The refined 1.9 A crystal structure of human alpha-thrombin: interaction with D-Phe-Pro-Arg chloromethylketone and significance of the Tyr-Pro-Pro-Trp insertion segment

Dynamic Consequences of Mutation of Tryptophan 215 in Thrombin

Thrombin normally cleaves fibrinogen to promote coagulation; however, binding of thrombomodulin to thrombin switches the specificity of thrombin toward protein C, triggering the anticoagulation pathway. The W215A thrombin mutant was reported to have decreased activity toward fibrinogen without significant loss of activity toward protein C. To understand how mutation of Trp215 may alter thrombin specificity, hydrogen-deuterium exchange experiments (HDXMS), accelerated molecular dynamics (AMD) simulations, and activity assays were carried out to compare the dynamics of Trp215 mutants with those of wild type (WT) thrombin. Variation in NaCl concentration had no detectable effect on the sodium-binding (220s_CT) loop, but appeared to affect other surface loops. Trp215 mutants showed significant increases in amide exchange in the 170s_CT loop consistent with a loss of H-bonding in this loop identified by the AMD simulations. The W215A thrombin showed increased amide exchange in the 220s_CT loop and in the N-terminus of the heavy chain. The AMD simulations showed that a transient conformation of the W215A thrombin has a distorted catalytic triad. HDXMS experiments revealed that mutation of Phe227, which engages in a π-stacking interaction with Trp215, also caused significantly increased amide exchange in the 170s_CT loop. Activity assays showed that only the F227V mutant had wild type catalytic activity, whereas all other mutants showed markedly lower activity. Taken together, the results explain the reduced pro-coagulant activity of the W215A mutant and demonstrate the allosteric connection between Trp215, the sodium-binding loop, and the active site.

High throughput protease profiling comprehensively defines active site specificity for thrombin and ADAMTS13

We have combined random 6 amino acid substrate phage display with high throughput sequencing to comprehensively define the active site specificity of the serine protease thrombin and the metalloprotease ADAMTS13. The substrate motif for thrombin was determined by >6,700 cleaved peptides, and was highly concordant with previous studies. In contrast, ADAMTS13 cleaved only 96 peptides (out of >10⁷ sequences), with no apparent consensus motif. However, when the hexapeptide library was substituted into the P3-P3′ interval of VWF73, an exosite-engaging substrate of ADAMTS13, 1670 unique peptides were cleaved. ADAMTS13 exhibited a general preference for aliphatic amino acids throughout the P3-P3′ interval, except at P2 where Arg was tolerated. The cleaved peptides assembled into a motif dominated by P3 Leu, and bulky aliphatic residues at P1 and P1′. Overall, the P3-P2′ amino acid sequence of von Willebrand Factor appears optimally evolved for ADAMTS13 recognition. These data confirm the critical role of exosite engagement for substrates to gain access to the active site of ADAMTS13, and define the substrate recognition motif for ADAMTS13. Combining substrate phage display with high throughput sequencing is a powerful approach for comprehensively defining the active site specificity of proteases.

Inhibition of thrombin by functionalized C60 nanoparticles revealed via in vitro assays and in silico studies

The studies on the human toxicity of nanoparticles (NPs) are far behind the rapid development of engineered functionalized NPs. Fullerene has been widely used as drug carrier skeleton due to its reported low risk. However, different from other kinds of NPs, fullerene-based NPs (C₆₀ NPs) have been found to have an anticoagulation effect, although the potential target is still unknown. In the study, both experimental and computational methods were adopted to gain mechanistic insight into the modulation of thrombin activity by nine kinds of C₆₀ NPs with diverse surface chemistry properties. In vitro enzyme activity assays showed that all tested surface-modified C₆₀ NPs exhibited thrombin inhibition ability. Kinetic studies coupled with competitive testing using 3 known inhibitors indicated that six of the C₆₀ NPs, of greater hydrophobicity and hydrogen bond (HB) donor acidity or acceptor basicity, acted as competitive inhibitors of thrombin by directly interacting with the active site of thrombin. A simple quantitative nanostructure-activity relationship model relating the surface substituent properties to the inhibition potential was then established for the six competitive inhibitors. Molecular docking analysis revealed that the intermolecular HB interactions were important for the specific binding of C₆₀ NPs to the active site canyon, while the additional stability provided by the surface groups through van der Waals interaction also play a key role in the thrombin binding affinity of the NPs. Our results suggest that thrombin is a possible target of the surface-functionalized C₆₀ NPs relevant to their anticoagulation effect.

Assessment of the protein interaction between coagulation factor XII and corn trypsin inhibitor by molecular docking and biochemical validation

Essentials Corn Trypsin Inhibitor (CTI) is a selective inhibitor of coagulation Factor XII (FXII). Molecular modelling of the CTI-FXIIa complex suggested a canonical inhibitor binding mode. Mutagenesis revealed the CTI inhibitory loop and helices α1 and α2 mediate the interaction. This confirms that CTI inhibits FXII in canonical fashion and validates the molecular model.

SUMMARY

Background Corn trypsin inhibitor (CTI) has selectivity for the serine proteases coagulation factor XII and trypsin. CTI is in widespread use as a reagent that specifically inhibits the intrinsic pathway of blood coagulation but not the extrinsic pathway. Objectives To investigate the molecular basis of FXII inhibition by CTI. Methods We performed molecular docking of CTI, using its known crystal structure, with a model of the activated FXII (FXIIa) protease domain. The interaction model was verified by use of a panel of recombinant CTI variants tested for their ability to inhibit FXIIa enzymatic activity in a substrate cleavage assay. Results The docking predicted that: (i) the CTI central inhibitory loop P1 Arg34 side chain forms a salt bridge with the FXIIa S1 pocket Asp189 side chain; (ii) Trp22 from CTI helix α1 interacts with the FXIIa S3 pocket; and (iii) Arg43 from CTI helix α2 forms a salt bridge with FXIIa H1 pocket Asp60A. CTI amino acid substitution R34A negated all inhibitory activity, whereas the G32W, L35A, W22A and R42A/R43A substitutions reduced activity by large degrees of 108-fold, 41-fold, 158-fold, and 100-fold, respectively; the R27A, W37A, W39A and R42A substitutions had no effect. Synthetic peptides spanning CTI residues 20-44 had inhibitory activity that was three-fold to 4000-fold less than that of full-length CTI. Conclusions The data confirm the validity of a canonical model of the FXIIa-CTI interaction, with helix α1 (Trp22), central inhibitory loop (Arg34) and helix α2 (Arg43) of CTI being required for effective binding by contacting the S1, S3 and H1 pockets of FXIIa, respectively.

Rendering factor Xa zymogen-like as a therapeutic strategy to treat bleeding

PURPOSE OF REVIEW

New therapies are needed to control bleeding in a range of clinical conditions. This review will discuss the biochemical properties of zymogen-like factor Xa, its preclinical assessment in different model systems, and future development prospects.

RECENT FINDINGS

Underlying many procoagulant therapeutic approaches is the rapid generation of thrombin to promote robust clot formation. Clinically tested prohemostatic agents (e.g., factor VIIa) can provide effective hemostasis to mitigate bleeding in hemophilia and other clinical situations. Over the past decade, we explored the possibility of using zymogen-like factor Xa variants to rapidly improve clot formation for the treatment of bleeding conditions. Compared to the wild-type enzyme, these variants adopt an altered, low activity, conformation which enables them to resist plasma protease inhibitors. However, zymogen-like factor Xa variants are conformationally dynamic and ligands such as its cofactor, factor Va, stabilize the molecule rescuing procoagulant activity. At the site of vascular injury, the variants in the presence of factor Va serve as effective prohemostatic agents. Preclinical data support their use to stop bleeding in a variety of clinical settings. Phase 1 studies suggest that zymogen-like factor Xa is safe and well tolerated, and a phase 1b is ongoing to assess safety in patients with intracerebral hemorrhage.

SUMMARY

Zymogen-like factor Xa is a unique prohemostatic agent for the treatment of a range of bleeding conditions.

Bio-orthogonal conjugation and enzymatically triggered release of proteins within multi-layered hydrogels

Hydrogels are facile architectures for the controlled presentation of proteins with far-reaching applications, from fundamental biological studies in three-dimensional culture to new regenerative medicine and therapeutic delivery strategies. Here, we demonstrate a versatile approach for spatially-defined presentation of engineered proteins within hydrogels through i) immobilization using bio-orthogonal strain-promoted alkyne-azide click chemistry and ii) dynamic protease-driven protein release using exogenously applied enzyme. Model fluorescent proteins were expressed using nonsense codon replacement to incorporate azide-containing unnatural amino acids in a site-specific manner toward maintaining protein activity: here, cyan fluorescent protein (AzCFP), mCherry fluorescent protein (AzmCh), and mCh decorated with a thrombin cut-site. (AzTMBmCh). Eight-arm poly(ethylene glycol) (PEG) was modified with dibenzylcyclooctyne (DBCO) groups and reacted with azide functionalized PEG in aqueous solution for rapid formation of hydrogels. Azide functionalized full-length fluorescent proteins were successfully incorporated into the hydrogel network by reaction with PEG-DBCO prior to gel formation. Temporal release and removal of select proteins (AzTMBmCh) was triggered with the application of thrombin and monitored in real-time with confocal microscopy, providing a responsive handle for controlling matrix properties. Hydrogels with regions of different protein compositions were created using a layering technique with thicknesses of hundreds of micrometers, affording opportunities for the creation of complex geometries on size scales relevant for controlling cellular microenvironments.

STATEMENT OF SIGNIFICANCE

Controlling protein presentation within biomaterials is important for modulating interactions with biological systems. For example, native tissues are composed of subunits with different matrix compositions (proteins, stiffness) that dynamically interact with cells, influencing function and fate. Toward mimicking such temporally-regulated and spatially-defined microenvironments, we utilize bio-orthogonal click chemistry and protein engineering to create hydrogels with distinct regions of proteins and modify them over time. Through nonsense codon replacement, we site-specifically functionalize large proteins with i) azides for covalent conjugation and ii) an enzymatic cleavage site for user-defined release from hydrogels. Our results exemplify not only the ability to create unique bio-functionalized hydrogels with controlled mechanical properties, but also the potential for creating interesting interfaces for cell culture and tissue engineering applications.

Bioengineering factor Xa to treat bleeding

There is a clinical need to develop safe and rapid therapeutic strategies to control bleeding arising from a host of emergent situations. Over the past several years our laboratory has developed novel zymogen-like FXa variants and tested their safety and efficacy using hemophilia as a model system. The variants have a spectrum of properties resulting from an amino acid change at the N-terminus of the heavy chain that alters a critical conformational change. These properties, which include resistance to plasma protease inhibitors, low activity in the absence of FVa, and rescue of low activity upon incorporation in prothrombinase, yield remarkably effective pro-hemostatic agents. The FVa-dependent restoration of activity is a key aspect to their efficacy and also contributes to localizing the variants to the site of vascular injury. While pre-clinical data support their use in the setting of hemophilia, they have the potential to act as rapid pro-hemostatic agents for the treatment of a range of bleeding conditions. This review will discuss the biochemical properties of these FXa zymogen-like variants and their in vivo characterization.

Aptamer based electrostatic-stimuli responsive surfaces for on-demand binding/unbinding of a specific ligand

We report an aptamer functionalized stimuli responsive surface that can controllably switch between binding and releasing its specific ligand α-thrombin under application of electrical stimuli.

Paradoxical bleeding and thrombotic episodes of dysprothrombinaemia due to a homozygous Arg382His mutation

We have characterised the pathogenic basis of dysprothrombinaemia in a patient exhibiting paradoxical bleeding and thrombotic defects during pregnancy and postpartum. Genetic analysis revealed that the proband is homozygous for the prothrombin Arg382His mutation, possessing only ~1 % clotting activity. The proband experienced severe bleeding episodes during her pregnancy, which required treatment with prothrombin complex concentrates, and then pulmonary embolism and deep-vein thrombosis at 28 days postpartum, which required treatment with LMWH and fresh frozen plasma. Analysis of haemostatic parameters revealed that the subject had elevated FDP and DD and decreased fibrinogen levels, indicating the presence of hyperfibrinolysis. Thrombin generation and clotting assays with the proband's plasma in the presence of soluble thrombomodulin and tissue-type plasminogen activator indicated a defect in activation of both protein C and thrombin activatable fibrinolysis inhibitor (TAFI). Unlike normal plasma, no TAFI activation could be detected in the patient's plasma. The expression and characterisation of recombinant prothrombin Arg382His indicated that zymogen activation by prothrombinase was markedly impaired and the activation of protein C and TAFI by thrombin-Arg382His was impaired 600-fold and 2500-fold, respectively. The recombinant thrombin mutant exhibited impaired catalytic activity toward both fibrinogen and PAR1 as determined by clotting and signalling assays. However, the mutant activated factor XI normally in both the absence and presence of polyphosphates. Arg382 is a key residue on (pro)exosite-1 of prothrombin and kinetic analysis of substrate activation suggested that the poor zymogenic activity of the mutant is due to its inability to bind factor Va in the prothrombinase complex.

Electrical Stimulus Controlled Binding/Unbinding of Human Thrombin-Aptamer Complex

The binding/unbinding of the human thrombin and its 15-mer single stranded DNA aptamer, under the application of external stimulus in the form of electrostatic potential/electric field, is investigated by a combination of continuum analysis and atomistic molecular dynamics simulation. In agreement with the experiments that demonstrate the influence of electrostatic potential on the thrombin/aptamer complex, our computations show that the application of positive electric field successfully unbinds the thrombin from the aptamer. Results from umbrella sampling simulations reveal that there is a decrease in the free energy of binding between the thrombin and aptamer in presence of positive electric fields. Hydrogen bonding and non-bonded interaction energies, and hence the free energy of binding, between the thrombin and its aptamer reduce as the applied electric field is shifted from negative to positive values. Our analyses demonstrate that application of electrical stimulus modifies the molecular interactions within the complex and consequently, electrical field can be used to modulate the association between the thrombin and its aptamer.

Design and characterization of an APC-specific serpin for the treatment of hemophilia

Hemophilia is a bleeding disorder caused by deficiency in factors VIII or IX, the two components of the intrinsic Xase complex. Treatment with replacement factor can lead to the development of inhibitory antibodies, requiring the use of bypassing agents such as factor VIIa and factor concentrates. An alternative approach to bypass the Xase complex is to inhibit endogenous anticoagulant activities. Activated protein C (APC) breaks down the complex that produces thrombin by proteolytically inactivating factor Va. Defects in this mechanism (eg, factor V Leiden) are associated with thrombosis but result in less severe bleeding when co-inherited with hemophilia. Selective inhibition of APC might therefore be effective for the treatment of hemophilia. The endogenous inhibitors of APC are members of the serpin family: protein C inhibitor (PCI) and α₁-antitrypsin (α₁AT); however, both exhibit poor reactivity and selectivity for APC. We mutated residues in and around the scissile P1-P1' bond in PCI and α₁AT, resulting in serpins with the desired specificity profile. The lead candidate was shown to promote thrombin generation in vitro and to restore fibrin and platelet deposition in an intravital laser injury model in hemophilia B mice. The power of targeting APC was further demonstrated by the complete normalization of bleeding after a severe tail clip injury in these mice. These results demonstrate that the protein C anticoagulant system can be successfully targeted by engineered serpins and that administration of such agents is effective at restoring hemostasis in vivo.

Loop-driven conformational transition between the alternative and collapsed form of prethrombin-2: targeted molecular dynamics study

Two distinct crystal structures of prethrombin-2, the alternative and collapsed forms, are elucidated by X-ray crystallogrphy. We analyzed the conformational transition from the alternative to the collapsed form employing targeted molecular dynamics (TMD) simulation. Despite small RMSD difference in the two X-ray crystal structures, some hydrophobic residues (W60d, W148, W215, and F227) show a significant difference between the two conformations. TMD simulation shows that the four hydrophobic residues undergo concerted movement from dimer to trimer transition via tetramer state in the conformational change from the alternative to the collapsed form. We reveal that the concerted movement of the four hydrophobic residues is controlled by movement of specific loop regions behind. In this paper, we propose a sequential scenario for the conformational transition from the alternative form to the collapsed form, which is partially supported by the mutant W148A simulation.

A rapid pro-hemostatic approach to overcome direct oral anticoagulants

Direct inhibitors of coagulation factor Xa (FXa) or thrombin are promising oral anticoagulants that are becoming widely adopted. The ability to reverse their anticoagulant effects is important when serious bleeding occurs or urgent medical procedures are needed. Here, using experimental mouse models of hemostasis, we show that a variant coagulation factor, FXa(I16L), rapidly restores hemostasis in the presence of the anticoagulant effects of these inhibitors. The ability of FXa(I16L) to reverse the anticoagulant effects of FXa inhibitor depends, at least in part, on the ability of the active site inhibitor to hinder antithrombin-dependent FXa inactivation, paradoxically allowing uninhibited FXa to persist in plasma. Because of its inherent catalytic activity, FXa(I16L) is more potent (by >50-fold) in the hemostasis models tested than a noncatalytic antidote that is currently in clinical development. FXa(I16L) also reduces the anticoagulant-associated bleeding in vivo that is induced by the thrombin inhibitor dabigatran. FXa(I16L) may be able to fill an important unmet clinical need for a rapid, pro-hemostatic agent to reverse the effects of several new anticoagulants.

Loop Electrostatics Asymmetry Modulates the Preexisting Conformational Equilibrium in Thrombin

Thrombin exists as an ensemble of active (E) and inactive (E*) conformations that differ in their accessibility to the active site. Here we show that redistribution of the E*-E equilibrium can be achieved by perturbing the electrostatic properties of the enzyme. Removal of the negative charge of the catalytic Asp102 or Asp189 in the primary specificity site destabilizes the E form and causes a shift in the 215-217 segment that compromises substrate entrance. Solution studies and existing structures of D102N document stabilization of the E* form. A new high-resolution structure of D189A also reveals the mutant in the collapsed E* form. These findings establish a new paradigm for the control of the E*-E equilibrium in the trypsin fold.

Identification of a thrombomodulin interaction site on thrombin-activatable fibrinolysis inhibitor that mediates accelerated activation by thrombin

BACKGROUND

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a human plasma zymogen that provides a molecular connection between coagulation and fibrinolysis. TAFI is activated through proteolytic cleavage by thrombin, thrombin in complex with the endothelial cell cofactor thrombomodulin (TM) or plasmin. Evidence from several studies suggests that TM and TAFI make direct contact at sites remote from the activating cleavage site to facilitate acceleration of thrombin-mediated TAFI activation. The elements of TAFI structure that allow accelerated activation of thrombin by TM are incompletely defined.

OBJECTIVES

To identify TM interaction regions on TAFI that mediate acceleration of activation by thrombin and therefore indicate TM binding sites on TAFI.

METHODS

We mutated selected surface-exposed charged residues on TAFI to alanine in order to identify sites that mediate acceleration of activation by TM. The kinetics of activation of the mutants by thrombin in the presence or absence of TM, as well as their thermal stabilities and antifibrinolytic potentials, were determined.

RESULTS

TAFI variants R15A, E28A, K59A, D75A/E77A/D78A, E99A and E106A all exhibited moderately reduced catalytic efficiencies of activation by thrombin-TM. TAFI variants R377A and, particularly, R12A and R12A/R15A exhibited severely reduced activation by thrombin-TM that was not explained by differences in activation by thrombin alone.

CONCLUSIONS

We have identified R12 as a critical residue for the activation of TAFI by thrombin-TM, extending a previous report that identified a role for this residue. R12 is likely to directly bind to TM while another key residue, R377, may affect the thrombin-TAFI interaction specifically in the presence of TM.

Design and Synthesis of Nonpeptide Inhibitors of Hepatocyte Growth Factor Activation

In this letter we report first nonpeptide inhibitors of hepatocyte growth factor (HGF) activation. These compounds inhibit the three proteases (matriptase, hepsin, and HGF activator) required for HGF maturation. We show that 6, 8a, 8b, and 8d block activation of fibroblast-derived pro-HGF, thus preventing fibroblast-induced scattering of DU145 prostate cancer cells. Compound 6 (SRI 31215) is very soluble (91 μM) and has excellent microsome stability (human t 1/2 = 162 min; mouse t 1/2 = 296 min). In mouse 6 has an in vivo t 1/2 = 5.8 h following IV administration. The high solubility of 6 and IV t 1/2 make this compound a suitable prototype "triplex inhibitor" for the study of the inhibition of HGF activation in vivo.

Design of Potent and Controllable Anticoagulants Using DNA Aptamers and Nanostructures

The regulation of thrombin activity offers an opportunity to regulate blood clotting because of the central role played by this molecule in the coagulation cascade. Thrombin-binding DNA aptamers have been used to inhibit thrombin activity. In the past, to address the low efficacy reported for these aptamers during clinical trials, multiple aptamers have been linked using DNA nanostructures. Here, we modify that strategy by linking multiple copies of various thrombin-binding aptamers using DNA weave tiles. The resulting constructs have very high anticoagulant activity in functional assays owing to their improved cooperative binding affinity to thrombin due to optimized spacing, orientation, and the high local concentration of aptamers. We also report the results of molecular dynamics simulations to gain insight into the solution conformations of the tiles. Moreover, by using DNA strand displacement, we were able to turn the coagulation cascade off and on as desired, thereby enabling significantly better control over blood coagulation.

The Dual Regulatory Role of Amino Acids Leu480 and Gln481 of Prothrombin

Prothrombin (FII) is activated to α-thrombin (IIa) by prothrombinase. Prothrombinase is composed of a catalytic subunit, factor Xa (fXa), and a regulatory subunit, factor Va (fVa), assembled on a membrane surface in the presence of divalent metal ions. We constructed, expressed, and purified several mutated recombinant FII (rFII) molecules within the previously determined fVa-dependent binding site for fXa (amino acid region 473-487 of FII). rFII molecules bearing overlapping deletions within this significant region first established the minimal stretch of amino acids required for the fVa-dependent recognition exosite for fXa in prothrombinase within the amino acid sequence Ser(478)-Val(479)-Leu(480)-Gln(481)-Val(482). Single, double, and triple point mutations within this stretch of rFII allowed for the identification of Leu(480) and Gln(481) as the two essential amino acids responsible for the enhanced activation of FII by prothrombinase. Unanticipated results demonstrated that although recombinant wild type α-thrombin and rIIa(S478A) were able to induce clotting and activate factor V and factor VIII with rates similar to the plasma-derived molecule, rIIa(SLQ→AAA) with mutations S478A/L480A/Q481A was deficient in clotting activity and unable to efficiently activate the pro-cofactors. This molecule was also impaired in protein C activation. Similar results were obtained with rIIa(ΔSLQ) (where rIIa(ΔSLQ) is recombinant human α-thrombin with amino acids Ser(478)/Leu(480)/Gln(481) deleted). These data provide new evidence demonstrating that amino acid sequence Leu(480)-Gln(481): 1) is crucial for proper recognition of the fVa-dependent site(s) for fXa within prothrombinase on FII, required for efficient initial cleavage of FII at Arg(320); and 2) is compulsory for appropriate tethering of fV, fVIII, and protein C required for their timely activation by IIa.

Boosting Affinity by Correct Ligand Preorganization for the S2 Pocket of Thrombin: A Study by Isothermal Titration Calorimetry, Molecular Dynamics, and High-Resolution Crystal Structures

Structural preorganization to fix bioactive conformations at protein binding sites is a popular strategy to enhance binding affinity during late-stage optimization. The rationale for this enhancement relates to entropic advantages assigned to rigidified versus flexible ligands. We analyzed a narrow series of peptidomimetics binding to thrombin. The individual ligands exhibit at P2 a conformationally flexible glycine, more restricted alanine, N-methylglycine, N-methylhomoalanine, and largely rigidified proline moiety. Overall, affinity was found to increase by a factor of 1000, explained partly by an entropic advantage. All ligands adopt the same binding mode with small deviations. The residual mobility of the bound ligands is decreased across the series, and a protein side chain differs in its order/disorder behavior along with changes in the surface-water network pattern established across the newly generated protein-ligand surfaces. The enthalpy/entropy inventory displays a rather complex picture and emphasizes that thermodynamics can only be compared in terms of relative differences within a structurally similar ligand series.

Discovery and characterization of an antibody directed against exosite I of thrombin

ESSENTIALS: An IgA paraprotein with anti-thrombin activity was not associated with a severe bleeding phenotype. This observation challenges the paradigm that anticoagulant therapy necessarily increases bleeding risk. Characterization of the antibody showed that it specifically binds to thrombin exosite I. A therapeutic drug with the properties of this antibody might be an antithrombotic that doesn't cause bleeding.

BACKGROUND

We report the case of a 54-year-old female who presented with a traumatic subdural hemorrhage. Coagulation tests were markedly prolonged due to the presence of an anti-thrombin IgA paraprotein at 3 g L(-1) . The patient made a complete recovery and has had no abnormal bleeding during a 7-year follow-up, despite the persistence of the paraprotein.

OBJECTIVES

To determine how the paraprotein prolonged clotting tests by defining its target and its epitope.

METHODS

The paraprotein was purified and added to normal pooled plasma for in vitro clotting assays. Binding studies were conducted to determine the affinity of the IgA for thrombin. The Fab was isolated and crystallized with thrombin.

RESULTS

The purified IgA was sufficient to confer the patient's in vitro coagulation profile in normal pooled plasma, and was found to bind specifically and with high affinity to thrombin. A crystal structure of the Fab fragment in complex with thrombin revealed an exosite I interaction involving CDRH3 of the antibody.

CONCLUSIONS

Although the patient originally presented with a subdural bleed, the hematoma resolved without intervention, and no other bleeding event occurred during the subsequent 7 years. During this period, the patient's IgA paraprotein levels have remained constant at 3 g L(-1) , suggesting that the presence of a high-affinity, exosite I-directed antibody is consistent with normal hemostasis. A therapeutic derivative of this antibody might therefore permit antithrombotic dose escalation without an associated increase in the risk of bleeding.

Molecular modeling studies, synthesis and biological evaluation of dabigatran analogues as thrombin inhibitors

In this work, 48 thrombin inhibitors based on the structural scaffold of dabigatran were analyzed using a combination of molecular modeling techniques. We generated three-dimensional quantitative structure-activity relationship (3D-QSAR) models based on three alignments for both comparative molecular field analysis (CoMFA) and comparative molecular similarity index analysis (CoMSIA) to highlight the structural requirements for thrombin protein inhibition. In addition to the 3D-QSAR study, Topomer CoMFA model also was established with a higher leave-one-out cross-validation q(2) and a non-cross-validation r(2), which suggest that the three models have good predictive ability. The results indicated that the steric, hydrophobic and electrostatic fields play key roles in QSAR model. Furthermore, we employed molecular docking and re-docking simulation explored the binding relationship of the ligand and the receptor protein in detail. Molecular docking simulations identified several key interactions that were also indicated through 3D-QSAR analysis. On the basis of the obtained results, two compounds were designed and predicted by three models, the biological evaluation in vitro (IC50) demonstrated that these molecular models were effective for the development of novel potent thrombin inhibitors.

Thrombomodulin Binding Selects the Catalytically Active Form of Thrombin

Human α-thrombin is a serine protease with dual functions. Thrombin acts as a procoagulant, cleaving fibrinogen to make the fibrin clot, but when bound to thrombomodulin (TM), it acts as an anticoagulant, cleaving protein C. A minimal TM fragment consisting of the fourth, fifth, and most of the sixth EGF-like domain (TM456m) that has been prepared has much improved solubility, thrombin binding capacity, and anticoagulant activity versus those of previous TM456 constructs. In this work, we compare backbone amide exchange of human α-thrombin in three states: apo, D-Phe-Pro-Arg-chloromethylketone (PPACK)-bound, and TM456m-bound. Beyond causing a decreased level of amide exchange at their binding sites, TM and PPACK both cause a decreased level of amide exchange in other regions including the γ-loop and the adjacent N-terminus of the heavy chain. The decreased level of amide exchange in the N-terminus of the heavy chain is consistent with the historic model of activation of serine proteases, which involves insertion of this region into the β-barrel promoting the correct conformation of the catalytic residues. Contrary to crystal structures of thrombin, hydrogen-deuterium exchange mass spectrometry results suggest that the conformation of apo-thrombin does not yet have the N-terminus of the heavy chain properly inserted for optimal catalytic activity, and that binding of TM allosterically promotes the catalytically active conformation.

Dynamics Govern Specificity of a Protein-Protein Interface: Substrate Recognition by Thrombin

Biomolecular recognition is crucial in cellular signal transduction. Signaling is mediated through molecular interactions at protein-protein interfaces. Still, specificity and promiscuity of protein-protein interfaces cannot be explained using simplistic static binding models. Our study rationalizes specificity of the prototypic protein-protein interface between thrombin and its peptide substrates relying solely on binding site dynamics derived from molecular dynamics simulations. We find conformational selection and thus dynamic contributions to be a key player in biomolecular recognition. Arising entropic contributions complement chemical intuition primarily reflecting enthalpic interaction patterns. The paradigm “dynamics govern specificity” might provide direct guidance for the identification of specific anchor points in biomolecular recognition processes and structure-based drug design.

Correction of human hemophilia A whole blood abnormalities with a novel bypass agent: zymogen-like FXa(I16L)

BACKGROUND

Approximately 30% of hemophilia A (HA) and 5% of hemophilia B patients develop inhibitors to protein replacement therapy, and this is the major cause of disease-related morbidity in the developed world. We previously developed zymogen-like factor Xa (FXa) molecules with impaired active site maturation, enabling a greater half-life than wild-type FXa while maintaining full procoagulant function in the prothrombinase complex. Here we evaluated the ability of zymogen-like FXa(I16L) to correct whole blood thromboelastometry abnormalities of severe HA subjects with and without inhibitors.

METHODS

Fourteen severe HA subjects without and five with inhibitors were enrolled at baseline (

FVIII

C < 1%) > 5 half-lives from factor or bypass therapy. The subjects' whole blood was evaluated by thromboelastography (ROTEM(®) ) using INTEM analysis with two concentrations of FXa(I16L) or recombinant factor VIIa (rFVIIa).

RESULTS

With 0.1 nm FXa(I16L) , clot time (CT, in minutes [min]) among HA subjects without and with inhibitors (mean = 2.87 min, 95% CI = 2.58-3.15 min, and mean = 2.9 min, 95% CI = 2.07-3.73 min, respectively) did not significantly differ from control CT (mean = 2.73 min, 95% CI = 2.62-2.85 min). Addition of 20 nm rFVIIa, simulating a 90-μg/kg dose, resulted in significantly prolonged CTs for HA subjects without and with inhibitors (mean = 5.43 min, 95% CI = 4.53-6.35 min, and mean = 4.25 min, 95% CI = 3.32-5.17 min, respectively) relative to controls.

CONCLUSIONS

FXa(I16L) restored thromboelastometry CT to control values in severe HA subjects with and without inhibitors. The findings corroborate previous animal data and demonstrate the first evidence of zymogen-like FXa(I16L) correcting human HA subjects' whole-blood abnormalities and support the use of FXa(I16L) as a novel hemostatic agent.

Fragment Binding Can Be Either More Enthalpy-Driven or Entropy-Driven: Crystal Structures and Residual Hydration Patterns Suggest Why

In lead optimization, small, enthalpically advantaged fragments have been suggested to be superior, as an entropic component will be added inevitably during late-stage optimization. Determination of thermodynamic signatures of weak-binding fragments is essential to support the decision-making process, to decide which fragment to take to further optimization. High-resolution crystal structures of six fragments binding to the S1 pocket of thrombin were determined and analyzed with respect to their thermodynamic profile. The two most potent fragments exhibiting an amidine-type scaffold are not the most enthalpic binders; instead a chloro-thiophene fragment binds more enthalpically. Two chemically very similar chloro-aromatic fragments differ strongly in their potency (430 μM vs 10 mM); their binding modes are related, but the surrounding residual water network differs. The more potent one recruits a water molecule and involves Glu192 in binding, thus succeeding in firmly capping the S1 pocket. Fragments exhibiting a rather perfect solvation pattern in their binding mode also experience the highest potency.

Expression and Characterization of Gly-317 Variants of Factor IX Causing Variable Bleeding in Hemophilia B Patients

We recently identified two hemophilia B patients who carried Gly-317 to Arg (FIX-G317R) or Gly-317 to Glu (FIX-G317E) substitutions in their FIX gene. The former mutation caused severe and the latter moderate bleeding in afflicted patients. To understand the molecular basis for the variable clinical manifestation of Gly-317 mutations, we prepared recombinant G317R and G317E derivatives of FIX and compared their kinetic properties to those of recombinant wild-type FIX in appropriate assay systems. Both physiological activators, factor XIa and extrinsic Tenase (factor VIIa-tissue factor), activated both zymogen variants with an ∼1.5-fold elevated K(m); however, extrinsic Tenase activated FIX-G317E with an ∼2-fold improved k(cat). By contrast to zymogen activation, the catalytic activities of both FIXa-G317R and FIXa-G317E enzymes toward the natural substrate, factor X, were dramatically (>4 orders of magnitude) impaired, but their apparent affinity for interaction with factor VIIIa was only slightly (<2-fold) decreased. Further studies revealed that the reactivity of FIXa-G317R and FIXa-G317E with antithrombin has been impaired 10- and 13-fold, respectively, in the absence and 166- and 500-fold, respectively, in the presence of pentasaccharide. As expected, the clotting activities of FIX variants could not be measured by the aPTT assay. These results implicate a critical role for Gly-317 in maintaining normal catalytic function for FIX/FIXa in the clotting cascade. The results further suggest that improved k(cat) of FIX-G317E activation in the extrinsic pathway together with dramatically impaired reactivity of FIXa-G317E with antithrombin may account for the less severe bleeding phenotype of a hemophilia B patient carrying the FIX-G317E mutation.

Dual aptamer-immobilized surfaces for improved affinity through multiple target binding in potentiometric thrombin biosensing

We developed a label-free and reagent-less potentiometric biosensor with improved affinity for thrombin. Two different oligomeric DNA aptamers that can recognize different epitopes in thrombin were introduced in parallel or serial manners on the sensing surface to capture the target via multiple contacts as found in many biological systems. The spacer and linker in the aptamer probes were optimized for exerting the best performance in molecular recognition. To gain the specificity of the sensor to the target, an antifouling molecule, sulfobeaine-3-undecanethiol (SB), was introduced on the sensor to form a self-assembled monolayer (SAM). Surface characterization revealed that the aptamer probe density was comparable to the distance of the two epitopes in thrombin, while the backfilling SB SAM was tightly aligned on the surface to resist nonspecific adsorption. The apparent binding parameters were obtained by thrombin sensing in potentiometry using the 1:1 Langmuir adsorption model, showing the improved dissociation constants (Kd) with the limit of detection of 5.5 nM on the dual aptamer-immobilized surfaces compared with single aptamer-immobilized ones. A fine control of spacer and linker length in the aptamer ligand was essential to realize the multivalent binding of thrombin on the sensor surface. The findings reported herein are effective for improving the sensitivity of potentiometric biosensor in an affordable way towards detection of tiny amount of biomolecules.

Hemostatic agents of broad applicability produced by selective tuning of factor Xa zymogenicity

There is a clinical need to develop safe therapeutic strategies to mitigate bleeding. Previously, we found that a novel zymogen-like factor Xa variant (FXa-I16L) was effective in correcting the coagulation defect in hemophilic mice. Here we expand the mutational framework to tune the FX(a) zymogen-like state. Alteration of FXa zymogenicity yields variants (V17M, I16L, I16M, V17T, V17S, and I16T) with a wide range (≤1000-fold) of reduced function toward physiologic substrates and inhibitors. The extent of zymogen-like character, including resistance to antithrombin III, correlates well with plasma half-life (<2 minutes to >4 hours). Importantly, biologic function, including that of the most zymogen-like variant (FXa-I16T), was greatly enhanced when bound to FVa membranes. This resulted in improvement of clotting times and thrombin generation in hemophilic plasma. The FXa variants were remarkably effective in mouse injury models. In these systems, the data show that the more active the protease, the more difficult it is to overcome the protective mechanism of circulating inhibitors to achieve a therapeutic benefit. Depending on the treatment situation, the more zymogen-like variants (V17S and I16T) were most useful when given before injury whereas variants exhibiting greater activity but shorter half-lives (I16L and I16M) were most effective when administered after injury. This new class of FXa variants provides a useful and flexible platform for selectively bioengineering biologic function and half-life to target different clinical bleeding scenarios.

Toward understanding allosteric activation of thrombin: a conjecture for important roles of unbound Na(+) molecules around thrombin

We shed light on important roles of unbound Na(+) molecules in enzymatic activation of thrombin. Molecular mechanism of Na(+)-activation of thrombin has been discussed in the context of allostery. However, the recent challenge to redesign K(+)-activated thrombin revealed that the allosteric interaction is insufficient to explain the mechanism. Under these circumstances, we have examined the roles of unbound Na(+) molecule in maximization of thrombin-substrate association reaction rate. We performed all-atomic molecular dynamics (MD) simulations of thrombin in the presence of three different cations; Li(+), Na(+), and Cs(+). Although these cations are commonly observed in the vicinity of the S1-pocket of thrombin, smaller cations are distributed more densely and extensively than larger ones. This suggests the two observation rules: (i) thrombin surrounded by Na(+) is at an advantage in the initial step of association reaction, namely, the formation of an encounter complex ensemble, and (ii) the presence of Na(+) molecules does not necessarily have an advantage in the final step of association reaction, namely, the formation of the stereospecific complex. In conclusion, we propose a conjecture that unbound Na(+) molecules also affect the maximization of rate constant of thrombin-substrate association reaction through optimally forming an encounter complex ensemble.

Structure and mechanism of cysteine peptidase gingipain K (Kgp), a major virulence factor of Porphyromonas gingivalis in periodontitis

Cysteine peptidases are key proteolytic virulence factors of the periodontopathogen Porphyromonas gingivalis, which causes chronic periodontitis, the most prevalent dysbiosis-driven disease in humans. Two peptidases, gingipain K (Kgp) and R (RgpA and RgpB), which differ in their selectivity after lysines and arginines, respectively, collectively account for 85% of the extracellular proteolytic activity of P. gingivalis at the site of infection. Therefore, they are promising targets for the design of specific inhibitors. Although the structure of the catalytic domain of RgpB is known, little is known about Kgp, which shares only 27% sequence identity. We report the high resolution crystal structure of a competent fragment of Kgp encompassing the catalytic cysteine peptidase domain and a downstream immunoglobulin superfamily-like domain, which is required for folding and secretion of Kgp in vivo. The structure, which strikingly resembles a tooth, was serendipitously trapped with a fragment of a covalent inhibitor targeting the catalytic cysteine. This provided accurate insight into the active site and suggested that catalysis may require a catalytic triad, Cys(477)-His(444)-Asp(388), rather than the cysteine-histidine dyad normally found in cysteine peptidases. In addition, a 20-Å-long solvent-filled interior channel traverses the molecule and links the bottom of the specificity pocket with the molecular surface opposite the active site cleft. This channel, absent in RgpB, may enhance the plasticity of the enzyme, which would explain the much lower activity in vitro toward comparable specific synthetic substrates. Overall, the present results report the architecture and molecular determinants of the working mechanism of Kgp, including interaction with its substrates.

Cleavage specificity analysis of six type II transmembrane serine proteases (TTSPs) using PICS with proteome-derived peptide libraries

Background

Type II transmembrane serine proteases (TTSPs) are a family of cell membrane tethered serine proteases with unclear roles as their cleavage site specificities and substrate degradomes have not been fully elucidated. Indeed just 52 cleavage sites are annotated in MEROPS, the database of proteases, their substrates and inhibitors.

Methodology/Principal Finding

To profile the active site specificities of the TTSPs, we applied Proteomic Identification of protease Cleavage Sites (PICS). Human proteome-derived database searchable peptide libraries were assayed with six human TTSPs (matriptase, matriptase-2, matriptase-3, HAT, DESC and hepsin) to simultaneously determine sequence preferences on the N-terminal non-prime (P) and C-terminal prime (P’) sides of the scissile bond. Prime-side cleavage products were isolated following biotinylation and identified by tandem mass spectrometry. The corresponding non-prime side sequences were derived from human proteome databases using bioinformatics. Sequencing of 2,405 individual cleaved peptides allowed for the development of the family consensus protease cleavage site specificity revealing a strong specificity for arginine in the P1 position and surprisingly a lysine in P1′ position. TTSP cleavage between R↓K was confirmed using synthetic peptides. By parsing through known substrates and known structures of TTSP catalytic domains, and by modeling the remainder, structural explanations for this strong specificity were derived.

Conclusions

Degradomics analysis of 2,405 cleavage sites revealed a similar and characteristic TTSP family specificity at the P1 and P1′ positions for arginine and lysine in unfolded peptides. The prime side is important for cleavage specificity, thus making these proteases unusual within the tryptic-enzyme class that generally has overriding non-prime side specificity.

Molecular basis of the clotting defect in a bleeding patient missing the Asp-185 codon in the factor X gene

Factor X (FX) is a vitamin K-dependent plasma zymogen, which following activation to factor Xa (FXa), converts prothrombin to thrombin in the blood clotting cascade. It was recently demonstrated that a natural variant of FX carrying the Asp-185 deletion (FX-D185del, chymotrypsinogen numbering) was associated with mild bleeding in a patient with severe FX deficiency. In this study, we expressed FX-D185del in mammalian cells and characterized its properties in appropriate kinetic assays in purified systems. We discovered that while the FX variant can be normally activated by physiological activators; both amidolytic and proteolytic activities of the mutant are dramatically impaired. Interestingly, factor Va (FVa) significantly improved the proteolytic defect when the mutant protease was assembled into the prothrombinase complex. Thus, in contrast to >50-fold catalytic defect in the absence of FVa, the variant activated prothrombin with only ~2.5-fold decreased catalytic efficiency in the presence of the cofactor. The FXa variant dramatically lost its susceptibility to inhibition by antithrombin and tissue factor pathway inhibitor, thus exhibiting ~2-3 orders of magnitude lower reactivity with the plasma inhibitors. Further studies revealed that Na(+) no longer activates the variant protease, suggesting that the functionally important allosteric linkage between the Na(+)-binding and the P1-binding sites of the protease has been eliminated. These results suggest that the lower catalytic efficiency of FXa-D185del in the bleeding patient may be partially compensated by the loss of its reactivity with plasma inhibitors, possibly explaining the basis for the paradoxical severe FX deficiency with only mild bleeding tendency for this mutation.

The N-terminal Arg residue is essential for autocatalytic activation of a lipopolysaccharide-responsive protease zymogen

Factor C, a serine protease zymogen involved in innate immune responses in horseshoe crabs, is known to be autocatalytically activated on the surface of bacterial lipopolysaccharides, but the molecular mechanism of this activation remains unknown. In this study, we show that wild-type factor C expressed in HEK293S cells exhibits a lipopolysaccharide-induced activity equivalent to that of native factor C. Analysis of the N-terminal addition, deletion, or substitution mutants shows that the N-terminal Arg residue and the distance between the N terminus and the tripartite of lipopolysaccharide-binding site are essential factors for autocatalytic activation, and that the positive charge of the N terminus may interact with an acidic amino acid(s) of the molecule to convert the zymogen into an active form. Chemical cross-linking experiments indicate that the N terminus is required to form a complex of the factor C molecules in a sufficiently close vicinity to be chemically cross-linked on the surface of lipopolysaccharides. We propose a molecular mechanism of the autocatalytic activation of the protease zymogen on lipopolysaccharides functioning as a platform to induce specific protein-protein interaction between the factor C molecules.

Directly investigating the interaction between aptamers and thrombin by atomic force microscopy

Aptamers are single-stranded nucleic acid molecules that can be used for protein recognition, detection, and inhibition. Over the past decades, two thrombin-binding aptamers (15apt and 27apt) were reported by systemic evolution of ligands by exponential enrichment technique. Though many studies have been reported about the interactions between the aptamers and thrombin by atomic force microscopy, the thrombins in those studies were all immobilized by chemical agents. Recently, we developed a new method using atomic force microscopy to directly investigate the specific interactions between thrombin and its two aptamers without immobilizing the thrombin. Furthermore, the unbinding dynamics and dissociation energy landscapes of aptamer/thrombin were discussed. The results indicate that the underlying interaction mechanisms of thrombin with its two aptamers will be similar despite that the structures of 15apt and 27apt are different in buffer solution.

Multiple roles of complement MASP-1 at the interface of innate immune response and coagulation

MASP-1 is a versatile serine protease that cleaves a number of substrates in human blood. In recent years it became evident that besides playing a crucial role in complement activation MASP-1 also triggers other cascade systems and even cells to mount a more powerful innate immune response. In this review we summarize the latest discoveries about the diverse functions of this multi-faceted protease. Recent studies revealed that among MBL-associated serine proteases, MASP-1 is the one responsible for triggering the lectin pathway via its ability to rapidly autoactivate then cleave MASP-2, and possibly MASP-3. The crystal structure of MASP-1 explains its more relaxed substrate specificity compared to the related complement enzymes. Due to the relaxed specificity, MASP-1 interacts with the coagulation cascade and the kinin generating system, and it can also activate endothelial cells eliciting pro-inflammatory signaling.

Computational characterization of ketone-ketal transformations at the active site of matrix metalloproteinases

We modeled the first steps of hydrolysis reactions of a natural oligopeptide substrate of matrix metalloproteinase MMP-2 as well as of a substrate analogue. In the latter, the scissile amide group is substituted by a ketomethylene group which can be transformed to the ketal group upon binding of this compound to the enzyme active site. According to our quantum mechanical-molecular mechanical (QM/MM) calculations, the reaction of the ketone-ketal transformation proceeds with a low energy barrier (3.4 kcal/mol) and a high equilibrium constant (10(4)). The reaction product with the ketal group formed directly at the active site of the enzyme works as an inhibitor that chelates the zinc ion. On the other hand, the oligopeptide mimetic retains molecular groups responsible for binding of this compound to the enzyme active site. This example illustrates a strategy to design MMP inhibitors in situ by using data on binding specificity of substrates to a particular type of MMP and details of the reaction mechanism.

Development of an aptamer-based impedimetric bioassay using microfluidic system and magnetic separation for protein detection

An aptamer-based impedimetric bioassay using the microfluidic system and magnetic separation was developed for the sensitive and rapid detection of protein. The microfluidic impedance device was fabricated through integrating the gold interdigitated array microelectrode into a flow cell made of polydimethylsiloxane (PDMS). Aptamer modified magnetic beads were used to capture and separate the target protein, and concentrated into a suitable volume. Then the complexes were injected into the microfluidic flow cell for impedance measurement. To demonstrate the high performance of this novel detection system, thrombin was employed as the target protein. The results showed that the impedance signals at the frequency of 90 kHz have a good linearity with the concentrations of thrombin in a range from 0.1 nM to 10nM and the detection limit is 0.01 nM. Compared with the reported impedimetric aptasensors for thrombin detection, this method possesses several advantages, such as the increasing sensitivity, improving reproducibility, reducing sample volume and assay time. All these demonstrate the proposed detection system is an alternative way to enable sensitive, rapid and specific detection of protein.

Silica nanowire arrays for diffraction-based bioaffinity sensing

Arrays of electrodeposited silica nanowires (SiO2 NWs) have been fabricated over large areas (cm(2)) on fluoropolymer thin films attached to glass substrates by a combination of photolithography and electrochemically triggered sol-gel nanoscale deposition. Optical and scanning electron microscopy (SEM) measurements revealed that the SiO2 NW arrays had an average spacing of ten micrometers and an average width of 700 nm with a significant grain structure that was a result of the sol-gel deposition process. The optical diffraction properties at 633 nm of the SiO2 NW arrays were characterized when placed in contact with solutions by using a prism-coupled total internal reflection geometry; quantification of changes in these diffraction properties was applied in various sensing applications. Bulk refractive index sensing by using the SiO2 NW grating was demonstrated with a sensitivity of 1.30×10(-5) RIU. Toposelectively chemically modified SiO2 NW arrays were used for diffraction biosensing measurements of surface binding events, such as the electrostatic adsorption of gold nanoparticles and the bioaffinity adsorption of streptavidin onto a biotin monolayer. Finally, the application of the SiO2 NW arrays for practical medical-diagnostic applications was demonstrated by monitoring the diffraction of SiO2 NW arrays functionalized with a single-stranded (ss)DNA aptamer to detect human α-thrombin from solutions at sub-pathologic nanomolar concentrations.

Selective albumin-binding surfaces modified with a thrombin-inhibiting peptide

Blood-contacting medical devices have been associated with severe clinical complications, such as thrombus formation, triggered by the activation of the coagulation cascade due to the adsorption of certain plasma proteins on the surface of biomaterials. Hence, the coating of such surfaces with antithrombotic agents has been used to increase biomaterial haemocompatibility. Biomaterial-induced clotting may also be decreased by albumin adsorption from blood plasma in a selective and reversible way, since this protein is not involved in the coagulation cascade. In this context, this paper reports that the immobilization of the thrombin inhibitor D-Phe-Pro-D-Arg-D-Thr-CONH2 (fPrt) onto nanostructured surfaces induces selective and reversible adsorption of albumin, delaying the clotting time when compared to peptide-free surfaces. fPrt, synthesized with two glycine residues attached to the N-terminus (GGfPrt), was covalently immobilized onto self-assembled monolayers (SAMs) having different ratios of carboxylate-hexa(ethylene glycol)- and tri(ethylene glycol)-terminated thiols (EG6-COOH/EG3) that were specifically designed to control GGfPrt orientation, exposure and density at the molecular level. In solution, GGfPrt was able to inactivate the enzymatic activity of thrombin and to delay plasma clotting time in a concentration-dependent way. After surface immobilization, and independently of its concentration, GGfPrt lost its selectivity to thrombin and its capacity to inhibit thrombin enzymatic activity against the chromogenic substrate n-p-tosyl-Gly-Pro-Arg-p-nitroanilide. Nevertheless, surfaces with low concentrations of GGfPrt could delay the capacity of adsorbed thrombin to cleave fibrinogen. In contrast, GGfPrt immobilized in high concentrations was found to induce the procoagulant activity of the adsorbed thrombin. However, all surfaces containing GGfPrt have a plasma clotting time similar to the negative control (empty polystyrene wells), showing resistance to coagulation, which is explained by its capacity to adsorb albumin in a selective and reversible way. This work opens new perspectives to the improvement of the haemocompatibility of blood-contacting medical devices.

Structural glycobiology of heparin dynamics on the exosite 2 of coagulation cascade proteases: Implications for glycosaminoglycans antithrombotic activity

fIIa and fXa are two of the main targets of antithrombin, a serine proteases inhibitor that plays a major role in the regulation of blood clotting. The formation of ternary complexes between such molecules and glycosaminoglycans, as heparin, is the main path for inhibiting those enzymes, which may occur through two distinct mechanisms of action. While these serine proteases present distinct susceptibilities to these paths, in which fIIa demands an interaction with heparin, neither the molecular basis of this differential inhibition nor the role of fIIa glycosylation on this process is fully understood. Thus, the present work evaluated through molecular dynamics simulations the effects of glycosylation on fIIa and the consequences of heparin binding to both proteases function and dynamics. Based on the obtained data, fIIa N-linked glycan promoted an increase in the active site pocket size by stabilizing regions that encircle it, while heparin binding was observed to reverse such an effect. Additionally, heparin orientation observed on the surface of fIIa, but not fXa, allows a linear long-chain heparin binding to antithrombin in ternary complexes. Finally, the enzymes catalytic triad organization was disrupted due to a strong glycosaminoglycan binding to the proteases exosite 2. Such data support an atomic-level explanation for the higher inhibition constant of the antithrombin-heparin complex over fIIa than fXa, as well as for the different susceptibilities of those enzymes for antithrombin mechanisms of action.