Tissue factor potentiates the factor VIIa-catalyzed hydrolysis of an ester substrate

Endometriosis, angiogenesis and tissue factor

Tissue factor (TF), is a cellular receptor that binds the factor VII/VIIa to initiate the blood coagulation cascade. In addition to its role as the initiator of the hemostatic cascade, TF is known to be involved in angiogenesis via intracellular signaling that utilizes the protease activated receptor-2 (PAR-2). We now review the physiologic expression of TF in the endometrium and its altered expression in multiple cell types derived from eutopic and ectopic endometrium from women with endometriosis compared with normal endometrium. Our findings suggest that TF might be an ideal target for therapeutic intervention in endometriosis. We have employed a novel immunoconjugate molecule known as Icon and were able to eradicate endometrial lesions in a mouse model of endometriosis without affecting fertility. These findings have major implications for potential treatment in humans.

The influence of different glycosylation patterns on factor VII biological activity

In this study the bioactivity of three differently glycosylated blood coagulation factor VII (FVII) variants (human plasma FVII, recombinant human FVII produced in CHO and BHK cell cultures) were analyzed and compared. Surface plasmon resonance studies of FVII interaction with soluble and full length TF together with FVII autoactivation assays revealed that BHK-derived FVII has the highest bioactivity, while human plasma and CHO-derived FVII showed very similar bioactivity. The affinity of FVII variants to TF correlates with FVII autoactivation rates--the higher the affinity, the faster the autoactivation rate.

An all-atom solution-equilibrated model for human extrinsic blood coagulation complex (sTF-VIIa-Xa): a protein-protein docking and molecular dynamics refinement study

Tissue factor (TF)-bound factor (F)VIIa plays a critical role in activating FX, an event that rapidly results in blood coagulation. Despite recent advances in the structural information about soluble TF (sTF)-bound VIIa and Xa individually, the atomic details of the ternary complex are not known. As part of our long-term goal to provide a structural understanding of the extrinsic blood coagulation pathway, we built an all atom solution-equilibrated model of the human sTF-VIIa-Xa ternary complex using protein-protein docking and molecular dynamics (MD) simulations. The starting structural coordinates of sTF-VIIa and Xa were derived from dynamically equilibrated solution structures. Due to the flexible nature of the light-chain of the Xa molecule, a three-stage docking approach was employed in which SP (Arg195-Lys448)/EGF2 (Arg86-Arg139), EGF1 (Asp46-Thr85) and GLA (Ala1-Lys45) domains were docked in a sequential manner. The rigid-body docking approach of the FTDOCK method in conjunction with filtering based on biochemical knowledge from experimental site-specific mutagenesis studies provided the strategy. The best complex obtained from the docking experiments was further refined using MD simulations for 3 ns in explicit water. In addition to explaining most of the known experimental site-specific mutagenesis data pertaining to sTF-VIIa, our model also characterizes likely enzyme-binding exosites on FVIIa and Xa that may be involved in the ternary complex formation. According to the equilibrated model, the 140s loop of VIIa serves as the key recognition motif for complex formation. Stable interactions occur between the FVIIa 140s loop and the FXa -strand B2 region near the sodium-binding domain, the 160 s loop and the N-terminal activation loop regions. The helical-hydrophobic stack region that connects the GLA and EGF1 domains of VIIa and Xa appears to play a potential role in the membrane binding region of the ternary complex. The proposed model may serve as a reasonable structural basis for understanding the exosite-mediated substrate recognition of sTF-VIIa and to advance understanding of the TFPI-mediated regulatory pathway of the extrinsic blood coagulation cascade.

The 99 and 170 loop-modified factor VIIa mutants show enhanced catalytic activity without tissue factor

To elucidate the functions of the surface loops of VIIa, we prepared two mutants, VII-30 and VII-39. The VII-30 mutant had all of the residues in the 99 loop replaced with those of trypsin. In the VII-39 mutant, both the 99 and 170 loops were replaced with those of trypsin. The k(cat)/K(m) value for hydrolysis of the chromogenic peptidyl substrate S-2288 by VIIa-30 (103 mm(-)1s(-)1) was 3-fold higher than that of wild-type VIIa (30.3 mm(-)1 s(-)1) in the presence of soluble tissue factor (sTF). This enhancement was due to a decrease in the K(m) value but not to an increase in the k(cat) value. On the other hand, the k(cat)/K(m) value for S-2288 hydrolysis by VIIa-39 (17.9 mm(-)1 s(-)1) was 18-fold higher than that of wild-type (1.0 mm(-)1 s(-)1) in the absence of sTF, and the value was almost the same as that of wild-type measured in the presence of sTF. This enhancement was due to not only a decrease in the K(m) value but also to an increase in the k(cat) value. These results were in good agreement with their susceptibilities to a subsite 1-directed serine protease inhibitor. In our previous paper (Soejima, K., Mizuguchi, J., Yuguchi, M., Nakagaki, T., Higashi, S., and Iwanaga, S. (2001) J. Biol. Chem. 276, 17229-17235), the replacement of the 170 loop of VIIa with that of trypsin induced a 10-fold enhancement of the k(cat) value for S-2288 hydrolysis as compared with that of wild-type VIIa in the absence of sTF. These results suggested that the 99 and the 170 loop structures of VIIa independently affect the K(m) and k(cat) values, respectively. Furthermore, we studied the effect of mutations on proteolytic activity toward S-alkylated lysozyme as a macromolecular substrate and the activation of natural macromolecular substrate factor X.

Factor VIIa modified in the 170 loop shows enhanced catalytic activity but does not change the zymogen-like property

Factor VIIa (VIIa) is an unusual trypsin-type serine proteinase that appears to exist in an equilibrium between minor active and dominant zymogen-like inactive conformational states. The binding of tissue factor to VIIa is assumed to shift the equilibrium into the active state. The proteinase domain of VIIa contains a unique structure: a loop formed by a disulfide bond between Cys310 and Cys329, which is five residues longer than those of other trypsin types. To examine the functional role of the loop region, we prepared two mutants of VIIa. One of the mutants, named VII-11, had five extra corresponding residues 316-320 of VII deleted. The other mutant, VII-31, had all of the residues in its loop replaced with those of trypsin. Functional analysis of the two mutants showed that VIIa-11 (Kd = 41 nm) and VIIa-31 (Kd = 160 nm) had lower affinities for soluble tissue factor as compared with the wild-type VIIa (Kd = 11 nm). The magnitude of tissue factor-mediated acceleration of amidolytic activities of VIIa-11 (7-fold) and that of VIIa-31 (2-fold) were also smaller than that of wild-type VIIa (30-fold). In the absence of tissue factor, VIIa-31 but not VIIa-11 showed enhanced activity; the catalytic efficiencies of VIIa-31 toward various chromogenic substrates were 2-18-fold greater than those of the wild-type VIIa. Susceptibility of the alpha-amino group of Ile-153 of VIIa-31 to carbamylation was almost the same as that of wild-type VIIa, suggesting that VIIa-31 as well as wild-type VIIa exist predominantly in the zymogen-like state. Therefore, the tested modifications in the loop region had adverse effects on affinity for tissue factor, disturbed the tissue factor-induced conformational transition, and changed the catalytic efficiency of VIIa, but they did not affect the equilibrium between active and zymogen-like conformational states.

Binding of Zn2+ to a Ca2+ loop allosterically attenuates the activity of factor VIIa and reduces its affinity for tissue factor

The protease domain of coagulation factor VIIa (FVIIa) is homologous to trypsin with a similar active site architecture. The catalytic function of FVIIa is regulated by allosteric modulations induced by binding of divalent metal ions and the cofactor tissue factor (TF). To further elucidate the mechanisms behind these transformations, the effects of Zn2+ binding to FVIIa in the free form and in complex with TF were investigated. Equilibrium dialysis suggested that two Zn2+ bind with high affinity to FVIIa outside the N-terminal gamma-carboxyglutamic acid (Gla) domain. Binding of Zn2+ to FVIIa, which was influenced by the presence of Ca2+, resulted in decreased amidolytic activity and slightly reduced affinity for TF. After binding to TF, FVIIa was less susceptible to zinc inhibition. Alanine substitutions for either of two histidine residues unique for FVIIa, His216, and His257, produced FVIIa variants with decreased sensitivity to Zn2+ inhibition. A search for putative Zn2+ binding sites in the crystal structure of the FVIIa protease domain was performed by Grid calculations. We identified a pair of Zn2+ binding sites in the Glu210-Glu220 Ca2+ binding loop adjacent to the so-called activation domain canonical to serine proteases. Based on our results, we propose a model that describes the conformational changes underlying the Zn2+-mediated allosteric down-regulation of FVIIa's activity.

Functional consequences of mutations in Ser-52 and Ser-60 in human blood coagulation factor VII

Human blood coagulation factor VII has unique carbohydrate moieties O-glycosidically linked to serine 52 and serine 60 residues in its first epidermal growth factor-like domain. To study the functional role of these glycosyl moieties in factor VII, we constructed, expressed, and purified site-specific recombinant mutants of human factor VII in which serine 52 and serine 60 were conservatively replaced with alanine residues. S52A factor VIIa (Ser-52-->Ala), S60A factor VIIa (Ser-60-->Ala), and S52,60A factor VIIa (Ser-52, Ser-60-->Ala) exhibited 56, 73, and 44%, respectively, of the clotting activity of wild-type factor VIIa using human brain thromboplastin as a source of tissue factor/phospholipids and 32, 43, and 14% of wild-type factor VIIa using a mixture of recombinant soluble tissue factor and mixed brain phospholipids. The tissue factor-dependent and -independent amidolytic activities of these mutants were essentially indistinguishable from that of wild-type factor VIIa. In addition, equilibrium dialysis experiments indicated that the profiles of 45Ca2+ binding to these mutants were identical with that of wild-type factor VII. In the presence of either Ca2+ or EGTA, the Kd values for the interaction of the three factor VIIa mutants to full-length tissue factor were 2- to 5-fold higher than that of wild-type factor VIIa, while the Kd values for the interaction of these mutants to soluble tissue factor were 4- to 15-fold higher than that of wild-type factor VIIa. Measurement of the association and dissociation rate constants for factor VIIa binding to relipidated tissue factor apoprotein revealed that the association rate constants of the three factor VII mutants were decreased in comparison with that of wild-type factor VIIa, while the dissociation rate constants of these three mutants were virtually identical to that of wild-type factor VIIa. These findings strongly suggest that glycosyl moieties attached to Ser-52 and Ser-60 in factor VII/VIIa provide unique structural elements that are important for the rapid association of factor VII/VIIa with its cellular receptor and cofactor.

Influence of cofactor binding and active site occupancy on the conformation of the macromolecular substrate exosite of factor VIIa

The catalytic activity of the trypsin-like serine protease coagulation factor VIIa is allosterically regulated. In this work, we employed monoclonal antibodies as probes to analyze conformational changes in the VII protease domain that are induced by zymogen activation, cofactor tissue factor (TF) binding, and active site occupancy. The epitopes of three monoclonal antibodies were mapped using a panel of 57 individual alanine replacement mutants in the protease domain. Two of the antibodies had typical "hot spot" epitopes in a basic cluster above the active site cleft and antibody binding to these epitopes was not affected by zymogen activation, TF binding, or active site occupancy. In contrast, the binding kinetics of VII/VIIa to a monoclonal antibody that mapped to an extended epitope overlapping with the macromolecular substrate exosite was affected by each of the conformational transitions of the VIIa protease domain. The changes in antibody affinity are consistent with a transition from zymogen VII to the TF.VIIa complex, with free enzyme VIIa as an intermediate that retains some zymogen-like features responsible for its low catalytic activity. In contrast, active site occupancy resulted in effects that were qualitatively different from the effects of zymogen activation on the antibody epitope. This provides novel insight into the conformational interdependence between the active site, the region for macromolecular substrate recognition, and the cofactor binding exosite of this allosterically regulated serine protease.

Tissue factor positions and maintains the factor VIIa active site far above the membrane surface even in the absence of the factor VIIa Gla domain. A fluorescence resonance energy transfer study

Coagulation factor VIIa (fVIIa), a soluble serine protease, exhibits full proteolytic activity only when bound to its cofactor, tissue factor (TF). Both proteins interact with membranes; TF is an integral membrane protein, while fVIIa binds reversibly to phospholipid surfaces via its Gla domain. In this study, we examine the extent to which the location of the fVIIa active site in the fVIIa.TF complex is determined by the fVIIa Gla domain. A fluorescein dye was covalently attached to the active site of fVIIa lacking the Gla domain (Gla domainless fVIIa, GD-fVIIa) via a tripeptide tether to yield fluorescein-D-Phe-Pro-Arg-GD-fVIIa (Fl-FPR-GD-fVIIa). The location of the active site of GD-fVIIa relative to the membrane surface was determined using fluorescence resonance energy transfer between the fluorescein dye in the active site of GD-fVIIa and octadecylrhodamine (OR) at the surface of phospholipid vesicles. As expected, no energy transfer was observed between Fl-FPR-GD-fVIIa and OR in vesicles composed of phosphatidylcholine/phosphatidylserine (PC/PS, 4:1) because the Gla domain is required for the binding of fVIIa to phospholipid. However, when Fl-FPR-GD-fVIIa was titrated with PC or PC/PS vesicles into which purified TF had been reconstituted, energy transfer was observed. Based on the dependence of fluorescence resonance energy transfer on OR density, the average distance of closest approach between fluorescein in the active site of Fl-FPR-GD-fVIIa.TF and OR at the vesicle surface was determined to be 78 A (kappa2 = (2)/(3)). Since this value is nearly the same as that obtained with intact Fl-FPR-fVIIa bound to TF, the presence or absence of the fVIIa Gla domain has only a small effect on the location of the active site in the fVIIa.TF complex. The extracellular domain of tissue factor therefore must be fairly rigid and fixed relative to the surface to position and maintain the fVIIa active site far above the membrane even in the absence of the fVIIa Gla domain.

Antithrombotic efficacy of inactivated active site recombinant factor VIIa is shear dependent in human blood

Several studies have indicated a profound role for factor VII(a) [FVII(a)] in venous and arterial thrombogenesis. In the present study, we quantified the inhibitory efficacy of dansyl-glutamyl-glycyl-arginyl-recombinant FVIIa (DEGR-rFVIIa) on acute thrombus formation. Thrombus formation was elicited by immobilized tissue factor (TF) in a parallel-plate perfusion chamber device at blood flow conditions characterized by wall shear rates of 100 S-1 (veins) and 650 S-1 (medium-sized healthy arteries). Native human blood was drawn directly from an antecubital vein by a pump into a heparin-coated mixing device in which DEGR-rFVIIa (0.09 to 880 nmol/L final plasma concentration) or buffer was mixed homogeneously with flowing blood. Subsequently, the blood was passed over a plastic coverslip coated with TF and phospholipids in the parallel-plate perfusion chamber. Fibrin deposition, platelet-fibrin adhesion, and platelet thrombus volume triggered by this surface were measured by morphometry. DEGR-rFVIIa inhibited thrombus formation in a dose-dependent manner, but the efficacy was shear rate dependent. At a wall shear rate of 100 S-1, the IC50 (50% inhibition) was 30 nmol/L, whereas at 650 S-1, the IC50 was 0.6 nmol/L. Binding studies to immobilized TF under flow conditions using surface plasmon resonance revealed a significantly higher on-rate for DEGR-rFVIIa and FVIIa than for FVII, 2.8 x 10(5), 2.6 x 10(5), and 1.8 x 10(5) M-1 S-1, respectively. This indicates that a contributing factor to the shear-dependent efficacy may be a differential importance of on-rates at arterial and venous blood flow conditions.

Conformation of factor VIIa stabilized by a labile disulfide bond (Cys-310-Cys-329) in the protease domain is essential for interaction with tissue factor

Unlike other trypsin-type serine proteases, zymogento-enzyme transition of conformation of factor VII apparently requires not only conversion of the zymogen to active form factor VIIa (VIIa) but also interaction of VIIa with tissue factor (TF). To determine the region of interaction that correlates with maturation of the VIIa active site, we modified intramolecular disulfide bonds in VIIa and examined the interaction of the modified VIIa with soluble TF (sTF). We found that partial reduction and S-carboxamidomethylation of disulfide bonds in VIIa led to losses of amidolytic activity and the binding ability to sTF. To determine the sites of modification that associate with the loss of functions, partially S-carboxamidomethylated VIIa was separated on a column of immobilized sTF. Each of the sTF-bound and sTF-unbound fractions and native VIIa was then digested by trypsin, and the digest was analyzed by reversed-phase high performance liquid chromatography. We found that reduction and S-carboxamidomethylation of a disulfide bond between Cys-310 and Cys-329 in the protease domain of VIIa led to loss of the binding ability with sTF, and the modification of a disulfide bond between Cys-340 and Cys-368 of VIIa led to loss of the amidolytic activity. In the three-dimensional structures of trypsinogen and trypsin, the disulfide bonds corresponding to Cys-340-Cys-368 and Cys-310-Cys-329 of VIIa are, respectively, in and adjacent to the activation domain, which has flexible conformation in trypsinogen but not in trypsin. Furthermore, the crystal structure of human VIIa.TF complex indicates that the region next to Cys-310-Cys-329 is in contact with sTF. We speculate that a regional flexibility, reflected by the labile nature of disulfide bonds of Cys-310-Cys-329 and Cys-340-Cys-368 in the protease domain, contributes to the inability of VIIa to attain the active conformation. Interaction of TF with this flexible region may stabilize the structure in a conformation similar to that of the active state of VIIa.

Characterization of the interaction between the light chain of factor VIIa and tissue factor

Factor VIIa (fVIIa) consists of a heavy chain (serine protease domain) and a light chain (gamma-carboxyglutamic acid (Gla)-rich and epidermal growth factor (EGF)-like domains). The light chain, primarily the first EGF-like domain, appears to provide most of the binding energy in the interaction with tissue factor (TF). The Ca2+-binding sites in the protease domain and in the first EGF-like domain influence activity and interaction with TF, but the contribution from the Ca2+-binding sites in the Gla domain has not been established. We have compared the soluble TF (sTF)-binding properties of intact fVIIa to those of a fragment comprising almost the entire light chain and a small disulphide-linked peptide from the protease domain. Half-maximal binding of fVIIa and the light chain to sTF occurred around 0.3 and 1 mM Ca2+, respectively. The Ca2+ dependence of light-chain binding indicates an influence of Ca2+ binding to the Gla domain on the interaction between fVIIa and sTF. Comparison of the sTF-binding properties of fVIIa and a truncated variant lacking the Gla domain suggests that this domain interferes with sTF association at suboptimal Ca2+ concentrations. The light chain of fVIIa associated 5-fold slower with sTF than did fVIIa at saturating Ca2+ concentrations, whereas the dissociation of its complex with sTF was at least 100-fold faster than that of fVIIa:sTF. This gave a dissociation constant of 1-2 microM for the interaction between the light chain and sTF compared to about 3 nM for the fVIIa:sTF interaction.

Ca2+ binding to the first epidermal growth factor-like domain of factor VIIa increases amidolytic activity and tissue factor affinity

Coagulation factor VIIa belongs to a family of homologous enzymes, including factors IXa and Xa and activated protein C, composed of two epidermal growth factor-like domains located between an N-terminal domain rich in gamma-carboxyglutamic acid residues and a C-terminal serine protease domain. The first epidermal growth factor-like domain in factor VIIa contains a Ca2+ binding site, the function of which is largely unknown. Site-directed mutagenesis of two Ca2+-liganding Asp residues in this domain abolished Ca2+ binding and resulted in a 2-3-fold decrease in amidolytic activity at optimal Ca2+ concentrations. The lower amidolytic activity persisted in complex with soluble tissue factor, apparently due to a lower kcat of the mutant factor VIIa. Mutant and wild-type factor VIIa bound to lipidated tissue factor were equally efficient activators of factor X. The dissociation constants, derived from amidolytic activity and surface plasmon resonance measurements, were 2-5 nM and 50-60 nM for the interactions between wild-type and mutant factor VIIa, respectively, and soluble tissue factor. Binding to lipidated tissue factor was characterized by dissociation constants of 7.5 pM for factor VIIa and 160 pM for the factor VIIa mutant. Hence, a functional Ca2+ binding site in the first epidermal growth factor-like domain added 7-8 kJ/mol to the total binding energy of the interaction with both lipidated and soluble tissue factor.

The location of the active site of blood coagulation factor VIIa above the membrane surface and its reorientation upon association with tissue factor. A fluorescence energy transfer study

The topography of membrane-bound blood coagulation factor VIIa (fVIIa) was examined by positioning a fluorescein dye in the active site of fVIIa via a tripeptide tether to yield fluorescein-D-phenylalanyl-L-prolyl-L-arginyl-fVIIa (Fl-FPR-fVIIa). The location of the active-site probe relative to the membrane surface was determined, both in the presence and absence of tissue factor (TF), using fluorescence energy transfer between the fluorescein dye and octadecylrhodamine (OR) at the phospholipid vesicle surface. When Fl-FPR-fVIIa was titrated with phospholipid vesicles containing OR, the magnitude of OR-, calcium ion-, and phosphatidylserine-dependent fluorescence energy transfer revealed that the average distance of closest approach between fluorescein in the active site of fVIIa and OR at the vesicle surface is 82 A assuming a random orientation of donor and acceptor dyes (kappa2 = 2/3; the orientational uncertainty totals approximately 10%). The active site of fVIIa is therefore located far above the membrane surface, and the elongated fVIIa molecule must bind at one end to the membrane and project approximately perpendicularly out of the membrane. When Fl-FPR-fVIIa was titrated with vesicles that contained TF, the efficiency of energy transfer was increased by a TF-dependent translational and/or rotational movement of the fVIIa protease domain relative to the membrane surface. If this movement was solely translational, the height of the active site of fVIIa was lowered by an average of 6 A after binding to TF. The association of fVIIa with TF on the membrane surface therefore causes a significant reorientation of the active site relative to the membrane surface. This cofactor-dependent realignment of the active-site groove presumably facilitates and optimizes fVIIa cleavage of its membrane-bound substrates.

Molecular mechanism of tissue factor-mediated acceleration of factor VIIa activity

The mechanism of the acceleration of the catalytic activity of factor VIIa (VIIa) in the presence of tissue factor (TF) was investigated. To explore the VIIa's site(s) that correlates with TF-mediated acceleration, zymogen VII, VIIa, and active site-modified VIIa were prepared, and dissociation constants (Kd) for their bindings to TF or soluble TF in solution were determined. We found that conversion of zymogen VII to VIIa led to an increase in affinity (DeltaDeltaG = 4.3-4.4 kJ/mol) for TFs. Dansyl-Glu-Gly-Arg chloromethyl ketone (DNS-EGRck) treatment of VIIa led to a further increase in the affinity (DeltaDeltaG = 7.3-12 kJ/mol). Neither removal of the Gla domain from VIIa nor truncation of the COOH-terminal membrane and cytoplasmic regions of TF affected the affinity enhanced after DNS-EGRck treatment of VIIa. Treatment of VIIa with (p-amidinophenyl)methanesulfonyl fluoride also enhanced its affinity for soluble TF, whereas treatment with 4-(2-aminoethyl)benzenesulfonyl fluoride, phenylmethylsulfonyl fluoride, or diisopropyl fluorophosphate had a slight effect on the affinity. On the other hand, DNS-EGRck and (p-amidinophenyl)methanesulfonyl fluoride treatments, but not diisopropyl fluorophosphate treatment, of VIIa led to protection of its alpha-amino group of Ile-153 from carbamylation. Protection of the alpha-amino group was consistent with formation of a critical salt bridge between Ile-153 and Asp-343 in the protease domain of VIIa. Therefore, TF may preferentially bind to the active conformational state of VIIa. When one assumes that free VIIa exists in equilibrium between minor active and dominant zymogen-like inactive conformational states, preferential binding of TF to the active state leads to a shift in equilibrium. We speculate that TF traps the active conformational state of VIIa and converts its zymogen-like state into an active state, thereby accelerating the VIIa activity.

Substrate recognition by tissue factor-factor VIIa. Evidence for interaction of residues Lys165 and Lys166 of tissue factor with the 4-carboxyglutamate-rich domain of factor X

Tissue factor (TF) is the protein cofactor for factor VIIa (FVIIa), the first serine protease of the clotting cascade. Previous studies using alanine mutagenesis have identified TF residues Lys165 and Lys166 as important for factor X (FX) activation, hypothesizing either that these residues interact with phospholipid head groups or that they directly or indirectly promote macromolecular substrate binding. In the recently reported x-ray crystal structure of the isolated extracellular domain of TF, both Lys165 and Lys166 are solvent-exposed and predicted to be near the phospholipid surface in intact TF. We hypothesized that these residues may in fact be ideally positioned to interact with the 4-carboxyglutamate-rich domain (Gla domain) of FX. We therefore predicted that mutations at Lys165 and Lys166 should have no effect on the activation of Gla domainless FX. To test this hypothesis, we mutated both residues Lys165 and Lys166 of TF to Ala, Glu, or Gln and examined the ability of these double mutants to support FVIIa-mediated activation of FX, Gla domainless FX, and factor IX (FIX). Each TF mutant was equivalent to wild-type TF in both FVIIa binding and promotion of FVIIa amidolytic activity. However, all three mutants were markedly deficient in supporting FIX and FX activation, with FX activation rates decreased more than FIX activation rates. In both reactions, the TF mutants exhibited different extents of activity: Gln165-Gln166 > Ala165-Ala166 > Glu165-Glu166. In sharp contrast, all three TF mutants were equivalent to wild-type TF in supporting activation of Gla domainless FX by FVIIa. Interestingly, the deficiency of the mutants in FX activation was less pronounced when Gla domainless FVIIa was used in place of native FVIIa. Together, these findings suggest that TF residues Lys165 and Lys166 contribute to a binding site for the Gla domain of FX (and perhaps other substrates) and that this interaction may be facilitated by the presence of the Gla domain of FVIIa.

Structural changes in factor VIIa induced by Ca2+ and tissue factor studied using circular dichroism spectroscopy

Factor VIIa (fVIIa) is composed of four discrete domains, a gamma-carboxyglutamic acid (Gla)-containing domain, two epidermal growth factor (EGF)-like domains, and a serine protease domain, all of which appear to be involved, to different extents, in an optimal interaction with tissue factor (TF). All except the second EGF-like domain contain at least one Ca2+ binding site and many properties of fVIIa, e.g., TF and phospholipid binding and amidolytic activity, are Ca(2+)-dependent. A CD study was performed to characterize and locate the conformational changes in fVIIa induced by Ca2+ and TF binding. In addition to intact fVIIa, derivatives lacking the Gla domain or the protease domain were used. Assignment of the Ca(2+)-induced changes in the far-UV region of the fVIIa spectrum to the Gla domain could be made by comparing the CD spectra obtained with these fVIIa derivatives. The changes primarily appeared to reflect a Ca(2+)-induced ordering of alpha-helices existing in the apo state of fVIIa. This was corroborated by models of the apo and Ca2+ forms of fVIIa, obtained as difference spectra between fVIIa derivatives, were very similar to those of isolated Gla peptides from other vitamin K-dependent plasma proteins. The near-UV CD spectrum of fVIIa was dominated by aromatic residues residing in the protease domain and specific bands affected by Ca2+ were indicative of tertiary structural alterations. The formation of a fVIIa:TF complex led to secondary structural changes that appeared to be restricted to the catalytic domain, possibly shedding light on the mechanism by which TF induces an enhancement of fVIIa catalytic activity.

The roles of factor VII's structural domains in tissue factor binding

Factor VIIa binds to tissue factor in one of the initial steps of blood clotting. In order to determine the role of the various domains of the factor VII molecule in this interaction, we made several chimeric factor VII proteins using recombinant DNA techniques. The molecules have factor IX domains substituted into factor VII and vice versa. The domains exchanged were the 4-carboxyglutamic acid plus aromatic stack domain (gla), the first epidermal growth factor-like domain (Egf-1), the second epidermal growth factor-like domain (Egf-2), and the catalytic domain. Using tissue factor-coated microtiter wells, competition binding studies with 125I-labeled factor VIIa indicated factor VIIa's Kd is 4.2 nM. Employing the same microtiter plate assay, koff and kon were determined and yielded a Kd of 1.5 nM. The results of competitive binding experiments and activation assays using chimeric proteins indicated the interaction between factor VIIa and tissue factor involves direct contact between tissue factor and factor VIIa's Egf-1 domain and catalytic domain. On the other hand, the gla and Egf-2 domains, while necessary for optimal binding, may merely impart structure to the rest of the molecule. However, either one or both of the latter domains might contribute a relatively small amount of energy to direct binding.

High affinity Ca(2+)-binding site in the serine protease domain of human factor VIIa and its role in tissue factor binding and development of catalytic activity

Factor VIIa, in the presence of Ca2+ and tissue factor (TF), initiates the extrinsic pathway of blood coagulation. The light chain (amino acids 1-152) of factor VIIa consists of an N-terminal gamma-carboxyglutamic acid (Gla) domain followed by two epidermal growth factor-like domains, whereas the heavy chain (amino acids 153-406) contains the serine protease domain. In this study, both recombinant factor VIIa (rVIIa) and factor VIIa lacking the Gla domain were found to contain two high-affinity (Kd approximately 150 microM) Ca2+ binding sites. The rVIIa also contained approximately 6-7 low-affinity (Kd approximately 1 mM) Ca(2+)-binding sites. By analogy to other serine proteases, one of the two high affinity Ca(2+)-binding sites in factor VIIa may be formed involving Glu-210 and Glu-220 of the protease domain. In support of this, a synthetic peptide composed of residues 206-242 of factor VIIa bound one Ca2+ with Kd approximately 230 microM; however, Ca2+ binding was observed only in Tris buffer (pH 7.5) containing 1 M NaCl and not in buffer containing 0.1 M NaCl. In both low or high salt +/- Ca2+, the peptide existed as a monomer as determined by sedimentation equilibrium measurements and had no detectable secondary structure as determined by CD measurements. This indicates that subtle changes undetectable by CD may occur in the conformation of the peptide that favor calcium binding in high salt. In the presence of recombinant TF and 5 mM Ca2+, the peptide inhibited the amidolytic activity of rVIIa toward the synthetic substrate, S-2288. The concentration of the peptide required for half-maximal inhibition was approximately 5-fold higher in the low salt buffer than that in the high salt buffer. From direct binding and competitive inhibition assays of active site-blocked 125I-rVIIa binding to TF, the Kd for peptide-TF interaction was calculated to be approximately 15 microM in the high salt and approximately 55 microM in the low salt buffer containing 5 mM Ca2+. Moreover, as inferred from S-2288 hydrolysis, the Kd for VIIa.TF interaction was approximately 1.5 microM in the absence of Ca2+, and, as inferred from factor X activation studies, it was approximately 10 pM in the presence of Ca2+. Thus, Ca2+ decreases the functional Kd of VIIa.TF interaction approximately 150,000-fold.(ABSTRACT TRUNCATED AT 400 WORDS)

Involvement of the hydrophobic stack residues 39-44 of factor VIIa in tissue factor interactions

Des(1-38) factor VIIa and des(1-44) factor VIIa were obtained by limited proteolysis. The binding of tissue factor to these factor VIIa-derivatives was assessed from its stimulation of the proteolytic activity on chromogenic oligopeptide substrates. Compared to native factor VIIa (KTF = 0.6 +/- 0.1 nM), Tissue factor binds to des(1-38) factor VIIa with a lower, but still significant affinity (KTF = 4.8 +/- 0.3 nM). The activity of des(1-44) factor VIIa was only slightly stimulated by TF (KTF approximately 200 nM). Binding of TF depends critically on the presence of Ca2+ ions. Ca2+ ions stimulated the activity of factor VIIa/TF with an apparent KCa = 0.16 +/- 0.02 mM. Factor VIIa in the absence of tissue factor was stimulated by Ca2+ with an apparent KCa = 0.05 +/- 0.01 mM, and similar KCa values were obtained for the truncated derivatives of factor VIIa. Measurements of Ca(2+)-induced changes in intrinsic protein fluorescence suggest a conformational change. The Ca2+ ion concentration at which this change occurred was higher for des(1-44) factor VIIa (apparent KCa = 0.14 mM) than for des(1-38)- and native factor VIIa (apparent KCa = 0.04 mM). The Tb3+ ion luminescence technique was used to further investigate the Ca2+ binding sites. Tb3+ ions bound with a lower affinity to des(1-44) factor VIIa than to des(1-38)-and native factor VIIa. The observed drastic decrease in affinity for tissue factor as a result of truncation of the 'hydrophobic stack' residues 39-44, suggest that this region of factor VIIa provides a structural determinant that together with other regions participates in tissue factor binding.

Activation peptide of human factor IX has oligosaccharides O-glycosidically linked to threonine residues at 159 and 169

O-Linked oligosaccharide chains were identified in the activation peptide (AP) of human blood coagulation factor IX. The peptide obtained from human factor IX was separated into three molecular species (AP alpha, AP beta, and AP gamma) by reversed-phase high-performance liquid chromatography. Amino acid analysis showed that AP alpha, but not AP beta and AP gamma, contained galactosamine in addition to glucosamine, thereby suggesting the presence of an O-linked sugar chain(s) in the molecule of AP alpha. A nonapeptide (AP alpha-D4, residues 157-165) and an undecapeptide (AP alpha-D5, 166-176) derived from AP alpha contained Thr-159 and Thr-169, neither of which could be identified using a gas-phase protein sequencer. All other serine and threonine residues present in AP alpha were identified by peptide sequencing. Component sugar and sialic acid analyses of AP alpha-D4 and AP alpha-D5 revealed that they contained 1 mol each of N-acetyl-D-galactosamine (GalNAc), D-galactose (Gal), and sialic acid. Fast atom bombardment tandem mass spectrometric analysis of AP alpha-D4 suggested the existence of Gal-GalNAc-Thr, NeuNAc-(Gal-)GalNAc-Thr, and NeuNAc-Gal-GalNAc-Thr structures. On the basis of amino acid analysis after the isolation of AP alpha, it accounted for approximately 35% of the total activation peptide obtained. From these results, it was concluded that a part of the activation peptide of human factor IX in circulating blood has tri- and tetrasaccharides O-glycosidically linked to the threonine residues at 159 and 169.

Synthetic substrates for human factor VIIa and factor VIIa-tissue factor

A series of 100 tripeptide fluorogenic substrates has been synthesized. These substrates contain Arg in the P1 position, various amino acids in the P2 and P3 positions, and different 6-amino-1-naphthalenesulfonamides (ANSN) as the detecting group (P'). The 38 compounds possessing the highest initial rates of factor VIIa hydrolysis were evaluated for substrate kinetic parameters in the presence and absence of tissue factor (TF) and by factor Xa. Most of these substrates had a higher kcat/KM (keff) value for the factor VIIa-TF complex than for factor Xa. Substitution of different amino acids in the P2 position showed that substrates with bulkier amino acids such as Leu, Pro, and Val have higher values for KM and kcat than those with smaller amino acids (Gly or Ser). The highest second-order rate constants were found for substrates with Val or Pro in the P2 position. A decrease or increase in volume of the P2 substituent (Gly, Ser, or Leu) resulted in a decrease in this constant. Substrates with the highest keff values have Phe in the P3 position. As the hydrophobicity and volume of the amino acid in the P3 position decreased, the keff was reduced. The efficiency of substrates for hydrolysis by factor VIIa was enhanced by an increase of hydrophobicity in the P' structure. TF enhanced the amidolytic activity of the "family" of 38 substrates with ANSN in the P' position on an average of 58-fold.