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

Cancer progression and tumor hypercoagulability: a platelet perspective

Venous thromboembolism, which is common in cancer patients and accompanies or even precedes malignant tumors, is known as cancer-related thrombosis and is an important cause of cancer- associated death. At present, the exact etiology of the elevated incidence of venous thrombosis in cancer patients remains elusive. Platelets play a crucial role in blood coagulation, which is intimately linked to the development of arterial thrombosis. Additionally, platelets contribute to tumor progression and facilitate immune evasion by tumors. Tumor cells can interact with the coagulation system through various mechanisms, such as producing hemostatic proteins, activating platelets, and directly adhering to normal cells. The relationship between platelets and malignant tumors is also significant. In this review article, we will explore these connections.

Role of sequence and position of the cleavage sites in prothrombin activation

In the penultimate step of the coagulation cascade, the multidomain vitamin-K-dependent zymogen prothrombin is converted to thrombin by the prothrombinase complex composed of factor Xa, cofactor Va, and phospholipids. Activation of prothrombin requires cleavage at two residues, R271 and R320, along two possible pathways generating either the intermediate prethrombin-2 (following initial cleavage at R271) or meizothrombin (following initial cleavage at R320). The former pathway is preferred in the absence of and the latter in the presence of cofactor Va. Several mechanisms have been proposed to explain this preference, but the role of the sequence and position of the sites of cleavage has not been thoroughly investigated. In this study, we engineered constructs where the sequences 261DEDSDRAIEGRTATSEYQT279 and 310RELLESYIDGRIVEGSDAE328 were swapped between the R271 and R320 sites. We found that in the absence of cofactor Va, the wild-type sequence at the R271 site is cleaved preferentially regardless of its position at the R271 or R320 site, whereas in the presence of cofactor Va, the R320 site is cleaved preferentially regardless of its sequence. Additional single-molecule FRET measurements revealed that the environment of R271 changes significantly upon cleavage at R320 due to the conformational transition from the closed form of prothrombin to the open form of meizothrombin. Detailed kinetics of cleavage at the R271 site were monitored by a newly developed assay based on loss of FRET. These findings show how sequence and position of the cleavage sites at R271 and R320 dictate the preferred pathway of prothrombin activation.

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.

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.

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.

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.

Structural transitions during prothrombin activation: On the importance of fragment 2

Prothrombin is activated to thrombin by the prothrombinase complex through sequential cleavage at two distinct sites. This occurs at sites of vascular injury in a highly regulated cascade of serine protease and cofactor activation, where activated platelets provide a suitable surface for protease/cofactor/substrate assembly. The precise structural and conformational changes undergone during the transition from prothrombin to thrombin have been studied for decades, and several structures of prothrombin fragments along the activation pathway have been solved. Here we present a new structure analyzed in context of other recent structures and biochemical studies. What emerges is an unexpected mechanism that involves a change in the mode of binding of the F2 domain (fragment 2) on the catalytic domain after cleavage at Arg320, and a subsequent reorientation of the linker between the F2 and catalytic domain to present the Arg271 site for cleavage.

•

The catalytic domain of thrombin precursors binds to its F2 domain by two distinct modes.

•

Cleavage of prothrombin at either Arg271 or Arg320 results in shift from mode 2 to mode 1.

•

After cleavage at Arg320, movement of F2 helps to present the second cleavage site at Arg271.

Multiple exosites distributed across the three domains of streptokinase co-operate to generate high catalytic rates in the streptokinase-plasmin activator complex

To examine the global function of the key surface-exposed loops of streptokinase, bearing substrate-specific exosites, namely, the 88-97 loop in the α domain, the 170 loop in the β domain, and the coiled-coil region (Leu321-Asn338) in the γ domain, mutagenic as well as peptide inhibition studies were carried out. Peptides corresponded to the primary structure of an exosite, either individual or stoichiometric mixtures of various disulfide-constrained synthetic peptide(s) inhibited plasminogen activation by streptokinase. Remarkably, pronounced inhibition of substrate plasminogen activation by the preformed streptokinase-plasmin activator complex was observed when complementary mixtures of different peptides were used compared to the same overall concentrations of individual peptides, suggesting co-operative interactions between the exosites. This observation was confirmed with streptokinase variants mutated at one, two, or three sites simultaneously. The single/double/triple exosite mutants of streptokinase showed a nonadditive, synergistic decline in kcat for substrate plasminogen activation in the order single > double > triple exosite mutant. Under the same conditions, zymogen activation by the various mutants remained essentially native- like in terms of nonproteolytic activation of partner plasminogen. Multisite mutants also retain affinity to form 1:1 stoichiometric activator complexes with plasmin when probed through sensitive equilibrium fluorescence studies. Thus, the present results strongly support a model of streptokinase action, wherein catalysis by the streptokinase-plasmin complex operates through a distributed network of substrate-interacting exosites resident across all three domains of the cofactor protein.

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.

The transition of prothrombin to thrombin

The proteolytic conversion of prothrombin to thrombin catalyzed by prothrombinase is one of the more extensively studied reactions of blood coagulation. Sophisticated biophysical and biochemical insights into the players of this reaction were developed in the early days of the field. Yet, many basic enzymological questions remained unanswered. I summarize new developments that uncover mechanisms by which high substrate specificity is achieved, and the impact of these strategies on enzymic function. Two principles emerge that deviate from conventional wisdom that has otherwise dominated thinking in the field. (i) Enzymic specificity is dominated by the contribution of exosite binding interactions between substrate and enzyme rather than by specific recognition of sequences flanking the scissile bond. Coupled with the regulation of substrate conformation as a result of the zymogen to proteinase transition, novel mechanistic insights result for numerous aspects of enzyme function. (ii) The transition of zymogen to proteinase following cleavage is not absolute and instead, thrombin can reversibly interconvert between zymogen-like and proteinase-like forms depending on the complement of ligands bound to it. This establishes new paradigms for considering proteinase allostery and how enzyme function may be modulated by ligand binding. These insights into the action of prothrombinase on prothrombin have wide-ranging implications for the understanding of function in blood coagulation.

A sequential mechanism for exosite-mediated factor IX activation by factor XIa

During blood coagulation, the protease factor XIa (fXIa) activates factor IX (fIX). We describe a new mechanism for this process. FIX is cleaved initially after Arg(145) to form fIXα, and then after Arg(180) to form the protease fIXaβ. FIXα is released from fXIa, and must rebind for cleavage after Arg(180) to occur. Catalytic efficiency of cleavage after Arg(180) is 7-fold greater than for cleavage after Arg(145), limiting fIXα accumulation. FXIa contains four apple domains (A1-A4) and a catalytic domain. Exosite(s) on fXIa are required for fIX binding, however, there is lack of consensus on their location(s), with sites on the A2, A3, and catalytic domains described. Replacing the A3 domain with the prekallikrein A3 domain increases K(m) for fIX cleavage after Arg(145) and Arg(180) 25- and ≥ 90-fold, respectively, and markedly decreases k(cat) for cleavage after Arg(180). Similar results were obtained with the isolated fXIa catalytic domain, or fXIa in the absence of Ca(2+). Forms of fXIa lacking the A3 domain exhibit 15-fold lower catalytic efficiency for cleavage after Arg(180) than for cleavage after Arg(145), resulting in fIXα accumulation. Replacing the A2 domain does not affect fIX activation. The results demonstrate that fXIa activates fIX by an exosite- and Ca(2+)-mediated release-rebind mechanism in which efficiency of the second cleavage is enhanced by conformational changes resulting from the first cleavage. Initial binding of fIX and fIXα requires an exosite on the fXIa A3 domain, but not the A2 or catalytic domain.

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.

P3-P3' residues flanking scissile bonds in factor VIII modulate rates of substrate cleavage and procofactor activation by thrombin

Thrombin-catalyzed activation of factor VIII (FVIII) occurs through proteolysis at three P1 Arg residues: Arg(372) and Arg(740) in the FVIII heavy chain and Arg(1689) in the FVIII light chain. Cleavage at the latter two sites is relatively fast compared with cleavage at Arg(372), which appears to be rate-limiting. Examination of the P3-P3' residues flanking each P1 site revealed that those sequences at Arg(740) and Arg(1689) are more optimal for thrombin cleavage than at Arg(372), suggesting these sequences may impact reaction rates. Recombinant FVIII variants were prepared with mutations swapping scissile bond flanking sequences in the heavy chain individually and in combination with a second swap or with a P1 point mutation. Rates of generation of A1 and A3-C1-C2 subunits were determined by Western blotting and correlated with rates of cleavage at Arg(372) and Arg(1689), respectively. Rates of thrombin cleavage at Arg(372) were increased ~10- and ~3-fold compared with that of wild-type FVIII when it was replaced with P3-P3' residues flanking Arg(740) and Arg(1689), respectively, and these values paralleled increased rates of A2 subunit generation and procofactor activation. Positioning of more optimal residues flanking Arg(372) abrogated the need for initial cleavage at Arg(740) to facilitate this step. These results show marked changes in cleavage rates correlate with the extent of cleavage-optimal residues flanking the scissile bond and modulate the mechanism for procofactor activation.

Active site-labeled prothrombin inhibits prothrombinase in vitro and thrombosis in vivo

Mouse and human prothrombin (ProT) active site specifically labeled with D-Phe-Pro-Arg-CH(2)Cl (FPR-ProT) inhibited tissue factor-initiated thrombin generation in platelet-rich and platelet-poor mouse and human plasmas. FPR-prethrombin 1 (Pre 1), fragment 1 (F1), fragment 1.2 (F1.2), and FPR-thrombin produced no significant inhibition, demonstrating the requirement for all three ProT domains. Kinetics of inhibition of ProT activation by the inactive ProT(S195A) mutant were compatible with competitive inhibition as an alternate nonproductive substrate, although FPR-ProT deviated from this mechanism, implicating a more complex process. FPR-ProT exhibited ∼10-fold more potent anticoagulant activity compared with ProT(S195A) as a result of conformational changes in the ProT catalytic domain that induce a more proteinase-like conformation upon FPR labeling. Unlike ProT and ProT(S195A), the pathway of FPR-ProT cleavage by prothrombinase was redirected from meizothrombin toward formation of the FPR-prethrombin 2 (Pre 2)·F1.2 inhibitory intermediate. Localization of ProT labeled with Alexa Fluor® 660 tethered through FPR-CH(2)Cl ([AF660]FPR-ProT) during laser-induced thrombus formation in vivo in murine arterioles was examined in real time wide-field and confocal fluorescence microscopy. [AF660]FPR-ProT bound rapidly to the vessel wall at the site of injury, preceding platelet accumulation, and subsequently to the thrombus proximal, but not distal, to the vessel wall. [AF660]FPR-ProT inhibited thrombus growth, whereas [AF660]FPR-Pre 1, lacking the F1 membrane-binding domain did not bind or inhibit. Labeled F1.2 localized similarly to [AF660]FPR-ProT, indicating binding to phosphatidylserine-rich membranes, but did not inhibit thrombosis. The studies provide new insight into the mechanism of ProT activation in vivo and in vitro, and the properties of a unique exosite-directed prothrombinase inhibitor.

The role of P4-P3' residues flanking Arg336 in facilitating activated protein C-catalyzed cleavage and inactivation of factor VIIIa

INTRODUCTION

Activated protein C (APC) inactivates factor VIIIa (FVIIIa) through cleavages at Arg336 in the A1 subunit and Arg562 in the A2 subunit. Proteolysis at Arg336 occurs 25-fold faster than at Arg562. Replacing residues flanking Arg336 en bloc with the corresponding residues surrounding Arg562 markedly reduced the rate of cleavage at Arg336, indicating a role for these residues in the catalysis mechanism.

MATERIALS AND METHODS

To assess the contributions of individual P4-P3' residues flanking the Arg336 site to cleavage efficiency, point mutations were made based upon those flanking Arg562 of FVIIIa (Pro333Val, Gln334Asp, Leu335Gln, Met337Gly, Lys338Asn, Asn339Gln) and selected residues flanking Arg506 of FVa (Leu335Arg, and Lys338Ile). APC-catalyzed inactivation of the FVIII variants and cleavage of FVIIIa subunits were monitored by FXa generation assays and Western blotting.

RESULTS

Specific activity values of the variants were 60-135% of the wild type (WT) value. APC-catalyzed rates of cleavage at Arg336 remained similar to WT for the Pro333Val and Lys338Ile variants and was modestly increased for the Asn339Gln variant; while rates were reduced ~2-3-fold for the Gln334Asp, Leu335Gln, Leu335Arg, and Lys338Asn variants, and 5-fold for the Met337Gly variant. Rates for cofactor inactivation paralleled cleavage at the A1 site. APC slowly cleaves Arg372 in FVIII, a site responsible for procofactor activation. Using FVIII as substrate for APC, the Met337Gly variant yielded significantly greater activation compared with WT FVIII.

CONCLUSIONS

These results show that individual P4-P3' residues surrounding Arg336 are in general more favorable to cleavage than those surrounding the Arg562 site.

Contribution of amino acid region 659-663 of Factor Va heavy chain to the activity of factor Xa within prothrombinase

Factor Va, the cofactor of prothrombinase, is composed of heavy and light chains associated noncovalently in the presence of divalent metal ions. The COOH-terminal region of the heavy chain contains acidic amino acid clusters that are important for cofactor activity. In this work, we have investigated the role of amino acid region 659−663, which contains five consecutive acidic amino acid residues, by site-directed mutagenesis. We have generated factor V molecules in which all residues were mutated to either lysine (factor V^5K) or alanine (factor V^5A). We have also constructed a mutant molecule with this region deleted (factor V^Δ659−663). The recombinant molecules along with wild-type factor V (factor V^WT) were transiently expressed in mammalian cells, purified, and assessed for cofactor activity. Two-stage clotting assays revealed that the mutant molecules had reduced clotting activities compared to that of factor Va^WT. Kinetic analyses of prothrombinase assembled with the mutant molecules demonstrated diminished k_cat values, while the affinity of all mutant molecules for factor Xa was similar to that for factor Va^WT. Gel electrophoresis analyses of plasma-derived and recombinant mutant prothrombin activation demonstrated delayed cleavage of prothrombin at both Arg³²⁰ and Arg²⁷¹ by prothrombinase assembled with the mutant molecules, resulting in meizothrombin lingering throughout the activation process. These results were confirmed after analysis of the cleavage of FPR-meizothrombin. Our findings provide new insights into the structural contribution of the acidic COOH-terminal region of factor Va heavy chain to factor Xa activity within prothrombinase and demonstrate that amino acid region 659−663 from the heavy chain of the cofactor contributes to the regulation of the rate of cleavage of prothrombin by prothrombinase.

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.

Fate of membrane-bound reactants and products during the activation of human prothrombin by prothrombinase

Membrane binding by prothrombin, mediated by its N-terminal fragment 1 (F1) domain, plays an essential role in its proteolytic activation by prothrombinase. Thrombin is produced in two cleavage reactions. One at Arg(320) yields the proteinase meizothrombin that retains membrane binding properties. The second, at Arg(271), yields thrombin and severs covalent linkage with the N-terminal fragment 1.2 (F12) region. Covalent linkage with the membrane binding domain is also lost when prethrombin 2 (P2) and F12 are produced following initial cleavage at Arg(271). We show that at the physiological concentration of prothrombin, thrombin formation results in rapid release of the proteinase into solution. Product release from the surface can be explained by the weak interaction between the proteinase and F12 domains. In contrast, the zymogen intermediate P2, formed following cleavage at Arg(271), accumulates on the surface because of a approximately 20-fold higher affinity for F12. By kinetic studies, we show that this enhanced binding adequately explains the ability of unexpectedly low concentrations of F12 to greatly enhance the conversion of P2 to thrombin. Thus, the utilization of all three possible substrate species by prothrombinase is regulated by their ability to bind membranes regardless of whether covalent linkage to the F12 region is maintained. The product, thrombin, interacts with sufficiently poor affinity with F12 so that it is rapidly released from its site of production to participate in its numerous hemostatic functions.

Factor Xa active site substrate specificity with substrate phage display and computational molecular modeling

Structural origin of substrate-enzyme recognition remains incompletely understood. In the model enzyme system of serine protease, canonical anti-parallel beta-structure substrate-enzyme complex is the predominant hypothesis for the substrate-enzyme interaction at the atomic level. We used factor Xa (fXa), a key serine protease of the coagulation system, as a model enzyme to test the canonical conformation hypothesis. More than 160 fXa-cleavable substrate phage variants were experimentally selected from three designed substrate phage display libraries. These substrate phage variants were sequenced and their specificities to the model enzyme were quantified with quantitative enzyme-linked immunosorbent assay for substrate phage-enzyme reaction kinetics. At least three substrate-enzyme recognition modes emerged from the experimental data as necessary to account for the sequence-dependent specificity of the model enzyme. Computational molecular models were constructed, with both energetics and pharmacophore criteria, for the substrate-enzyme complexes of several of the representative substrate peptide sequences. In contrast to the canonical conformation hypothesis, the binding modes of the substrates to the model enzyme varied according to the substrate peptide sequence, indicating that an ensemble of binding modes underlay the observed specificity of the model serine protease.

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.

Exosites in the substrate specificity of blood coagulation reactions

The specificity of blood coagulation proteinases for substrate, inhibitor, and effector recognition is mediated by exosites on the surfaces of the catalytic domains, physically separated from the catalytic site. Some thrombin ligands bind specifically to either exosite I or II, while others engage both exosites. The involvement of different, overlapping constellations of exosite residues enables binding of structurally diverse ligands. The flexibility of the thrombin structure is central to the mechanism of complex formation and the specificity of exosite interactions. Encounter complex formation is driven by electrostatic ligand-exosite interactions, followed by conformational rearrangement to a stable complex. Exosites on some zymogens are in low affinity proexosite states and are expressed concomitant with catalytic site activation. The requirement for exosite expression controls the specificity of assembly of catalytic complexes on the coagulation pathway, such as the membrane-bound factor Xa*factor Va (prothrombinase) complex, and prevents premature assembly. Substrate recognition by prothrombinase involves a two-step mechanism with initial docking of prothrombin to exosites, followed by a conformational change to engage the FXa catalytic site. Prothrombin and its activation intermediates bind prothrombinase in two alternative conformations determined by the zymogen to proteinase transition that are hypothesized to involve prothrombin (pro)exosite I interactions with FVa, which underpin the sequential activation pathway. The role of exosites as the major source of substrate specificity has stimulated development of exosite-targeted anticoagulants for treatment of thrombosis.

Residues Surrounding Arg336 and Arg562 Contribute to the Disparate Rates of Proteolysis of Factor VIIIa Catalyzed by Activated Protein C

Activated Protein C (APC) inactivates factor VIIIa by cleavage at Arg(336) and Arg(562) within the A1 and A2 subunits, respectively, with reaction at the former site occurring at a rate approximately 25-fold faster than the latter. Recombinant factor VIII variants possessing mutations within the P4-P3' sequences were used to determine the contributions of these residues to the disparate cleavage rates at the two P1 sites. Specific activity values for 336(P4-P3')562, 336(P4-P2)562, and 336(P1'-P3')562 mutants, where indicated residues surrounding the Arg(336) site were replaced with those surrounding Arg(562), were similar to wild type (WT) factor VIII; whereas 562(P4-P3')336 and 562(P4-P2)336 mutants showed specific activity values <1% the WT value. Inactivation rates for the 336 site mutants were reduced approximately 6-11-fold compared with WT factor VIIIa, and approached values attributed to cleavage at Arg(562). Cleavage rates at Arg(336) were reduced approximately 100-fold for 336(P4-P3')562, and approximately 9-16-fold for 336(P4-P2)562 and 336(P1'-P3')562 mutants. Inhibition kinetics revealed similar affinities of APC for WT factor VIIIa and 336(P4-P3')562 variant. Alternatively, the 562(P4-P3')336 variant showed a modest increase in cleavage rate ( approximately 4-fold) at Arg(562) compared with WT, whereas these rates were increased by approximately 27- and 6-fold for 562(P4-P3')336 and 562(P4-P2)336, respectively, using the factor VIII procofactor form as substrate. Thus the P4-P3' residues surrounding Arg(336) and Arg(562) make significant contributions to proteolysis rates at each site, apparently independent of binding affinity. Efficient cleavage at Arg(336) by APC is attributed to favorable P4-P3' residues at this site, whereas cleavage at Arg(562) can be accelerated following replacement with more optimal P4-P3' residues.

Expression of allosteric linkage between the sodium ion binding site and exosite I of thrombin during prothrombin activation

The specificity of thrombin for procoagulant and anticoagulant substrates is regulated allosterically by Na+. Ordered cleavage of prothrombin (ProT) at Arg320 by the prothrombinase complex generates proteolytically active, meizothrombin (MzT), followed by cleavage at Arg271 to produce thrombin and fragment 1.2. The alternative pathway of initial cleavage at Arg271 produces the inactive zymogen form, the prethrombin 2 (Pre 2).fragment 1.2 complex, which is cleaved subsequently at Arg320. Cleavage at Arg320 of ProT or prethrombin 1 (Pre 1) activates the catalytic site and the precursor form of exosite I (proexosite I). To determine the pathway of expression of Na+-(pro)exosite I linkage during ProT activation, the effects of Na+ on the affinity of fluorescein-labeled hirudin-(54-65) ([5F]Hir-(54-65)(SO-3)) for the zymogens, ProT, Pre 1, and Pre 2, and for the proteinases, MzT and MzT-desfragment 1 (MzT(-F1)) were quantitated. The zymogens showed no significant linkage between proexosite I and Na+, whereas cleavage at Arg320 caused the affinities of MzT and MzT(-F1) for [5F]Hir-(54-65)(SO-3) to be enhanced by Na+ 8- to 10-fold and 5- to 6-fold, respectively. MzT and MzT(-F1) showed kinetically different mechanisms of Na+ enhancement of chromogenic substrate hydrolysis. The results demonstrate for the first time that MzT is regulated allosterically by Na+. The results suggest that the distinctive procoagulant substrate specificity of MzT, in activating factor V and factor VIII on membranes, and the anticoagulant, membrane-modulated activation of protein C by MzT bound to thrombomodulin are regulated by Na+-induced allosteric transition. Further, the Na+ enhancement in MzT activity and exosite I affinity may function in directing the sequential ProT activation pathway by accelerating thrombin formation from the MzT fast form.

Role of P1 residues Arg336 and Arg562 in the activated-Protein-C-catalysed inactivation of Factor VIIIa

APC (activated Protein C) inactivates human Factor VIIIa following cleavage at residues Arg336 and Arg562 within the A1 and A2 subunits respectively. The role of the P1 arginine in APC-catalysed inactivation of Factor VIIIa was examined by employing recombinant Factor VIIIa molecules where residues 336 and 562 were replaced with alanine and/or glutamine. Stably expressed Factor VIII proteins were activated by thrombin and resultant Factor VIIIa was reacted at high concentration with APC to minimize cofactor inactivation due to A2 subunit dissociation. APC cleaved wild-type Factor VIIIa at the A1 site with a rate approximately 25-fold greater than that for the A2 site. A1 mutants R336A and R336Q were inactivated approximately 9-fold slower than wild-type Factor VIIIa, whereas the A2 mutant R562A was inactivated approximately 2-fold slower. No cleavage at the mutated sites was observed. Taken together, these results suggested that cleavage at the A1 site was the dominant mechanism for Factor VIIIa inactivation catalysed by the proteinase. On the basis of cleavage at Arg336, a K(m) value for wild-type Factor VIIIa of 102 nM was determined, and this value was significantly greater than K(i) values (approximately 9-18 nM) obtained for an R336Q/R562Q Factor VIIIa. Furthermore, evaluation of a series of cluster mutants in the C-terminal region of the A1 subunit revealed a role for acidic residues in segment 341-345 in the APC-catalysed proteolysis of Arg336. Thus, while P1 residues contribute to catalytic efficiency, residues removed from these sites make a primary contribution to the overall binding of APC to Factor VIIIa.

Mutating factor VIII: lessons from structure to function

Factor VIII, a metal ion-dependent heterodimer, circulates in complex with von Willebrand factor. At sites of vessel wall damage, this procofactor is activated to factor VIIIa by limited proteolysis and assembles onto an anionic phospholipid surface in complex with factor IXa to form the intrinsic factor Xase; an enzyme complex that efficiently converts factor X to factor Xa during the propagation phase of coagulation. Factor Xase activity is down-regulated by mechanisms that include self-dampening by dissociation of a critical factor VIIIa subunit and proteolytic inactivation by the activated protein C pathway. Recent studies identify putative metal ion coordination sites as well as ligands involved in the catabolism of the activated and procofactor forms of the protein. Our knowledge of these multiple intra- and inter-molecular interactions has been facilitated by the application of naturally occurring and site-directed mutations to study factor VIII structure and function. In this review, we document important and novel contributions following this line of investigation.

Exosite-mediated substrate recognition of factor IX by factor XIa. The factor XIa heavy chain is required for initial recognition of factor IX

Studies of the mechanisms of blood coagulation zymogen activation demonstrate that exosites (sites on the activating complex distinct from the protease active site) play key roles in macromolecular substrate recognition. We investigated the importance of exosite interactions in recognition of factor IX by the protease factor XIa. Factor XIa cleavage of the tripeptide substrate S2366 was inhibited by the active site inhibitors p-aminobenzamidine (Ki 28 +/- 2 microM) and aprotinin (Ki 1.13 +/- 0.07 microM) in a classical competitive manner, indicating that substrate and inhibitor binding to the active site was mutually exclusive. In contrast, inhibition of factor XIa cleavage of S2366 by factor IX (Ki 224 +/- 32 nM) was characterized by hyperbolic mixed-type inhibition, indicating that factor IX binds to free and S2366-bound factor XIa at exosites. Consistent with this premise, inhibition of factor XIa activation of factor IX by aprotinin (Ki 0.89 +/- 0.52 microM) was non-competitive, whereas inhibition by active site-inhibited factor IXa beta was competitive (Ki 0.33 +/- 0.05 microM). S2366 cleavage by isolated factor XIa catalytic domain was competitively inhibited by p-aminobenzamidine (Ki 38 +/- 14 microM) but was not inhibited by factor IX, consistent with loss of factor IX-binding exosites on the non-catalytic factor XI heavy chain. The results support a model in which factor IX binds initially to exosites on the factor XIa heavy chain, followed by interaction at the active site with subsequent bond cleavage, and support a growing body of evidence that exosite interactions are critical determinants of substrate affinity and specificity in blood coagulation reactions.

Exosite-interactive regions in the A1 and A2 domains of factor VIII facilitate thrombin-catalyzed cleavage of heavy chain

Thrombin catalyzes the proteolytic activation of factor VIII, cleaving two sites in the heavy chain and one site in the light chain of the procofactor. Evaluation of thrombin binding the reaction products from heavy chain cleavage by steady state fluorescence energy transfer using a fluorophore-labeled, active site-modified thrombin as well as by solid phase binding assays using a thrombin Ser(205) --> Ala mutant indicated a high affinity site in the A1 subunit (K(d) approximately 5 nm) that was dependent upon the Na(+)-bound form of thrombin, whereas a moderate affinity site in the A2 subunit (K(d) approximately 100 nm) was observed for both Na(+)-bound and -free forms. The solid phase assay also indicated that hirudin blocked thrombin interaction with the A1 subunit and had little, if any, effect on its interaction with the A2 subunit. Conversely, heparin blocked thrombin interaction with the A2 subunit and showed a marginal effect on A1 binding. Evaluation of the A2 sequence revealed two regions rich in acidic residues that are localized close to the N and C termini of this domain. Peptides encompassing these clustered acidic regions, residues 373-395 and 719-740, blocked thrombin cleavage of the isolated heavy chain at Arg(372) and Arg(740) and inhibited A2 binding to thrombin Ser(205) --> Ala, suggesting that both A2 domain regions potentially support interaction with thrombin. A B-domainless, factor VIII double mutant Asp(392) --> Ala/Asp(394) --> Ala was constructed, expressed, and purified and possessed specific activity equivalent to a severe hemophilia phenotype. This mutant was resistant to cleavage at Arg(740), whereas cleavage at Arg(372) was not affected. These data suggest the acidic region comprising residues 389-394 in factor VIII A2 domain interacts with thrombin via its heparin-binding exosite and facilitates cleavage at Arg(740) during procofactor activation.

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.

Exosite-driven substrate specificity and function in coagulation

Macromolecular substrate recognition and serine proteinase specificity lie at the heart of the tightly regulated hemostatic response. Mechanisms established for the less specific serine proteinases of digestion have played a dominant role in guiding investigations of the basis for the narrow specificities exhibited by the coagulation enzymes. These concepts have also dominated the development of specific inhibitors of coagulation for therapeutic purposes. Studies of the enzymology and physical biochemistry of prothrombinase challenge these prevailing ideas by establishing a principal role for exosites within the enzyme in determining substrate recognition and directing the action of the enzyme on its biological substrate. Mechanisms by which narrow protein substrate specificity is achieved by prothrombinase also apply to several other reactions of coagulation. These strategies are increasingly evident in the action of other families of enzymes that act with high specificity on protein substrates. Exosite-driven enzymic function probably represents a widely employed biological strategy for the achievement of high macromolecular substrate specificity.

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.

The blood coagulation cascade

PURPOSE OF REVIEW

The biochemistry of blood coagulation has been well defined over the past 50 years. Although much is known about the sequence of the proteolytic cascade and its regulation in the pathway to fibrin generation, many important questions remain unsolved about the mechanism of initiation and the structure of the protein complexes that form during blood coagulation.

RECENT FINDINGS

This article summarizes some of the advances that have been made in this field from the last quarter of 2002 and during 2003. The papers, which vary in rigor and content, have been selected on the basis of their interest and possible contribution to knowledge in this field. Summaries are given of new findings on the source of factor V and the synthesis of factor VIII, the mechanism of tissue factor action in the initiation of blood coagulation, the structure and membrane-binding properties of the protein complexes formed, and regulation of the blood coagulation cascade.

SUMMARY

Continued progress in this field offers opportunity for understanding the basis of thrombotic diseases and bleeding disorders, with the potential for defining novel targets for therapeutic applications. Some of the conclusions reviewed are conflicting, and further work will be necessary to place the results in the context of what has already been established. The structural biology of the coagulation proteins and understanding of hemostasis and thrombosis in a physiologic context have important implications for future work.

Active Site-independent Recognition of Substrates and Product by Bovine Prothrombinase

The conversion of prothrombin to thrombin is catalyzed by prothrombinase, an enzyme complex composed of the serine proteinase factor Xa and a cofactor protein, factor Va, assembled on membranes. Kinetic studies indicate that interactions with extended macromolecular recognition sites (exosites) rather than the active site of prothrombinase are the principal determinants of binding affinity for substrate or product. We now provide a model-independent evaluation of such ideas by physical studies of the interaction of substrate derivatives and product with prothrombinase. The enzyme complex was assembled using Xa modified with a fluorescent peptidyl chloromethyl ketone to irreversibly occlude the active site. Binding was inferred by prethrombin 2-dependent perturbations in the fluorescence of Oregon Green(488) at the active site of prothrombinase. Active site-independent binding was also unequivocally established by fluorescence resonance energy transfer between 2,6-dansyl tethered to the active site of Xa and eosin tethered to the active sites of either thrombin or meizothrombin des fragment 1. Comparable interprobe distances obtained from these measurements suggest that substrate and product interact equivalently with the enzyme. Competition established the ability of a range of substrate or product derivatives to bind in a mutually exclusive fashion to prothrombinase. Equilibrium dissociation constants obtained for the active site-independent binding of prothrombin, prethrombin 2, meizothrombin des fragment 1 and thrombin to prothrombinase were comparable with their affinities inferred from kinetic studies using active enzyme. Our findings directly establish that binding affinity is principally determined by the exosite-mediated interaction of either the substrate, both possible intermediates, or product with prothrombinase. A single type of exosite binding interaction evidently drives affinity and binding specificity through the stepwise reactions necessary for the two cleavage reactions of prothrombin activation and product release.

The Contribution of Amino Acid Region Asp695-Tyr698 of Factor V to Procofactor Activation and Factor Va Function

There is strong evidence that a functionally important cluster of amino acids is located on the COOH-terminal portion of the heavy chain of factor Va, between amino acid residues 680 and 709. To ascertain the importance of this region for cofactor activity, we have synthesized five overlapping peptides representing this amino acid stretch (10 amino acids each, HC1-HC5) and tested them for inhibition of prothrombinase assembly and function. Two peptides, HC3 (spanning amino acid region 690-699) and HC4 (containing amino acid residues 695-704), were found to be potent inhibitors of prothrombinase activity with IC(50) values of approximately 12 and approximately 10 microm, respectively. The two peptides were unable to interfere with the binding of factor Va to active site fluorescently labeled Glu-Gly-Arg human factor Xa, and kinetic analyses showed that HC3 and HC4 are competitive inhibitors of prothrombinase with respect to prothrombin with K(i) values of approximately 6.3 and approximately 5.3 microm, respectively. These data suggest that the peptides inhibit prothrombinase because they interfere with the incorporation of prothrombin into prothrombinase. The shared amino acid motif between HC3 and HC4 is composed of Asp(695)-Tyr-Asp-Tyr-Gln(699) (DYDYQ). A pentapeptide with this sequence inhibited both prothrombinase function with an IC(50) of 1.6 microm (with a K(D) for prothrombin of 850 nm), and activation of factor V by thrombin. Peptides HC3, HC4, and DYDYQ were also found to interact with immobilized thrombin. A recombinant factor V molecule with the mutations Asp(695) --> Lys, Tyr(696) --> Phe, Asp(697) --> Lys, and Tyr(698) --> Phe (factor V(2K2F)) was partially resistant to activation by thrombin but could be readily activated by RVV-V activator (factor Va(RVV)(2K2F)) and factor Xa (factor Va(Xa)(2K2F)). Factor Va(RVV)(2K2F) and factor Va(Xa)(2K2F) had impaired cofactor activity within prothrombinase in a system using purified reagents. Our data demonstrate for the first time that amino acid sequence 695-698 of factor Va heavy chain is important for procofactor activation and is required for optimum prothrombinase function. These data provide functional evidence for an essential and productive contribution of factor Va to the activity of prothrombinase.

Role of prothrombin fragment 1 in the pathway of regulatory exosite I formation during conversion of human prothrombin to thrombin

Prothrombin (Pro) activation by factor Xa generates the thrombin catalytic site and exosites I and II. The role of fragment 1 (F1) in the pathway of exosite I expression during Pro activation was characterized in equilibrium binding studies using hirudin(54-65) labeled with 6-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)hexanoate ([NBD]Hir(54-65)(SO3-)) or 5-(carboxy)fluorescein ([5F]Hir(54-65)(SO3-)). [NBD]Hir(54-65)(SO3-) distinguished exosite I environments on Pro, prethrombin 1 (Pre 1), and prethrombin 2 (Pre 2) but bound with the same affinities as [5F]Hir(54-65)(SO3-). Conversion of Pro to Pre 1 caused a 7-fold increase in affinity for the peptides. Conversely, fragment 1.2 (F1.2) decreased the affinity of Pre 2 for [5F]Hir(54-65)(SO3-) by 3-fold. This was correlated with a 16-fold increased affinity of F1.2 for Pre 2 in comparison to thrombin, demonstrating an enhancing effect of F1 on F1.2 binding. The active intermediate, meizothrombin, demonstrated a 50- to 220-fold increase in exosite affinity. Free thrombin and thrombin.F1.2 complex bound [5F]Hir(54-65)(SO3-) with indistinguishable affinity, indicating that the effect of F1 on peptide binding was eliminated upon expression of catalytic activity and exosite I. The results demonstrate a new zymogen-specific role for F1 in modulating the affinity of ligands for exosite I. This may reflect a direct interaction between the F1 and Pre 2 domains in Pro that is lost upon folding of the zymogen activation domain. The effect of F1 on (pro)exosite I and the role of (pro)exosite I in factor Va-dependent substrate recognition suggest that the Pro activation pathway may be regulated by (pro)exosite I interactions with factor Va.

Specificity of coagulation factor signaling

Coagulation serine proteases signal through protease-activated receptors (PARs). Thrombin-dependent PAR signaling on platelets is essential for the hemostatic response and vascular thrombosis, but regulation of inflammation by PAR signaling is now recognized as an important aspect of the pro- and anti-coagulant pathways. In tissue factor (TF)-dependent initiation of coagulation, factor (F) Xa is the PAR-1 or PAR-2-activating protease when associated with the transient TF-FVIIa-FXa complex. In the anticoagulant protein C (PC) pathway, the thrombin-thrombomodulin complex activates PC bound to the endothelial cell PC receptor (EPCR), which functions as a required coreceptor for activated PC-mediated signaling through endothelial cell PAR-1. Thus, the pro- and anti-inflammatory receptor cascades are mechanistically coupled to immediate cell signaling, which precedes systemic coagulant or anticoagulant effects. In contrast to the substrate-like recognition of PARs by thrombin, TF- or EPCR-targeted activation of PARs generates cell-type specificity, PAR selectivity and protease receptor cosignaling with the G-protein-coupled PAR response. Protease receptors are thus major determinants of the biological outcome of coagulation factor signaling on vascular cells.