Identification of the factor VIIa binding site on tissue factor by homologous loop swap and alanine scanning mutagenesis

Orphan Three-Finger Toxins Bind at Tissue Factor-Factor VIIa Interface to Inhibit Factor X Activation: Identification of Functional Site by Docking

Three-finger toxins (3FTxs) contribute to toxicity of venomous snakes belonging to the family Elapidae. Currently, functions of a considerable proportion of 3FTxs are still unknown. Here, we describe the function of orphan group I 3FTxs consisting of four members. We also identified a new member of this group by sequencing a transcript isolated from Naja naja venom. This transcript, named najalexin, is identical to that previously described 3FTx from Naja atra venom gland, and shared high sequence identity with ringhalexin from Hemachatus haemachatus and a hypothetical protein from Ophiophagus hannah (here named as ophiolexin). The three-dimensional structure, as predicted by molecular modeling, showed that najalexin and ophiolexin share the same conserved structural organization as ringhalexin and other 3FTxs. Since ringhalexin inhibits the activation of factor X by the tissue factor-factor VIIa complex (TF-FVIIa), we evaluated the interaction of this group of 3FTxs with all components using in silico protein-protein docking studies. The binding of orphan group I 3FTxs to TF-FVIIa complex appears to be driven by their interaction with TF. They bind to fibronectin domain closer to the 170-loop of the FVIIa heavy chain to inhibit factor X activation. The docking studies reveal that functional site residues Tyr7, Lys9, Glu12, Lys26, Arg34, Leu35, Arg40, Val55, Asp56, Cys57, Cys58, and Arg65 on these 3FTxs are crucial for interaction. In silico replacement of these residues by Ala resulted in significant effects in the binding energies. Furthermore, these functional residues are not found in other groups of 3FTxs, which exhibit distinct pharmacological properties.

Synergies of phosphatidylserine and protein disulfide isomerase in tissue factor activation

Tissue factor (TF), the cellular receptor and cofactor for factor VII/VIIa, initiates haemostasis and thrombosis. Initial tissue distribution studies suggested that TF was sequestered from the circulation and only present at perivascular sites. However, there is now clear evidence that TF also exists as a blood-borne form with critical contributions not only to arterial thrombosis following plaque rupture and to venous thrombosis following endothelial perturbation, but also to various other clotting abnormalities associated with trauma, infection, or cancer. Because thrombin generation, fibrin deposition, and platelet aggregation in the contexts of haemostasis, thrombosis, and pathogen defence frequently occur without TF de novo synthesis, considerable efforts are still directed to understanding the molecular events underlying the conversion of predominantly non-coagulant or cryptic TF on the surface of haematopoietic cells to a highly procoagulant molecule following cellular injury or stimulation. This article will review some of the still controversial mechanisms implicated in cellular TF activation or decryption with particular focus on the coordinated effects of outer leaflet phosphatidylserine exposure and thiol-disulfide exchange pathways involving protein disulfide isomerase (PDI). In this regard, our recent findings of ATP-triggered stimulation of the purinergic P2X7 receptor on myeloid and smooth muscle cells resulting in potent TF activation and shedding of procoagulant microparticles as well as of rapid monocyte TF decryption following antithymocyte globulin-dependent membrane complement fixation have delineated specific PDI-dependent pathways of cellular TF activation and thus illustrated additional and novel links in the coupling of inflammation and coagulation.

Structural biology of factor VIIa/tissue factor initiated coagulation

Factor VII (FVII) consists of an N-terminal gamma-carboxyglutamic acid domain followed by two epidermal growth factor-like (EGF1 and EGF2) domains and the C-terminal protease domain. Activation of FVII results in a two-chain FVIIa molecule consisting of a light chain (Gla-EGF1-EGF2 domains) and a heavy chain (protease domain) held together by a single disulfide bond. During coagulation, the complex of tissue factor (TF, a transmembrane glycoprotein) and FVIIa activates factor IX (FIX) and factor X (FX). FVIIa is structurally "zymogen-like" and when bound to TF, it is more "active enzyme-like." FIX and FX share structural homology with FVII. Three structural biology aspects of FVIIa/TF are presented in this review. One, regions in soluble TF (sTF) that interact with FVIIa as well as mapping of Ca2+, Mg2+, Na+ and Zn2+ sites in FVIIa and their functions; two, modeled interactive regions of Gla and EGF1 domains of FXa and FIXa with FVIIa/sTF; and three, incompletely formed oxyanion hole in FVIIa/sTF and its induction by substrate/inhibitor. Finally, an overview of the recognition elements in TF pathway inhibitor is provided.

Mammalian Mip/LIN-9 interacts with either the p107, p130/E2F4 repressor complex or B-Myb in a cell cycle-phase-dependent context distinct from the Drosophila dREAM complex

Mammalian Mip/LIN-9 is a cell cycle regulatory protein that is negatively regulated by CDK4/cyclin D. It has been demonstrated that Mip/LIN-9 collaborates with B-Myb during S and G(2)/M in the induction of cyclins A and B, and CDK1. The ortholog of Mip/LIN-9 in Drosophila, Mip130, is part of a large multisubunit protein complex that includes RBF, repressor E2Fs and Myb, in what was termed the dREAM complex. A similar complex, although lacking B-Myb, was also described in Caenorhabditis elegans. Here, we demonstrate that unlike Drosophila, Mip/LIN-9 has mutually exclusive and cell cycle-phase-specific interactions with the mammalian orthologs of the dREAM complex. In G(0)/early G(1), Mip/LIN-9 forms a complex with E2F4 and p107 or p130, while in late G(1)/S phase, it associates with B-Myb. The separation of Mip/LIN-9 from p107,p130/E2F4 is likely driven by phosphorylation of the pocket proteins by CDK4 since Mip/LIN-9 fails to interact with phosphorylated forms of p107,p130. Importantly, the repressor complex that Mip/LIN-9 forms with p107 takes functional precedence over the transcriptional activation linked to the Mip/LIN-9 and B-Myb interaction since expression of p107 blocks the activation of the cyclin B promoter triggered by B-Myb and Mip/LIN-9.

Tissue factor: (patho)physiology and cellular biology

The transmembrane glycoprotein tissue factor (TF) is the initiator of the coagulation cascade in vivo. When TF is exposed to blood, it forms a high-affinity complex with the coagulation factors factor VII/activated factor VIIa (FVII/VIIa), activating factor IX and factor X, and ultimately leading to the formation of an insoluble fibrin clot. TF plays an essential role in hemostasis by restraining hemorrhage after vessel wall injury. An overview of biological and physiological aspects of TF, covering aspects consequential for thrombosis and hemostasis such as TF cell biology and biochemistry, blood-borne (circulating) TF, TF associated with microparticles, TF encryption-decryption, and regulation of TF activity and expression is presented. However, the emerging role of TF in the pathogenesis of diseases such as sepsis, atherosclerosis, certain cancers and diseases characterized by pathological fibrin deposition such as disseminated intravascular coagulation and thrombosis, has directed attention to the development of novel inhibitors of tissue factor for use as antithrombotic drugs. The main advantage of inhibitors of the TF*FVIIa pathway is that such inhibitors have the potential of inhibiting the coagulation cascade at its earliest stage. Thus, such therapeutics exert minimal disturbance of systemic hemostasis since they act locally at the site of vascular injury.

Transition state analysis of the complex between coagulation factor VIIa and tissue factor: suggesting a sequential domain-binding pathway

Injury of a blood vessel exposes membrane-bound tissue factor (TF) to blood, which allows binding of coagulation factor VIIa (FVIIa). This initiation of the coagulation cascade is dictated by a specific multi-domain interaction between FVIIa and TF. To examine the energies involved in the transition state of the FVIIa:TF complex, various residues in the extracellular part of TF (sTF) that are known to interact with FVIIa were replaced with a smaller cysteine residue. Determination of Phi values in each of the positions using surface plasmon resonance measurements enabled us to characterize the transition state complex between the resulting sTF variants and FVIIa. We found that the interactions in the transition state seemed to be most pronounced between the protease domain of FVIIa and sTF while detailed specific interactions between the Gla-domain and sTF were missing. Thus, the transition state energy data indicate a sequential binding event between these two macromolecules.

The cofactor function of the N-terminal domain of tissue factor

Tissue factor (TF) is an integral membrane protein cofactor for factor VIIa (fVIIa) that initiates the blood coagulation cascade during vascular injury. TF has two fibrinonectin type III-like domains, both of which make extensive interactions with both the light and heavy chains of fVIIa. In addition to interaction with fVIIa, the membrane proximal C-terminal domain of TF is also known to bind the natural substrates factors IX and X, thereby facilitating their assembly and recognition by fVIIa in the activation complex. Both fVIIa and TF are elongated proteins, and their complex appears to be positioned nearly perpendicular to the membrane surface. It is possible that, similar to fVIIa, the N-terminal domain of TF also contacts the natural substrates. To investigate this possibility, we substituted all 23 basic and acidic residues of the N-terminal domain of TF with Ala or Asn and expressed the mutants as soluble TF(2-219) in a novel expression/purification vector system in the periplasmic space of bacteria. Following purification to homogeneity, the cofactor properties of mutants in promoting the amidolytic and proteolytic activity of fVIIa were analyzed in appropriate kinetic assays. The amidolytic activity assays indicated that several charged residues spatially clustered at the junction of the N- and C-terminal domains of TF are required for high affinity interaction with fVIIa. On the other hand, the proteolytic activity assays revealed that none of the residues under study may be an interactive site for either factor IX or factor X. However, it was discovered the Arg(74) mutant of TF was defective in enhancing both the amidolytic and proteolytic activity of fVIIa, suggesting that this residue may be required for the allosteric activation of the protease.

Spin and fluorescent probing of the binding interface between tissue factor and factor VIIa at multiple sites

The specific complex between the extracellular part of tissue factor (sTF) and factor VIIa (FVIIa) was chosen as a model for studies of the binding interface between two interacting proteins. Six surface-exposed positions in sTF, residues known to contribute to the sTF-FVIIa interaction, were selected for cysteine mutation and site-directed labeling with spin and fluorescent probes. The binding interface was characterized by spectral data from electron paramagnetic resonance (EPR) and steady-state and time-domain fluorescence spectroscopy. The labels reported on compact local environments at positions 158 and 207 in the interface region between sTF and the gamma-carboxyglutamic acid (Gla) domain of FVIIa, and at positions 22 and 140 in the interface region between sTF and the first epidermal growth factor-like (EGF1) domain of FVIIa. The tightness of the local interactions in these parts of the interface is similar to that seen in the interior of globular proteins. This was further emphasized by the reduced local polarity detected by the fluorescent label upon FVIIa binding, especially in the sTF-Gla region. There were indications of structural rigidity also at positions 45 and 94 in the interface region between sTF and the protease domain (PD) of FVIIa, despite the perturbed cofactor function of these sTF variants. The results of the present study indicate that the multi-probing approach enables comparison of the tightness and characteristics of interaction along the binding interface of a protein complex. This approach also increases the probability of acquiring reliable structural data that are descriptive of the wild-type proteins.

Four loops of the catalytic domain of factor viia mediate the effect of the first EGF-like domain substitution on factor viia catalytic activity

The presence of tissue factor is essential for factor VIIa (FVIIa) to reach its full catalytic potential. The previous work in this laboratory demonstrated that substitution of the EGF1 domain of factor VIIa with that of factor IX (FVII((IXegf1))a) results in a substantial decrease in TF-binding affinity and catalytic activity. Supporting simulations of the solution structures of Ca(2+)-bound factor VIIa and FVII((IXegf1))a with tissue factor are provided. Mutants are generated, based on the simulation model, to study the effect of EGF1 substitution on catalytic activity. The simulations show larger Gla-EGF1 and EGF1-EGF2 inter-domain motions for FVII((IXegf1))a than for factor VIIa. The catalytic domain of the chimeric factor VIIa has been disturbed and several surface loops in the catalytic domain of FVII((IXegf1))a (Loop 170s (170-182), Loop 1 (185-188) and Loop 2 (221A-225)) manifest larger position fluctuations than wild-type. The position of Loop 140s (142-152) of FVII((IXegf1))a, near the N terminus insertion site of the catalytic domain, shifts relative to factor VIIa, resulting in a slight alteration of the active site. The results suggest that these four loops mediate the effect of the EGF1 domain substitution on the S1 site and catalytic residues. To test the model, we prepared mutations of these surface loops, including four FVII mutants, D186A, K188A, L144A and R147A, a FVII mutant with multiple mutations (MM3: L144A+R147A+D186A) and a FVII mutant with Loop 170s partially deleted, Loop 170s(del). The catalytic activities towards a small peptidyl substrate decreased 2.4, 4.5 and 9-fold for Loop 170s(del)a (a, activated), L144Aa and D186Aa, respectively, while MM3a lost almost all catalytic activity. The combined results of the simulations and mutants provide insight into the mechanism by which tissue factor enhances factor VIIa catalytic activity.

Spectroscopic probing of the influence of calcium and the gla domain on the interaction between the first EGF domain in factor VIIa and tissue factor

The binding of factor VIIa (FVIIa) to tissue factor (TF) initiates blood coagulation. The binary complex is dependent on Ca2+ binding to several sites in FVIIa and is maintained by multiple contacts distributed throughout the various domains. Although the contributions from various residues and domains, including the Ca2+ coordination, to the global binding energy have been characterized, their importance for specific local interactions is virtually unknown. To address this aspect, we have attached four spectroscopic probes to an engineered Cys residue replacing Phe140 in soluble TF (sTF). This allows the monitoring of local changes in hydrophobicity and rigidity upon complex formation at the interface between the first epidermal growth factor-like (EGF1) domain of FVIIa and sTF. The fluorescent labels used sense a more hydrophobic environment and the spin labels are dramatically immobilized when FVIIa binds sTF. The results obtained with a 4-carboxyglutamic acid (Gla)-domainless derivative of FVIIa indicate that the Gla domain has no or minimal influence on the interaction between EGF1 and sTF. However, there is a difference in local Ca2+ dependence between Gla-domainless and full-length FVIIa.

Properties of spin and fluorescent labels at a receptor-ligand interface

Site-directed labeling was used to obtain local information on the binding interface in a receptor-ligand complex. As a model we have chosen the specific association of the extracellular part of tissue factor (sTF) and factor VIIa (FVIIa), the primary initiator of the blood coagulation cascade. Different spectroscopic labels were covalently attached to an engineered cysteine in position 140 in sTF, a position normally occupied by a Phe residue previously characterized as an important contributor to the sTF:FVIIa interaction. Two spin labels, IPSL [N-(1-oxyl-2,2,5, 5-tetramethyl-3-pyrrolidinyl)iodoacetamide] and MTSSL [(1-oxyl-2,2,5, 5-tetramethylpyrroline-3-methyl)methanethiosulfonate], and two fluorescent labels, IAEDANS [5-((((2-iodoacetyl)amino) ethyl)amino)naphthalene-1-sulfonic acid] and BADAN [6-bromoacetyl-2-dimethylaminonaphthalene], were used. Spectral data from electron paramagnetic resonance (EPR) and fluorescence spectroscopy showed a substantial change in the local environment of all labels when the sTF:FVIIa complex was formed. However, the interaction was probed differently by each label and these differences in spectral appearance could be attributed to differences in label properties such as size, polarity, and/or flexibility. Accordingly, molecular modeling data suggest that the most favorable orientations are unique for each label. Furthermore, line-shape simulations of EPR spectra and calculations based on fluorescence depolarization measurements provided additional details of the local environment of the labels, thereby confirming a tight protein-protein interaction between FVIIa and sTF when the complex is formed. The tightness of this local interaction is similar to that seen in the interior of globular proteins.

Anatomy of hot spots in protein interfaces

Binding of one protein to another is involved in nearly all biological functions, yet the principles governing the interaction of proteins are not fully understood. To analyze the contributions of individual amino acid residues in protein-protein binding we have compiled a database of 2325 alanine mutants for which the change in free energy of binding upon mutation to alanine has been measured (available at http://motorhead. ucsf.edu/thorn/hotspot). Our analysis shows that at the level of side-chains there is little correlation between buried surface area and free energy of binding. We find that the free energy of binding is not evenly distributed across interfaces; instead, there are hot spots of binding energy made up of a small subset of residues in the dimer interface. These hot spots are enriched in tryptophan, tyrosine and arginine, and are surrounded by energetically less important residues that most likely serve to occlude bulk solvent from the hot spot. Occlusion of solvent is found to be a necessary condition for highly energetic interactions.

Coagulation factor VII Gln100 --> Arg. Amino acid substitution at the epidermal growth factor 2-protease domain interface results in severely reduced tissue factor binding and procoagulant function

We have used recombinant mammalian expression and purification of the factor VII (FVII) variant Gln100 --> Arg (Q100RFVII) to study FVII deficiency in subjects with this mutation. Q100RFVII was secreted poorly in comparison with wild-type FVII (WTFVII) in a stable mammalian expression system, and purified variant protein was found to have undetectable clotting activity. Following activation by immobilized factor Xa, Q100RFVIIa had amidolytic activity similar to WTFVIIa in the absence of soluble tissue factor (sTF); however, unlike WTFVIIa no typical increase in activity was seen after addition of sTF. In a factor X activation assay using relipidated transmembrane truncated tissue factor (residues 1-243), Q100RFVIIa showed less than 5% of the ability of WTFVIIa to activate factor X. We performed direct binding analysis of WT and Q100RFVII/FVIIa to immobilized sTF using surface plasmon resonance, and severely reduced binding of both non-activated and activated Q100RFVII to sTF was seen, indicating a pronounced defect in tissue factor (TF) interaction with this variant. In the sTF-FVIIa crystal structure the candidate residue Gln100 is not in contact with TF but is at the epidermal growth factor 2-protease domain interface. We suggest that the mutation results in a global fold change severely reducing tissue factor interaction; mutation of FVII residues not directly involved in the interaction with TF may still result in variant FVII unable to take part in the initiation of coagulation.

Ca2+ binding to the first epidermal growth factor module of coagulation factor VIIa is important for cofactor interaction and proteolytic function

Epidermal growth factor-like (EGF) domain Ca2+ binding sites in the homologous coagulation factors VII, IX, and X stabilize the structural orientation of the gamma-carboxyglutamic acid-rich (Gla) domain relative to EGF-1. Site-directed mutagenesis was employed here to analyze the functional importance of Ca2+ binding to EGF-1 in factor VIIa (VIIa), which initiates coagulation in complex with its cofactor, tissue factor (TF). Ala replacements for Asp63 or Gln49 resulted in reduced TF affinity concordant with the number of eliminated Ca2+-coordinating oxygen atoms in the respective side chains. Ca2+ binding to EGF-1 had no major direct effect on contacts with TF residue Gln110 or on interactions of VIIa residues Arg79 and Phe40, suggesting that the stabilized Gla-EGF-1 orientation affects overall docking. Gly, Ala, and Glu replacements at Asp46, which is a Ca2+-coordinating residue at the Gla aromatic stack carboxyl terminus, are consistent with the notion that an increased flexibility of the Gla domain relative to EGF-1 contributes significantly to loss of function. Certain mutants in the EGF-1 Ca2+ site had reduced proteolytic function, suggesting the importance of the high affinity Ca2+ binding site for macromolecular substrate interaction.

A Soluble Tissue Factor Mutant Is a Selective Anticoagulant and Antithrombotic Agent

One approach to developing safer and more efficacious agents for the treatment of thrombotic disease involves the design and testing of inhibitors that block specific steps in the coagulation cascade. We describe here the development of a mutant of human tissue factor (TF ) as a specific antagonist of the extrinsic pathway of blood coagulation and the testing of this mutant in a rabbit model of arterial thrombosis. Alanine substitutions of Lys residues 165 and 166 in human TF have been shown previously to diminish the cofactor function of TF in support of factor X (FX) activation catalyzed by factor VIIa (FVIIa). The K165A:K166A mutations have been incorporated into soluble TF (sTF; residues 1-219) to generate the molecule “hTFAA.” hTFAA binds FVIIa with kinetics and affinity equivalent to wild-type sTF, but the hTFAA⋅FVIIa complex shows a 34-fold reduction in catalytic efficiency for FX activation relative to the activity measured for sTF⋅FVIIa. hTFAA inhibits the activation of FX catalyzed by the complex formed between FVIIa and relipidated TF(1-243). hTFAA prolongs prothrombin time (PT) determined with human plasma and relipidated TF(1-243) or membrane bound TF, and has no effect on activated partial thromboplastin time, but is 70-fold less potent as an inhibitor of PT with rabbit plasma. The rabbit homologue of this mutant (“rTFAA”) was produced and shown to have greater potency with rabbit plasma. Both hTFAA and rTFAA display an antithrombotic effect in a rabbit model of arterial thrombosis with rTFAA giving full efficacy at a lower dose than hTFAA. Compared to heparin doses of equal antithrombotic potential, hTFAA and rTFAA cause less bleeding as judged by measurements of the cuticle bleeding time. These results indicate that TF⋅FVIIa is a good target for the development of new anticoagulant drugs for the treatment of thrombotic disease.

A peptide sequence from the EGF-2 like domain of FVII inhibits TF-dependent FX activation

We have found that synthetic peptides derived from the two epidermal growth factor-like domains of factor VII are inhibitors of tissue factor dependent factor X activation. Inhibition was most pronounced for a constrained sequence of amino acids corresponding to positions 91-102 of factor VII, Cys-Val-Asn-Glu-Asn-Gly-Gly-Cys-Glu-Gin-Tyr-Cys. The biological activity appeared to be localized to the tripeptide 'motif', Glu-Gln-Tyr, within the larger sequence. The cyclic peptide was also an inhibitor of tissue factor induced coagulation of plasma, using lipidated tissue factor or tissue factor expressed on the surface of living cells. However, it did not interfere with intrinsic coagulation. Inhibition of factor X activation was dose-dependent with an IC50 value of 350 microM. Kinetic analyses revealed non-competitive inhibition with respect to factor X and suggested that the peptide sequence interferes with the factor VII/tissue factor/factor X complex formation and function. A pentapeptide analog of the putative pharmacophore was also a dose-dependent inhibitor of factor X activation with an IC50 value of 560 microM, but the tripeptide, Glu-Gin-Tyr, alone was without effect. Our results suggest a direct role for the second epidermal growth factor-like domain of factor VII, and in particular its loop I, in the formation and function of the factor VII/tissue factor/factor X complex.

Synthetic peptide analogs of tissue factor and factor VII which inhibit factor Xa formation by the tissue factor/factor VIIa complex

Factor VII (FVII) and tissue factor (TF) form a binary complex which initiates the extrinsic pathway of the blood coagulation cascade. The infrequent tripeptide motif Trp-Lys-Ser (WKS) is found three times in TF. It has been suggested that the motif is involved in binding of TF to FVII(a). Also. Lys165 and Lys166 of TF have been reported to be important for factor X activation. To elucidate the molecular interactions between TF and FVIIa, and the interactions between the binary complex and FX, we examined the inhibitory effect of synthetic TF and FVII peptide analogs. One- and two-stage chromogenic assays were employed, as well as one-stage coagulation assay. The peptide analogs of TF possessed the WKS motif, the double lysine residues or other regions of TF. Synthetic peptides of FVII encompassing sequences of the FVII285-305 region were included for comparative purposes. TF154-167 and FVII300-305 significantly inhibited both FX activation and plasma coagulation. FVII285-294 acted synergistically, increasing that effect observed by FVII300-305 on FX activation. However, TF163-175 possessing the double lysine residues did not inhibit FX activation, indicating that inhibition of FXa formation and coagulation by TF154-167 is due to the region 154-162 of TF. None of the peptides, including the WKS tripeptide, interfered with the FVIIa activity of the TF/FVIIa complex. Thus, the results do not suggest that the WKS motifs are necessary for binding of TF to FVIIa but that the third WKS motif may be of importance for the activation of FX.

Initiation of blood coagulation: the tissue factor/factor VIIa complex

Tissue factor (TF), a transmembrane glycoprotein, functions as an essential activator of the serine protease factor VIIa. This enzymatic complex is considered to be the principal initiator of in vivo coagulation. Recent studies emphasize the role of the TF/VIIa complex in a number of pathophysiological processes, such as Gram-negative sepsis, coronary artery disease and neointimal hyperplasia after angioplasty. Monocytes/macrophages are important contributors to some of these diseases and there have been new insights into the biology of TF regulation in monocytes. In the light of its structural similarity to cytokine receptors, there has been frequent speculation that TF has a role in intracellular signaling, a suggestion that is supported by some recent studies that propose a true receptor function for TF.

Tissue Factor residue Asp44 regulates catalytic function of the bound proteinase Factor VIIa

The coagulation pathways are initiated by the cell-surface receptor Tissue Factor (TF), which binds the serine proteinase coagulation Factor VIIa (VIIa), resulting in enhanced catalytic function, both amidolytic, towards small pseudo-substrates, and proteolytic, towards macromolecular substrates. Here we implicate Asp44 in TF as a ligand-interactive residue that, in contrast with previously characterized binding residues, is involved in the enhancement of VIIa catalytic function. Whereas charge neutralization by replacement of Asp44 with Asn did not reduce function of human TF, the exchange by Ala resulted in mutants with 8-fold reduced affinity for binding of VIIa. Enhancement of VIIa amidolytic function by TF Ala44 was reduced by 20-25% relative to wild-type and support of proteolytic function was diminished 6-fold indicating that this cofactor residue is significantly enhancing proteolysis of the macromolecular substrate by VIIa. Replacement of Asp44 by Glu, Thr, and Arg exhibited a less severe phenotype with an approx. 4-fold reduced affinity for VIIa and a 2-3 fold diminished activation of Factor X. The improved activity of these mutants as compared with the Ala replacement is consistent with functional importance of an extended side chain at this position. The specific influence of the Asp44 exchange on catalytic function of the TF x VIIa complex indicates fine specificity of the TF ligand interface in mediating receptor and cofactor function.

The crystal structure of the complex of blood coagulation factor VIIa with soluble tissue factor

Blood coagulation is initiated when tissue factor binds to coagulation factor VIIa to give an enzymatically active complex which then activates factors IX and X, leading to thrombin generation and clot formation. We have determined the crystal structure at 2.0-A degrees resolution of active-site-inhibited factor VIIa complexed with the cleaved extracellular domain of tissue factor. In the complex, factor VIIa adopts an extended conformation. This structure provides a basis for understanding many molecular aspects of the initiation of coagulation.

Analysis of the factor VIIa binding site on human tissue factor: effects of tissue factor mutations on the kinetics and thermodynamics of binding

Surface plasmon resonance (SPR) measurements on a BIAcore instrument have been used to measure the effects of mutations in human tissue factor (TF), the initiator of blood coagulation, on the kinetics and affinity of binding to human FVIIa. TF mutant proteins were produced in soluble form by expression of the extracellular domain (sTF) in Escherichia coli followed by immunoaffinity purification. Mutants were designed and analyzed on the basis of the structure of sTF recently determined by X-ray crystallography [Muller et al. (1994) Biochemistry 33, 10864-10870]. Wild-type sTF binding to immobilized FVIIa has k(on) = 3.4 +/- 0.8 x 10(5) M-1 s-1 and k(off) = 2.1 +/- 0.1 x 10(-3) s-1 with a calculated KD of 6.3 +/- 1.2 nM and delta G of -11.2 +/- 0.1 kcal mol-1. Calorimetric measurements indicate that binding occurs with a favorable delta H of -32 kcal mol-1, an unfavorable delta S of -70 cal K-1 mol-1, and a delta Cp of -730 cal K mol-1. The value of delta Cp is consistent with burial of a large nonpolar surface area upon binding. Five residues on TF, Lys20, Trp45, Asp58, Tyr94, and Phe140, make a large contribution (delta delta G = 1-2.5 kcal mol-1) to FVIIa binding, a set of 17 mutations result in modest decreases in affinity (delta delta G = 0.3-1 kcal mol-1), and 40 mutations have delta delta G smaller than the experimental uncertainty (+/- 0.3 kcal mol-1). Mutations at four sites result in small (0.3-0.5 kcal mol-1) increases in affinity. Decreases in affinity result primarily from increased rates of dissociation. These data define a putative FVIIa binding site on one face of the TF structure with most of the contacts contributed by the N-terminal fibronectin type III domain. The critical binding residues are found on beta-strands. An additional set of residues located on the surface of the C-terminal fibronectin type III domain opposite the FVIIa binding site have a role in the procoagulant activity of sTF but are not involved in FVIIa binding.

Activation of blood coagulation factor VIIa with cleaved tissue factor extracellular domain and crystallization of the active complex

Exposure of blood to tissue factor leads to the formation of a high affinity tissue factor/factor VIIa complex which initiates blood coagulation. As a first step toward obtaining structural information of this enzyme system, a complex of active-site inhibited factor VIIa (F.VIIai) and soluble tissue factor (sTF) was prepared for crystallization. Crystals were obtained, but only after long incubation times. Analysis by SDS-PAGE and mass spectrometry indicated the presence of sTF fragments similar to those formed by proteolytic digestion with subtilisin (Konigsberg, W., Nemerson, Y., Fang, C., Lin, T.-C. Thromb. Haemost. 69:1171, 1993). To test the hypothesis that limited proteolysis of sTF facilitated the crystallization of the complex, sTF fragments were generated by subtilisin digestion and purified. Analysis by tandem mass spectrometry showed the presence of nonoverlapping N- and C-terminal sTF fragments encompassing more than 90% of the tissue factor extracellular domain. Enzymatic assays and binding studies demonstrated that an equimolar mixture of N- and C-terminal fragments bound to factor VIIa and fully restored cofactor activity. A complex of F.VIIai and sTF fragments was prepared for crystallization. Crystals were obtained using microseeding techniques. The best crystals had maximum dimensions of 0.12 x 0.12 x 0.6 mm and showed diffraction to a resolution of 3 A.