Role of the activation peptide domain in human factor X activation by the extrinsic Xase complex

Membrane-bound model of the ternary complex between factor VIIa/tissue factor and factor X

ABSTRACT

Formation of the extrinsic complex (EC) on cell surfaces is the event that triggers the coagulation cascade. Tissue factor (TF) and factor VIIa (FVIIa) form the EC together with FX on phosphatidylserine-containing membranes, leading to FX activation by TF:FVIIa. This lipid dependence has made experimental characterization of the EC structure challenging. Using a novel computational methodology combining rigid-body protein-protein docking and extensive nonequilibrium molecular dynamics simulations in the explicit presence of a membrane, we developed, to our knowledge, the first atomic-level model of the EC, taking full account of the role of the membrane. Rigid-body docking generated 1 000 000 protein-only structures that predict the binding of key EC domains. Residue-residue contact information was then used in nonequilibrium simulations to drive the formation of the EC on a phosphatidylserine/phosphatidylcholine membrane surface, providing, to our knowledge, the first membrane-bound model for the EC. Strikingly, in our model, FX makes contact with TF:FVIIa chiefly via its γ-carboxyglutamate-rich (GLA) domain and protease domain, with the majority of the FX light chain (ie, its 2 epidermal growth factor-like domains) out in the solvent, making no direct contact with TF:FVIIa. The TF exosite makes substantial contacts with both the FX- and FVIIa-GLA domains, in which TF residue K165 engages directly with the FVIIa-GLA domain, whereas K166 plays a central role in binding to the FX-GLA domain. These findings underscore the substrate-binding exosite of TF as being pivotal in the formation of the EC, serving as a critical interface linking the GLA domains of both FVIIa and FX.

Activated protein C has a regulatory role in factor VIII function

Mechanisms thought to regulate activated factor VIII (FVIIIa) cofactor function include A2-domain dissociation and activated protein C (APC) cleavage. Unlike A2-domain dissociation, there is no known phenotype associated with altered APC cleavage of FVIII, and biochemical studies have suggested APC plays a marginal role in FVIIIa regulation. However, the in vivo contribution of FVIIIa inactivation by APC is unexplored. Here we compared wild-type B-domainless FVIII (FVIII-WT) recombinant protein with an APC-resistant FVIII variant (FVIII-R336Q/R562Q; FVIII-QQ). FVIII-QQ demonstrated expected APC resistance without other changes in procoagulant function or A2-domain dissociation. In plasma-based studies, FVIII-WT/FVIIIa-WT demonstrated dose-dependent sensitivity to APC with or without protein S, whereas FVIII-QQ/FVIIIa-QQ did not. Importantly, FVIII-QQ demonstrated approximately fivefold increased procoagulant function relative to FVIII-WT in the tail clip and ferric chloride injury models in hemophilia A (HA) mice. To minimize the contribution of FV inactivation by APC in vivo, a tail clip assay was performed in homozygous HA/FV Leiden (FVL) mice infused with FVIII-QQ or FVIII-WT in the presence or absence of monoclonal antibody 1609, an antibody that blocks murine PC/APC hemostatic function. FVIII-QQ again demonstrated enhanced hemostatic function in HA/FVL mice; however, FVIII-QQ and FVIII-WT performed analogously in the presence of the PC/APC inhibitory antibody, indicating the increased hemostatic effect of FVIII-QQ was APC specific. Our data demonstrate APC contributes to the in vivo regulation of FVIIIa, which has the potential to be exploited to develop novel HA therapeutics.

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

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

Exosite binding drives substrate affinity for the activation of coagulation factor X by the intrinsic Xase complex

Factor X activation by the intrinsic Xase complex, composed of factor IXa bound to factor VIIIa on membranes, is essential for the amplified blood coagulation response. The biological significance of this step is evident from bleeding arising from deficiencies in factors VIIIa or IXa in hemophilia. Here, we assess the mechanism(s) that enforce the distinctive specificity of intrinsic Xase for its biological substrate. Active-site function of IXa was assessed with a tripeptidyl substrate (PF-3688). The reversible S1 site binder, 4-aminobenzamidine (pAB), acted as a classical competitive inhibitor of PF-3688 cleavage by Xase. In contrast, pAB acted as a noncompetitive inhibitor of factor X activation. This disconnect between peptidyl substrate and protein substrate cleavage indicates a major role for interactions between factor X and extended sites on Xase in determining substrate affinity. Accordingly, an uncleavable factor X variant, not predicted to engage the active site of IXa within Xase, acted as a classical competitive inhibitor of factor X activation. Fluorescence studies confirmed the binding of factor X to Xase assembled with IXa with a covalently blocked active site. Our findings suggest that the recognition of factor X by the intrinsic Xase complex occurs through a multistep "dock-and-lock" pathway in which the initial interaction between factor X and intrinsic Xase occurs at exosites distant from the active site, followed by active-site docking and bond cleavage.

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.

Occlusion of anion-binding exosite 2 in meizothrombin explains its impaired ability to activate factor V

The proteolytic conversion of factor V to factor Va is central for amplified flux through the blood coagulation cascade. Heterodimeric factor Va is produced by cleavage at three sites in the middle of factor V by thrombin, yielding an N terminus-derived heavy chain and a C terminus-derived light chain. Here, we show that light chain formation resulting from the C-terminal cleavage is the rate-limiting step in the formation of fully cleaved Va. This rate-limiting step also corresponded to and was sufficient for the ability of cleaved factor V to bind Xa and assemble into the prothrombinase complex. Meizothrombin, the proteinase intermediate in thrombin formation, cleaves factor V more slowly than does thrombin, resulting in a pronounced defect in the formation of the light chain. A ∼100-fold reduced rate of meizothrombin-mediated light chain formation by meizothrombin corresponded to equally slow production of active cofactor and an impaired ability to amplify flux through the coagulation cascade initiated in plasma. We show that this defect arises from the occlusion of anion-binding exosite 2 in the catalytic domain by the covalently retained propiece in meizothrombin. Our findings provide structural insights into the prominent role played by exosite 2 in the rate-limiting step of factor V activation. They also bear on how factor V is converted into a cofactor capable of assembling into prothrombinase.

The carboxyl-terminal region of human coagulation factor X as a natural linker for fusion strategies

Fusion with human serum albumin (HSA), which represents a well-established technique to extend half-life of therapeutic proteins, commonly exploits intervening peptide linkers as key components. Here, we explored the human coagulation factor X (FX) carboxyl-terminal region, previously demonstrated by us to be dispensable for secretion and coagulant activity, as a natural linker for fusion purposes. To test our hypothesis, we compared direct FX-HSA fusion with the designed FX-HSA fusion proteins mimicking the recombinant activated factor VII (rFVIIa)-HSA or factor IX (FIX)-HSA chimeras, both strongly dependent from artificial linkers. Three constructs were produced by direct tandem fusion (FX-HSA) and through flexible (glycine/serine; FX-GS-HSA, mimicking rFVIIa-HSA) or cleavable (incorporating the FX activation site; FX-CL-HSA, mimicking FIX-HSA) linkers. The FX-HSA was efficiently secreted and displayed prolonged plasma persistence in mice. All chimeras possessed remarkable pro-coagulant activity, comparable to FX for FX-HSA (88.7 ± 6.0%) and FX-CL-HSA (98.0 ± 16.4%) or reduced for FX-GS-HSA (55.8 ± 5.4%). Upon incubation with activators, FX-HSA and FX-CL-HSA displayed a correct activation profile while the FX-GS-HSA activation was slightly defective. In fluorogenic-based assays, FX-HSA showed normal activity over time and a specific amidolytic activity (1.0 ± 0.12) comparable to that of FX. Overall, the FX-HSA features indicate that the FX carboxyl-terminal region represents an intrinsic sequence allowing direct tandem fusion. Our results provide the first experimental evidence for i) a coagulation factor fusion protein with biological properties independent from artificial linkers, ii) the suitability of FX carboxyl-terminal region as a natural linker for fusion purposes.

Combination of aptamer and drug for reversible anticoagulation in cardiopulmonary bypass

Unfractionated heparin (UFH), the standard anticoagulant for cardiopulmonary bypass (CPB) surgery, carries a risk of post-operative bleeding and is potentially harmful in patients with heparin-induced thrombocytopenia-associated antibodies. To improve the activity of an alternative anticoagulant, the RNA aptamer 11F7t, we solved X-ray crystal structures of the aptamer bound to factor Xa (FXa). The finding that 11F7t did not bind the catalytic site suggested that it could complement small-molecule FXa inhibitors. We demonstrate that combinations of 11F7t and catalytic-site FXa inhibitors enhance anticoagulation in purified reaction mixtures and plasma. Aptamer-drug combinations prevented clot formation as effectively as UFH in human blood circulated in an extracorporeal oxygenator circuit that mimicked CPB, while avoiding side effects of UFH. An antidote could promptly neutralize the anticoagulant effects of both FXa inhibitors. Our results suggest that drugs and aptamers with shared targets can be combined to exert more specific and potent effects than either agent alone.

Exactin: A specific inhibitor of Factor X activation by extrinsic tenase complex from the venom of Hemachatus haemachatus

Unwanted clots lead to heart attack and stroke that result in a large number of deaths. Currently available anticoagulants have some drawbacks including their non-specific actions. Therefore novel anticoagulants that target specific steps in the coagulation pathway are being sought. Here we describe the identification and characterization of a novel anticoagulant protein from the venom of Hemachatus haemachatus (African Ringhals cobra) that specifically inhibits factor X (FX) activation by the extrinsic tenase complex (ETC) and thus named as exactin. Exactin belongs to the three-finger toxin (3FTx) family, with high sequence identity to neurotoxins and low identity to the well-characterized 3FTx anticoagulants-hemextin and naniproin. It is a mixed-type inhibitor of ETC with the kinetic constants, Ki’ and Ki determined as 30.62 ± 7.73 nM and 153.75 ± 17.96 nM, respectively. Exactin does not bind to the active site of factor VIIa and factor Xa based on its weak inhibition (IC₅₀ ≫ 300 μM) to the amidolytic activities of these proteases. Exactin shows exquisite macromolecular specificity to FX activation as compared to factor IX activation by ETC. Exactin thus displays a distinct mechanism when compared to other anticoagulants targeting ETC, with its selective preference to ETC-FX [ES] complex.

The Fragment 1 Region of Prothrombin Facilitates the Favored Binding of Fragment 12 to Zymogen and Enforces Zymogen-like Character in the Proteinase

Thrombin is produced from the C-terminal half of prothrombin following its proteolytic activation. The N-terminal half, released as the propiece Fragment 12 (F12), is composed of an N-terminal γ-carboxyglutamate domain (Gla) followed by two kringles (K1 and K2). The propiece plays essential roles in regulating prothrombin activation and proteinase function. The latter results from the ability of F12 to reversibly bind to the (pro)catalytic domain through K2 with high affinity and highly favorable thermodynamic constants when it is a zymogen in comparison to proteinase. Such discrimination is lost for K2 binding after proteolytic removal of the N-terminal Gla-K1 region of F12. The Ca(2+)-stabilized structure of the Gla domain is not required for F12 to bind the zymogen form more favorably. Enhanced binding to zymogen versus proteinase correlates with the ability of the propiece to enforce zymogen-like character in the proteinase. This is evident in variants of meizothrombin, an intermediate of prothrombin activation that contains the propiece covalently attached. This phenomenon is also independent of the Gla domain. Thus, the presence of K1 in covalent linkage with K2 in the propiece governs the ability of K2 to bind the (pro)catalytic domain in favor of zymogen, thereby enforcing zymogen-like character in the proteinase.

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

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

Membrane binding by prothrombin mediates its constrained presentation to prothrombinase for cleavage

Long-standing dogma proposes a profound contribution of membrane binding by prothrombin in determining the rate at which it is converted to thrombin by prothrombinase. We have examined the action of prothrombinase on full-length prothrombin variants lacking γ-carboxyglutamate modifications (desGla) with impaired membrane binding. We show an unexpectedly modest decrease in the rate of thrombin formation for desGla prothrombin but with a major effect on the pathway for substrate cleavage. Using desGla prothrombin variants in which the individual cleavage sites have been singly rendered uncleavable, we find that loss of membrane binding and other Gla-dependent functions in the substrate leads to a decrease in the rate of cleavage at Arg(320) and a surprising increase in the rate of cleavage at Arg(271). These compensating effects arise from a loss in the membrane component of exosite-dependent tethering of substrate to prothrombinase and a relaxation in the constrained presentation of the individual cleavage sites for active site docking and catalysis. Loss of constraint is evident as a switch in the pathway for prothrombin cleavage and the intermediate produced but without the expected profound decrease in rate. Extension of these findings to the action of prothrombinase assembled on platelets and endothelial cells on fully carboxylated prothrombin reveals new mechanistic insights into function on physiological membranes. Cell-dependent enzyme function is probably governed by a differential ability to support prothrombin binding and the variable accumulation of intermediates from the two possible pathways of prothrombin activation.

Thrombin activity propagates in space during blood coagulation as an excitation wave

Injury-induced bleeding is stopped by a hemostatic plug formation that is controlled by a complex nonlinear and spatially heterogeneous biochemical network of proteolytic enzymes called blood coagulation. We studied spatial dynamics of thrombin, the central enzyme of this network, by developing a fluorogenic substrate-based method for time- and space-resolved imaging of thrombin enzymatic activity. Clotting stimulation by immobilized tissue factor induced localized thrombin activity impulse that propagated in space and possessed all characteristic traits of a traveling excitation wave: constant spatial velocity, constant amplitude, and insensitivity to the initial stimulation once it exceeded activation threshold. The parameters of this traveling wave were controlled by the availability of phospholipids or platelets, and the wave did not form in plasmas from hemophilia A or C patients who lack factors VIII and XI, which are mediators of the two principal positive feedbacks of coagulation. Stimulation of the negative feedback of the protein C pathway with thrombomodulin produced nonstationary patterns of wave formation followed by deceleration and annihilation. This indicates that blood can function as an excitable medium that conducts traveling waves of coagulation.

Meizothrombin is an unexpectedly zymogen-like variant of thrombin

Thrombin is produced by the ordered action of prothrombinase on two cleavage sites in prothrombin. Meizothrombin, a proteinase precursor of thrombin, is a singly cleaved species that accumulates abundantly as an intermediate. We now show that covalent linkage of the N-terminal propiece with the proteinase domain in meizothrombin imbues it with exceptionally zymogen-like character. Meizothrombin exists in a slowly reversible equilibrium between two equally populated states, differing by as much as 140-fold in their affinity for active site-directed ligands. The distribution between the two forms, designated zymogen-like and proteinase-like, is affected by Na(+), thrombomodulin binding, or active site ligation. In rapid kinetic measurements with prothrombinase, we also show that the zymogen-like form is produced following the initial cleavage reaction and slowly equilibrates with the proteinase-like form in a previously unanticipated rate-limiting step before it can be further cleaved to thrombin. The reversible equilibration of meizothrombin between zymogen- and proteinase-like states provides new insights into its ability to selectively exhibit the anticoagulant function of thrombin and the mechanistic basis for its accumulation during prothrombin activation. Our findings also provide unexpected insights into the regulation of proteinase function and how the formation of meizothrombin may yield a long lived intermediate with an important regulatory role in coagulation.

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

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

Antibodies associated with heparin-induced thrombocytopenia (HIT) inhibit activated protein C generation: new insights into the prothrombotic nature of HIT

Heparin-induced thrombocytopenia (HIT) is caused by antibodies that recognize complexes between platelet factor 4 (PF4) and heparin or glycosaminoglycan side chains. These antibodies can lead to a limb- and life-threatening prothrombotic state. We now show that HIT antibodies are able to inhibit generation of activated protein C (aPC) by thrombin/thrombomodulin (IIa/TM) in the presence of PF4. Tetrameric PF4 potentiates aPC generation by formation of complexes with chondroitin sulfate (CS) on TM. Formation of these complexes occurs at a specific molar ratio of PF4 to glycosaminoglycan. This observation and the finding that the effect of heparin on aPC generation depends on the concentration of PF4 suggest similarity between PF4/CS complexes and those that bind HIT antibodies. HIT antibodies reduced the ability of PF4 to augment aPC formation. Cationic protamine sulfate, which forms similar complexes with heparin, also enhanced aPC generation, but its activity was not blocked by HIT antibodies. Our studies provide evidence that complexes formed between PF4 and TM's CS may play a physiologic role in potentiating aPC generation. Recognition of these complexes by HIT antibodies reverses the PF4-dependent enhancement in aPC generation and may contribute to the prothrombotic nature of HIT.

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

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

Regulated cleavage of prothrombin by prothrombinase: repositioning a cleavage site reveals the unique kinetic behavior of the action of prothrombinase on its compound substrate

Prothrombinase converts prothrombin to thrombin via cleavage at Arg(320) followed by cleavage at Arg(271). Exosite-dependent binding of prothrombin to prothrombinase facilitates active site docking by Arg(320) and initial cleavage at this site. Precise positioning of the Arg(320) site for cleavage is implied by essentially normal cleavage at Arg(320) in recombinant prothrombin variants bearing additional Arg side chains either one or two residues away. However, mutation of Arg(320) to Gln reveals that prothrombinase can cleave prothrombin following Arg side chains shifted by as many as two residues N-terminal to the 320 position at near normal rates. Further repositioning leads to a loss in cleavage at this region with an abrupt shift toward slow cleavage at Arg(271). In contrast, the binding constant for the active site docking step is strongly dependent on the sequence preceding the scissile bond as well as position. Large effects on binding only yield minor changes in rate until the binding constant passes a threshold value. This behavior is expected for a substrate that can engage the enzyme through mutually exclusive active site docking reactions followed by cleavage to yield different products. Cleavage site specificity as well as the ordered action of prothrombinase on its compound substrate is regulated by the thermodynamics of active site engagement of the individual sites as well as competition between alternate cleavage sites for active site docking.

Functional role of O-linked and N-linked glycosylation sites present on the activation peptide of factor X

BACKGROUND

There are two O-linked and two N-linked glycosylation sites on the activation peptide of factor X (FX) involving residues Thr-17, Thr-29, Asn-39 and Asn-49.

OBJECTIVES

The purpose of this study was to explore the contribution of carbohydrates of the FX activation peptide to zymogen recognition by physiological activators.

METHODS

The putative glycosylation sites were substituted individually or in combinations with Ala and mutants were expressed in mammalian cells. The entire activation peptide up to the P3 residue was deleted in another construct.

RESULTS

It was discovered that activation of zymogen mutants by both FVIIa and FIXa on negatively charged phospholipid vesicles has been improved 2-40-fold independent of a cofactor. These mutants were activated with slightly lower catalytic efficiency (k(cat)/K(m)) by FVIIa in the extrinsic Xase complex, though both K(m) and k(cat) constants for mutants were elevated. With the exception of approximately 3-fold improvement in the activation of N49A, the catalytic efficiency of FIXa toward mutants was decreased 2-5-fold in the intrinsic Xase complex.

CONCLUSIONS

The carbohydrate chains of the FX activation peptide play an important role in restricting the specificity of zymogen recognition by both FVIIa and FIXa, thereby preventing the cofactor-independent activation of FX by these proteases. On the other hand, the carbohydrates contribute to the cofactor-dependent recognition of the zymogen by both extrinsic and intrinsic Xase complexes.

The conformational switch from the factor X zymogen to protease state mediates exosite expression and prothrombinase assembly

Zymogens of the chymotrypsin-like serine protease family are converted to the protease state following insertion of a newly formed, highly conserved N terminus. This transition is accompanied by active site formation and ordering of several surface loops in the catalytic domain. Here we show that disruption of this transition in factor X through mutagenesis (FXa(I16L) and FXa(V17A)) not only alters active site function, but also significantly impairs Na(+) and factor Va binding. Active site binding was improved in the presence of high NaCl or with saturating amounts of factor Va membranes, suggesting that allosteric linkage exists between these sites. In line with this, irreversible stabilization of FXa(I16L) with Glu-Gly-Arg-chloromethyl ketone fully rescued FVa binding. Furthermore, the K(m) for prothrombin conversion with the factor Xa variants assembled into prothrombinase was unaltered, whereas the k(cat) was modestly reduced (3- to 4-fold). These findings show that intramolecular activation of factor X following the zymogen to protease transition not only drives catalytic site activation but also contributes to the formation of the Na(+) and factor Va binding sites. This structural plasticity of the catalytic domain plays a key role in the regulation of exosite expression and prothrombinase assembly.

Restricted active site docking by enzyme-bound substrate enforces the ordered cleavage of prothrombin by prothrombinase

The preferred pathway for prothrombin activation by prothrombinase involves initial cleavage at Arg(320) to produce meizothrombin, which is then cleaved at Arg(271) to liberate thrombin. Exosite binding drives substrate affinity and is independent of the bond being cleaved. The pathway for cleavage is determined by large differences in V(max) for cleavage at the two sites within intact prothrombin. By fluorescence binding studies in the absence of catalysis, we have assessed the ability of the individual cleavage sites to engage the active site of Xa within prothrombinase at equilibrium. Using a panel of recombinant cleavage site mutants, we show that in intact prothrombin, the Arg(320) site effectively engages the active site in a 1:1 interaction between substrate and enzyme. In contrast, the Arg(271) site binds to the active site poorly in an interaction that is approximately 600-fold weaker. Perceived substrate affinity is independent of active site engagement by either cleavage site. We further show that prior cleavage at the 320 site or the stabilization of the uncleaved zymogen in a proteinase-like state facilitates efficient docking of Arg(271) at the active site of prothrombinase. Therefore, we establish direct relationships between docking of either cleavage site at the active site of the catalyst, the V(max) for cleavage at that site, substrate conformation, and the resulting pathway for prothrombin cleavage. Exosite tethering of the substrate in either the zymogen or proteinase conformation dictates which cleavage site can engage the active site of the catalyst and enforces the sequential cleavage of prothrombin by prothrombinase.

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

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

Role of Hirudin-like factor Va heavy chain sequences in prothrombinase function

Proexosite I on prothrombin has been implicated in providing a recognition site for factor Va within prothrombinase. To examine whether hirudin-like sequences (659-698) on the cofactor contribute to this interaction, we expressed and purified two-chain FVa derivatives that were intracellularly truncated at the C terminus of the heavy chain: FVa709 (des710-1545), FVa699 (des700-1545), FVa(692 (des693-1545), FVa678 (des679-1545), and FVa658 (des659-1545). We found that FVa709, FVa699, FVa692, and FVa678 exhibited specific clotting activities that were comparable with plasma-derived and recombinant FVa. Additionally, kinetic studies using prothrombin revealed that the Km and kcat values for these derivatives were unaltered. Fluorescent measurements and chromatography studies indicated that FVa709, FVa699, FVa692, and FVa678 bound to FXa membranes and thrombin-agarose in a manner that was comparable with the wild-type cofactors. In contrast, FVa658 had an approximately 1% clotting activity and reduced affinity for FXa membranes (approximately 20-fold) and did not bind to thrombin-agarose. Surprisingly, however, FVa(658) exhibited essentially normal kinetic parameters for prothrombin when the variant was fully saturated with FXa membranes. Overall our results are consistent with the interpretation that any possible binding interactions between prothrombin and the C-terminal region of the FVa heavy chain do not contribute in a detectable way to the enhanced function of prothrombinase.

Thrombin-activable factor X re-establishes an intrinsic amplification in tenase-deficient plasmas

Classical hemophilia results from a defect of the intrinsic tenase complex, the main factor X (FX) activator. Binding of factor VIIa to tissue factor triggers coagulation, but little amplification of thrombin production occurs. Handling of hemophilia by injection of the deficient or missing (thus foreign) factor often causes immunological complications. Several strategies have been designed to bypass intrinsic tenase complex, but none induce true auto-amplification of thrombin production. In an attempt to re-establish a cyclic amplification of prothrombin activation in the absence of tenase, we prepared a chimera of FX having fibrinopeptide A for the activation domain (FX(FpA)). We reasoned that cascade initiation would produce traces of thrombin that would activate FX(FpA) (contrary to its normal homologue). Given that the activation domain of FX is released upon activation, thrombin cleavage would produce authentic FXa that would produce more thrombin, which in turn would activate more chimeras. FX(FpA) was indeed activable by thrombin, albeit at a relatively low rate (5 x 10(3) M(-1) s(-1)). Nevertheless, FX(FpA) allowed in vitro amplification of thrombin production, and 100 nM efficiently corrected thrombin generation in tenase-deficient plasmas. A decisive advantage of FX(FpA) could be that the artificial cascade is self-regulating: FX(FpA) had little influence on the clotting time of normal plasma, yet corrected that of tenase deficiency. Another advantage could be the half-life of FX(FpA) in blood; FX has a half-life of about 30 h (less than 3 h for FVIIa). It is also reasonable to expect little or no immunogenicity, because FX and fibrinopeptide A both circulate normally in the blood of hemophiliacs.

Comparative Analysis of Prothrombin Activators from the Venom of Australian Elapids

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

The affinity of protein C for the thrombin.thrombomodulin complex is determined in a primary way by active site-dependent interactions

The interaction of thrombin (IIa) with thrombomodulin (TM) is essential for the efficient activation of protein C (PC). Interactions between PC and extended surfaces, likely contributed by TM within the IIa.TM complex, have been proposed to play a key role in PC activation. Initial velocities of PC activation at different concentrations of PC and TM could be accounted for by a model that did not require consideration of direct binding interactions between PC and TM. Reversible inhibitors directed toward the active site of IIa within the IIa.TM complex behaved as classic competitive inhibitors of both peptidyl substrate cleavage as well as PC activation. The ability of these small molecule inhibitors to block PC binding to the enzyme points to a principal role for active site-dependent substrate recognition in determining the affinity of IIa.TM for its protein substrate. Selective abrogation of active site docking by mutation of the P1 Arg in PC to Gln yielded an uncleavable derivative (PC(R15Q)). PC(R15Q) was a poor inhibitor (K(i) >or= 30 microm) of PC activation as well as peptidyl substrate cleavage by IIa.TM. Thus, inhibition by PC(R15Q) most likely results from its ability to weakly interfere with active site function rather than by blocking extended interactions with the enzyme complex. The data suggest a primary role for active site-dependent substrate recognition in driving the affinity of the IIa.TM complex for its protein substrate. Interactions between PC and extended surfaces contributed by IIa and/or TM within the IIa.TM complex likely contribute in a secondary or minor way to protein substrate affinity.

Binding of substrate in two conformations to human prothrombinase drives consecutive cleavage at two sites in prothrombin

Thrombin formation results from cleavage of prothrombin following Arg(271) and Arg(320). Both bonds are accessible for cleavage, yet the sequential action of prothrombinase on Arg(320) followed by Arg(271) is implied by the intermediate observed during prothrombin activation. We have studied the individual cleavage reactions catalyzed by prothrombinase by using a series of recombinant derivatives: wild type prothrombin (II(WT)) contained both cleavage sites; II(Q271) contained a single cleavable site at Arg(320); II(Q320) and II(A320) contained a single cleavable site at Arg(271); and II(QQ) was resistant to cleavage. Cleavage at Arg(320) in II(Q271) could account for the initial cleavage reaction leading to the consumption of either plasma prothrombin or II(WT), whereas cleavage at Arg(271) in either II(Q320) or II(A320) was found to be approximately 30-fold slower. Equivalent kinetic constants were obtained for three of the four possible half-reactions. Slow cleavage at Arg(271) in intact prothrombin resulted from an approximately 30-fold reduction in V(max). Thus, the observed pathway of bond cleavage by prothrombinase can be explained by the kinetic constants for the four possible individual cleavage reactions. II(Q320) was a competitive inhibitor of II(Q271) cleavage, and II(QQ) was a competitive inhibitor for each reaction with K(i) approximately K(m). The data are inconsistent with previous proposals and suggest a model in which substrates for each of the four possible half-reactions bind in a mutually exclusive manner and with equal affinity to prothrombinase in a cleavage site-independent way. Despite equivalent exosite binding interactions between all four possible substrates and the enzyme, we propose that ordered bond cleavage results from the constraints associated with the binding of substrates in one of two conformations to a single form of prothrombinase.

Zymogenic and enzymatic properties of the 70-80 loop mutants of factor X/Xa

The Ca(2+) binding 70-80 loop of factor X (fX) contains one basic (Arg(71)) and three acidic (Glu(74), Glu(76), and Glu(77)) residues whose contributions to the zymogenic and enzymatic properties of the protein have not been evaluated. We prepared four Ala substitution mutants of fX (R71A, E74A, E76A, and E77A) and characterized their activation kinetics by the factor VIIa and factor IXa in both the absence and presence of cofactors. Factor VIIa exhibited normal activity toward E74A and E76A and less than a twofold impaired activity toward R71A and E77A in both the absence and presence of tissue factor. Similarly, factor IXa in the absence of factor VIIIa exhibited normal activity toward both E74A and E76A; however, its activity toward R71A and E77A was impaired approximately two- to threefold. In the presence of factor VIIIa, factor IX activated all mutants with approximately two- to fivefold impaired catalytic efficiency. In contrast to changes in their zymogenic properties, all mutant enzymes exhibited normal affinities for factor Va, and catalyzed the conversion of prothrombin to thrombin with normal catalytic efficiencies. However, further studies revealed that the affinity of mutant enzymes for interaction with metal ions Na(+) and Ca(2+) was impaired. These results suggest that although charged residues of the 70-80 loop play an insignificant role in fX recognition by the factor VIIa-tissue factor complex, they are critical for the substrate recognition by factor IXa in the intrinsic Xase complex. The results further suggest that mutant residues do not play a specific role in the catalytic function of fXa in the prothrombinase complex.

Removal of B-domain sequences from factor V rather than specific proteolysis underlies the mechanism by which cofactor function is realized

Factor V, the precursor of factor Va, circulates in plasma with little or no procoagulant activity. Activity is generated following limited proteolysis indicating that the conversion of factor V to factor Va results in appropriate structural changes, which impart cofactor function. We have produced recombinant partial B-domain-truncated derivatives of factor V (FV(des811-1491) and FV(des811-1491) with Arg(709) and Arg(1545) mutated to Gln) to investigate whether discrete proteolysis within the B-domain followed by a conformational transition is responsible for activation. Direct binding fluorescence measurements as well as steady-state kinetic assays were employed to assess the ability of these factor V derivatives to assemble and function in prothrombinase. In contrast to human factor V, single-chain B-domain-truncated factor V bound to FXa membranes with an affinity that was identical to factor Va. Additionally, it was found that, once this modified derivative was assembled in prothrombinase, it functioned in an equivalent manner to factor Va. Taken together these data support the hypothesis that proteolysis within the B-domain of factor V, although necessary, is incidental to the mechanism by which cofactor function is realized. Instead, our results are more consistent with the interpretation that proteolytic activation of factor V simply eliminates steric and/or conformational constraints contributed by the B-domain that otherwise interfere with discrete binding interactions that govern the eventual function of factor Va.

Formation of factors IXa and Xa by the extrinsic pathway: differential regulation by tissue factor pathway inhibitor and antithrombin III

The activation of factor X by VIIa/TF and the Xa-dependent inhibition of the enzyme complex by tissue factor pathway inhibitor (TFPI) are considered primary steps in the initiation of coagulation. IX activation by VIIa/TF is considered to contribute catalyst necessary for further Xa production in the ensuing amplification phase. We have investigated Xa and IXabeta production by VIIa-TF in a system reconstituted with both X and IX and the principal physiologic inhibitors of this pathway TFPI and antithrombin III (AT). Kinetic studies without inhibitors established that IX and X functioned as competitive alternate substrates for VIIa/TF with similar kinetic constants. When both IX and X were present, TFPI significantly inhibited the extent of formation of either IXabeta or Xa. In contrast, AT rapidly depleted active Xa with a small effect on IXabeta formation. When both AT and TFPI were present, active IXabeta formation significantly exceeded the formation of active Xa regardless of the VIIa/TF concentration. These findings could be quantitatively accounted for by a model encompassing the kinetics of the individual activation and inhibition steps. Active Xa formation by this pathway is regulated in a principal way by its rapid inactivation by AT. In contrast, the Xa-dependent inhibitory reactions of TFPI play a primary role in limiting zymogen consumption and the formation of active IXabeta. These regulatory phenomena yield active IXabeta as a major rather than secondary product of VIIa/TF. Our findings raise the possibility that IXabeta produced by the extrinsic pathway, and its ability to function within the intrinsic Xase complex to activate X may play a significant role in producing Xa necessary for both the initiation and sustained phases of the procoagulant response following vascular damage.

Extended interactions with prothrombinase enforce affinity and specificity for its macromolecular substrate

The specific action of serine proteinases on protein substrates is a hallmark of blood coagulation and numerous other physiological processes. Enzymic recognition of substrate sequences preceding the scissile bond is considered to contribute dominantly to specificity and function. We have investigated the contribution of active site docking by unique substrate residues preceding the scissile bond to the function of prothrombinase. Mutagenesis of the authentic P(1)-P(3) sequence in prethrombin 2/fragment 1.2 yielded substrate variants that could be converted to thrombin by prothrombinase. Proteolytic activation was also observed with a substrate variant containing the P(1)-P(3) sequence found in a coagulation zymogen not known to be activated by prothrombinase. Lower rates of activation of the variants derived from a decrease in maximum catalytic rate but not in substrate affinity. Replacement of the P(1) residue with Gln yielded an uncleavable derivative that retained the affinity of the wild type substrate for prothrombinase but did not engage the active site of the enzyme. Thus, active site docking of the substrate contributes to catalytic efficiency, but it is does not determine substrate affinity nor does it fully explain the specificity of prothrombinase. Therefore, extended interactions between prothrombinase and substrate regions removed from the cleavage site drive substrate affinity and enforce the substrate specificity of this enzyme complex.

Nematode anticoagulant protein c2 reveals a site on factor Xa that is important for macromolecular substrate binding to human prothrombinase

The binding of recombinant nematode anticoagulant protein c2 (NAPc2) to either factor X or Xa is a requisite step in the pathway for the potent inhibition of VIIa tissue factor. We have used NAPc2 as a tight binding probe of human Xa to investigate protein substrate recognition by the human prothrombinase complex. NAPc2 binds with high affinity (K(d) approximately 1 nm) to both X and Xa in a way that does not require or occlude the active site of the enzyme. In contrast, NAPc2 is a tight binding, competitive inhibitor of protein substrate cleavage by human Xa incorporated into prothrombinase with saturating concentrations of membranes and Va. By fluorescence binding studies we show that NAPc2 does not interfere with the assembly of human prothrombinase. These are properties expected of an inhibitor that blocks protein substrate recognition by targeting extended macromolecular recognition sites (exosites) on the enzyme complex. A weaker interaction (K(d) = 260-500 nm) observed between NAPc2 and bovine X was restored to a high affinity one in a recombinant chimeric bovine X derivative containing 25 residues from the COOH terminus of the proteinase domain of human X. This region implicated in binding NAPc2 is spatially adjacent to a site previously identified as a potential exosite. Despite the weaker interaction with bovine Xa, NAPc2 was a tight binding competitive inhibitor of protein substrate cleavage by bovine prothrombinase as well. Extended enzymic surfaces elucidated with exosite-directed probes, such as NAPc2, may define a unique region of factor Xa that is modulated following its assembly into prothrombinase and in turn determines the binding specificity of the enzyme complex for its protein substrate.

A model for the stoichiometric regulation of blood coagulation

We have developed a model of the extrinsic blood coagulation system that includes the stoichiometric anticoagulants. The model accounts for the formation, expression, and propagation of the vitamin K-dependent procoagulant complexes and extends our previous model by including: (a) the tissue factor pathway inhibitor (TFPI)-mediated inactivation of tissue factor (TF).VIIa and its product complexes; (b) the antithrombin-III (AT-III)-mediated inactivation of IIa, mIIa, factor VIIa, factor IXa, and factor Xa; (c) the initial activation of factor V and factor VIII by thrombin generated by factor Xa-membrane; (d) factor VIIIa dissociation/activity loss; (e) the binding competition and kinetic activation steps that exist between TF and factors VII and VIIa; and (f) the activation of factor VII by IIa, factor Xa, and factor IXa. These additions to our earlier model generate a model consisting of 34 differential equations with 42 rate constants that together describe the 27 independent equilibrium expressions, which describe the fates of 34 species. Simulations are initiated by "exposing" picomolar concentrations of TF to an electronic milieu consisting of factors II, IX, X, VII, VIIa, V, and VIIII, and the anticoagulants TFPI and AT-III at concentrations found in normal plasma or associated with coagulation pathology. The reaction followed in terms of thrombin generation, proceeds through phases that can be operationally defined as initiation, propagation, and termination. The generation of thrombin displays a nonlinear dependence upon TF, AT-III, and TFPI and the combination of these latter inhibitors displays kinetic thresholds. At subthreshold TF, thrombin production/expression is suppressed by the combination of TFPI and AT-III; for concentrations above the TF threshold, the bolus of thrombin produced is quantitatively equivalent. A comparison of the model with empirical laboratory data illustrates that most experimentally observable parameters are captured, and the pathology that results in enhanced or deficient thrombin generation is accurately described.

Differences between human and rabbit coagulation factor X-implications for in vivo models of thrombosis

The activation of factor X (fX) to factor Xa (fXa) marks the penultimate step in the coagulation cascade and modulating fXa activity may be effective for antithrombotic therapy. Even though fXa inhibitors are screened using in vitro inhibition of human fXa (HfXa) while subsequent evaluation uses in vivo rabbit models, there is limited knowledge of species differences between the coagulation proteins. When comparing amino acid sequences for the human (HfX) and rabbit (RafX) protein, differences are found in the activation peptide and active site regions. In order to study the relative functional characteristics of HfX and RafX, we asked (1) whether fX from the two species is immunologically related, (2) whether the two proteins are activated to fXa in a similar manner, (3) whether HfXa and rabbit factor Xa (RafXa) have similar catalytic activities toward tripeptide substrates. To answer (1), we expressed RafX-glutathione S-transferase (RafX-GST) fusion protein in bacteria and purified the protein for use as an antigen. The resulting monoclonal antibodies were suitable for affinity purification of plasma RafX and for effective anticoagulation in rabbit plasma clotting assays. We found two antibodies (mAb 214 and mAb 290) that anticoagulated rabbit plasma in a dose responsive manner but did not cross-react with human plasma. At a concentration of 500 nM, mAb 214 attained a two-fold extension of rabbit plasma activated partial thromboplastin time (aPTT). To answer (2), we purified plasma RafX and compared the activation of HfX and RafX with Russell's viper venom (RVV-X). Under equivalent reaction conditions, conversion was 30% slower for the rabbit protein. To answer (3), amidolytic activity of HfXa and RafXa were assayed by cleavage of three para-nitroanilide (pNA) substrates (S2222 [Bz-Ile-Glu(gamma-OR)-Gly-Arg-pNA.HCl], S2765 [Z-D-Arg-Gly-Arg-pNA.HCl] and Spectrozyme Xa [MeO-CO-D-CHG-Gly-Arg-pNA.AcOH]). Michaelis constants (K(m)) for the rabbit protein were 187, 72 and 69 microM, respectively, and for the human analog, 255, 63 and 135 microM, respectively. Comparing the extent of substrate turnover (V(max)) for HfXa and RafXa, the latter was shown to cleave all three substrates at a reduced rate. Based on these observations, it can be speculated that the relative antithrombotic potency of active site directed fXa inhibitors might be different between the two species. Predicted human therapeutic doses derived from in vivo results in rabbit models should therefore take species variation into consideration.

Tissue factor pathway inhibitor: a possible mechanism of action

We have analyzed several mathematical models that describe inhibition of the factor VIIa-tissue factor complex (VIIa-TF) by tissue factor pathway inhibitor (TFPI). At the core of these models is a common mechanism of TFPI action suggesting that only the Xa-TFPI complex is the inhibitor of the extrinsic tenase activity. However, the model based on this hypothesis could not explain well all the available experimental data. Here, we show that a good quantitative description of all experimental data could be achieved in a model that contains two more assumptions. The first assumption is based on the hypothesis originally proposed by Baugh et al. [Baugh, R.J., Broze, G.J. Jr & Krishnaswamy, S. (1998) J. Biol. Chem. 273, 4378-4386], which suggests that TFPI could inhibit the enzyme-product complex Xa-VIIa-TF. The second assumption proposes an interaction between the X-VIIa-TF complex and the factor Xa-TFPI complex. Experiments to test these hypotheses are suggested.

The contribution of factor Xa to exosite-dependent substrate recognition by prothrombinase

Kinetic studies support the concept that protein substrate recognition by the prothrombinase complex of coagulation is achieved by interactions at extended macromolecular recognition sites (exosites), distinct from the active site of factor Xa within the complex. We have used this formal kinetic model and a monoclonal antibody directed against Xa (alphaBFX-2b) to investigate the contributions of surfaces on the proteinase to exosite-mediated protein substrate recognition by prothrombinase. alphaBFX-2b bound reversibly to a fluorescent derivative of factor Xa (K(d) = 17.1 +/- 5.6 nm) but had no effect on active site function of factor Xa or factor Xa saturably assembled into prothrombinase. In contrast, alphaBFX-2b was a slow, tight binding inhibitor of the cleavage of either prethrombin 2 or meizothrombin des-fragment 1 by prothrombinase (K(i)(*) = 0.55 +/- 0.05 nm). Thus, alphaBFX-2b binding to factor Xa within prothrombinase selectively leads to the inhibition of protein substrate cleavage without interfering with active site function. Inhibition kinetics could adequately be accounted for by a kinetic model in which prethrombin 2 and alphaBFX-2b bind in a mutually exclusive way to prothrombinase. These are properties expected of an exosite-directed inhibitor. The site(s) on factor Xa responsible for antibody binding were evaluated by identification of immunoreactive fragments following chemical digestion of human and bovine Xa and were further confirmed with a series of recombinantly expressed fragments. These approaches suggest that residues 82-91 and 102-116 in the proteinase domain contribute to alphaBFX-2b binding. The data establish this antibody as a prototypic exosite-directed inhibitor of prothrombinase and suggest that the occlusion of a surface on factor Xa, spatially removed from the active site, is sufficient to block exosite-dependent recognition of the protein substrate by prothrombinase.

Exosite interactions determine the affinity of factor X for the extrinsic Xase complex

The initiation of coagulation results from the activation of factor X by an enzyme complex (Xase) composed of the trypsin-like serine proteinase, factor VIIa, bound to tissue factor (TF) on phospholipid membranes. We have investigated the basis for the protein substrate specificity of Xase using TF reconstituted into vesicles of phosphatidylcholine, phosphatidylserine, or pure phosphatidylcholine. We show that occupation of the active site of VIIa within Xase by a reversible inhibitor or an alternate peptidyl substrate is sufficient to exclude substrate interactions at the active site but does not alter the affinity of Xase for factor X. This is evident as classical competitive inhibition of peptidyl substrate cleavage but as classical noncompetitive inhibition of factor X activation by active site-directed ligands. This implies that the productive recognition of factor X by Xase arises from a multistep reaction requiring an initial interaction at sites on the enzyme complex distinct from the active site (exosites), followed by active site interactions and bond cleavage. Exosite interactions determine protein substrate affinity, whereas the second binding step influences the maximum catalytic rate for the reaction. We also show that competitive inhibition can be achieved by interfering with exosite binding using factor X derivatives that are expected to have limited or abrogated interactions with the active site of VIIa within Xase. Thus, substrate interactions at exosites, sites removed from the active site of VIIa within the enzyme complex, determine affinity and binding specificity in the productive recognition of factor X by the VIIa-TF complex. This may represent a prevalent strategy through which distinctive protein substrate specificities are achieved by the homologous enzymes of coagulation.

Regions remote from the site of cleavage determine macromolecular substrate recognition by the prothrombinase complex

The proteolytic formation of thrombin is catalyzed by the prothrombinase complex of blood coagulation. The kinetics of prethrombin 2 cleavage was studied to delineate macromolecular substrate structures necessary for recognition at the exosite(s) of prothrombinase. The product, alpha-thrombin, was a linear competitive inhibitor of prethrombin 2 activation without significantly inhibiting peptidyl substrate cleavage by prothrombinase. Prethrombin 2 and alpha-thrombin compete for binding to the exosite without restricting access to the active site of factor Xa within prothrombinase. Inhibition by alpha-thrombin was not altered by saturating concentrations of low molecular weight heparin. Furthermore, proteolytic removal of the fibrinogen recognition site in alpha-thrombin only had a modest effect on its inhibitory properties. Both alpha-thrombin and prethrombin 2 were cleaved with chymotrypsin at Trp148 and separated into component domains. The C-terminal-derived zeta2 fragment retained the ability to selectively inhibit macromolecular substrate cleavage by prothrombinase, while the zeta1 fragment was without effect. As the zeta2 fragment lacks the fibrinogen recognition site, the P1-P3 residues or the intact cleavage site, specific recognition of the macromolecular substrate by the exosite in prothrombinase is achieved through substrate regions, distinct from the fibrinogen recognition or heparin-binding sites, and spatially removed from structures surrounding the scissile bond.

Regulation of extrinsic pathway factor Xa formation by tissue factor pathway inhibitor

Tissue factor (TF) pathway inhibitor (TFPI) regulates factor X activation through the sequential inhibition of factor Xa and the VIIa.TF complex. Factor Xa formation was studied in a purified, reconstituted system, at plasma concentrations of factor X and TFPI, saturating concentrations of factor VIIa, and increasing concentrations of TF reconstituted into phosphatidylcholine:phosphatidylserine membranes (TF/PCPS) or PC membranes (TF/PC). The initial rate of factor Xa formation was equivalent in the presence or absence of 2.4 nM TFPI. However, reaction extent was small (<20%) relative to that observed in the absence of TFPI, implying the rapid inhibition of VIIa.TF during factor X activation. Initiation of factor Xa formation using increasing concentrations of TF/PCPS or TF/PC in the presence of TFPI yielded families of progress curves where both initial rate and reaction extent were linearly proportional to the concentration of VIIa.TF. These observations were consistent with a kinetic model in which the rate-limiting step represents the initial inhibition of newly formed factor Xa. Numerical analyses of progress curves yielded a rate constant for inhibition of VIIa.TF by Xa.TFPI (>10(8) M-1.s-1) that was substantially greater than the value (7.34 +/- 0.8 x 10(6) M-1.s-1) directly measured. Thus, VIIa.TF is inhibited at near diffusion-limited rates by Xa.TFPI formed during catalysis which cannot be explained by studies of the isolated reaction. We propose that the predominant inhibitory pathway during factor X activation may involve the initial inhibition of factor Xa either bound to or in the near vicinity of VIIa.TF on the membrane surface. As a result, VIIa.TF inhibition is unexpectedly rapid, and the concentration of active factor Xa that escapes regulation is linearly dependent on the availability of TF.

Identification of surface residues mediating tissue factor binding and catalytic function of the serine protease factor VIIa

Factor VIIa (VIIa), the serine protease that initiates the coagulation pathways, is catalytically activated upon binding to its cell surface receptor and cofactor tissue factor (TF). This study provides a comprehensive analysis of the functional surface of VIIa by alanine scanning mutagenesis of 112 residues. Residue side chains were defined which contribute to TF binding and factor X hydrolysis. Energetically important binding contacts at the interface with TF were identified in the first epidermal growth factor domain of VIIa (Gln-64, Ile-69, Phe-71, Arg-79) and in the protease domain (Arg-277, Met-306, Asp-309). The observed energetic defects are in good agreement with the corresponding residues in TF, suggesting that the VIIa light chain plays a prominent role in high affinity binding of cofactor. Mutation of protease domain interface residues indicated that TF allosterically influences the active site of VIIa. Stabilization of a labile zymogen to enzyme transition could explain the activating effect of TF on VIIa catalytic function. Residues important for factor X hydrolysis were found in three regions of the protease domain: (i) specificity determinants in the catalytic cleft and adjacent loops, (ii) an exosite near the TF binding site, and (iii) a large electronegative exosite which is in a position analogous to the basic exosite I of thrombin. TF regions involved in factor X activation are positioned on the same face of the TF-VIIa complex as the two exosites identified on the protease domain surface, providing evidence for an extended interaction of TF-VIIa with macromolecular substrate.