The Phosphatidylserine Binding Site of the Factor Va C2 Domain Accounts for Membrane Binding but Does Not Contribute to the Assembly or Activity of a Human Factor Xa−Factor Va Complex

Factors V(a) and X(a) (FV(a) and FX(a), respectively) assemble on phosphatidylserine (PS)-containing platelet membranes to form the essential "prothrombinase" complex of blood coagulation. The C-terminal domain (C2) of FV(a) (residues 2037-2196 in human FV(a)) contains a soluble phosphatidylserine (C6PS) binding pocket flanked by a pair of tryptophan residues, Trp(2063) and Trp(2064). Mutating these tryptophans abolishes FV(a) membrane binding. To address both the roles of these tryptophans in C6PS or membrane binding and the role of the C2 domain lipid binding site in regulation of FV(a) cofactor activity, we expressed W(2063,2064)A mutants of the recombinant C2 domain (rFV(a2)-C2) and of a B domain-deleted factor V light isoform (rFV(a2)) in Hi-5 and COS cells, respectively. Intrinsic fluorescence showed that wild-type rFV(a2)-C2 binds to C6PS and to 20% PS/PC membranes with apparent K(d) values of 2.8 microM and 9 nM, respectively, while mutant rFV(a2)-C2 does not. Equilibrium dialysis confirmed that mutant rFV(a2)-C2 does not bind to C6PS. Mutant rFV(a2) binds to C6PS (K(d) approximately 37 microM) with an affinity comparable to that of wild-type rFV(a2) (K(d) approximately 20 microM), although it does not bind to PS/PC membranes to which wild-type rFV(a2) binds with native affinity (K(d) approximately 3 nM). Both wild-type and mutant rFV(a2) bind to active site-labeled FX(a) (DEGR-X(a)) in the presence of 400 microM C6PS with native affinity (K(d) approximately 3-4 nM) to produce a solution rFV(a2)-FX(a) complex of native activity. We conclude that (1) the C2 domain PS site provides all but approximately 1 kT of the free energy of FV(a) membrane binding, (2) tryptophans lining the C2 lipid binding pocket are critical to C6PS and membrane binding and insert into the bilayer interface during membrane binding, (3) occupancy of the C2 lipid binding pocket is not necessary for C6PS-induced formation of the FX(a)-FV(a) complex or its activity, but (4) another PS site on FV(a) does have a regulatory role.

The crystal structure of activated protein C-inactivated bovine factor Va: Implications for cofactor function

In vertebrate hemostasis, factor Va serves as the cofactor in the prothrombinase complex that results in a 300,000-fold increase in the rate of thrombin generation compared with factor Xa alone. Structurally, little is known about the mechanism by which factor Va alters catalysis within this complex. Here, we report a crystal structure of protein C inactivated factor Va (A1.A3-C1-C2) that depicts a previously uncharacterized domain arrangement. This orientation has implications for binding to membranes essential for function. A high-affinity calcium-binding site and a copper-binding site have both been identified. Surprisingly, neither shows a direct involvement in chain association. This structure represents the largest physiologically relevant fragment of factor Va solved to date and provides a new scaffold for the future generation of models of coagulation cofactors.