The anticoagulant activation of antithrombin by heparin

A Mechanistic Investigation of Methylene Blue and Heparin Interactions and Their Photoacoustic Enhancement

We recently reported a real-time method to measure heparin in human whole blood based on the photoacoustic change of methylene blue (MB). Intriguingly, the MB behaved unlike other "turn on" photoacoustic probes-the absorbance decreased as the photoacoustic signal increased. The underlying mechanism was not clear and motivated this study. We studied the binding mechanism of MB and heparin in water and phosphate buffer saline (PBS) with both experimental and computational methods. We found that the photoacoustic enhancement of the MB-heparin mixture was a result of MB-heparin aggregation due to charge neutralization and resulting sequestration of MB in these aggregates. The sequestration of MB in the MB-heparin aggregates led to decreased absorbance-there was simply less free dye in solution to absorb light. The highest photoacoustic signal and aggregation occurred when the number of negatively charged sulfate groups on heparin was approximately equal to the number of positively charged MB molecule. The MB-heparin aggregates dissociated when there were more sulfated groups from heparin than MB molecules because of the electrostatic repulsion between negatively charged sulfate groups. PBS facilitated MB dimer formation regardless of heparin concentration and reprecipitated free MB in aggregates due to ionic strength and ionic shielding. Further molecular dynamics experiments found that binding of heparin occurred at the sulfates and glucosamines in heparin. Phosphate ions could interact with the heparin via sodium ions to impair the MB-heparin binding. Finally, our model found 3.7-fold more MB dimerization upon addition of heparin in MB solution confirming that heparin facilitates MB aggregation. We conclude that the addition of heparin in MB decreases the absorbance of the sample because of MB-heparin aggregation leading to fewer MB molecules in solution; however, the aggregation also increases the PA intensity because the MB molecules in the MB-heparin aggregate have reduced degrees of freedom and poor heat transfer to solvent.

Replacement of the L-iduronic acid unit of the anticoagulant pentasaccharide idraparinux by a 6-deoxy-L-talopyranose - Synthesis and conformational analysis

One critical part of the synthesis of heparinoid anticoagulants is the creation of the L-iduronic acid building block featured with unique conformational plasticity which is crucial for the anticoagulant activity. Herein, we studied whether a much more easily synthesizable sugar, the 6-deoxy-L-talose, built in a heparinoid oligosaccharide, could show a similar conformational plasticity, thereby can be a potential substituent of the L-idose. Three pentasaccharides related to the synthetic anticoagulant pentasaccharide idraparinux were prepared, in which the L-iduronate was replaced by a 6-deoxy-L-talopyranoside unit. The talo-configured building block was formed by C4 epimerisation of the commercially available L-rhamnose with high efficacy at both the monosaccharide and the disaccharide level. The detailed conformational analysis of these new derivatives, differing only in their methylation pattern, was performed and the conformationally relevant NMR parameters, such as proton-proton coupling constants and interproton distances were compared to the corresponding ones measured in idraparinux. The lack of anticoagulant activity of these novel heparin analogues could be explained by the biologically not favorable 1C4 chair conformation of their 6-deoxy-L-talopyranoside residues.

Methods for Determining and Understanding Serpin Structure and Function: X-Ray Crystallography

Deciphering the X-ray crystal structures of serine protease inhibitors (serpins) and serpin complexes has been an integral part of understanding serpin function and inhibitory mechanisms. In addition, high-resolution structural information of serpins derived from the three domains of life (bacteria, archaea, and eukaryotic) and viruses has provided valuable insights into the hereditary and evolutionary history of this unique superfamily of proteins. This chapter will provide an overview of the predominant biophysical method that has yielded this information, X-ray crystallography. In addition, details of up-and-coming methods, such as neutron crystallography, cryo-electron microscopy, and small- and wide-angle solution scattering, and their potential applications to serpin structural biology will be briefly discussed. As serpins remain important both biologically and medicinally, the information provided in this chapter will aid in future experiments to expand our knowledge of this family of proteins.

Chemical and pharmacological aspects of neutralization of heparins from different animal sources by protamine

Essentials Bovine (HBI) and porcine (HPI) heparins differ in structure and anticoagulant activity. Protamine-neutralization was evaluated on a variety of physical-chemical methods. HBI requires more protamine than HPI to fully neutralize its anticoagulant activity. Protamine preferentially removes higher-sulfated chains of HBI while HPI is evenly precipitated.

SUMMARY

Background Protamine neutralization is an essential step for the safe use and inactivation of the unfractionated heparin (UFH) that is widely employed in surgical and non-surgical procedures involving extracorporeal circulation. Objective To compare protamine neutralization of different pharmaceutical-grade UFHs prepared from porcine or bovine intestine (HPI and HBI, respectively). HBI has approximately half the anticoagulant potency of HPI, mostly as consequence of its fraction enriched with N-sulfated α-glucosamine disaccharides. Methods Protamine neutralization of HPI and HBI was evaluated with in vitro, ex vivo and in vivo assays. We also performed in-depth assessments of the complexation of protamine with these distinct UFHs by using nuclear magnetic resonance and mass spectroscopy. Results HPI and HBI interact similarly with protamine on a mass/mass basis; however, HBI requires more protamine than HPI to have its anticoagulant activity fully neutralized, because of its lower potency, which entails the use of higher doses. Nuclear magnetic resonance spectra revealed that HPI precipitates homogeneously with protamine. On the other hand, the low-sulfated fraction of HBI, enriched with N-sulfated α-glucosamine, precipitates at higher concentrations of protamine than the fraction more like HPI, with a preponderance of N,6-disulfated α-glucosamine disaccharides. Finally, mass spectroscopy spectra showed that some of the different peptide components of protamine interact preferentially with the heparins, irrespective of their animal origin. Conclusion Our results have important medical implications, indicating that protamine neutralization of HBI, determined exclusively by point-of-care coagulation assessments, must fail because of its lower-sulfated fraction with reduced anticoagulant activity that could remain in the circulation after the neutralization procedure.

Heparin: role in protein purification and substitution with animal-component free material

Heparin is a highly sulfated polysaccharide which belongs to the family of glycosaminoglycans. It is involved in various important biological activities. The major biological purpose is the inhibition of the coagulation cascade to maintain the blood flow in the vasculature. These properties are employed in several therapeutic drugs. Heparin’s activities are associated with its interaction to various proteins. To date, the structural heparin-protein interactions are not completely understood. This review gives a general overview of specific patterns and functional groups which are involved in the heparin-protein binding. An understanding of the heparin-protein interactions at the molecular level is not only advantageous in the therapeutic application but also in biotechnological application of heparin for downstreaming. This review focuses on the heparin affinity chromatography. Diverse recombinant proteins can be successfully purified by this method. While effective, it is disadvantageous that heparin is an animal-derived material. Animal-based components carry the risk of contamination. Therefore, they are liable to strict quality controls and the validation of effective good manufacturing practice (GMP) implementation. Hence, adequate alternatives to animal-derived components are needed. This review examines strategies to avoid these disadvantages. Thereby, alternatives for the provision of heparin such as chemical synthesized heparin, chemoenzymatic heparin, and bioengineered heparin are discussed. Moreover, the usage of other chromatographic systems mimetic the heparin effect is reviewed.

Antithrombotic potential of esculin 7, 3', 4', 5', 6'-O-pentasulfate (EPS) for its role in thrombus reduction using rat thrombosis model

Currently available anticoagulants for prevention and treatment of thrombosis have several limitations, thus, small organic scaffolds that can dissolve clots in vivo in a dose dependent manner with lesser side effects are highly desirable. Here we report the synthesis of esculin pentasulfate (EPS) and assessment of its in vitro, in vivo and ex vivo anticoagulant and antithrombotic potential. Assessment of in vitro clotting times showed prolonged activated partial thromboplastin time (APTT), prothrombin time (PT) and thrombin time (TT) in the presence of EPS. EPS also showed remarkable reduction in thrombus formation when administered in occlusion induced thrombotic rats at a low dose (2.5 mg/kg). Further, assessment of clot rate with plasma isolated from EPS treated rats confirmed its anticoagulation potential. EPS at varying concentrations showed no significant cytotoxic effect on HEK293 cell line. Further, molecular docking analysis of EPS with known anticoagulant proteins [(antithrombin (ATIII) and heparin cofactor II (HCF II)] that require heparin revealed good binding affinity (-7.9 kcal/mol) with ATIII but not with HCF II. ATIII when incubated with EPS showed increased fluorescence intensity, with no change in secondary structure. Overall, our results clearly show the in vivo modulation of thrombus formation using a modified natural scaffold EPS.

So you think computational approaches to understanding glycosaminoglycan-protein interactions are too dry and too rigid? Think again!

Glycosaminoglycans (GAGs) play key roles in virtually all biologic responses through their interaction with proteins. A major challenge in understanding these roles is their massive structural complexity. Computational approaches are extremely useful in navigating this bottleneck and, in some cases, the only avenue to gain comprehensive insight. We discuss the state-of-the-art on computational approaches and present a flowchart to help answer most basic, and some advanced, questions on GAG-protein interactions. For example, firstly, does my protein bind to GAGs?; secondly, where does the GAG bind?; thirdly, does my protein preferentially recognize a particular GAG type?; fourthly, what is the most optimal GAG chain length?; fifthly, what is the structure of the most favored GAG sequence?; and finally, is my GAG-protein system 'specific', 'non-specific', or a combination of both? Recent advances show the field is now poised to enable a non-computational researcher perform advanced experiments through the availability of various tools and online servers.

Cooperative Interactions of Three Hotspot Heparin Binding Residues Are Critical for Allosteric Activation of Antithrombin by Heparin

Heparin allosterically activates the anticoagulant serpin, antithrombin, by binding through a sequence-specific pentasaccharide and inducing activating conformational changes in the protein. Three basic residues of antithrombin, Lys114, Lys125, and Arg129, have been shown to be hotspots for binding the pentasaccharide, but the molecular basis for such hotspot binding has been unclear. To determine whether this results from cooperative interactions, we analyzed the effects of single, double, and triple mutations of the hotspot residues on pentasaccharide binding and activation of antithrombin. Double-mutant cycles revealed that the contribution of each residue to pentasaccharide binding energy was progressively reduced when one or both of the other residues were mutated, indicating strong coupling between each pair of residues that was dependent on the third residue and reflective of the three residues acting as a cooperative unit. Rapid kinetic studies showed that the hotspot residue mutations progressively abrogated the ability of the pentasaccharide to bind productively to native antithrombin and to conformationally activate the serpin by engaging the hotspot residues in an induced-fit interaction. Examination of the antithrombin-pentasaccharide complex structure revealed that the hotspot residues form two adjoining binding pockets for critical sulfates of the pentasaccharide that structurally link these residues. Together, these findings demonstrate that cooperative interactions of Lys114, Lys125, and Arg129 are critical for the productive induced-fit binding of the heparin pentasaccharide to antithrombin that allosterically activates the anticoagulant function of the serpin.

Recognition and Conformational Properties of an Alternative Antithrombin Binding Sequence Obtained by Chemoenzymatic Synthesis

Heparin is a highly sulfated glycosaminoglycan (GAG) of natural origin used as an anticoagulant and antithrombotic drug. These properties are principally based on the binding and activation of antithrombin (AT) through the pentasaccharide sequence GlcNAc/NS,6S-GlcA-GlcNS,3,6S-IdoA2S-GlcNS,6S (AGA*IA). Literature data show that the population of the 2 S0 ring conformation of the 2-O-sulfo-α-l-iduronic acid (IdoA2S) motif correlates with the affinity and activation of AT. It was recently demonstrated that two synthetic AGA*IA-containing hexasaccharides (one G unit added at the reducing end), differing in the degree of sulfation of the IdoA unit, show comparable affinity and ability to activate AT, despite a different conformation of the IdoA residue. In this paper, the binding of these two glycans to AT was studied by isothermal titration microcalorimetry (ITC), transferred (tr-) NOESY, saturation transfer difference (STD) NMR spectroscopy and molecular dynamics (MD) simulations. Results indicated that both the IdoA2S and the IdoA units assume a 2 S0 conformation when bound with AT, and so present a common binding epitope for the two glycans, centred on the AGA*IA sequence.

Expression and functional characterization of two natural heparin-binding site variants of antithrombin

Essentials Heparin-binding site (HBS) variants of antithrombin (AT) are associated with thrombosis risk. HSB variants have, in general, normal progressive inhibitory activity but reduced heparin affinity. Thrombosis in HSB carriers has been primarily attributed to the loss of heparin cofactor activity. Results here demonstrate that HSB variants of AT also lack anti-inflammatory signaling functions.

SUMMARY

Background Several heparin-binding site (HBS) variants of antithrombin (AT) have been identified that predispose carriers to a higher incidence of thrombosis. Thrombosis in carriers of HBS variants has been primarily attributed to a loss in their heparin-dependent anticoagulant function. Objective The objective of this study was to determine whether HSB mutations affect the anti-inflammatory functions of variants. Methods Two HBS variants of AT (AT-I7N and AT-L99F), which are known to be associated with a higher incidence of thrombosis, were expressed in mammalian cells and purified to homogeneity. These variants were characterized by kinetic assays followed by analysis of their activities in established cellular and/or in vivo inflammatory models. The possible effects of mutations on AT structure were also evaluated by molecular modeling. Results The results indicated that, whereas progressive inhibitory activities of variants were minimally affected, their heparin affinity and inhibitory activity in the presence of heparin were markedly decreased. Unlike wild-type AT, neither AT variant was capable of inhibiting activation of nuclear factor-κB or downregulation of expression of cell adhesion molecules in response to lipopolysaccharide (LPS). Similarly, neither variant elicited barrier protective activity in response to LPS. Structural analysis suggested that the L99F substitution locally destabilizes AT structure. Conclusions It is concluded that the L99F mutation of AT is associated with destabilization of the serpin structure, and that the loss of anti-inflammatory signaling function of the HBS variants may also contribute to enhanced thrombosis in carriers of HBS mutations.

Covalently linking heparin to antithrombin enhances prothrombinase inhibition on activated platelets

Factor (F)Xa within the prothrombinase complex is protected from inhibition by unfractionated heparin (UFH), enoxaparin and fondaparinux. We have developed a covalent antithrombin-heparin complex (ATH) with enhanced anticoagulant activity. We have also demonstrated that ATH is superior at inhibiting coagulation factors when assembled on artificial surfaces. The objective of the present study is to determine the ability of ATH vs AT+UFH to inhibit FXa within the prothrombinase complex when the enzyme complex is assembled on the more native platelet system. Discontinuous inhibition assays were performed to determine final k 2-values for inhibition of FXa, FXa within the platelet-prothrombinase, or FXa within prothrombinase devoid of various components. Thrombin generation and plasma clotting was also assayed in the presence of resting/activated platelets ± inhibitors. Protection of FXa was not observed for ATH, whereas a moderate 60% protection was observed for AT+UFH. ATH inhibited platelet-prothrombinase ∼4-fold faster than AT+UFH. Relative to intact prothrombinase, rates for FXa inhibition by AT+UFH in prothrombinase complexes devoid of either prothrombin (II)/activated platelets/FVa were higher. However, inhibition by AT+UFH of prothrombinase devoid of FII yielded slightly lower rates compared to free FXa inhibition. Thrombin generation and plasma clotting was enhanced with activated platelets, while inhibition was better by ATH compared to AT+UFH, thus suggesting an overall enhanced anticoagulant activity of ATH against platelet-bound prothrombinase complexes.

Gene analysis of inherited antithrombin deficiency and functional analysis of abnormal antithrombin protein (N87D)

Inherited antithrombin (AT) deficiency is one of the most clinically significant forms of congenital thrombophilia and follows an autosomal dominant mode of inheritance. We analyzed SERPINC1 in a patient who developed deep-vein thrombosis and low AT activity during pregnancy, and identified a novel missense mutation c.259A>G (p.Asn87Asp; N87D). Surprisingly, analysis of the parents' DNA showed that they did not possess this mutant, and thus, it may have been due to a de novo mutation. We also expressed this mutant AT protein in COS-1 cells and compared its intracellular localization and intracellular and extracellular antigen levels with that of wild-type AT. The expression experiment did not reveal a significant difference in the antigen levels of the mutant and wild-type AT in the cell lysate, but the mutant AT antigen level was markedly lower than that of its wild-type counterpart in the COS-1 cell supernatant. Immunofluorescence did not indicate any difference between the mutant and wild-type AT in terms of cytoplasmic localization of fluorescence signals. Our findings suggest that the patient's AT deficiency may have been caused by impaired extracellular secretion of mutant AT protein p.Asn87Asp.

Characterization of structural motifs for interactions between glycosaminoglycans and proteins

Glycosaminoglycans (GAGs) are a family of linear and anionic polysaccharides that play essential roles in many biological and physiological processes. Interactions between GAGs and proteins regulate function in many proteins and are related to many human diseases and disorders. The structural motifs and mechanisms for interactions between GAGs and proteins are not fully understood. Specific bindings, including minor but unique sequences sporadically distributed along the GAG chains or variably sulfated domains interspersed by undersulfated regions, may be specifically recognized by defined domains of a variety of proteins. Understanding the molecular basis of these interactions will provide a template for developing novel glycotherapeutic agents. The present article reviews recent methodologies and progress on the characterization of structural motifs in both GAGs and proteins involved in GAG-protein interactions. The analytical approaches are categorized into three groups: affinity-based methods; molecular docking, nuclear magnetic resonance (NMR) spectroscopy and X-ray crystallography; and mass spectrometry (MS) techniques. The advantages and limitations of each category of methods are discussed and are based on examples of using these techniques to investigate binding between GAGs and proteins.

Extended Physicochemical Characterization of the Synthetic Anticoagulant Pentasaccharide Fondaparinux Sodium by Quantitative NMR and Single Crystal X-ray Analysis

Fondaparinux sodium is a synthetic pentasaccharide representing the high affinity antithrombin III binding site in heparin. It is the active pharmaceutical ingredient of the anticoagulant drug Arixtra^®. The single crystal X-ray structure of Fondaparinux sodium is reported, unequivocally confirming both structure and absolute configuration. The iduronic acid adopts a somewhat distorted chair conformation. Due to the presence of many sulfur atoms in the highly sulfated pentasaccharide, anomalous dispersion could be applied to determine the absolute configuration. A comparison with the conformation of Fondaparinux in solution, as well as complexed with proteins is presented. The content of the solution reference standard was determined by quantitative NMR using an internal standard both in 1999 and in 2016. A comparison of the results allows the conclusion that this method shows remarkable precision over time, instrumentation and analysts.

Anti-inflammatory Effect of Self-assembling Glycol-Split Glycosaminoglycan-Stearylamine Conjugates in Lipopolysaccharide-Stimulated Macrophages

Glycosaminoglycans (GAGs) play important roles in various biological processes such as cell adhesion and signal transduction, as well as promote anti-inflammatory activity. We previously revealed that glycol-split heparin (HP)-aliphatic amine conjugates form self-assembled nanoparticles and suppress the production of pro-inflammatory cytokines such as tumor necrosis factor (TNF)-α, interleukin (IL)-6, and IL-1β in lipopolysaccharide (LPS)-stimulated macrophages much more strongly than native HP (J.

CONTROL

Release, 194, 2014, Babazada et al.). Considering that HP is not the only GAG to have anti-inflammatory activity, the present study was initiated to examine whether conjugation of GAGs with aliphatic amines is generally effective in their activity augmentation against LPS-stimulated macrophages. We newly synthesized the stearylamine conjugates of chondroitin sulfate (CS), hyaluronic acid (HA), and low-molecular-weight heparin (LH), and investigated the effect of the position and degree of sulfation and molecular weight of GAGs on their anti-inflammatory activity. All of the conjugates formed self-assembled nanoparticles in aqueous solution. The IC₅₀ value for suppression of TNF-α production from the macrophages was the smallest with the derivative of LH, followed by HP, CS, and HA. The degree of sulfation appeared to be important in determining their anti-inflammatory activity, which would correspond to previous results using the derivatives of site-selectively desulfated HP. Comparison of HP and LH derivatives revealed that fractionated smaller heparin has greater anti-inflammatory activity.

Targeting heparin and heparan sulfate protein interactions

Heparin and heparan sulfate glycosaminoglycans are long, linear polysaccharides that are made up of alternating dissacharide sequences of sulfated uronic acid and amino sugars. Unlike heparin, which is only found in mast cells, heparan sulfate is ubiquitously expressed on the cell surface and in the extracellular matrix of all animal cells. These negatively-charged glycans play essential roles in important cellular functions such as cell growth, adhesion, angiogenesis, and blood coagulation. These biomolecules are also involved in pathophysiological conditions such as pathogen infection and human disease. This review discusses past and current methods for targeting these complex biomolecules as a novel therapeutic strategy to treating disorders such as cancer, neurodegenerative diseases, and infection.

A Comprehensive Docking and MM/GBSA Rescoring Study of Ligand Recognition upon Binding Antithrombin

Background:

A high-throughput virtual screening pipeline has been extended from single energetically minimized structure Molecular Mechanics/Generalized Born Surface Area (MM/GBSA) rescoring to ensemble-average MM/GBSA rescoring. The correlation coefficient (R2) of calculated and experimental binding free energies for a series of antithrombin ligands has been improved from 0.36 to 0.69 when switching from the single-structure MM/GBSA rescoring to ensemble-average one. The electrostatic interactions in both solute and solvent are identified to play an important role in determining the binding free energy after the decomposition of the calculated binding free energy. The increasing negative charge of the compounds provides a more favorable electrostatic energy change but creates a higher penalty for the solvation free energy. Such a penalty is compensated by the electrostatic energy change, which results in a better binding affinity. A highly hydrophobic ligand is determined by the docking program to be a non-specific binder.

Results:

Our results have demonstrated that it is very important to keep a few top poses for rescoring, if the binding is non-specific or the binding mode is not well determined by the docking calculation.

Inhibitory serpins. New insights into their folding, polymerization, regulation and clearance

Serpins are a widely distributed family of high molecular mass protein proteinase inhibitors that can inhibit both serine and cysteine proteinases by a remarkable mechanism-based kinetic trapping of an acyl or thioacyl enzyme intermediate that involves massive conformational transformation. The trapping is based on distortion of the proteinase in the complex, with energy derived from the unique metastability of the active serpin. Serpins are the favoured inhibitors for regulation of proteinases in complex proteolytic cascades, such as are involved in blood coagulation, fibrinolysis and complement activation, by virtue of the ability to modulate their specificity and reactivity. Given their prominence as inhibitors, much work has been carried out to understand not only the mechanism of inhibition, but how it is fine-tuned, both spatially and temporally. The metastability of the active state raises the question of how serpins fold, whereas the misfolding of some serpin variants that leads to polymerization and pathologies of liver disease, emphysema and dementia makes it clinically important to understand how such polymerization might occur. Finally, since binding of serpins and their proteinase complexes, particularly plasminogen activator inhibitor-1 (PAI-1), to the clearance and signalling receptor LRP1 (low density lipoprotein receptor-related protein 1), may affect pathways linked to cell migration, angiogenesis, and tumour progression, it is important to understand the nature and specificity of binding. The current state of understanding of these areas is addressed here.

Synthesis of 3-O-Sulfated Oligosaccharides to Understand the Relationship between Structures and Functions of Heparan Sulfate

The sulfation at the 3-OH position of glucosamine is an important modification in forming structural domains for heparan sulfate to enable its biological functions. Seven 3-O-sulfotransferase isoforms in the human genome are involved in the biosynthesis of 3-O-sulfated heparan sulfate. As a rare modification present in heparan sulfate, the availability of 3-O-sulfated oligosaccharides is very limited. Here, we report the use of a chemoenzymatic synthetic approach to synthesize six 3-O-sulfated oligosaccharides, including three hexasaccharides and three octasaccharides. The synthesis was achieved by rearranging the enzymatic modification sequence to accommodate the substrate specificity of 3-O-sulfotransferase 3. We studied the impact of 3-O-sulfation on the conformation of the pyranose ring of 2-O-sulfated iduronic acid using NMR, and on the correlation between ring conformation and anticoagulant activity. We identified a novel octasaccharide that interacts with antithrombin and displays anti factor Xa activity. Interestingly, the octasaccharide displays a faster clearance rate than fondaparinux, an FDA-approved pentasaccharide drug, in a rat model, making this octasaccharide a potential short-acting anticoagulant drug candidate that could reduce bleeding risk. Having access to a set of critically important 3-O-sulfated oligosaccharides offers the potential to develop new heparan sulfate-based therapeutics.

Thrombotic risk according to SERPINC1 genotype in a large cohort of subjects with antithrombin inherited deficiency

Inherited quantitative (type I) or qualitative (type II) antithrombin deficiency (ATD) due to mutations in the SERPINC1 gene is a well-known risk factor for venous thromboembolism. ATD may also increase risk for arterial thrombosis. Few studies have investigated risk for thrombosis according to mutations. We addressed this topic in a large retrospective cohort study of 540 heterozygous carriers of SERPINC1 mutations and compared risk for first venous or arterial thrombosis associated with carrying of different type II or type I mutations. No clear difference in risk for first venous thrombotic event was observed among type I (missense or null), type IIRS or type IIPE mutation carriers except for a few variants that displayed lower risk [all events, adjusted relative risk: Cambridge II: 0.42 (95 %CI 0.25-0.70), Dublin: 0.35 (95 %CI 0.13-0.99)]. IIHBS mutation carrying was associated with a clearly lower risk than type I mutation carrying [0.28 (95 %CI 0.20-0.40)]. These differences in risk were observed for both all venous thrombotic events and pulmonary embolism associated with deep venous thrombosis. The HBS group was also heterogeneous, with AT Budapest 3 carriers displaying a non-significantly different risk [0.61 (95 %CI 0.31-1.20)] compared to type I mutation carriers. We also studied risk for arterial thrombosis and found no significant influence of mutation type. Altogether, our findings suggest a place for SERPINC1 genotyping in the diagnosis of ATD.

A Hexasaccharide Containing Rare 2-O-Sulfate-Glucuronic Acid Residues Selectively Activates Heparin Cofactor II

Glycosaminoglycan (GAG) sequences that selectively target heparin cofactor II (HCII), a key serpin present in human plasma, remain unknown. Using a computational strategy on a library of 46 656 heparan sulfate hexasaccharides we identified a rare sequence consisting of consecutive glucuronic acid 2-O-sulfate residues as selectively targeting HCII. This and four other unique hexasaccharides were chemically synthesized. The designed sequence was found to activate HCII ca. 250-fold, while leaving aside antithrombin, a closely related serpin, essentially unactivated. This group of rare designed hexasaccharides will help understand HCII function. More importantly, our results show for the first time that rigorous use of computational techniques can lead to discovery of unique GAG sequences that can selectively target GAG-binding protein(s), which may lead to chemical biology or drug discovery tools.

Peptidomic Analysis of Rat Plasma: Proteolysis in Hemorrhagic Shock

It has been previously shown that intestinal proteases translocate into the circulation during hemorrhagic shock and contribute to proteolysis in distal organs. However, consequences of this phenomenon have not previously been investigated using high-throughput approaches. Here, a shotgun label-free quantitative proteomic approach was utilized to compare the peptidome of plasma samples from healthy and hemorrhagic shock rats to verify the possible role of uncontrolled proteolytic activity in shock. Plasma was collected from rats after hemorrhagic shock (HS) consisting of 2-h hypovolemia followed by 2-h reperfusion, and from healthy control (CTRL) rats. A new two-step enrichment method was applied to selectively extract peptides and low molecular weight proteins from plasma, and directly analyze these samples by tandem mass spectrometry. One hundred twenty-six circulating peptides were identified in CTRL and 295 in HS animals. Ninety-six peptides were present in both conditions; of these, 57 increased and 30 decreased in shock. In total, 256 peptides were increased or present only in HS confirming a general increase in proteolytic activity in shock. Analysis of the proteases that potentially generated the identified peptides suggests that the larger relative contribution to the proteolytic activity in shock is due to chymotryptic-like proteases. These results provide quantitative confirmation that extensive, system-wide proteolysis is part of the complex pathologic phenomena occurring in hemorrhagic shock.

Synthesis and Solution-Phase Characterization of Sulfonated Oligothioetheramides

Nature has long demonstrated the importance of chemical sequence to induce structure and tune physical interactions. Investigating macromolecular structure and dynamics is paramount to understand macromolecular binding and target recognition. To that end, we have synthesized and characterized flexible sulfonated oligothioetheramides (oligo-TEAs) by variable temperature pulse field gradient (PFG) NMR, double electron-electron resonance (DEER), and molecular dynamics (MD) simulations to capture their room temperature structure and dynamics in water. We have examined the contributions of synthetic length (2-12mer), pendant group charge, and backbone hydrophobicity. We observe significant entropic collapse, driven in part by backbone hydrophobicity. Analysis of individual monomer contributions revealed larger changes due to the backbone compared to pendant groups. We also observe screening of intramolecular electrostatic repulsions. Finally, we comment on the combination of DEER and PFG NMR measurements via Stokes-Einstein-Sutherland diffusion theory. Overall, this sensitive characterization holds promise to enable de novo development of macromolecular structure and sequence-structure-function relationships with flexible, but biologically functional macromolecules.

Thrombin-Responsive Transcutaneous Patch for Auto-Anticoagulant Regulation

A thrombin-responsive closed-loop patch is developed for prolonged heparin delivery in a feedback-controlled manner. This microneedle-based patch can sense activated thrombin and subsequently releases heparin to prevent coagulation in the blood flow. This "smart" heparin patch can be transcutaneously inserted into skin without drug leakage and can sustainably regulate blood coagulation in response to thrombin.

Light up detection of heparin based on aggregation-induced emission and synergistic counter ion displacement

An easily accessible fluorescent light up probe HPQ-TBP-I is developed for sensitive and selective detection of heparin based on a synergistic strategy of aggregation-induced emission (AIE) and displacement of the fluorescence quencher iodide ion.

How serpins transport hormones and regulate their release

The adaptation of the serpin framework and its mechanism to perform diverse functions is epitomised in the hormone carriers of the blood. Thyroxine and the corticosteroids are transported bound in a 1:1 ratio on almost identical sites in the two homologous binding-globulins, TBG and CBG. Recent structural findings show an equilibrated, rather than on-and-off, release of the hormones from the carriers, reflecting small reversible movements of the hinge region of the reactive loop that modify the conformational flexibility of the underlying hormone-binding site. Consequently, contrary to previous concepts, the binding affinities of TBG and CBG are not fixed but can be allosterically modified to allow differential hormone delivery. Notably, the two carriers function like protein thermocouples with a surge in hormone release as body temperatures rise in fevers, and conversely a large diminution in free hormone levels at hibernation temperatures. By comparison angiotensinogen, the source of the angiotensin peptides that control blood pressure, does not appear to utilise the serpin mechanism. It has instead evolved a 63 residue terminal extension containing the buried angiotensin cleavage site, which on interaction moves into the active cleft of the renin. The conformational shift involved is critically linked by a labile disulphide bridge. The observation of changes in the redox status of this S-S bridge, in the hypertensive complication of pregnancy, pre-eclampsia, has opened an unexpected level of regulation at what is the initial stage in the control of blood pressure.

Non-anticoagulant effects of low molecular weight heparins in inflammatory disorders: A review

Low molecular weight heparins (LMWHs) are produced by chemical or enzymatic depolymerization of unfractionated heparin (UFH). Besides their well-known anticoagulant effects, LMWHs have also been reported to exhibit numerous anti-inflammatory properties. Previous studies have, however, shown that different production processes result in unique structural characteristics of LMWHs. The structural variations may help explain the different therapeutic spectrums in disease treatment for non-anticoagulant effects. In the present review, we summarize major advances in understanding and exploiting the anti-inflammatory disorder activities of LMWHs, based on mechanistic studies, preclinical experiments and clinical trials. We highlight differences in these activities of commercially available LMWHs produced using different manufacturing processes. We stress the importance of structure-activity relationship (SAR) studies on the non-anticoagulant effects of LMWHs and discuss strategies for exploring new clinical indications.

Basic Residues of β-Sheet A Contribute to Heparin Binding and Activation of Vaspin (Serpin A12)

Many members of the serine protease inhibitor (serpin) family are activated by glycosaminoglycans (GAGs). Visceral adipose tissue-derived serpin (vaspin), serpin A12 of the serpin family, and its target protease kallikrein 7 (KLK7) are heparin-binding proteins, and inhibition of KLK7 by vaspin is accelerated by heparin. However, the nature of GAG binding to vaspin is not known. Here, we measured vaspin binding of various glycosaminoglycans and low molecular weight heparins by microscale thermophoresis and analyzed acceleration of protease inhibition by these molecules. In addition, basic residues contributing to heparin binding and heparin activation were identified by a selective labeling approach. Together, these data show that vaspin binds heparin with high affinity (K_D = 21 ± 2 nm) and that binding takes place at a basic patch on top of β-sheet A and is different from other heparin-binding serpins. Mutation of basic residues decreased heparin binding and activation of vaspin. Similarly, reactive center loop insertion into sheet A decreased heparin binding because it disturbs the basic cluster. Finally, using vaspin-overexpressing keratinocyte cells, we show that a significant part of secreted vaspin is bound in the extracellular matrix on the cell surface. Together, basic residues of central β-sheet A contribute to heparin binding and activation of vaspin. Thus, binding to GAGs in the extracellular matrix can direct and regulate vaspin interaction with target proteases or other proteins and may play an important role in the various beneficial functions of vaspin in different tissues.

1.45 Å resolution structure of SRPN18 from the malaria vector Anopheles gambiae

Serine protease inhibitors (serpins) in insects function within development, wound healing and immunity. The genome of the African malaria vector, Anopheles gambiae, encodes 23 distinct serpin proteins, several of which are implicated in disease-relevant physiological responses. A. gambiae serpin 18 (SRPN18) was previously categorized as non-inhibitory based on the sequence of its reactive-center loop (RCL), a region responsible for targeting and initiating protease inhibition. The crystal structure of A. gambiae SRPN18 was determined to a resolution of 1.45 Å, including nearly the entire RCL in one of the two molecules in the asymmetric unit. The structure reveals that the SRPN18 RCL is extremely short and constricted, a feature associated with noncanonical inhibitors or non-inhibitory serpin superfamily members. Furthermore, the SRPN18 RCL does not contain a suitable protease target site and contains a large number of prolines. The SRPN18 structure therefore reveals a unique RCL architecture among the highly conserved serpin fold.

A small deletion in SERPINC1 causes type I antithrombin deficiency by promoting endoplasmic reticulum stress

Antithrombin (AT) deficiency is an autosomal dominant disorder, and identification of mutation AT variants would improve our understanding of the anticoagulant function of this serine protease inhibitor (SERPIN) and the molecular pathways underlying this disorder. In the present study, we performed whole-exome sequencing of a Chinese family with deep vein thrombosis, and identified a new small deletion that eliminates four amino acids (INEL) from exon 4 of SERPINC1 gene. This causes type I AT deficiency by enhancing the intracellular retention of this protein. AT retention leads to endoplasmic reticulum (ER) stress, which further inhibits AT release. In addition, ER stress activates ER-associated degradation, which promotes AT degradation. Suppression of ER stress enhanced the secretion of AT, while inhibition of ER-associated degradation suppressed AT release. Thus, our study identified a new mutation (INEL deletion) causing type I AT deficiency, and uncovered a novel mechanism for AT retention through enhanced ER stress, which may provide an innovative approach for treating AT deficiency.

Causative genetic mutations for antithrombin deficiency and their clinical background among Japanese patients

We summarize causative genetic mutations for antithrombin (AT) deficiency and their clinical background in Japanese patients. A total of 19 mutations, including seven novel mutations, were identified. We also summarize clinical symptoms of thrombosis, age at onset, family history, and contributing factors for thrombosis, and review the use of prophylactic anticoagulation in pregnant women with heterozygous type II heparin binding site defects (HBS) AT deficiency. The prevalence of thrombosis in probands with type I AT deficiency (88%) was double that observed in those with type II AT deficiency (50%). The prevalence of thrombotic episodes among family members was also higher for type I AT deficiency subjects (82%) than for those with type II AT deficiency (0%). The most common contributing factor for thrombosis among women with type I AT deficiency was pregnancy. Forty-five percent of women with type I AT deficiency developed thrombotic events before the 20th week of gestation. In contrast, women with type II (HBS) AT deficiency appear to be at a lower risk of thrombosis during pregnancy. In conclusion, thrombotic risk varies among different subtypes. Risk assessments based on genetic/clinical backgrounds may contribute to appropriate diagnosis, treatment, and prophylaxis for patients with AT deficiency.

Subtypes of SERPINC1 mutations and the thrombotic phenotype of inherited antithrombin deficient individuals in Chinese Han population

Inherited antithrombin (AT) deficiency is a rare autosomal disease that could increase the risk of venous thromboembolism (VTE) and usually caused by mutations of SERPINC1. Although a number of mutations of SERPINC1 have been reported in Chinese Han population, the impact of different subtypes of these mutations on the thrombotic phenotype is still unknown. Here, we performed a retrospective cohort study including 169 AT patients from 63 families to compare the clinical features between null mutation carriers and missense mutation carriers. We found that patients carrying null mutations have a higher risk of VTE (HR 2.29, 95% CI 1.16-4.69, P=0.02 adjusted for sex and VTE family history) and earlier median onset age of VTE (27 vs. 32years, P=0.045) as well as lower AT activities (47.6±1.0% vs. 59.1±2.3%, P<0.001) than those with missense mutations. We also observed that thrombus location sites showed no difference between null mutation carriers and missense mutation carriers, gene locations of the mutations did not relate with the incidence rate of VTE. This study demonstrated that different types of SERPINC1 mutations may play different roles in the development of VTE and should be considered in the prevention of VTE.

A Modular Synthetic Approach to Isosteric Sulfonic Acid Analogues of the Anticoagulant Pentasaccharide Idraparinux

Heparin-based anticoagulants are drugs of choice in the therapy and prophylaxis of thromboembolic diseases. Idraparinux is a synthetic anticoagulant pentasaccharide based on the heparin antithrombin-binding domain. In the frame of our ongoing research aimed at the synthesis of sulfonic acid-containing heparinoid anticoagulants, we elaborated a modular pathway to obtain a series of idraparinux-analogue pentasaccharides bearing one or two primary sulfonic acid moieties. Five protected pentasaccharides with different C-sulfonation patterns were prepared by two subsequent glycosylation reactions, respectively, using two monosaccharide and four disaccharide building blocks. Transformation of the protected derivatives into the fully O-sulfated, O-methylated sulfonic acid end-products was also studied.

Structural and haemostatic features of pharmaceutical heparins from different animal sources: challenges to define thresholds separating distinct drugs

Heparins extracted from different animal sources have been conventionally considered effective anticoagulant and antithrombotic agents despite of their pharmacological dissimilarities. We performed herein a systematic analysis on the physicochemical properties, disaccharide composition, in vitro anticoagulant potency and in vivo antithrombotic and bleeding effects of several batches of pharmaceutical grade heparins obtained from porcine intestine, bovine intestine and bovine lung. Each of these three heparin types unambiguously presented differences in their chemical structures, physicochemical properties and/or haemostatic effects. We also prepared derivatives of these heparins with similar molecular weight differing exclusively in their disaccharide composition. The derivatives from porcine intestinal and bovine lung heparins were structurally more similar with each other and hence presented close anticoagulant activities whereas the derivative from bovine intestinal heparin had a higher proportion of 6-desulfated α-glucosamine units and about half anticoagulant activity. Our findings reasonably indicate that pharmaceutical preparations of heparin from different animal sources constitute distinct drugs, thus requiring specific regulatory rules and therapeutic evaluations.

Characterization of conformers and dimers of antithrombin by capillary electrophoresis-quadrupole-time-of-flight mass spectrometry

Antithrombin (AT) is a plasma glycoprotein which possesses anticoagulant and anti-inflammatory properties. AT exhibits various forms, among which are native, latent and heterodimeric ones. We studied the potential of capillary electrophoresis-mass spectrometry (CE-MS) using a sheath liquid interface, electrospray ionization (ESI), and a quadrupole-time-of-flight (Q-TOF) mass spectrometer to separate and quantify the different AT forms. For CE separation, a neutral polyvinyl alcohol (PVA) coated capillary was employed. The protein conformation was preserved by using a background electrolyte (BGE) at physiological pH. A sheath liquid of isopropanol-water 50:50 (v/v) with 14 mM ammonium acetate delivered at a flow rate of 120 μL h^-1 resulted in optimal signal intensities. Each AT form exhibited a specific mass spectrum, allowing unambiguous distinction. Several co-injection experiments proved that latent AT had a higher electrophoretic mobility (μ_ep) than native AT, and that these conformers could associate to form a heterodimer during the CE analysis. The developed CE-MS method enabled the detection and quantitation of latent and heterodimeric forms in a commercial AT preparation stored at room temperature for three weeks.

Heparan Sulfate Regulates the Structure and Function of Osteoprotegerin in Osteoclastogenesis

Osteoprotegerin (OPG), a decoy receptor secreted by osteoblasts, is a major negative regulator of bone resorption. It functions by neutralizing the receptor activator of nuclear factor κB ligand (RANKL), which plays a central role in promoting osteoclastogenesis. OPG is known to be a high-affinity heparan sulfate (HS)-binding protein. Presumably, HS could regulate the function of OPG and affect how it inhibits RANKL. However, the molecular detail of HS-OPG interaction remains poorly understood, which hinders our understanding of how HS functions in osteoclastogenesis. Here we report mapping of the HS-binding site of OPG. The HS-binding site, identified by mutagenesis study, consists of eight basic residues that are located mostly at the junction of the second death domain and the C-terminal domain. We further show that heparin-derived dodecasaccharide is able to induce dimerization of OPG monomers with a stoichiometry of 1:1. Small-angle X-ray scattering analysis revealed that upon binding of HS, OPG undergoes a dramatic conformational change, resulting in a more compact and less flexible structure. Importantly, we present here three lines of evidence that HS, OPG, and RANKL form a stable ternary complex. Using a HS binding-deficient OPG mutant, we further show that in an osteoblast/bone marrow macrophage co-culture system, immobilization of OPG by HS at the osteoblast cell surface substantially lowers the inhibitory threshold of OPG toward RANKL. These discoveries strongly suggest that HS plays an active role in regulating OPG-RANKL interaction and osteoclastogenesis.

An antibody that prevents serpin polymerisation acts by inducing a novel allosteric behaviour

Serpins are important regulators of proteolytic pathways with an antiprotease activity that involves a conformational transition from a metastable to a hyperstable state. Certain mutations permit the transition to occur in the absence of a protease; when associated with an intermolecular interaction, this yields linear polymers of hyperstable serpin molecules, which accumulate at the site of synthesis. This is the basis of many pathologies termed the serpinopathies. We have previously identified a monoclonal antibody (mAb4B12) that, in single-chain form, blocks α1-antitrypsin (α1-AT) polymerisation in cells. Here, we describe the structural basis for this activity. The mAb4B12 epitope was found to encompass residues Glu32, Glu39 and His43 on helix A and Leu306 on helix I. This is not a region typically associated with the serpin mechanism of conformational change, and correspondingly the epitope was present in all tested structural forms of the protein. Antibody binding rendered β-sheet A - on the opposite face of the molecule - more liable to adopt an 'open' state, mediated by changes distal to the breach region and proximal to helix F. The allosteric propagation of induced changes through the molecule was evidenced by an increased rate of peptide incorporation and destabilisation of a preformed serpin-enzyme complex following mAb4B12 binding. These data suggest that prematurely shifting the β-sheet A equilibrium towards the 'open' state out of sequence with other changes suppresses polymer formation. This work identifies a region potentially exploitable for a rational design of ligands that is able to dynamically influence α1-AT polymerisation.

Heparin Binds Lamprey Angiotensinogen and Promotes Thrombin Inhibition through a Template Mechanism

Lamprey angiotensinogen (l-ANT) is a hormone carrier in the regulation of blood pressure, but it is also a heparin-dependent thrombin inhibitor in lamprey blood coagulation system. The detailed mechanisms on how angiotensin is carried by l-ANT and how heparin binds l-ANT and mediates thrombin inhibition are unclear. Here we have solved the crystal structure of cleaved l-ANT at 2.7 Å resolution and characterized its properties in heparin binding and protease inhibition. The structure reveals that l-ANT has a conserved serpin fold with a labile N-terminal angiotensin peptide and undergoes a typical stressed-to-relaxed conformational change when the reactive center loop is cleaved. Heparin binds l-ANT tightly with a dissociation constant of ∼10 nm involving ∼8 monosaccharides and ∼6 ionic interactions. The heparin binding site is located in an extensive positively charged surface area around helix D involving residues Lys-148, Lys-151, Arg-155, and Arg-380. Although l-ANT by itself is a poor thrombin inhibitor with a second order rate constant of 500 m^-1 s^-1, its interaction with thrombin is accelerated 90-fold by high molecular weight heparin following a bell-shaped dose-dependent curve. Short heparin chains of 6-20 monosaccharide units are insufficient to promote thrombin inhibition. Furthermore, an l-ANT mutant with the P1 Ile mutated to Arg inhibits thrombin nearly 1500-fold faster than the wild type, which is further accelerated by high molecular weight heparin. Taken together, these results suggest that heparin binds l-ANT at a conserved heparin binding site around helix D and promotes the interaction between l-ANT and thrombin through a template mechanism conserved in vertebrates.