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

Evolutionary Adaptations in Pseudonaja Textilis Venom Factor X Induce Zymogen Activity and Resistance to the Intrinsic Tenase Complex

The venom of the Australian snake Pseudonaja textilis comprises powerful prothrombin activators consisting of factor X (v-ptFX)- and factor V-like proteins. While all vertebrate liver-expressed factor X (FX) homologs, including that of P. textilis, comprise an activation peptide of approximately 45 to 65 residues, the activation peptide of v-ptFX is significantly shortened to 27 residues. In this study, we demonstrate that exchanging the human FX activation peptide for the snake venom ortholog impedes proteolytic cleavage by the intrinsic factor VIIIa-factor IXa tenase complex. Furthermore, our findings indicate that the human FX activation peptide comprises an essential binding site for the intrinsic tenase complex. Conversely, incorporation of FX into the extrinsic tissue factor-factor VIIa tenase complex is completely dependent on exosite-mediated interactions. Remarkably, the shortened activation peptide allows for factor V-dependent prothrombin conversion while in the zymogen state. This indicates that the active site of FX molecules comprising the v-ptFX activation peptide partially matures upon assembly into a premature prothrombinase complex. Taken together, the shortened activation peptide is one of the remarkable characteristics of v-ptFX that has been modified from its original form, thereby transforming FX into a powerful procoagulant protein. Moreover, these results shed new light on the structural requirements for serine protease activation and indicate that catalytic activity can be obtained without formation of the characteristic Ile¹⁶-Asp¹⁹⁴ salt bridge via modification of the activation peptide.

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.

A frequent human coagulation Factor VII mutation (A294V, c152) in loop 140s affects the interaction with activators, tissue factor and substrates

Activated Factor VII (FVIIa) is a vitamin-K-dependent serine protease that initiates blood clotting after interacting with its cofactor tissue factor (TF). The complex FVIIa-TF is responsible for the activation of Factor IX (FIX) and Factor X (FX), leading ultimately to the formation of a stable fibrin clot. Activated FX (FXa), a product of FVIIa enzymic activity, is also the most efficient activator of zymogen FVII. Interactions of FVII/FVIIa with its activators, cofactor and substrates have been investigated extensively to define contact regions and residues involved in the formation of the complexes. Site-directed mutagenesis and inhibition assays led to the identification of sites removed from the FVIIa active site that influence binding specificity and affinity of the enzyme. In this study we report the characterization of a frequent naturally occurring human FVII mutant, A294V (residue 152 in the chymotrypsin numbering system), located in loop 140s. This region undergoes major rearrangements after FVII activation and is relevant to the development of substrate specificity. FVII A294V shows delayed activation by FXa as well as reduced activity towards peptidyl and macromolecular substrates without impairing the catalytic efficiency of the triad. Also, the interaction of this FVII variant with TF was altered, suggesting that this residue, and more likely loop 140s, plays a pivotal role not only in the recognition of FX by the FVIIa-TF complex, but also in the interaction of FVII with both its activators and cofactor TF.

Predicted solution structure of zymogen human coagulation FVII

A model solution structure for the complete tissue factor-free calcium ion-bound human zymogen FVII (residues 1-406) (FVII) has been constructed to study possible conformational changes associated with the activation process and tissue factor (TF) binding. The initial structure for the present model was constructed using the X-ray crystallographic structure of human coagulation FVIIa/TF complex bound with calcium ions (Banner et al., Nature 1996, 380, 41-46). This model was subsequently subjected to lengthy molecular dynamics simulations. The Amber force field in conjunction with the PME electrostatic summation method was employed. The estimated TF free solution structure was then compared with the currently available X-ray crystal structures of FVIIa (with or without TF, variable inhibitor bound) to estimate the restructuring of FVII due to TF binding and activation. The solution structure of the zymogen FVII in the absence of TF is predicted to be an extended domain structure similar to that of the TF-bound X-ray crystal structure. An additional extension of the serine protease (SP) domain of the zymogen above a reference lipid surface by approximately 7 A was in agreement with experiment. Significant Gla-EGF1 and EGF1-EGF2 interdomain motions in the zymogen were observed. Carbohydrate dimers attached to Ser-52 and Ser-60 did not cause restructuring in this domain. Minimal restructuring of the SP domain is found upon inference of the zymogen from the activated form. The catalytic triad residues maintain the H-bonded network while Lys-341 occupies the S1 specific site in the zymogen.

A new mutation in the HNF4 binding region of the factor VII promoter in a patient with severe factor VII deficiency

Investigation of the molecular basis of a severe factor VII (fVII) deficiency revealed compound heterozygosity in the fVII gene. On the paternal allele the patient had 3 structural gene abnormalities frequently associated with fVII deficiency. A new mutation, a C to T transition at position −55 relative to the translational start site, was found on the maternal allele. The study demonstrates that this mutation partially impeded binding of the transcriptional activator, hepatic nuclear factor 4, to the fVII promoter while greatly reducing reporter gene expression in hepatic cells.

A new mutation in the HNF4 binding region of the factor VII promoter in a patient with severe factor VII deficiency

Investigation of the molecular basis of a severe factor VII (fVII) deficiency revealed compound heterozygosity in the fVII gene. On the paternal allele the patient had 3 structural gene abnormalities frequently associated with fVII deficiency. A new mutation, a C to T transition at position −55 relative to the translational start site, was found on the maternal allele. The study demonstrates that this mutation partially impeded binding of the transcriptional activator, hepatic nuclear factor 4, to the fVII promoter while greatly reducing reporter gene expression in hepatic cells.

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.

Peptides corresponding to the second epidermal growth factor-like domain of human blood coagulation factor VII: synthesis, folding and biological activity

Factor VIIa (FVIIa) is the enzymatically active constituent of the FVIIa/tissue factor (TF) complex, the initiator of the extrinsic pathway of blood coagulation. The zymogen FVII and FVIIa are composed of discrete domains, two of which are homologous to the epidermal growth factor (EGF). This investigation examined the significance of the FVII EGF-2 domain in the processes leading to activation of factor X (FX). Peptides 47 residues in length and corresponding to the amino acid sequence of the EGF-2 domain of human FVII were prepared by solid-phase synthesis methods. Peptide variants with all six Cys residues replaced by L-2-aminobutyryl residues (1), or containing one (2a-c), two (3a,b) or three (4) disulfide bonds, were obtained by application of various S-protecting groups and oxidation methods. Peptide 4, containing the cystine bridge arrangement corresponding to that found in the native protein, was prepared by a two-step regioselective disulfide bond formation method. An evaluation of the anti-coagulant properties of peptides 1-4 revealed that all peptides, with the exception of the two-cystine isomer containing non-native disulfide pairings (3b), were potent inhibitors of TF/FVIIa-mediated activation of FX. The fully constrained peptide 4 was found to be twice as active as its completely non-constrained counterpart 1, the two peptides showing IC50 values of 1.6 +/- 0.5 microM (1) and 0.8 +/- 0.2 microM (4) with respect to TF/FVIIa-dependent FX activation. The results of this study demonstrate the functional importance of the EGF-2 domain of FVII in the induction of coagulation by the extrinsic pathway.

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.

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.

Mutation pattern in clinically asymptomatic coagulation factor VII deficiency

A total of 122 subjects, referred after presurgery screening or checkup for prolonged prothrombin time, were characterized for the presence of coagulation factor VII deficiency. Fourteen subjects carried a partial and asymptomatic deficiency, and in half of them dysfunctional molecules were detected in plasma. In nine subjects we found five missense mutations differing from those previously found in factor VII deficient patients. The others were homozygous for a common polymorphism (R353Q) that affects factor VII levels. A new codon dimorphism (A330) was also found in exon 8. Four mutations (R223W, M298I, R304Q, and R353Q) located at FVII-specific residues point out protein regions that are important for coagulation factor evolution, and two mutations (G342E and E265K) affect generic or partially generic residues. The newly reported mutations were combined with those we previously found, totalling 17 independent mutations responsible for FVII deficiency in 27 Italian pedigrees. We observed several similarities with the mutation pattern determined in factor IX, which include a high percentage of transitions at CpG doublets, the presence of hot spot sites affected by multiple substitutions, and of several topologically equivalent mutations.

Structurally and functionally distinct Ca2+ binding sites in the gamma-carboxyglutamic acid-containing domain of factor VIIa

The structural and functional effects of Ca2+ binding to vitamin-K-dependent coagulation factor VIIa were investigated. Conformational changes with a midpoint around 0.7 mM Ca2+ quenched the intrinsic protein fluorescence of a fragment of factor VIIa comprising only the light chain and this coincided with an increase in factor VIIa amidolytic activity in the absence of tissue factor. Ca2+ binding to sites in factor VIIa and in the fragment with an apparent dissociation constant of 1.3-1.4 mM induced binding to phospholipids. A similar Ca2+ dependency was not observed with factor VIIa lacking the N-terminal 38 or 44 residues of the light chain and the observed effects could thus be attributed to gamma-carboxyglutamic-acid-dependent Ca2+ binding. Mg2+ appeared to bind to the site(s) of relatively higher affinity since, although it was less efficient than Ca2+, it stimulated the amidolytic activity and induced quenching of the intrinsic fluorescence. In contrast, Mg2+ did not induce expression of the phospholipid-interactive structure. The binding properties of two monoclonal antibodies that recognized epitopes in the gamma-carboxy-glutamic-acid-rich domain of factor VIIa corroborated the occurrence of two Ca(2+)-induced, sequential structural changes and only one of the antibodies recognized the Mg(2+)-induced structure. Thus Ca2+ binding to the gamma-carboxyglutamic-acid-containing domain appeared to result in at least two distinct structural transitions with different functional consequences. The two (sets of) sites responsible for the observed effects could be distinguished based upon differences in Ca2+ affinity and metal ion selectivity. The interaction between factor VIIa and tissue factor was monitored by means of a direct binding assay and an amidolytic assay. In both systems, half-maximal Ca2+ enhancement was observed at 0.25 mM. This coincided with a Ca(2+)-induced conformational change in factor VIIa associated with fluorescence quenching. The same effect on amidolytic activity was observed with the two N-terminally truncated forms of factor VIIa and it is presumably mediated by Ca2+ binding to a site located in the serine protease part.

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.

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.