Molecular recognition sites on factor Xa which participate in the prothrombinase complex

Blood coagulation factor Va's key interactive residues and regions for prothrombinase assembly and prothrombin binding

Blood coagulation factor Va serves an indispensable role in hemostasis as cofactor for the serine protease factor Xa. In the presence of an anionic phospholipid membrane and calcium ions, factors Va and Xa assemble into the prothrombinase complex. Following formation of the ternary complex with the macromolecular zymogen substrate prothrombin, the latter is rapidly converted into thrombin, the key regulatory enzyme of coagulation. Over the years, multiple binding sites have been identified in factor Va that play a role in the interaction of the cofactor with factor Xa, prothrombin, or the anionic phospholipid membrane surface. In this review, an overview of the currently available information on these interactive sites in factor Va is provided, and data from biochemical approaches and 3D structural protein complex models are discussed. The structural models have been generated in recent years and provide novel insights into the molecular requirements for assembly of both the prothrombinase and the ternary prothrombinase-prothrombin complexes. Integrated knowledge of functionally important regions in factor Va will allow for a better understanding of factor Va cofactor activity.

Asymmetric processing of mutant factor X Arg386Cys reveals differences between intrinsic and extrinsic pathway activation

Alterations in coagulation factor X (FX) activation, mediated by the extrinsic VIIa/tissue factor (FVIIa/TF) or the intrinsic factor IXa/factor VIIIa (FIXa/FVIIIa) complexes, can result in hemorrhagic/prothrombotic tendencies. However, the molecular determinants involved in substrate recognition by these enzymes are poorly defined. Here, we investigated the role of arginine 386 (chymotrypsin numbering c202), a surface-exposed residue on the FX catalytic domain. The naturally occurring FX386Cys mutant and FX386Ala variant were characterized. Despite the unpaired cysteine, recombinant (r)FX386Cys was efficiently secreted (88.6±21.3% of rFXwt) and possessed normal clearance in mice. rFX386Cys was also normally activated by FVIIa/TF and displayed intact amidolytic activity. In contrast, rFX386Cys activation by the FIXa/FVIIIa complex was 4.5-fold reduced, which was driven by a decrease in the kcat (1.6∗10(-4) s(-1) vs 5.8∗10(-4) s(-1), rFXwt). The virtually unaltered Km (70.6 nM vs 55.6nM, rFXwt) suggested no major alterations in the FX substrate exosite. Functional assays in plasma supplemented with rFX386Cys indicated a remarkable reduction in the thrombin generation rate and thus in coagulation efficiency. Consistently, the rFX386Ala variant displayed similar biochemical features suggesting that global changes at position 386 impact the intrinsic pathway activation. These data indicate that the FXArg386 is involved in FIXa/FVIIIa-mediated FX activation and help in elucidating the bleeding tendency associated with the FX386Cys in a rare FX deficiency case. Taking advantage of the unpaired cysteine, the rFX386Cys mutant may be efficiently targeted by thiol-specific ligands and represent a valuable tool to study FX structure-function relationships both in vitro and in vivo.

The hemostatic system as a regulator of inflammation in atherosclerosis

Atherosclerosis is a chronic inflammatory disease of the arterial vessel wall. As part of a tightly connected cross-talk between inflammation and coagulation, there is growing evidence that the coagulation system plays a pivotal role in the development and progression of atherosclerosis. We here discuss the presence of coagulation factors in atherosclerotic lesions and the overall effects of hypercoagulability and hypocoagulability on atherosclerotic lesion formation. Moreover, we focus on the unifying common pathway of coagulation, which can be initiated by the intrinsic and extrinsic pathway of coagulation, and discuss the functions of the coagulation factors FX, thrombin, and FXIII as regulators of inflammation in atherosclerosis. In particular, we review the non-hemostatic and immune-modulatory functions of these factors in endothelial and smooth muscle cells, as well as monocytes/macrophages, but also other cells, such as dendritic cells and T cells, that may control the inflammatory process of atherosclerosis. Their multiple roles in coagulation, but also their non-hemostatic functions in different cell types in inflammation and immunity, may harbor great potential for the development of novel therapeutic approaches for treating cardiovascular disease.

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.

An anticoagulant RNA aptamer that inhibits proteinase-cofactor interactions within prothrombinase

The interaction of factor Xa with factor Va on membranes to form prothrombinase profoundly increases the rate of the proteolytic conversion of prothrombin to thrombin. We present the characterization of an RNA aptamer (RNA(11F7t)) selected from a combinatorial library based on its ability to bind factor Xa. We show that RNA(11F7t) inhibits thrombin formation catalyzed by prothrombinase without obscuring the active site of Xa within the enzyme complex. Selective inhibition of protein substrate cleavage arises from the ability of the aptamer to bind to factor Xa and exclude interactions between the proteinase and cofactor within prothrombinase. Competition for enzyme complex assembly results from the binding of RNA(11F7t) to factor Xa with nanomolar affinity in a Ca(2+)-dependent interaction. RNA(11F7t) binds equivalently to the zymogen factor X as well as derivatives lacking gamma-carboxyglutamic acid residues. We suggest that the ability of RNA(11F7t) to compete for the Xa-Va interaction with surprisingly high affinity likely reflects a significant contribution from its ability to indirectly impact regions of Xa that participate in the proteinase-cofactor interaction. Thus, despite the complexity of the macromolecular interactions that underlie the assembly of prothrombinase, efficient inhibition of enzyme complex assembly and thrombin formation can be achieved by tight binding ligands that target factor Xa in a discrete manner.

Membrane-dependent interaction of factor Xa and prothrombin with factor Va in the prothrombinase complex

Because all three protein components of prothrombinase, factors (f) Xa and Va and prothrombin, bind to negatively charged membrane phospholipids, the exact role of the membrane in the prothrombinase reaction has not been fully understood. In this study, we prepared deletion derivatives of fXa and prothrombin in which both the Gla and first EGF-like domains of the protease (E2-fXa) as well as the Gla and both kringle domains of the substrate (prethrombin-2) had been deleted. The fVa-mediated catalytic activity of E2-fXa toward prethrombin-2 was analyzed in both the absence and presence of phospholipids composed of 80% phosphatidylcholine (PC) and 20% phosphatidylserine (PS). PCPS markedly accelerated the initial rate of prethrombin-2 activation by E2-fXa, with the cofactor exhibiting saturation only in the presence of phospholipids (apparent K(d) of approximately 60 nM). Competitive kinetic studies in the presence of the two exosite-1-specific ligands Tyr(63)-sulfated hirudin(54-65) and TM456 suggested that while both peptides are highly effective inhibitors of the fVa-mediated activation of prethrombin-2 by E2-fXa in the absence of PCPS, they are ineffective competitors in the presence of phospholipids. Since neither E2-fXa nor prethrombin-2 can interact with membranes, these results suggest that interaction of fVa with PCPS improves the affinity of the activation complex for proexosite-1 of the substrate. Direct binding studies employing OG(488)-EGR-labeled fXa and E2-fXa revealed that the interaction of the Gla domain of fXa with PCPS also induces conformational changes in the protease to facilitate its high-affinity interaction with fVa.

Functional and structural characterization of factor Xa dimer in solution

Previous studies showed that binding of water-soluble phosphatidylserine (C6PS) to bovine factor Xa (FXa) leads to Ca2+-dependent dimerization in solution. We report the effects of Ca2+, C6PS, and dimerization on the activity and structure of human and bovine FXa. Both human and bovine dimers are 10(6)- to 10(7)-fold less active toward prothrombin than the monomer, with the decrease being attributed mainly to a substantial decrease in k(cat). Dimerization appears not to block the active site, since amidolytic activity toward a synthetic substrate is largely unaffected. Circular dichroism reveals a substantial change in tertiary or quaternary structure with a concomitant decrease in alpha-helix upon dimerization. Mass spectrometry identifies a lysine (K(270)) in the catalytic domain that appears to be buried at the dimer interface and is part of a synthetic peptide sequence reported to interfere with factor Va (FVa) binding. C6PS binding exposes K(351) (part of a reported FVa binding region), K(242) (adjacent to the catalytic triad), and K(420) (part of a substrate exosite). We interpret our results to mean that C6PS-induced dimerization produces substantial conformational changes or domain rearrangements such that structural data on PS-activated FXa is required to understand the structure of the FXa dimer or the FXa-FVa complex.

Proposed structural models of the prothrombinase (FXa-FVa) complex

Activated coagulation factor V (FVa) functions as a cofactor to factor Xa (FXa) in the conversion of prothrombin (PT) to thrombin. This essential procoagulant reaction, despite being the subject of extensive investigation, is not fully understood structurally and functionally. To elucidate the structure of the FXa-FVa complex, we have performed protein:protein (Pr:Pr) docking simulation with the pseudo-Brownian Pr:Pr docking ICM package and with the shape-complementarity Pr:Pr docking program PPD. The docking runs were carried out using a new model of full-length human FVa and the X-ray structure of human FXa. Five representative models of the FXa-FVa complex were in overall agreement with some of the available experimental data, but only one model was found to be consistent with almost all of the reported experimental results. The use of hybrid docking approach (theoretical plus experimental) is definitively important to study such large macromolecular complexes. The FXa-FVa model we have created will be instrumental for further investigation of this macromolecular system and will guide future site directed mutagenesis experiments.

Ixolaris: a factor Xa heparin-binding exosite inhibitor

Ixolaris is a two-Kunitz TFPI (tissue factor pathway inhibitor) from the tick salivary gland. In contrast with human TFPI, Ixolaris binds tightly to the zymogen FX (Factor X) and to dansyl-Glu-Gly-Arg-chloromethyl ketone-treated FXa (DEGR-FXa; active-site-blocked FXa), indicating that exosites are involved in the FX(a)-Ixolaris interaction. Here we provide evidence that Ixolaris binds specifically to the FXa HBE (heparin-binding exosite), since (i) it markedly decreases the inhibition of FXa by the antithrombin-heparin but not the antithrombin-pentasaccharide complex, (ii) it impairs FXa binding to Sepharose-immobilized heparin, and (iii) it allosterically modulates the catalytic activity of FXa for small chromogenic substrates (S-2765). By using a series of recombinant FXa mutants in which the HBE is mutated, we have identified the importance of amino acids involved in the enzyme-inhibitor interaction as being in the following order: Arg-93>>Arg-165> or =Lys-169>Lys-236>Lys-96>Arg-240>Arg-125. Ixolaris at appropriate concentrations also inhibits thrombin formation in vitro by the assembled prothrombinase complex, a process that is critically dependent on the FXa HBE. Ixolaris is the first inhibitor characterized to date that binds specifically to the FXa HBE.

Comparative Analysis of Prothrombin Activators from the Venom of Australian Elapids

A key component of the venom of many Australian snakes belonging to the elapid family is a toxin that is structurally and functionally similar to that of the mammalian prothrombinase complex. In mammals, this complex is responsible for the cleavage of prothrombin to thrombin and is composed of factor Xa in association with its cofactors calcium, phospholipids, and factor Va. The snake prothrombin activators have been classified on the basis of their requirement for cofactors for activity. The two major subgroups described in Australian elapid snakes, groups C and D, are differentiated by their requirement for mammalian coagulation factor Va. In this study, we describe the cloning, characterization, and comparative analysis of the factor X- and factor V-like components of the prothrombin activators from the venom glands of snakes possessing either group C or D prothrombin activators. The overall domain arrangement in these proteins was highly conserved between all elapids and with the corresponding mammalian clotting factors. The deduced protein sequence for the factor X-like protease precursor, identified in elapids containing either group C or D prothrombin activators, demonstrated a remarkable degree of relatedness to each other (80%-97%). The factor V-like component of the prothrombin activator, present only in snakes containing group C complexes, also showed a very high degree of homology (96%-98%). Expression of both the factor X- and factor V-like proteins determined by immunoblotting provided an additional means of separating these two groups at the molecular level. The molecular phylogenetic analysis described here represents a new approach for distinguishing group C and D snake prothrombin activators and correlates well with previous classifications.

The intriguing world of prothrombin activators from snake venom

Activation of prothrombin to mature thrombin occurs by the proteolytic action of the prothrombinase complex consisting of a serine proteinase factor Xa, and cofactors factor Va, Ca(2+) ions and phospholipids. Several exogenous prothrombin activators are found in snake venom. They are classified into four groups based on their cofactor requirements. Group A and B prothrombin activators are metalloproteinases whereas group C and D prothrombin activators are serine proteinases. Group C prothrombin activators resemble the mammalian factor Xa-factor Va complex, while Group D activators are structurally and functionally similar to factor Xa. This review provides a detailed description of the current knowledge on all prothrombin activators and highlights several intriguing questions that are yet to be answered.

Genetic analysis of hereditary factor X deficiency in a French patient of Sri Lankan ancestry

We investigated a new family with cross-reactive material-positive factor X (FX) deficiency. The proband was an 11-year-old French girl from Sri Lanka with a tendency towards severe bleeding. The FX antigen level was 67%, although the activity with extrinsic pathway was 1 U/dl. The complete nucleotide sequences of all exons and exon/intron junctions of the patient's genomic DNA revealed a homozygous G <-- A substitution in exon 8, which would result in replacement of Gly366 with Ser. The proband is the first reported case of homozygote for the FX Gly366Ser mutation. Heterozygosity for Gly366Ser substitution was previously reported in a Japanese patient (FX Nagoya 2). We studied the functional consequences by expressing mutant FX Gly366Ser protein in HEK293 cells. FX Gly366Ser was secreted into the culture media at levels similar to wild-type FX; however, mutant FX activities were only 0.04, 1.05, and 0.75% of wild-type FX upon activation by the extrinsic system, the intrinsic system, and Russell's viper venom, respectively. Moreover, the activity of FX Gly366Ser was undetectable when analyzed with chromogenic-activated FX and thrombin generation assays. These data suggest that the Gly366Ser substitution would cause a major defect in function of the FX molecule.

Identification of distinct sequences in human blood coagulation factor Xa and prothrombin essential for substrate and cofactor recognition in the prothrombinase complex

To identify amino acid sequences in factor Xa (fXa) and prothrombin (fII) that may be involved in prothrombinase complex (fXa.factor Va.fII.phospholipids) assembly, synthetic peptides based on fXa and fII sequences were prepared and screened for their ability to inhibit fXa-induced clotting of normal plasma. One fII peptide (PT557-571 homologous to chymotrypsin (CHT) residues 225-239) and two fXa peptides (X404-418, CHT231-244, and X415-429, CHT241-252C) potently inhibited plasma clotting and prothrombinase activity with 50% inhibition between 41 and 115 microM peptide. Inhibition of prothrombinase by PT557-571 and X415-429 was fVa-independent, whereas the inhibition by X404-418 was fVa-dependent. X404-418 inhibited the binding of fVa to fluorescein-labeled, inhibited fXai in the presence of phosphatidylcholine/phosphatidylserine vesicles, whereas X415-429 inhibited binding of fII to phospholipid-bound fluorescein-labeled, inhibited fXai. PT557-571 altered the fluorescence emission of fluorescein-labeled fXai, showing that PT557-571 binds to fXai. These data suggest that residues 404-418 in fXa provide fVa binding sites, whereas residues 557-571 in fII and 415-429 in fXa mediate interactions between fXa and fII in the prothrombinase complex.

Structure and dynamics of zymogen human blood coagulation factor X

The solution structure and dynamics of the human coagulation factor X (FX) have been investigated to understand the key structural elements in the zymogenic form that participates in the activation process. The model was constructed based on the 2.3-A-resolution x-ray crystallographic structure of active-site inhibited human FXa (PDB:1XKA). The missing gamma-carboxyglutamic acid (GLA) and part of epidermal growth factor 1 (EGF1) domains of the light chain were modeled based on the template of GLA-EGF1 domains of the tissue factor (TF)-bound FVIIa structure (PDB:1DAN). The activation peptide and other missing segments of FX were introduced using homology modeling. The full calcium-bound model of FX was subjected to 6.2 ns of molecular dynamics simulation in aqueous medium using the AMBER6.0 package. We observed significant reorientation of the serine-protease (SP) domain upon activation leading to a compact multi-domain structure. The solution structure of zymogen appears to be in a well-extended conformation with the distance between the calcium ions in the GLA domain and the catalytic residues estimated to be approximately 95 A in contrast to approximately 83 A in the activated form. The latter is in close agreement with fluorescence studies on FXa. The S1-specificity residues near the catalytic triad show significant differences between the zymogen and activated structures.

Binding of Plasminogen and Tissue Plasminogen Activator to Plasmin-Modulated Factor X and Factor Xa

Previous work in our laboratory has suggested that the fibrinolytic enzyme plasmin (Pn) inactivates coagulation factors X (FX) and Xa (FXa) in the presence of Ca(2+) and anionic phospholipid (aPL), producing fragments which bind plasminogen (Pg) and accelerate tissue plasminogen activator (t-PA). Our goals here were to determine if the Pn-mediated fragments of FX or FXa remain associated, whether they directly bind t-PA, and to quantify their interaction with Pg. Binding to aPL, benzamidine-Sepharose, or the active-site inhibitor dansyl-Glu-Gly-Arg-chloromethyl ketone demonstrated that Pn cleavage yielded noncovalent heterodimers of a fragment containing the aPL-binding domain (FXgamma(47) or FXagamma(33)) and a 13-kDa fragment (FXgamma(13) or FXagamma(13)). Both ligand blotting and surface plasmon resonance (SPR) showed that Pn-cleaved FX and FXa bound t-PA directly when Pn-treatment was effected in the presence of aPL and Ca(2+). Using SPR, apparent K(d) values of 1-3 microM and 0.3-0.4 microM were measured directly and by competition for the FXgamma(47/13)-Pg and FXagamma(33/13)-Pg interactions, respectively. For the first time, Pg-binding to a receptor was shown to be Ca(2+) enhanced, although primarily mediated by C-terminal lysine residues. Mathematical modeling of kinetic data suggesting two Pg per FXgamma(47/13) or FXagamma(33/13) was consistent with our conclusion that each subunit of FXgamma(47/13) or FXagamma(33/13) contains a C-terminal lysine. Earlier X-ray structures show that these Lys residues are distal from each other and the membrane, supporting the model where each interacts with a separate Pg. t-PA acceleration by FXgamma(47/13) or FXagamma(33/13) may therefore involve simultaneous presentation of two substrate molecules.

Definition of a factor Va binding site in factor Xa

We reported previously that residue 347 in activated fX (fXa) contributes to binding of the cofactor, factor Va (fVa) (Rudolph, A. E., Porche-Sorbet, R. and Miletich, J. P. (2000) Biochemistry 39, 2861-2867). Four additional residues that participate in fVa binding have now been identified by mutagenesis. All five resulting fX species, fX(R306A), fX(E310N), fX(R347N), fX(K351A), and fX(K414A), are activated and inhibited normally. However, the rate of inhibition by antithrombin III in the presence of submaximal concentrations of heparin is reduced for all the enzymes. In the absence of fVa, all of the enzymes bind and activate prothrombin similarly except fXa(E310N), which has a reduced apparent affinity ( approximately 3-fold) for prothrombin compared with wild type fXa (fXa(WT)). In the absence of phospholipid, fVa enhances the catalytic activity of fXa(WT) significantly, but the response of the variant enzymes was greatly diminished. On addition of 100 nm PC:PS (3:1) vesicles, fVa enhanced fXa(WT), fXa(R306A), and fXa(E310N) similarly, whereas fXa(R347N), fXa(K351A), and fXa(K414A) demonstrated near-normal catalytic activity but reduced apparent affinity for fVa under these conditions. All enzymes function similarly to fXa(WT) on activated platelets, which provide saturating fVa on an ideal surface. Loss of binding affinity for fVa as a result of the substitutions in residues Arg-347, Lys-351, and Lys-414 was verified by a competition binding assay. Thus, Arg-347, Lys-351, and Lys-414 are likely part of a core fVa binding site, whereas Arg-306 and Glu-310 serve a less critical role.

A dysfunctional factor X (factor X San Giovanni Rotondo) present at homozygous and double heterozygous level: identification of a novel microdeletion (delC556) and missense mutation (Lys(408)-->Asn) in the factor X gene. A study of an Italian family

Low levels of factor X (F.X) were detected in a 4-year-old boy who experienced acute lymphoblastic leukemia and bleeding manifestations. Laboratory data suggested the presence of a dysfunctional F.X molecule. Two novel F.X gene mutations were identified in the proband that was double heterozygous for both: a microdeletion (delC556) in exon VI resulting in a frameshift leading to a termination codon at position 226. This deletion was found in six family members with reduced F.X antigen and activity levels. A second mutation characterised by a G(1344)-->C transversion in exon VIII was detected in the proband resulting in a Lys(408)-->Asn substitution. This latter mutation was present in several asymptomatic family members from the paternal and the maternal side. The proband's sister was homozygous for the Lys(408)-->Asn substitution and exhibited low F.X activity with a normal antigen level. The naturally occurring F.X Lys(408)-->Asn (F.X(K408N)) variant was isolated from plasma of either homozygous or double heterozygous individuals. NH(2)-terminal sequencing of the heavy chain of F.X(K408N) failed to show any sequence abnormality in patients who were also carriers of the delC556, suggesting that this latter lesion accounted for the lack of F.X synthesis. Purified F.X Lys(408)-->Asn had an identical behaviour to normal F.X as judged by SDS-PAGE and immunoblotting. Clotting assay using purified F.X(K408N) and F.X-deficient plasma resulted in a laboratory phenotype similar to that observed in a homozygous subject for F.X Lys(408)-->Asn substitution. This is the first characterisation of a naturally occurring F.X variant with a mutation at the COOH-terminal end of the molecule.

Identification of basic residues in the heparin-binding exosite of factor Xa critical for heparin and factor Va binding

We recently demonstrated that a template mechanism makes a significant contribution to the heparin-accelerated inactivation of factor Xa (FXa) by antithrombin at physiologic Ca(2+), suggesting that FXa has a potential heparin-binding site. Structural data indicate that 7 of the 11 basic residues of the heparin-binding exosite of thrombin are conserved at similar three-dimensional locations in FXa. These residues, Arg(93), Lys(96), Arg(125), Arg(165), Lys(169), Lys(236), and Arg(240) were substituted with Ala in separate constructs in Gla domainless forms. It was found that all derivatives cleave Spectrozyme FXa with similar catalytic efficiencies. Antithrombin inactivated FXa derivatives with a similar second-order association rate constant (k(2)) in both the absence and presence of pentasaccharide. In the presence of heparin, however, k(2) with certain mutants were impaired up to 25-fold. Moreover, these mutants bound to heparin-Sepharose with lower affinities. Heparin concentration dependence of the inactivation revealed that only the template portion of the cofactor effect of heparin was affected by the mutagenesis. The order of importance of these residues for binding heparin was as follows: Arg(240) > Lys(236) > Lys(169) > Arg(165) > Lys(96) > Arg(93) >/= Arg(125). Interestingly, further study suggested that certain basic residues of this site, particularly Arg(165) and Lys(169), play key roles in factor Va and/or prothrombin recognition by FXa in prothrombinase.

C-terminal residues 621-635 of protein S are essential for binding to factor Va

Protein S is anticoagulant in the absence of activated protein C because of direct interactions with coagulation Factors Xa and Va. Synthetic peptides corresponding to amino acid sequences of protein S were tested for their ability to inhibit prothrombinase activity. The peptide containing the C-terminal sequence of protein S, residues 621-635 (PSP14), reversibly inhibited prothrombinase activity in the presence but not in the absence of Factor Va (K(i) approximately 2 microM). PSP14 inhibition of prothrombinase was independent of phospholipids but could be competitively overcome by increasing Factor Xa concentrations, suggesting that the C-terminal region of protein S may compete for a Factor Xa binding site on Factor Va. Studies using peptides with amino acid substitutions suggested that lysines 630, 631, and 633 were critical residues. PSP14 inhibited Factor Va activity in Factor Xa-one-stage clotting assays. PSP14 inhibited protein S binding to immobilized Factor Va. When preincubated with protein S, antibodies raised against PSP14 inhibited binding of protein S to Factor Va and blocked inhibition of prothrombinase activity by protein S. These results show that the C-terminal region of protein S containing residues 621-635 is essential for binding of protein S to Factor Va and that this interaction contributes to anticoagulant action.

Surface loop 199-204 in blood coagulation factor IX is a cofactor-dependent site involved in macromolecular substrate interaction

In factor IX residues 199-204 encompass one of six surface loops bordering its substrate-binding groove. To investigate the contribution of this loop to human factor IX function, a series of chimeric factor IX variants was constructed, in which residues 199-204 were replaced by the corresponding sequence of factor VII, factor X, or prothrombin. The immunopurified and activated chimeras were indistinguishable from normal factor IXa in hydrolyzing a small synthetic substrate, indicating that this region is not involved in the interaction with substrate residues on the N-terminal side of the scissile bond. In contrast, replacement of loop 199-204 resulted in a 5-25-fold reduction in reactivity toward the macromolecular substrate factor X. This reduction was due to a combination of increased K(m) and reduced k(cat). In the presence of factor VIIIa the impaired reactivity toward factor X was largely restored for all factor IXa variants, resulting in a more pronounced stimulation by factor VIIIa compared with normal factor IXa (3 to 5 x 10(4)-fold versus 5 x 10(3)-fold). Inhibition by antithrombin was only slightly affected for the factor IXa variant with the prothrombin loop sequence, whereas factor IXa variants containing the analogous residues of factor VII or factor X were virtually insensitive to antithrombin inhibition. In the presence of heparin, however, all chimeric factor IXa variants formed complexes with antithrombin. Thus the cofactors heparin and factor VIIIa have in common that they both alleviate the deleterious effects of mutations in the factor IX loop 199-204. Collectively, our data demonstrate that loop 199-204 plays an important role in the interaction of factor IXa with macromolecular substrates.

Amino Acid Sequence of Trocarin, a Prothrombin Activator FromTropidechis carinatus Venom: Its Structural Similarity to Coagulation Factor Xa

Among snake venom procoagulant proteins, group II prothrombin activators are functionally similar to blood coagulation factor Xa. We have purified and partially characterized the enzymatic properties of trocarin, the group II prothrombin activator from the venom of the Australian elapid, Tropidechis carinatus (rough-scaled snake). Prothrombin activation by trocarin is enhanced by Ca2+, phospholipids, and factor Va, similar to that by factor Xa. However, its amidolytic activity on peptide substrate S-2222 is significantly lower. We have determined the complete amino acid sequence of trocarin. It is a 46,515-Dalton glycoprotein highly homologous to factor Xa and shares the same domain architecture. The light chain possesses an N-terminal Gla domain containing 11 γ-carboxyglutamic acid residues, followed by two epidermal growth factor (EGF)-like domains; the heavy chain is a serine proteinase. Both chains are likely glycosylated: the light chain at Ser 52 and the heavy chain at Asn 45. Unlike other types of venom procoagulants, trocarin is the first true structural homologue of a coagulation factor. It clots snake plasma and thus may be similar, if not identical, to snake blood coagulation factor Xa. Unlike blood factor Xa, it is expressed in high quantities and in a nonhepatic tissue, making snake venom the richest source of factor Xa-like proteins. It induces cyanosis and death in mice at 1 mg/kg body weight. Thus, trocarin acts as a toxin in venom and a similar, if not identical, protein plays a critical role in hemostasis.

Amino Acid Sequence of Trocarin, a Prothrombin Activator FromTropidechis carinatus Venom: Its Structural Similarity to Coagulation Factor Xa

Among snake venom procoagulant proteins, group II prothrombin activators are functionally similar to blood coagulation factor Xa. We have purified and partially characterized the enzymatic properties of trocarin, the group II prothrombin activator from the venom of the Australian elapid, Tropidechis carinatus (rough-scaled snake). Prothrombin activation by trocarin is enhanced by Ca2+, phospholipids, and factor Va, similar to that by factor Xa. However, its amidolytic activity on peptide substrate S-2222 is significantly lower. We have determined the complete amino acid sequence of trocarin. It is a 46,515-Dalton glycoprotein highly homologous to factor Xa and shares the same domain architecture. The light chain possesses an N-terminal Gla domain containing 11 γ-carboxyglutamic acid residues, followed by two epidermal growth factor (EGF)-like domains; the heavy chain is a serine proteinase. Both chains are likely glycosylated: the light chain at Ser 52 and the heavy chain at Asn 45. Unlike other types of venom procoagulants, trocarin is the first true structural homologue of a coagulation factor. It clots snake plasma and thus may be similar, if not identical, to snake blood coagulation factor Xa. Unlike blood factor Xa, it is expressed in high quantities and in a nonhepatic tissue, making snake venom the richest source of factor Xa-like proteins. It induces cyanosis and death in mice at 1 mg/kg body weight. Thus, trocarin acts as a toxin in venom and a similar, if not identical, protein plays a critical role in hemostasis.

Binding site for blood coagulation factor Xa involving residues 311-325 in factor Va

Factor Va inactivation by activated protein C is associated with cleavages at Arg306, Arg506, and Arg679 with Arg306 cleavage causing the major activity loss. To study functional roles of the Arg306 region, overlapping 15-mer peptides representing the sequence of factor Va residues 271-345 were synthesized and screened for anticoagulant activities. The peptide containing residues 311-325 (VP311) noncompetitively inhibited prothrombin activation by factor Xa, but only in the presence of factor Va. Fluorescence studies showed that VP311 bound to fluorescence-labeled 5-dimethylaminonaphthalene-1-sulfonyl-Glu-Gly-Arg factor Xa in solution with a Kd of 70 microM. Diisopropylphosphoryl factor Xa and factor Xa but not factor VII/VIIa or prothrombin bound to immobilized VP311 with relatively high affinity. These results support the hypothesis that residues 311-325, which are positioned between the A1 and A2 domains of factor Va and likely exposed to solvent, contribute to the binding of factor Xa by factor Va. Based on this hypothesis, it is suggested that cleavage by activated protein C at Arg306 in factor Va not only severs the covalent connection between the A1 and A2 domains but also disrupts the environment and structure of residues 311-325, thereby down-regulating the binding of factor Xa to factor Va.

alpha-enolase, a novel strong plasmin(ogen) binding protein on the surface of pathogenic streptococci

The plasmin(ogen) binding property of group A streptococci is incriminated in tissue invasion processes. We have characterized a novel 45-kDa protein displaying strong plasmin(ogen) binding activity from the streptococcal surface. Based on its biochemical properties, we confirmed the identity of this protein as alpha-enolase, a key glycolytic enzyme. Dose-dependent alpha-enolase activity, immune electron microscopy of whole streptococci using specific antibodies, and the opsonic nature of polyclonal and monoclonal antibodies concluded the presence of this protein on the streptococcal surface. We, henceforth, termed the 45-kDa protein, SEN (streptococcal surface enolase). SEN is found ubiquitously on the surface of most streptococcal groups and serotypes and showed significantly greater plasmin(ogen) binding affinity compared with previously reported streptococcal plasminogen binding proteins. Both the C-terminal lysine residue of SEN and a region N-terminal to it play a critical role in plasminogen binding. Results from competitive plasminogen binding inhibition assays and cross-linking studies with intact streptococci indicate that SEN contributes significantly to the overall streptococcal ability to bind plasmin(ogen). Our findings, showing both the protected protease activity of SEN-bound plasmin and SEN-specific immune responses, provide evidence for an important role of SEN in the disease process and post-streptococcal autoimmune diseases.

Interaction of calcium with native and decarboxylated human factor X. Effect of proteolysis in the autolysis loop on catalytic efficiency and factor Va binding

Human factor X is a two-chain, 58-kDa, vitamin K-dependent blood coagulation zymogen. The light chain of factor X consists of an NH2-terminal gamma-carboxyglutamic acid (Gla) domain, followed by a few helical hydrophobic residues and the two epidermal growth factor-like domains, whereas the heavy chain contains the serine protease domain. In this study, native factor X was found to contain three classes of Ca2+-binding sites: two high affinity (Kd 100 +/- 30 microM), four intermediate affinity (Kd 450 +/- 70 microM), and five to six low affinity (Kd 2 +/- 0.2 mM). Decarboxylated factor X in which the Gla residues were converted to Glu retained the two high affinity sites (Kd 140 +/- 20 microM). In contrast, factor X lacking the Gla domain as well as a part of the helical hydrophobic residues (des-44-X) retained only one high affinity Ca2+-binding site (Kd 130 +/- 20 microM). Moreover, a synthetic peptide composed of residues 238-277 (58-97 in chymotrypsinogen numbering) from the protease domain of factor X bound one Ca2+ with high affinity (Kd 150 +/- 20 microM). From competitive inhibition assays for binding of active site-blocked factor Xa to factor Va in the prothrombinase complex, the Kd for peptide-Va interaction was calculated to be approximately 10 microM as compared with 30 pM for factor Xa and approximately 1.5 microM for decarboxylated factor Xa. A peptide containing residues 238-262(58-82) bound Ca2+ with reduced affinity (Kd approximately 600 microM) and did not inhibit Xa:Va interaction. In contrast, a peptide containing residues 253-277(73-97) inhibited Xa:Va interaction (Kd approximately 10 microM) but did not bind Ca2+. In additional studies, Ca2+ increased the amidolytic activity of native and des-44-Xa toward a tetrapeptide substrate (benzoyl-Ile-Glu-Gly-Arg-p-nitroanilide) by approximately 1.6-fold. The half-maximal increase was observed at approximately 150 microM Ca2+ and the effect was primarily on the kcat. Ca2+ also significantly protected cleavage at Arg-332-Gln-333(150-151) in the protease domain autolysis loop. Des-44-Xa in which the autolysis loop was cleaved possessed </=5% of the amidolytic activity of the noncleaved form; however, the S1 binding site was not affected, as determined by the p-aminobenzamidine binding. Additionally, autolysis loop-cleaved, active site-blocked native factor Xa was calculated to have approximately 10-fold reduced affinity for factor Va as compared with that of the noncleaved form.

Characterization of a full-length cDNA for rabbit factor X

A 300 bp probe generated by the PCR was derived from rabbit genomic DNA using primers from a highly conserved region of the DNA for human factor X (HFX). The probe was used in northern blot analysis of liver RNA to demonstrate an mRNA species of 1.6 kb for the rabbit factor X and subsequently for isolation and characterization of the cDNA for rabbit factor X (RFX) from a lambda Zap II cDNA library generated from rabbit liver mRNA. The cDNA contains 22 bases upstream from the 5'-translation initiation codon, 1470 nucleotides of open reading frame, a stop codon and a 3' poly (A) tail. The cDNA codes for a 40-residue signal/propeptide region, followed by a 447-residue mature protein. The deduced amino acid sequence shows a high degree of homology with the sequence of HFX. Inhibitory peptides derived from interactive sites of HFX for activators, cofactor and substrate exerted degrees of inhibition of RFX activation which showed a dependence on extent of homology with the corresponding regions of RFX.

X-ray structure of active site-inhibited clotting factor Xa. Implications for drug design and substrate recognition

The 3.0-A resolution x-ray structure of human des-Gla-coagulation factor Xa (fXa) has been determined in complex with the synthetic inhibitor DX-9065a. The binding geometry is characterized primarily by two interaction sites: the naphthamidine group is fixed in the S1 pocket by a typical salt bridge to Asp-189, while the pyrrolidine ring binds in the unique aryl-binding site (S4) of fXa. Unlike the large majority of inhibitor complexes with serine proteinases, Gly-216 (S3) does not contribute to hydrogen bond formation. In contrast to typical thrombin binding modes, the S2 site of fXa cannot be used by DX-9065a since it is blocked by Tyr-99, and the aryl-binding site (S4) of fXa is lined by carbonyl oxygen atoms that can accommodate positive charges. This has implications for natural substrate recognition as well as for drug design.

Kinetics of blood coagulation factor Xaalpha autoproteolytic conversion to factor Xabeta. Effect on inhibition by antithrombin, prothrombinase assembly, and enzyme activity

Autoproteolysis of blood coagulation factor Xa (FXa) results in the excision of a 4-kDa fragment (beta-peptide) from the intact subform, factor Xaalpha (FXaalpha), to yield factor Xabeta (FXabeta). In the preceding paper, we showed that generation of FXabeta leads to expression of a plasminogen binding site. FXabeta may consequently participate in fibrinolysis; therefore, the timing of subform conversion compared with thrombin production is important. In the current study we evaluated the kinetics of FXabeta generation, which showed that autoproteolysis of FXaalpha followed a second order mechanism where FXaalpha and FXabeta behaved as identical enzymes. Rate constants of 9 and 172 M-1 s-1 were derived, respectively, in the absence and presence of FXaalpha binding to procoagulant phospholipid. Under identical conditions the latter is estimated to be 6 orders of magnitude slower than thrombin generation by prothrombinase. Since heparin binding and prothrombin recognition have been previously attributed to a region of FXaalpha proximal to the beta-peptide, functional comparisons were conducted using homogeneous and stabilized preparations of FXaalpha and FXabeta. Comparisons included 1) the recognition of small substrates; 2) the rate of interaction with antithrombin/heparin; 3) the assembly of prothrombinase; and 4) the activation of prothrombin by prothrombinase. Although the beta-peptide neighbors a probable functional region in FXaalpha, conversion to FXabeta was not observed to influence these functions. The data support a model where FXaalpha is predominantly responsible for thrombin generation and where slow conversion to FXabeta coordinates coagulation and the initiation of fibrinolysis at sites of prothrombinase assembly.

Functional consequences of the Ser334-->Pro mutation in a human factor X variant (factor XMarseille)

A factor X molecular variant was identified in a 55-year-old woman at a routine preoperative coagulation screening. Plasma factor X antigen was normal, whereas factor X activity was decreased when factor X was activated by either the extrinsic pathway (21%), the intrinsic pathway (21%) or the factor X activator from Russell viper venom, RVV-X (26%). Factor XMarseille was isolated from plasma by immunoaffinity chromatography and compared with normal factor X purified by the same method. Activation of factor XMarseille by factor IXa or by RVV-X in a purified system showed that the rate of cleavage was decreased, whereas once produced, factor XaMarseille had a normal catalytic efficiency for either the peptide substrate S-2765 (D-Arg-Gly-Arg-NH-Np) or prothrombin. The rate of inhibition of factor XaMarseille by antithrombin III was also normal. Defective proteolysis of factor XMarseille by factor IXa or by RVV-X was the consequence of a threefold decrease in the kcat for the activation of factor XMarseille while the Km of RVV-X or factor IXa for factor X was normal. We have determined the molecular basis of the defect in the factor XMarseille gene by amplification of all eight exons, single-strand conformational polymorphism analysis of the amplified exons and subsequent sequence analysis. The patient was homozygous for a T-->C mutation in exon VIII, resulting in the substitution of Ser334 by proline. From comparison of three-dimensional models of various serine proteases, it appears that Ser334 is located within a surface-exposed variable region of factor X. This observation suggests that the Ser334-->Pro mutation either is responsible for a misalignment of the active sites of specific factor X activators in close proximity to the cleavage site, or that the Ser-->Pro mutation alters the spatial orientation of the cleavage site by nonlocal modifications of factor X structure.

Molecular bases of CRM+ factor X deficiency: a frequent mutation (Ser334Pro) in the catalytic domain and a substitution (Glu102Lys) in the second EGF-like domain

The presence of gene lesions in coagulation factor X (FX, Stuart factor) was investigated in asymptomatic subjects with FX deficiency characterized by the presence of dysfunctional molecules in plasma, as demonstrated by the discrepancy between clotting activity and antigen level. A missense mutation (Ser334Pro) in the catalytic domain was found in three unrelated families in both the homozygous and the heterozygous conditions, and also in the compound heterozygous form with the substitution of Lys for 102 Glu. None of the mutations was detected in 40 unrelated subjects from the same geographic area. The Ser334Pro mutation affects a serine protease region characterized by extensive variation in the coagulation factors but conserved in mammalian factor X molecules. The Glu102Lys mutation affects a residue of the second EGF-like module also conserved in protein C. Both mutated residues are surface-exposed and found in protein regions suggested to be involved in macromolecular interactions which are impaired in the dysfunctional molecules.

Specific molecular interaction sites on factor VII involved in factor X activation

Factor VII, a serine-protease zymogen, and tissue factor, the cellular receptor/coenzyme, are the protein components of the macromolecular complex which initiates the extrinsic pathway of the coagulation cascade. Previous studies were directed to the identification of functional sites on factor VII which mediate factor X activation, employing a series of potentially inhibitory synthetic peptides representing the primary structure of factor VII and antibodies to selected peptides. The involvement of at least four high-affinity interactive regions [factor VII (44-50), (196-229), (285-305) and (376-396) peptides] on the surface of factor VII was clearly demonstrated. The minimal sequences for the expression of inhibitory activity of these four molecular recognition domains on factor VII were identified using short and overlapping peptides. The short factor VII-(206-218)-peptide (most inhibitory peptide in the sequence 196-229 on factor VII) inhibited the binding of factor VII to the tissue-factor-expressing J82 cell line. Furthermore, radiolabeled [Tyr201] factor VII-(199-221)-peptide, with a tyrosine substituted for the normal tryptophan residue, was specifically bound to J82 cells, and also the binding of the radiolabeled peptide to this cell line was specifically inhibited by a monoclonal antibody to tissue factor, confirming that the interaction site for tissue factor on factor VII is present within the peptide sequence 196-229. Kinetic analyses suggested that the regions represented by factor VII-(285-305)- and factor VII-(376-396)-peptides are involved in factor X recognition and the chemical cross-linking of the radiolabeled peptides resulted in specific binding to factor X, confirming that these two regions on factor VII represent the substrate-recognition site. Furthermore, these radiolabeled peptides specifically interact with the heavy chain of factor X, suggesting that the complementary binding region for the substrate-recognition site on factor VII are present on the heavy chain of factor X.

Interactions and inhibition of blood coagulation factor Va involving residues 311-325 of activated protein C

Activated protein C (APC) exerts its physiologic anticoagulant role by proteolytic inactivation of the blood coagulation cofactors Va and VIIIa. The synthetic peptide-(311-325) (KRNRTFVLNFIKIPV), derived from the heavy chain sequence of APC, potently inhibited APC anticoagulant activity in activated partial thromboplastin time (APTT) and Xa-1-stage coagulation assays in normal and in protein S-depleted plasma with 50% inhibition at 13 microM peptide. In a system using purified clotting factors, peptide-(311-325) inhibited APC-catalyzed inactivation of factor Va in the presence or absence of phospholipids with 50% inhibition at 6 microM peptide. However, peptide-(311-325) had no effect on APC amidolytic activity or on the reaction of APC with the serpin, recombinant [Arg358]alpha 1-antitrypsin. Peptide-(311-325) surprisingly inhibited factor Xa clotting activity in normal plasma, and in a purified system it inhibited prothrombinase activity in the presence but not in the absence of factor Va with 50% inhibition at 8 microM peptide. The peptide had no significant effect on factor Xa or thrombin amidolytic activity and no effect on the clotting of purified fibrinogen by thrombin, suggesting it does not directly inhibit these enzymes. Factor Va bound in a dose-dependent manner to immobilized peptide-(311-325). Peptide-(311-315) inhibited the binding of factor Va to immobilized APC or factor Xa.(ABSTRACT TRUNCATED AT 250 WORDS)