Comparison of heparin- and dermatan sulfate-mediated catalysis of thrombin inactivation by heparin cofactor II

Influence of Anticoagulants and Heparin Contaminants on the Suitability of MMP-9 as a Blood-Derived Biomarker

In contrast to other common anticoagulants such as citrate and low-molecular-weight heparin (LMWH), high-molecular-weight heparin (HMWH) induces the expression of matrix metalloproteinase (MMP)-9, which is also measured as a biomarker for stroke in blood samples. Mechanistically, HMWH-stimulated T cells produce cytokines that induce monocytic MMP-9 expression. Here, the influence of further anticoagulants (Fondaparinux, Hirudin, and Alteplase) and the heparin-contaminating glycosaminoglycans (GAG) hyaluronic acid (HA), dermatan sulfate (DS), chondroitin sulfate (CS), and over-sulfated CS (OSCS) on MMP-9 was analyzed to assess its suitability as a biomarker under various conditions. Therefore, starved Jurkat T cells were stimulated with anticoagulants/contaminants. Subsequently, starved monocytic THP-1 cells were incubated with the conditioned Jurkat supernatant, and MMP-9 mRNA levels were monitored (quantitative (q)PCR). Jurkat-derived mediators secreted in response to anticoagulants/contaminants were also assessed (proteome profiler array). The supernatants of HMWH-, Hirudin-, CS-, and OSCS-treated Jurkat cells comprised combinations of activating mediators and led to a significant (in the case of OSCS, dramatic) MMP-9 induction in THP-1. HA induced MMP-9 only in high concentrations, while LMWH, Fondaparinux, Alteplase, and DS had no effect. This indicates that depending on molecular weight and charge (but independent of anticoagulant activity), anticoagulants/contaminants provoke the expression of T-cell-derived cytokines/chemokines that induce monocytic MMP-9 expression, thus potentially impairing the diagnostic validity of MMP-9.

Anticoagulant SERPINs: Endogenous Regulators of Hemostasis and Thrombosis

Appropriate activation of coagulation requires a balance between procoagulant and anticoagulant proteins in blood. Loss in this balance leads to hemorrhage and thrombosis. A number of endogenous anticoagulant proteins, such as antithrombin and heparin cofactor II, are members of the serine protease inhibitor (SERPIN) family. These SERPIN anticoagulants function by forming irreversible inhibitory complexes with target coagulation proteases. Mutations in SERPIN family members, such as antithrombin, can cause hereditary thrombophilias. In addition, low plasma levels of SERPINs have been associated with an increased risk of thrombosis. Here, we review the biological activities of the different anticoagulant SERPINs. We further consider the clinical consequences of SERPIN deficiencies and insights gained from preclinical disease models. Finally, we discuss the potential utility of engineered SERPINs as novel therapies for the treatment of thrombotic pathologies.

Chemoenzymatically prepared heparan sulfate containing rare 2-O-sulfonated glucuronic acid residues

The structural diversity of natural sulfated glycosaminoglycans (GAGs) presents major promise for discovery of chemical biology tools or therapeutic agents. Yet, few GAGs have been identified so far to exhibit this promise. We reasoned that a simple approach to identify such GAGs is to explore sequences containing rare residues, for example, 2-O-sulfonated glucuronic acid (GlcAp2S). Genetic algorithm-based computational docking and filtering suggested that GlcAp2S containing heparan sulfate (HS) may exhibit highly selective recognition of antithrombin, a key plasma clot regulator. HS containing only GlcAp2S and 2-N-sulfonated glucosamine residues, labeled as HS2S2S, was chemoenzymatically synthesized in just two steps and was found to preferentially bind antithrombin over heparin cofactor II, a closely related serpin. Likewise, HS2S2S directly inhibited thrombin but not factor Xa, a closely related protease. The results show that a HS containing rare GlcAp2S residues exhibits the unusual property of selective antithrombin activation and direct thrombin inhibition. More importantly, HS2S2S is also the first molecule to activate antithrombin nearly as well as the heparin pentasaccharide although being completely devoid of the critical 3-O-sulfonate group. Thus, this work shows that novel functions and mechanisms may be uncovered by studying rare GAG residues/sequences.

Marine sulfated glycans with serpin-unrelated anticoagulant properties

Marine organisms are a rich source of sulfated polysaccharides with unique structures. Fucosylated chondroitin sulfate (FucCS) from the sea cucumber Ludwigothurea grisea and sulfated galactan from the red alga Botryocladia occidentalis are one of these unusual molecules. Besides their uncommon structures, they also exhibit high anticoagulant and antithrombotic effects. Earlier, it was considered that the anticoagulant activities of these two marine glycans were driven mainly by a catalytic serpin-dependent mechanism likewise the mammalian heparins. Its serpin-dependent anticoagulant action relies on promoting thrombin and/or factor Xa inhibition by their specific natural inhibitors (the serpins antithrombin and heparin cofactor II). However, as opposed to heparins, these two previously mentioned marine glycans were proved still capable in promoting coagulation inhibition using serpin-free plasmas. This puzzle observation was further investigated and clearly demonstrated that the cucumber FucCS and the red algal sulfated galactan have an unusual serpin-independent anticoagulant effect by inhibiting the formation of factor Xa and/or thrombin through the procoagulants tenase and prothrombinase complexes, respectively. These marine polysaccharides with unusual anticoagulant effects open clearly new perspectives for the development of new antithrombotic drugs as well as push the glycomics project.

Fusion of the C-terminal triskaidecapeptide of hirudin variant 3 to alpha1-proteinase inhibitor M358R increases the serpin-mediated rate of thrombin inhibition

BACKGROUND

Alpha-1 proteinase inhibitor (API) is a plasma serpin superfamily member that inhibits neutrophil elastase; variant API M358R inhibits thrombin and activated protein C (APC). Fusing residues 1-75 of another serpin, heparin cofactor II (HCII), to API M358R (in HAPI M358R) was previously shown to accelerate thrombin inhibition over API M358R by conferring thrombin exosite 1 binding properties. We hypothesized that replacing HCII 1-75 region with the 13 C-terminal residues (triskaidecapeptide) of hirudin variant 3 (HV354-66) would further enhance the inhibitory potency of API M358R fusion proteins. We therefore expressed HV3API M358R (HV354-66 fused to API M358R) and HV3API RCL5 (HV354-66 fused to API F352A/L353V/E354V/A355I/I356A/I460L/M358R) API M358R) as N-terminally hexahistidine-tagged polypeptides in E. coli.

RESULTS

HV3API M358R inhibited thrombin 3.3-fold more rapidly than API M358R; for HV3API RCL5 the rate enhancement was 1.9-fold versus API RCL5; neither protein inhibited thrombin as rapidly as HAPI M358R. While the thrombin/Activated Protein C rate constant ratio was 77-fold higher for HV3API RCL5 than for HV3API M358R, most of the increased specificity derived from the API F352A/L353V/E354V/A355I/I356A/I460L API RCL 5 mutations, since API RCL5 remained 3-fold more specific than HV3API RCL5. An HV3 54-66 peptide doubled the Thrombin Clotting Time (TCT) and halved the binding of thrombin to immobilized HCII 1-75 at lower concentrations than free HCII 1-75. HV3API RCL5 bound active site-inhibited FPR-chloromethyl ketone-thrombin more effectively than HAPI RCL5. Transferring the position of the fused HV3 triskaidecapeptide to the C-terminus of API M358R decreased the rate of thrombin inhibition relative to that mediated by HV3API M358R by 11-to 14-fold.

CONCLUSIONS

Fusing the C-terminal triskaidecapeptide of HV3 to API M358R-containing serpins significantly increased their effectiveness as thrombin inhibitors, but the enhancement was less than that seen in HCII 1-75-API M358R fusion proteins. HCII 1-75 was a superior fusion partner, in spite of the greater affinity of the HV3 triskaidecapeptide, manifested both in isolated and API-fused form, for thrombin exosite 1. Our results suggest that HCII 1-75 binds thrombin exosite 1 and orients the attached serpin scaffold for more efficient interaction with the active site of thrombin than the HV3 triskaidecapeptide.

Determination of galactosamine impurities in heparin sodium using fluorescent labeling and conventional high-performance liquid chromatography

Heparin is a sulfated glycosaminoglycan (GAG), which contains N-acetylated or N-sulfated glucosamine (GlcN). Heparin, which is generally obtained from the healthy porcine intestines, is widely used as an anticoagulant during dialysis and treatments of thrombosis such as disseminated intravascular coagulation. Dermatan sulfate (DS) and chondroitin sulfate (CS), which are galactosamine (GalN)-containing GAGs, are major process-related impurities of heparin products. The varying DS and CS contents between heparin products can be responsible for the different anticoagulant activities of heparin. Therefore, a test to determine the concentrations of GalN-containing GAG is essential to ensure the quality and safety of heparin products. In this study, we developed a method for determination of relative content of GalN from GalN-containing GAG in heparin active pharmaceutical ingredients (APIs). The method validation and collaborative study with heparin manufacturers and suppliers showed that our method has enough specificity, sensitivity, linearity, repeatability, reproducibility, and recovery as the limiting test for GalN from GalN-containing GAGs. We believe that our method will be useful for ensuring quality, efficacy, and safety of pharmaceutical heparins. On July 30, 2010, the GalN limiting test based on our method was adopted in the heparin sodium monograph in the Japanese Pharmacopoeia.

The complete N-terminal extension of heparin cofactor II is required for maximal effectiveness as a thrombin exosite 1 ligand

Background

Heparin cofactor II (HCII) is a circulating protease inhibitor, one which contains an N-terminal acidic extension (HCII 1-75) unique within the serpin superfamily. Deletion of HCII 1-75 greatly reduces the ability of glycosaminoglycans (GAGs) to accelerate the inhibition of thrombin, and abrogates HCII binding to thrombin exosite 1. While a minor portion of HCII 1-75 can be visualized in a crystallized HCII-thrombin S195A complex, the role of the rest of the extension is not well understood and the affinity of the HCII 1-75 interaction has not been quantitatively characterized. To address these issues, we expressed HCII 1-75 as a small, N-terminally hexahistidine-tagged polypeptide in E. coli.

Results

Immobilized purified HCII 1-75 bound active α-thrombin and active-site inhibited FPR-ck- or S195A-thrombin, but not exosite-1-disrupted γ_T-thrombin, in microtiter plate assays. Biotinylated HCII 1-75 immobilized on streptavidin chips bound α-thrombin and FPR-ck-thrombin with similar K_D values of 330-340 nM. HCII 1-75 competed thrombin binding to chip-immobilized HCII 1-75 more effectively than HCII 54-75 but less effectively than the C-terminal dodecapeptide of hirudin (mean K_i values of 2.6, 8.5, and 0.29 μM, respectively). This superiority over HCII 54-75 was also demonstrated in plasma clotting assays and in competing the heparin-catalysed inhibition of thrombin by plasma-derived HCII; HCII 1-53 had no effect in either assay. Molecular modelling of HCII 1-75 correctly predicted those portions of the acidic extension that had been previously visualized in crystal structures, and suggested that an α-helix found between residues 26 and 36 stabilizes one found between residues 61-67. The latter region has been previously shown by deletion mutagenesis and crystallography to play a crucial role in the binding of HCII to thrombin exosite 1.

Conclusions

Assuming that the K_D value for HCII 1-75 of 330-340 nM faithfully predicts that of this region in intact HCII, and that 1-75 binding to exosite 1 is GAG-dependent, our results support a model in which thrombin first binds to GAGs, followed by HCII addition to the ternary complex and release of HCII 1-75 for exosite 1 binding and serpin mechanism inhibition. They further suggest that, in isolated or transferred form, the entire HCII 1-75 region is required to ensure maximal binding of thrombin exosite 1.

Fluorescent reporters of thrombin, heparin cofactor II, and heparin binding in a ternary complex

Thrombin inactivation by heparin cofactor II (HCII) is accelerated by ternary complex formation with heparin. The novel active-site-labeled thrombins, [4'F]FPR-T and [6F]FFR-T, and the exosite I probe, Hir-(54-65)(SO₃⁻), characterized thrombin exosite I and II interactions with HCII and heparin in the complex. HCII binding to exosite I of heparin-bound [4'F]FPR-T caused a saturable fluorescence increase, absent with antithrombin. Heparin binding to exosite II and a second weaker site caused fluorescence quenching of [6F]-FFR-T, attenuated by simultaneous Hir-(54-65)(SO₃⁻) binding. Stopped-flow analysis demonstrated ordered assembly of HCII and the [6F]FFR-T·heparin complex, in agreement with tighter heparin binding to thrombin than to HCII. Saturating HCII dependences and bell-shaped heparin dependences of the fluorescence change reported ternary complex formation, consistent with a template mechanism in which the thrombin·heparin complex binds HCII and allowing for interaction of thrombin·(heparin)₂ complexes with HCII. Hir-(54-65)(SO₃⁻) displacement in reactions with FPR-blocked and active thrombin indicated a concerted action of the active site and exosite I during ternary complex formation. These studies demonstrate that binding of HCII to the thrombin·heparin complex is dramatically enhanced compared with heparin binding alone and that exosite I is still available for ligand or HCII binding when both heparin binding sites on thrombin are saturated.

Retention of thrombin inhibitory activity by recombinant serpins expressed as integral membrane proteins tethered to the surface of mammalian cells

BACKGROUND

Serpins form a widely distributed protein superfamily, but no integral membrane serpins have been described.

OBJECTIVES

To anchor three serpins -α(1) -proteinase inhibitor (α(1) PI) (M358R), antithrombin (AT), and heparin cofactor II (HCII) - in the plasma membranes of transfected mammalian cells and assess their ability to inhibit thrombin.

METHODS

Serpin cDNAs were altered to include N-terminal, non-cleavable plasma membrane-targeting sequences from the human transferrin receptor (TR) (TR-serpin) or the human asialoglycoprotein receptor (AR) (AR-serpin), and used to transfect COS-1 or HEK 293 cells. Cells were analyzed for serpin expression by immunoblotting of subcellular fractions, by immunofluorescence microscopy, or by flow cytometry, with or without exposure to exogenous thrombin; AR-serpins and TR-serpins were also compared with their soluble recombinant counterparts.

RESULTS

Both TR-α(1) PI (M358R) and AR-α(1) PI (M358R) were enriched in the integral membrane fraction of transfected COS-1 or HEK 293 cells, and formed inhibitory complexes with thrombin, although less rapidly than soluble α(1) PI (M358R). Thrombin inhibition was abrogated by an additional T345R mutation in AR-α(1) PI (M358R). Surface-displayed AR-AT also formed serpin-enzyme complexes with thrombin, but to a lesser extent than AR-α(1) PI (M358R); AR-HCII inhibitory function was not detected. Immunofluorescence detection and flow cytometric quantification of bound thrombin also supported the status of AR-α(1) PI (M358R) and AR-AT as thrombin inhibitors.

CONCLUSIONS

Two of three thrombin-inhibitory serpins retained functionality when expressed as integral membrane proteins. Our findings could be applied to create and screen hypervariable serpin libraries expressed in mammalian cells, or to confer protease resistance on engineered cells in vivo.

Heparin is a major activator of the anticoagulant serpin, protein Z-dependent protease inhibitor

Protein Z-dependent protease inhibitor (ZPI) is a recently identified member of the serpin superfamily that functions as a cofactor-dependent regulator of blood coagulation factors Xa and XIa. Here we provide evidence that, in addition to the established cofactors, protein Z, lipid, and calcium, heparin is an important cofactor of ZPI anticoagulant function. Heparin produced 20-100-fold accelerations of ZPI reactions with factor Xa and factor XIa to yield second order rate constants approaching the physiologically significant diffusion limit (k(a) = 10(6) to 10(7) M(-1) s(-1)). The dependence of heparin accelerating effects on heparin concentration was bell-shaped for ZPI reactions with both factors Xa and XIa, consistent with a template-bridging mechanism of heparin rate enhancement. Maximal accelerations of ZPI-factor Xa reactions required calcium, which augmented the heparin acceleration by relieving Gla domain inhibition as previously shown for heparin bridging of the antithrombin-factor Xa reaction. Heparin acceleration of both ZPI-protease reactions was optimal at heparin concentrations and heparin chain lengths comparable with those that produce physiologically significant rate enhancements of other serpin-protease reactions. Protein Z binding to ZPI minimally affected heparin rate enhancements, indicating that heparin binds to a distinct site on ZPI and activates ZPI in its physiologically relevant complex with protein Z. Taken together, these results suggest that whereas protein Z, lipid, and calcium cofactors promote ZPI inhibition of membrane-associated factor Xa, heparin activates ZPI to inhibit free factor Xa as well as factor XIa and therefore may play a physiologically and pharmacologically important role in ZPI anticoagulant function.

Serpin-glycosaminoglycan interactions

Serpins (serine protease inhibitors) have traditionally been grouped together based on structural homology. They share common structural features of primary sequence, but not all serpins require binding to cofactors in order to achieve maximal protease inhibition. In order to obtain physiologically relevant rates of inhibition of target proteases, some serpins utilize the unbranched sulfated polysaccharide chains known as glycosaminoglycans (GAGs) to enhance inhibition. These GAG-binding serpins include antithrombin (AT), heparin cofactor II (HCII), and protein C inhibitor (PCI). The GAGs heparin and heparan sulfate have been shown to bind AT, HCII, and PCI, while HCII is also able to utilize dermatan sulfate as a cofactor. Other serpins such as PAI-1, kallistatin, and α(1)-antitrypsin also interact with GAGs with different endpoints, some accelerating protease inhibition while others inhibit it. There are many serpins that bind or carry ligands that are unrelated to GAGs, which are described elsewhere in this work. For most GAG-binding serpins, binding of the GAG occurs in a conserved region of the serpin near or involving helix D, with the exception of PCI, which utilizes helix H. The binding of GAG to serpin can lead to a conformational change within the serpin, which can lead to increased or tighter binding to the protease, and can accelerate the rates of inhibition up to 10,000-fold compared to the unbound native serpin. In this chapter, we will discuss three major GAG-binding serpins with known physiological roles in modulating coagulation: AT (SERPINC1), HCII (SERPIND1), and PCI (SERPINA5). We will review methodologies implemented to study the structure of these serpins and those used to study their interactions with GAG's. We discuss novel techniques to examine the serpin-GAG interaction and finally we review the biological roles of these serpins by describing the mouse models used to study them.

Understanding Dermatan Sulfate-Heparin Cofactor II Interaction through Virtual Library Screening

Dermatan sulfate, an important member of the glycosaminoglycan family, interacts with heparin cofactor II, a member of the serpin family of proteins, to modulate antithrombotic response. Yet, the nature of this interaction remains poorly understood at a molecular level. We report the genetic algorithm-based combinatorial virtual library screening study of a natural, high-affinity dermatan sulfate hexasaccharide with heparin cofactor II. Of the 192 topologies possible for the hexasaccharide, only 16 satisfied the “high-specificity” criteria used in computational study. Of these, 13 topologies were predicted to bind in the heparin-binding site of heparin cofactor II at a ∼60° angle to helix D, a novel binding mode. This new binding geometry satisfies all known solution and mutagenesis data and supports thrombin ternary complexation through a template mechanism. The study is expected to facilitate the design of allosteric agonists of heparin cofactor II as antithrombotic agents.

Investigating serpin-enzyme complex formation and stability via single and multiple residue reactive centre loop substitutions in heparin cofactor II

INTRODUCTION

Following thrombin cleavage of the reactive centre (P1-P1'; L444-S445) of the serpin heparin cofactor II (HCII), HCII traps thrombin (IIa) in a stable inhibitory complex. To compare HCII to other serpins we substituted: the P13-P5' residues of HCII with those of alpha(1)-proteinase inhibitor (alpha(1)-PI), alpha(1)-PI (M358R), or antithrombin (AT); the P4-P1, P3-P1, and P2-P1 residues of HCII with those of AT; and made L444A/H/K/M or R point mutations. We also combined L444R with changes in the glycosaminoglycan binding domain collectively termed MutD.

MATERIALS AND METHODS

Variants were made by site-directed mutagenesis, expressed in bacteria, purified and characterized electrophoretically and kinetically.

RESULTS AND CONCLUSIONS

Of the P13-P5' mutants, only the alpha(1)-PI-loop variant retained anti-IIa activity, but less than the corresponding L444M. Heparin-catalyzed rate constants for IIa inhibition were reduced vs. wild-type (WT) by at most three-fold for all P1 mutants save L444A (reduced 20-fold). L444R and L444K inhibited IIa>50- and >6-fold more rapidly than WT in heparin-free reactions, but stoichiometries of inhibition were increased for all variants. HCII-IIa complexes of all P1 variants were stable in the absence of heparin, but those of the L444K and L444R variants released active IIa over time with heparin. Limited proteolysis of these two groups of HCII-IIa complexes produced different fragmentation patterns consistent with conformational differences. The combination of either substituted AT residues at P2, P3, and P4, or the MutD mutations with L444R resulted in complex instability with or without heparin. This is the first description of HCII-IIa complexes of transient stability forming in the absence of heparin, and may explain the extent to which the reactive centre loop of HCII differs from that of AT.

GLYCOMICS APPROACH TO STRUCTURE-FUNCTION RELATIONSHIPS OF GLYCOSAMINOGLYCANS

Extracellular modulation of phenotype is an emerging paradigm in this current postgenomics age of molecular and cell biology. Glycosaminoglycans (GAGs) are primary components of the cell surface and the cell-extracellular matrix (ECM) interface. Advances in the technology to analyze GAGs and in whole-organism genetics have led to a dramatic increase in the known important biological role of these complex polysaccharides. Owing to their ubiquitous distribution at the cell-ECM interface, GAGs interact with numerous proteins and modulate their activity, thus impinging on fundamental biological processes such as cell growth and development. Many recent reviews have captured important aspects of GAG structure and biosynthesis, GAG-protein interactions, and GAG biology. GAG research is currently at a stage where there is a need for an integrated systems or glycomics approach, which involves an integration of all of the above concepts to define their structure-function relationships. Focusing on heparin/heparan (HSGAGs) and chondroitin/dermatan sulfate (CSGAGs), this review highlights the important aspects of GAGs and summarizes these aspects in the context of taking a glycomics approach that integrates the different technologies to define structure-function relationships of GAGs.

Biochemical characterization of the chondroitinase ABC I active site

cABC I (chondroitinase ABC I) from Proteus vulgaris is a GalAG (galactosaminoglycan) depolymerizing lyase that cleaves its substrates at the glycosidic bond via beta-elimination. cABC I cleaves a particularly broad range of GalAG substrates, including CS (chondroitin sulphate), DS (dermatan sulphate) and hyaluronic acid. We recently cloned and recombinantly expressed cABC I in Escherichia coli, and completed a preliminary biochemical characterization of the enzyme. In the present study, we have coupled site-directed mutagenesis of the recombinant cABC I with a structural model of the enzyme-substrate complex in order to investigate in detail the roles of active site amino acids in the catalytic action of the enzyme. The putative catalytic residues His-501, Tyr-508, Arg-560 and Glu-653 were probed systematically via mutagenesis. Assessment of these mutants in kinetic and end-point assays provided direct evidence on the catalytic roles of these active-site residues. The crystal structure of the native enzyme provided a framework for molecular docking of representative CS and DS substrates. This enabled us to construct recombinant enzyme-substrate structural complexes. These studies together provided structural insights into the effects of the mutations on the catalytic mechanism of cABC I and the differences in its processing of CS and DS substrates. All His-501 mutants were essentially inactive and thereby implicating this amino acid to play the critical role of proton abstraction during catalysis. The kinetic data for Glu-653 mutants indicated that it is involved in a hydrogen bonding network in the active site. The proximity of Tyr-508 to the glycosidic oxygen of the substrate at the site of cleavage suggested its potential role in protonating the leaving group. Arg-560 was proximal to the uronic acid C-5 proton, suggesting its possible role in the stabilization of the carbanion intermediate formed during catalysis.

Intimatan (dermatan 4,6-O-disulfate) prevents rethrombosis after successful thrombolysis in the canine model of deep vessel wall injury

INTRODUCTION

Intimatan (dermatan 4,6-O-disulfate), a heparin cofactor II (HCII) agonist, inhibits both the fluid phase and thrombus bound thrombin. The efficacy of Intimatan as an adjunctive anticoagulant during thrombolysis was evaluated in the canine model of arterial injury.

MATERIALS AND METHODS

After forming an occlusive thrombus in the right carotid artery (RCA), twenty-one dogs were administered recombinant tissue plasminogen activator (rt-PA) intra-arterially to achieve thrombolysis in the presence of either 0.9% NaCl or Intimatan (9 mg/kg bolus+300 mug/kg/min i.v. infusion). Next, the left carotid arteries (LCA) of the same animals were injured in the presence of either Intimatan or 0.9% NaCl.

RESULTS

The incidence of RCA rethrombosis between the Intimatan and control groups was 2/9 and 8/12, respectively. The quality of RCA blood flow, i.e., patency score (Scale of 0-3, i.e., no flow to high flow, respectively), was 2.3+/-0.4 (Intimatan) versus 0.9+/-0.4 (0.9% NaCl). The incidence of primary thrombosis was determined among the groups as 0/9 (Intimatan) versus 7/12 (0.9% NaCl); the patency score was 2.8+/-0.1 (Intimatan) versus 0.9+/-0.4 (0.9% NaCl). Intimatan resulted in a >90% ex vivo inhibition of gamma-thrombin-induced platelet aggregation whereas 0.9% NaCl had no inhibitory effect. Clot-bound thrombin activity was reduced significantly by Intimatan. Intimatan induced <2-fold change in aPTT and bleeding time (BT) when corrected for the 0.9% NaCl group.

CONCLUSIONS

Intimatan significantly reduces the incidence of both primary and secondary arterial thrombosis while maintaining a high-grade vessel patency score with only moderate increases in BT and aPTT.

Control of the coagulation system by serpins. Getting by with a little help from glycosaminoglycans

Members of the serine protease inhibitor (serpin) superfamily play important roles in the inhibition of serine proteases involved in complex systems. This is evident in the regulation of coagulation serine proteases, especially the central enzyme in this system, thrombin. This review focuses on three serpins which are known to be key players in the regulation of thrombin: antithrombin and heparin cofactor II, which inhibit thrombin in its procoagulant role, and protein C inhibitor, which primarily inhibits the thrombin/thrombomodulin complex, where thrombin plays an anticoagulant role. Several structures have been published in the past few years which have given great insight into the mechanism of action of these serpins and have significantly added to a wealth of biochemical and biophysical studies carried out previously. A major feature of these serpins is that they are under the control of glycosaminoglycans, which play a key role in accelerating and localizing their action. While further work is clearly required to understand the mechanism of action of the glycosaminoglycans, the biological mechanisms whereby cognate glycosaminoglycans for each serpin come into contact with the inhibitors also requires much further work in this important field.

Reformable intramolecular cross-linking of the N-terminal domain of heparin cofactor II

The crystal structure of a heparin cofactor II (HCII)-thrombin Michaelis complex has revealed extensive contacts encompassing the N-terminal domain of HCII and exosite I of the proteinase. In contrast, the location of the N-terminal extension in the uncomplexed inhibitor was unclear. Using a disulfide cross-linking strategy, we demonstrate that at least three different sites (positions 52, 54 and 68) within the N terminus may be tethered in a reformable manner to position 195 in the loop region between helix D and strand s2A of the HCII molecule, suggesting that the N-terminal domain may interact with the inhibitor scaffold in a permissive manner. Cross-linking of the N terminus to the HCII body does not strongly affect the inhibition of alpha-chymotrypsin, indicating that the reactive site loop sequences of the engineered inhibitor variants, required for interaction with one of the HCII target enzymes, are normally accessible. In contrast, intramolecular tethering of the N-terminal extension results in a drastic decrease of alpha-thrombin inhibitory activity, both in the presence and in the absence of glycosaminoglycans. Treatment with dithiothreitol and iodoacetamide restores activity towards alpha-thrombin, suggesting that release of the N terminus of HCII is an important component of the multistep interaction between the inhibitor and alpha-thrombin.

Studies on the effect of calcium in interactions between heparin and heparin cofactor II using surface plasmon resonance

Heparin is the most acidic polysaccharide in the human body and as a result interacts with many cationic species, including ions and proteins, giving rise to myriad biologic activities. Heparin cofactor II (HCII) is a serine protease inhibitor that resembles antithrombin (ATIII) in its ability to be activated by heparin. The interaction of heparin with HCII has been the focus of many studies using affinity chromatography and fluorescence spectroscopy. In this study, surface plasmon resonance (SPR) spectroscopy was used to quantitatively measure the interaction of heparin and HCII using a heparin biochip prepared by covalently immobilizing preformed albumin-heparin conjugate. HCII contains multiple EF hand domains that represent putative calcium ion binding sites. The interactions of HCII with heparin, low-molecular-weight heparin, and heparin oligosaccharides (disaccharide, tetrasaccharide, hexasaccharide) were examined in solution competition experiments using SPR. The results also showed while calcium ions enhanced the heparin/HCII interaction, the activity of heparin-HCII complex against thrombin was not calcium dependent but can be enhanced by the presence of calcium.

Molecular Mapping of the Thrombin-Heparin Cofactor II Complex

We used 55 Ala-scanned recombinant thrombin molecules to define residues important for inhibition by the serine protease inhibitor (serpin) heparin cofactor II (HCII) in the absence and presence of glycosaminoglycans. We verified the importance of numerous basic residues in anion-binding exosite-1 (exosite-1) and found 4 additional residues, Gln24, Lys65, His66, and Tyr71 (using the thrombin numbering system), that were resistant to HCII inhibition with and without glycosaminoglycans. Inhibition rate constants for these exosite-1 (Q24A, K65A, H66A, Y71A) thrombin mutants (0.02-0.38 x 10(8) m(-1) min(-1) for HCII-heparin when compared with 2.36 x 10(8) m(-1) min(-1) with wild-type thrombin and 0.03-0.53 x 10(8) m(-1) min(-1) for HCII-dermatan sulfate when compared with 5.23 x 10(8) m(-1) min(-1) with wild-type thrombin) confirmed that the structural integrity of thrombin exosite-1 is critical for optimal HCII-thrombin interactions in the presence of glycosaminoglycans. However, our results are also consistent for HCII-glycosaminoglycan-thrombin ternary complex formation. Ten residues surrounding the active site of thrombin were implicated in HCII interactions. Four mutants (Asp51, Lys52, Lys145/Thr147/Trp148, Asp234) showed normal increased rates of inhibition by HCII-glycosaminoglycans, whereas four mutants (Trp50, Glu202, Glu229, Arg233) remained resistant to inhibition by HCII with glycosaminoglycans. Using 11 exosite-2 thrombin mutants with 20 different mutated residues, we saw no major perturbations of HCII-glycosaminoglycan inhibition reactions. Collectively, our results support a "double bridge" mechanism for HCII inhibition of thrombin in the presence of glycosaminoglycans, which relies in part on ternary complex formation but is primarily dominated by an allosteric process involving contact of the "hirudin-like" domain of HCII with thrombin exosite-1.

The heparin binding properties of heparin cofactor II suggest an antithrombin-like activation mechanism

The serpin heparin cofactor II (HCII) is a glycosaminoglycan-activated inhibitor of thrombin that circulates at a high concentration in the blood. The antithrombotic effect of heparin, however, is due primarily to the specific interaction of a fraction of heparin chains with the related serpin antithrombin (AT). What currently prevents selective therapeutic activation of HCII is the lack of knowledge of the determinants of glycosaminoglycan binding specificity. In this report we investigate the heparin binding properties of HCII and conclude that binding is nonspecific with a minimal heparin length of 13 monosaccharide units required and affinity critically dependent on ionic strength. Rapid kinetics of heparin binding indicate an induced fit mechanism that involves a conformational change in HCII. Thus, HCII binds to heparin in a manner analogous to the interaction of AT with low affinity heparin. A fully allosteric 2000-fold heparin activation of thrombin inhibition by HCII is demonstrated for heparin chains up to 26 monosaccharide units in length. We conclude that the heparin-binding mechanism of HCII is closely analogous to that of AT and that the induced fit mechanism suggests the potential design or discovery of specific HCII agonists.

RNA aptamer to thrombin binds anion-binding exosite-2 and alters protease inhibition by heparin-binding serpins

We studied the RNA aptamer Toggle-25/thrombin interaction during inhibition by antithrombin (AT), heparin cofactor II (HCII) and protein C inhibitor (PCI). Thrombin inhibition was reduced 3-fold by Toggle-25 for AT and HCII, but it was slightly enhanced for PCI. In the presence of glycosaminoglycans, AT and PCI had significantly reduced thrombin inhibition with Toggle-25, but it was only reduced 3-fold for HCII. This suggested that the primary effect of aptamer binding was through the heparin-binding site of thrombin, anion-binding exosite-2 (exosite-2). We localized the Toggle-25 binding site to Arg 98, Glu 169, Lys 174, Asp 175, Arg 245, and Lys 248 of exosite-2. We conclude that a RNA aptamer to thrombin exosite-2 might provide an effective clinical reagent to control heparin's anticoagulant action.

The Preferred Pathway of Glycosaminoglycan-accelerated Inactivation of Thrombin by Heparin Cofactor II

Thrombin (T) inactivation by the serpin, heparin cofactor II (HCII), is accelerated by the glycosaminoglycans (GAGs) dermatan sulfate (DS) and heparin (H). Equilibrium binding and thrombin inactivation kinetics at pH 7.8 and ionic strength (I) 0.125 m demonstrated that DS and heparin bound much tighter to thrombin (K(T(DS)) 1-5.8 microm; K(T(H)) 0.02-0.2 microm) than to HCII (K(HCII(DS)) 236-291 microm; K(HCII(H)) 25-35 microm), favoring formation of T.GAG over HCII.GAG complexes as intermediates for T.GAG.HCII complex assembly. At [GAG] << K(HCII(GAG)) the GAG and HCII concentration dependences of the first-order inactivation rate constants (k(app)) were hyperbolic, reflecting saturation of T.GAG complex and formation of the T.GAG.HCII complex from T.GAG and free HCII, respectively. At [GAG] >> K(HCII(GAG)), HCII.GAG complex formation caused a decrease in k(app). The bell-shaped logarithmic GAG dependences fit an obligatory template mechanism in which free HCII binds GAG in the T.GAG complex. DS and heparin bound fluorescently labeled meizothrombin(des-fragment 1) (MzT(-F1)) with K(MzT(-F1)(GAG)) 10 and 20 microm, respectively, demonstrating a binding site outside of exosite II. Exosite II ligands did not attenuate the DS-accelerated thrombin inactivation markedly, but DS displaced thrombin from heparin-Sepharose, suggesting that DS and heparin share a restricted binding site in or nearby exosite II, in addition to binding outside exosite II. Both T.DS and MzT(-F1).DS interactions were saturable at DS concentrations substantially below K(HCII(DS)), consistent with DS bridging T.DS and free HCII. The results suggest that GAG template action facilitates ternary complex formation and accommodates HCII binding to GAG and thrombin exosite I in the ternary complex.

Targeting thrombin – rational drug design from natural mechanisms

It is difficult to overstate the medical importance of the serine protease thrombin. Thrombin is involved in many diverse processes, such as cell signaling and memory, but it is the crucial role that it plays in blood coagulation that commands the interest of the medical community. Thrombosis is the most common cause of death in the industrialized world and, whether through venous thromboembolism, myocardial infarction or stroke, ultimately involves the inappropriate activity of thrombin. The number and type of intrinsic and extrinsic natural mechanisms of targeting thrombin that have evolved validate thrombin as an important physiological target, and provide strategies to knock it out. The more we learn about the natural mechanisms that determine thrombin specificity the more likely we are to develop compounds that selectively alter thrombin activity. In this article, we review the natural mechanisms that regulate thrombin activity and novel approaches to inhibit thrombin based on these mechanisms.

Mechanism of activation of heparin cofactor II by calcium spirulan

Calcium spirulan (Ca-SP), a novel sulfated polysaccharide, increases the rate of thrombin inhibition by heparin cofactor II (HCII) more than 1000-fold through a mechanism not requiring the amino-terminal acidic domain of HCII. Activation of HCII by Ca-SP was molecular-weight dependent. Furthermore, HD22, an aptamer that binds exosite II of thrombin, produced a concentration-dependent, 15-fold reduction at 5 microM in the rate of thrombin inhibition by HCII with Ca-SP, suggesting that Ca-SP interacts with exosite II of thrombin. Mutations of Lys173 to Leu (K173L) and Arg189 to Leu (R189L) in the HCII molecule resulted in large decreases in the rate of thrombin inhibition mediated by Ca-SP and in the NaCl concentration needed for elution from Ca-SP-Toyopearl. Mutations of Lys173 to Arg (K173R) and Arg189 to Lys (R189K) showed inhibition of thrombin similar to wild-type rHCII (wt-rHCII). These results indicate that Ca-SP binds to the positive charges of Lys173 and Arg189 on the HCII molecule. In the thrombin inhibitory process by HCII, Ca-SP appears to play as a template by binding to both thrombin and HCII.

Zinc ions promote the interaction between heparin and heparin cofactor II

The effects of bivalent cations on heparin binding, structure, and thrombin inhibition rates of heparin cofactor II were examined. Zn(2+) - and to a lesser extent Cu(2+) and Ni(2+) - enhanced the interaction between heparin cofactor II and heparin as demonstrated by heparin affinity chromatography and surface plasmon resonance experiments. Metal chelate chromatography and increased intrinsic protein fluorescence in the presence of Zn(2+) indicated that heparin cofactor II has metal ion-binding properties. The results are compatible with the hypothesis that Zn(2+) induces a conformational change in heparin cofactor II that favors its interaction with heparin.

Crystal structure of protein C inhibitor provides insights into hormone binding and heparin activation

Protein C inhibitor (PCI) is a member of the serpin family that has many biological functions. In blood it acts as a procoagulant, and, in the seminal vesicles, it is required for spermatogenesis. The activity of PCI is affected by heparin binding in a manner unique among the heparin binding serpins, and, in addition, PCI binds hydrophobic hormones with apparent specificity for retinoids. Here we present the 2.4 A crystallographic structure of reactive center loop (RCL) cleaved PCI. A striking feature of the structure is a two-turn N-terminal shortening of helix A, which creates a large hydrophobic pocket that docking studies indicate to be the retinoid binding site. On the basis of surface electrostatic properties, a novel mechanism for heparin activation is proposed.

New antithrombin-based anticoagulants

Clinically used anticoagulants are inhibitors of enzymes involved in the coagulation pathway, primarily thrombin and factor Xa. These agents can be either direct or indirect inhibitors of clotting enzymes. Heparin-based anticoagulants are indirect inhibitors that enhance the proteinase inhibitory activity of a natural anticoagulant, antithrombin. Despite its phenomenal success, current anticoagulation therapy suffers from the risk of serious bleeding. The need for safer and more effective antithrombotic agents clearly exists. The past decade has seen enormous effort directed toward discovering and/or designing new molecules with anticoagulant activity. These new molecules can be classified into (a). antithrombin and its mutants, (b). natural polysaccharides, (c). synthetic modified heparins and heparin-mimics, (d). synthetic oligosaccharides, and (e). synthetic non-sugar antithrombin activators. This review focuses on these efforts in designing or discovering new molecules that act through the antithrombin pathway of anticoagulation.

Crystal structures of native and thrombin-complexed heparin cofactor II reveal a multistep allosteric mechanism

The serine proteases sequentially activated to form a fibrin clot are inhibited primarily by members of the serpin family, which use a unique beta-sheet expansion mechanism to trap and destroy their targets. Since the discovery that serpins were a family of serine protease inhibitors there has been controversy as to the role of conformational change in their mechanism. It now is clear that protease inhibition depends entirely on rapid serpin beta-sheet expansion after proteolytic attack. The regulatory advantage afforded by the conformational mobility of serpins is demonstrated here by the structures of native and S195A thrombin-complexed heparin cofactor II (HCII). HCII inhibits thrombin, the final protease of the coagulation cascade, in a glycosaminoglycan-dependent manner that involves the release of a sequestered hirudin-like N-terminal tail for interaction with thrombin. The native structure of HCII resembles that of native antithrombin and suggests an alternative mechanism of allosteric activation, whereas the structure of the S195A thrombin-HCII complex defines the molecular basis of allostery. Together, these structures reveal a multistep allosteric mechanism that relies on sequential contraction and expansion of the central beta-sheet of HCII.

Molecular and functional characterization of a natural homozygous Arg67His mutation in the prothrombin gene of a patient with a severe procoagulant defect contrasting with a mild hemorrhagic phenotype

In a patient who presented with a severe coagulation deficiency in plasma contrasting with a very mild hemorrhagic diathesis a homozygous Arg67His mutation was identified in the prothrombin gene. Wild-type (factor IIa [FIIa]-WT) and mutant Arg67His thrombin (FIIa-MT67) had similar amidolytic activity. By contrast, the k(cat)/K(m) value of fibrinopeptide A hydrolysis by FIIa-WT and FIIa-MT67 was equal to 2.1 x 10(7) M(-1)s(-1) and 9 x 10(5) M(-1)s(-1). Decreased activation of protein C (PC) correlated with the 33-fold decreased binding affinity for thrombomodulin (TM; K(d) = 65.3 nM vs 2.1 nM, in FIIa-MT67 and in FIIa-WT, respectively). In contrast, hydrolysis of PC in the absence of TM was normal. The Arg67His mutation had a dramatic effect on the cleavage of protease-activated G protein-coupled receptor 1 (PAR-1) 38-60 peptide (k(cat/)K(m) = 4 x 10(7) M(-1)s(-1) to 1.2 x 10(6) M(-1)s(-1)). FIIa-MT67 showed a weaker platelet activating capacity, attributed to a defective PAR-1 interaction, whereas the interaction with glycoprotein Ib was normal. A drastic decrease (up to 500-fold) of the second-order rate constant pertaining to heparin cofactor II (HCII) interaction, especially in the presence of dermatan sulfate, was found for the FIIa-MT67 compared with FIIa-WT, suggesting a severe impairment of thrombin inhibition by HCII in vivo. Finally, the Arg67His mutation was associated with a 5-fold decrease of prothrombin activation by the factor Xa-factor Va complex, perhaps through impairment of the prothrombin-factor Va interaction. These experiments show that the Arg67His substitution affects drastically both the procoagulant and the anticoagulant functions of thrombin as well as its inhibition by HCII. The mild hemorrhagic phenotype might be explained by abnormalities that ultimately counterbalance each other.

Contribution of basic residues of the A helix of heparin cofactor II to heparin- or dermatan sulfate-mediated thrombin inhibition

Inhibition of thrombin by heparin cofactor II (HCII) is accelerated 1000-fold by heparin or dermatan sulfate. To investigate the contribution of basic residues of the A helix of HCII to this activation, we constructed amino acid substitutions (K101Q, R103L, and R106L) by site-directed mutagenesis. K101Q greatly reduced heparin cofactor activity and required a more than 10-fold higher concentration of dermatan sulfate to accelerate thrombin inhibition compared with wild-type recombinant HCII. Thrombin inhibition by R106L was not significantly stimulated by dermatan sulfate. These results provide evidence that basic residues of the A helix of HCII (Lys(101) and Arg(106)) are necessary for heparin- or dermatan sulfate-accelerated thrombin inhibition.

Intimatan prevents arterial and venous thrombosis in a canine model of deep vessel wall injury

Resistance of fibrin-bound thrombin to inactivation by the heparin/antithrombin III complex is considered a limitation in the use of heparin as an antithrombotic agent. Intimatan (dermatan 4,6-di-O-sulfate) is a heparin cofactor II agonist that inhibits both free and bound forms of thrombin. The present study examines the hypothesis that Intimatan prevents thrombotic occlusion in response to vascular wall injury in a canine model of carotid artery/jugular vein thrombosis. The left carotid artery and right jugular vein served as vehicle-treated control vessels, whereas the right carotid artery and left jugular vein were subjected to electrolytic injury after administration of Intimatan (9 mg/kg bolus + 300 microg/kg/min infusion, i.v.) or dalteparin (Fragmin) (400 IU/kg, s.c.). Intimatan significantly increased time to carotid artery (226.0 +/- 14.0 min) and jugular vein (240.0 +/- 0.0 min) thrombosis, compared with control vessels (carotid artery, 87.1 +/- 7.9 min; jugular vein, 60.6 +/- 7.4 min). Vessel patency was maintained in eight of eight jugular veins and seven of eight carotid arteries during treatment with Intimatan. Dalteparin significantly increased time to carotid artery thrombosis (122.1 +/- 17.5 min) compared with control (64.3 +/- 8.2 min), but did not change the time to thrombosis in the jugular vein. Only one carotid artery remained patent at the end of the dalteparin protocol. The two drugs produced minimal increases in bleeding times, and Intimatan increased the activated partial thromboplastin time above that observed with dalteparin. The results demonstrate that Intimatan is effective in preventing occlusive arterial and venous thrombosis in an experimental model of deep vascular wall injury.

Aspartic acid residues 72 and 75 and tyrosine-sulfate 73 of heparin cofactor II promote intramolecular interactions during glycosaminoglycan binding and thrombin inhibition

We used site-directed mutagenesis to investigate the role of Glu(69), Asp(70), Asp(71), Asp(72), Tyr-sulfate(73), and Asp(75) in the second acidic region (AR2) of the serpin heparin cofactor II (HCII) during formation of the thrombin.HCII complex with and without glycosaminoglycans. E69Q/D70N/D71N recombinant (r)HCII, D72N/Y73F/D75N rHCII, and E69Q/D70N/D71N/D72N/Y73F/D75N rHCII were prepared to localize acidic residues important for thrombin inhibition. Interestingly, D72N/Y73F/D75N rHCII had significantly enhanced thrombin inhibition without glycosaminoglycan (4-fold greater) and with heparin (6-fold greater), showing maximal activity at 2 microg/ml heparin compared with wild-type recombinant HCII (wt-rHCII) with maximal activity at 20 microg/ml heparin. The other rHCII mutants had lesser-enhanced activities, but they all eluted from heparin-Sepharose at significantly higher ionic strengths compared with wt-rHCII. Neutralizing and reversing the charge of Asp(72), Tyr-sulfate(73), and Asp(75) were done to characterize their individual contribution to HCII activity. Only Y73K rHCII and D75K rHCII have significantly increased heparin cofactor activity compared with wt-rHCII; however, all of the individual rHCII mutants required substantially less glycosaminoglycan at maximal inhibition than did wt-rHCII. Inhibition of either alpha-thrombin/hirugen or gamma(T)-thrombin (both with an altered anion-binding exosite-1) by the AR2 rHCII mutants was similar to wt-rHCII. D72N/Y73F/D75N rHCII and D75K rHCII were significantly more active than wt-rHCII in a plasma-based thrombin inhibition assay with glycosaminoglycans. These results indicate that improved thrombin inhibition in the AR2 HCII mutants is mediated by enhanced interactions between the acidic domain and anion-binding exosite-1 of thrombin and that AR2 may be a "molecular rheostat" to promote thrombin inhibition in the presence of glycosaminoglycans.

Capillary electrophoresis for the analysis of glycosaminoglycans and glycosaminoglycan-derived oligosaccharides

Glycosaminoglycans are a family of polydisperse, highly sulfated complex mixtures of linear polysaccharides that are involved in many life processes. Defining the structure of glycosaminoglycans is an important factor in elucidating their structure-activity relationship. Capillary electrophoresis has emerged as a highly promising technique consuming an extremely small amount of sample and capable of rapid, high-resolution separation, characterization and quantitation of analytes. Numerous capillary electrophoresis methods for analysis of intact glycosaminoglycans and glycosaminoglycan-derived oligosaccharides have been developed. These methods allow for both qualitative and quantitative analysis with a high level of sensitivity. This review is concerned with separation methods of capillary electrophoresis, detection methods and applications to several aspects of research into glycosaminoglycans and glycosaminoglycan-derived oligosaccharides. The importance of capillary electrophoresis in biological and pharmaceutical samples in glycobiology and carbohydrate biochemistry and its possible applications in disease diagnosis and monitoring chemical synthesis are described.

Tyrosine sulfation and N-glycosylation of human heparin cofactor II from plasma and recombinant Chinese hamster ovary cells and their effects on heparin binding

The structure of post-translational modifications of human heparin cofactor II isolated from human serum and from recombinant Chinese hamster ovary cells and their effects on heparin binding have been characterized. Oligosaccharide chains were found attached to all three potential N-glycosylation sites in both protein preparations. The carbohydrate structures of heparin cofactor II circulating in blood are complex-type diantennary and triantennary chains in a ratio of 6 : 1 with the galactose being > 90% sialylated with alpha 2-->6 linked N-acetylneuraminic acid. About 50% of the triantennary structures contain one sLe(x) motif. Proximal alpha 1-->6 fucosylation of oligosacharides from Chinese hamster ovary cell-derived HCII was detected in > 90% of the diantennary and triantennary glycans, the latter being slightly less sialylated with exclusively alpha 2-->3-linked N-acetylneuraminic acid units. Applying the ESI-MS/ MS-MS technique, we demonstrate that the tryptic peptides comprising tyrosine residues in positions 60 and 73 were almost completely sulfated irrespective of the protein's origin. Treatment of transfected Chinese hamster ovary cells with chlorate or tunicamycin resulted in the production of heparin cofactor II molecules that eluted with higher ionic strength from heparin-Sepharose, indicating that tyrosine sulfation and N-linked glycans may affect the inhibitor's interaction with glycosaminoglycans.

Conformational changes in thrombin when complexed by serpins

Thrombin possesses two positively charged surface domains, termed exosites, that orient substrates and inhibitors for reaction with the enzyme. Because the exosites also allosterically modulate thrombin's activity, we set out to determine whether the structure or function of the exosites changes when thrombin forms complexes with antithrombin, heparin cofactor II, or alpha(1)-antitrypsin (M358R), serpins that utilize both, one, or neither of the exosites, respectively. Using a hirudin-derived peptide to probe the integrity of exosite 1, no binding was detected when thrombin was complexed with heparin cofactor II or alpha(1)-antitrypsin (M358R), and the peptide exhibited a 55-fold lower affinity for the thrombin-antithrombin complex than for thrombin. Bound peptide or HD-1, an exosite 1-binding DNA aptamer, was displaced from thrombin by each of the three serpins. Thrombin binding to fibrin also was abrogated when the enzyme was complexed with serpins. These data reveal that, regardless of the initial mode of interaction, the function of exosite 1 is lost when thrombin is complexed by serpins. In contrast, the integrity of exosite 2 is largely retained when thrombin is complexed by serpins, because interaction with heparin or an exosite 2-directed DNA aptamer was only modestly altered. The disorganization of exosite 1 that occurs when thrombin is complexed by serpins is consistent with results of protease sensitivity studies and crystallographic analysis of a homologous enzyme-serpin complex.

Molecular basis for the susceptibility of fibrin-bound thrombin to inactivation by heparin cofactor ii in the presence of dermatan sulfate but not heparin

Although fibrin-bound thrombin is resistant to inactivation by heparin.antithrombin and heparin.heparin cofactor II complexes, indirect studies in plasma systems suggest that the dermatan sulfate.heparin cofactor II complex can inhibit fibrin-bound thrombin. Herein we demonstrate that fibrin monomer produces a 240-fold decrease in the heparin-catalyzed rate of thrombin inhibition by heparin cofactor II but reduces the dermatan sulfate-catalyzed rate only 3-fold. The protection of fibrin-bound thrombin from inhibition by heparin.heparin cofactor II reflects heparin-mediated bridging of thrombin to fibrin that results in the formation of a ternary heparin.thrombin.fibrin complex. This complex, formed as a result of three binary interactions (thrombin.fibrin, thrombin.heparin, and heparin.fibrin), limits accessibility of heparin-catalyzed inhibitors to thrombin and induces conformational changes at the active site of the enzyme. In contrast, dermatan sulfate binds to thrombin but does not bind to fibrin. Although a ternary dermatan sulfate. thrombin.fibrin complex forms, without dermatan sulfate-mediated bridging of thrombin to fibrin, only two binary interactions exist (thrombin.fibrin and thrombin. dermatan sulfate). Consequently, thrombin remains susceptible to inactivation by heparin cofactor II. This study explains why fibrin-bound thrombin is susceptible to inactivation by heparin cofactor II in the presence of dermatan sulfate but not heparin.

New tricks for old dogs: nonthrombotic effects of thrombin in vessel wall biology

Thrombin is a serine protease that potently activates platelets and catalyzes the conversion of fibrinogen to fibrin. Thrombin also exerts direct effects on vascular cells, such as smooth muscle cells, via interactions with members of the protease-activated receptor family. Evidence in several animal models implicates thrombin-mediated signaling events in the response to injury that typifies vascular lesion formation in atherosclerosis and restenosis. In this review, we examine the activation of protease-activated receptors by thrombin, the downstream signaling events mediated by these receptors, and the physiological role of thrombin in vascular cells and vascular disease.

Inhibition of thrombin by sulfated polysaccharides isolated from green algae

Eight different sulfated polysaccharides were isolated from Chlorophyta. All exhibited thrombin inhibition through a heparin cofactor II (HCII)-dependent pathway, and their effects on the inhibition of thrombin were more potent than those of heparin or dermatan sulfate. In particular, remarkably potent thrombin inhibition was found for the sulfated polysaccharides isolated from the Codiales. In the presence of these sulfated polysaccharides, both the recombinant HCII (rHCII) variants Lys(173)-->Leu and Arg(189)-->His, which are defective in interactions with heparin and dermatan sulfate, respectively, inhibited thrombin in a manner similar to native rHCII. This result indicates that the binding site of HCII for each of these eight sulfated polysaccharides is different from the heparin- or dermatan sulfate-binding site. All the sulfated polysaccharides but RS-2 significantly stimulated the inhibition of thrombin by an N-terminal deletion mutant of HCII (rHCII-Delta74). Furthermore, hirudin(54-65) decreased only 2-5-fold the rate of thrombin inhibition by HCII stimulated by the sulfated polysaccharides, while HD22, a single-stranded DNA aptamer that binds exosite II of thrombin, produced an approximately 10-fold reduction in this rate. These results suggest that, unlike heparin and dermatan sulfate, the sulfated polysaccharides isolated from Chlorophyta activate HCII primarily by an allosteric mechanism different from displacement and template mechanisms.