Synthesis and structural study of two new heparin-like hexasaccharides

Design and Synthesis of 6-O-Phosphorylated Heparan Sulfate Oligosaccharides to Inhibit Amyloid β Aggregation

Dysregulation of amyloidogenic proteins and their abnormal processing and deposition in tissues cause systemic and localized amyloidosis. Formation of amyloid β (Aβ) fibrils that deposit as amyloid plaques in Alzheimer's disease (AD) brains is an earliest pathological hallmark. The polysulfated heparan sulfate (HS)/heparin (HP) is one of the non-protein components of Aβ deposits that not only modulates Aβ aggregation, but also acts as a receptor for Aβ fibrils to mediate their cytotoxicity. Interfering the interaction between HS/HP and Aβ could be a therapeutic strategy to arrest amyloidosis. Here we have synthesized the 6- O -phosphorylated HS/HP oligosaccharides and reported their competitive effects on the inhibition of HP-mediated Aβ fibril formation in vitro using a thioflavin T fluorescence assay and a tapping mode atomic force microscopy.

Strategies in Synthesis of Heparin/Heparan Sulfate Oligosaccharides: 2000-Present

Heparin and heparan sulfate are members of the glycosaminoglycan family that are involved in a multitude of biological processes. The great interests in the anticoagulant properties of heparin have stimulated major advances in synthetic strategies toward clinically effective analogues, as demonstrated importantly by the approval of the fully synthetic pentasaccharide fragment, termed fondaparinux (Arixtra®), of the heparin macromolecule for treatment of deep-vein thrombosis. Given the highly complex nature of heparin and heparan sulfate, the chemical synthesis of their components is a challenging endeavor. In the past decade, multiple approaches have been developed to improve the overall synthetic efficiency. New strategies have emerged that can generate libraries of oligosaccharide components of heparin and heparan sulfate. This article discusses recent developments in the assembly of heparin and heparan sulfate oligosaccharides and the associated challenges in their synthesis.

Synthetic Oligosaccharide Libraries and Microarray Technology: A Powerful Combination for the Success of Current Glycosaminoglycan Interactomics

Glycosaminoglycans (GAGs) are extracellular matrix and/or cell-surface sulfated glycans crucial to the regulation of various signaling proteins, the functions of which are essential in many pathophysiological systems. Because structural heterogeneity is high in GAG chains and purification is difficult, the use of structurally defined GAG oligosaccharides from natural sources as molecular models in both biophysical and pharmacological assays is limited. To overcome this obstacle, GAG-like oligosaccharides of well-defined structures are currently being synthesized by chemical and/or enzymatic means in many research groups around the world. These synthetic GAG oligosaccharides serve as useful molecular tools in studies of GAG-protein interactions. In this review, besides discussing the commonest routes used for the synthesis of GAG oligosaccharides, we also survey some libraries of these synthetic models currently available for research and discuss their activities in interaction studies with functional proteins, especially through the microarray approach.

Chemoenzymatic synthesis of heparan sulfate and heparin oligosaccharides and NMR analysis: paving the way to a diverse library for glycobiologists

A library of diverse heparan sulfate (HS) oligosaccharides was chemoenzymatically synthesized and systematically studied using NMR.

Heparan sulfate (HS) is a member of the glycosaminoglycans (GAG) family that plays essential roles in biological processes from animal sources. Heparin, a highly sulfated form of HS, is widely used as anticoagulant drug worldwide. The high diversity and complexity of HS and heparin represent a roadblock for structural characterization and biological activity studies. Access to structurally defined oligosaccharides is critical for the successful development of HS and heparin structure–activity relationships. In this study, a library of 66 HS and heparin oligosaccharides covering different sulfation patterns and sizes was prepared through an efficient method of chemoenzymatic synthesis. A systematic nuclear magnetic resonance spectroscopy study was firstly undertaken for every oligosaccharide in the library. In addition to the availability of different oligosaccharides, this work also provides spectroscopic data helpful for characterizing more complicated polysaccharide structures providing a safeguard to ensure the quality of the drug heparin. This HS/heparin library will be useful for activity screening and facilitate future structure–activity relationship studies.

Interactions between a Heparin Trisaccharide Library and FGF-1 Analyzed by NMR Methods

FGF-1 is a potent mitogen that, by interacting simultaneously with Heparan Sulfate Glycosaminoglycan HSGAG and the extracellular domains of its membrane receptor (FGFR), generates an intracellular signal that finally leads to cell division. The overall structure of the ternary complex Heparin:FGF-1:FGFR has been finally elucidated after some controversy and the interactions within the ternary complex have been deeply described. However, since the structure of the ternary complex was described, not much attention has been given to the molecular basis of the interaction between FGF-1 and the HSGAG. It is known that within the complex, the carbohydrate maintains the same helical structure of free heparin that leads to sulfate groups directed towards opposite directions along the molecular axis. The precise role of single individual interactions remains unclear, as sliding and/or rotating of the saccharide along the binding pocket are possibilities difficult to discard. The HSGAG binding pocket can be subdivided into two regions, the main one can accommodate a trisaccharide, while the other binds a disaccharide. We have studied and analyzed the interaction between FGF-1 and a library of trisaccharides by STD-NMR and selective longitudinal relaxation rates. The library of trisaccharides corresponds to the heparin backbone and it has been designed to interact with the main subsite of the protein.

Extension and validation of the GLYCAM force field parameters for modeling glycosaminoglycans

Glycosaminoglycans (GAGs) are an important class of carbohydrates that serve critical roles in blood clotting, tissue repair, cell migration and adhesion, and lubrication. The variable sulfation pattern and iduronate ring conformations in GAGs influence their polymeric structure and nature of interaction. This study characterizes several heparin-like GAG disaccharides and tetrasaccharides using NMR and molecular dynamics simulations to assist in the development of parameters for GAGs within the GLYCAM06 force field. The force field additions include parameters and charges for a transferable sulfate group for O- and N-sulfation, neutral (COOH) forms of iduronic and glucuronic acid, and Δ4,5-unsaturated uronate (ΔUA) residues. ΔUA residues frequently arise from the enzymatic digestion of heparin and heparin sulfate. Simulations of disaccharides containing ΔUA reveal that the presence of sulfation on this residue alters the relative populations of 1H2 and 2H1 ring conformations. Simulations of heparin tetrasaccharides containing N-sulfation in place of N-acetylation on glucosamine residues influence the ring conformations of adjacent iduronate residues.

Uncovering the Relationship between Sulphation Patterns and Conformation of Iduronic Acid in Heparan Sulphate

The L-iduronic acid (IdoA) residue is a critically important structural component in heparan sulphate polysaccharide for the biological functions. The pyranose ring of IdoA is present in (1)C4-chair, (2)SO-skew boat, and less frequently, in (4)C1-chair conformations. Here, we analyzed the conformation of IdoA residue in eight hexasaccharides by NMR. The data demonstrate a correlation between the conformation of IdoA and sulphations in the surrounding saccharide residues. For the 2-O-sulpho IdoA residue, a high degree of sulphation on neighboring residues drives ring dynamics towards the (2)SO-skew boat conformer. In contrast, the nonsulphated IdoA residue is pushed towards the (1)C4-chair conformer when the neighboring residues are highly sulphated. Our data suggest that the conformation of IdoA is regulated by the sulphation pattern of nearby saccharides that is genetically controlled by the heparan sulphate biosynthetic pathway.

Solid-phase assembly of glycosaminoglycan oligosaccharide precursors

A high yielding procedure for the assembly of heparan and dermatan sulfate oligosaccharide precursors on the solid-phase has been developed.

Effect of the substituents of the neighboring ring in the conformational equilibrium of iduronate in heparin-like trisaccharides

Based on the structure of the regular heparin, we have prepared a smart library of heparin-like trisaccharides by incorporating some sulfate groups in the sequence α-D-GlcNS- (1-4)-α-L-Ido2S-(1-4)-α-D-GlcN. According to the 3D structure of heparin, which features one helix turn every four residues, this fragment corresponds to the minimum binding motif. We have performed a complete NMR study and found that the trisaccharides have a similar 3D structure to regular heparin itself, but their spectral properties are such that allow to extract very detailed information about distances and coupling constants as they are isotropic molecules. The characteristic conformational equilibrium of the central iduronate ring has been analyzed combining NMR and molecular dynamics and the populations of the conformers of the central iduronate ring have been calculated. We have found that in those compounds lacking the sulfate group at position 6 of the reducing end glucosamine, the population of (2)S(0) of the central iduronate residue is sensitive to the temperature decreasing to 19% at 278 K. On the contrary, the trisaccharides with 6-O-sulfate in the reducing end glucosamine keep the level of population constant with temperature circa 40% of (2)S(0) similar to that observed at room temperature. Another structural feature that has been revealed through this analysis is the larger flexibility of the L-IdoAS- D-GlcN glycosidic linkage, compared with the D-GlcNS-L-IdoA. We propose that this is the point where the heparin chain is bended to form structures far from the regular helix known as kink that have been proposed to play an important role in the specificity of the heparin-protein interaction.

Structure and physicochemical characterisation of heparin

Heparin is a member of the heparan sulphate family of glycosaminoglycans, a linear polysaccharide with a complex sequence resulting from the action of post-polymerisation enzymes on a regular repeating disaccharide background. Its overall conformation is rod-like in solution as well as in the solid state, but the conformational fluctuations of iduronate residues give rise to considerable internal motion and variation in local three-dimensional structure. Structure/function relationships and their relation to sequence are still the subject of argument, but new methodologies to tackle the subject are emerging. Heparin as a therapeutic agent and as the object of research may be characterised by numerous physico-chemical techniques. These include chromatographic methods for measurement of molecular weight; a variety of spectroscopic techniques; separation methods for whole polysaccharides, as well as for oligo- and monosaccharides; and mass spectrometric methods for mapping and sequence analysis. The impetus provided by the discovery of heparin contamination with oversulphated chondroitin sulphate has been influential in bringing combinations of many old and new techniques into use to ensure that heparin is sufficiently consistent and pure to be used safely. Synthetic and semi-synthetic heparins are in development and may become reality in the relatively near future.

Molecular architecture of heparin and heparan sulfate: Recent developments in solution structural studies

The study of the relationship between the complex structures and numerous physiological functions of the glycosaminoglycans (GAGs) heparin and heparan sulfate (HS) has continued to thrive in the past decade. Though it is clear that the monosaccharide sequences of these polysaccharides must determine their ability to modulate the action of growth factors, morphogens, chemokines, cytokines, and many other extracellular proteins, the exact details of this dependence still prove elusive. Sequence determines the 3D structure of GAGs at more than one level; detailed sequences of highly sulfated regions may influence affinity for specific proteins in some cases, but in addition attention has been called to the importance of the length and spacing of these highly sulfated sequences, which are separated by unsulfated domains. Within the sulfated “S-domains”, the internal dynamics of the conformationally flexible iduronate pyranose ring have continued to interest NMR spectroscopists and molecular modelers. New studies of the relative degrees of flexibility of sulfated and unsulfated domains lead to an overall model of heparin/HS in which protein-binding, highly sulfated S-domains with well-defined conformations are separated by more flexible NA-domains.

Conformational degeneracy restricts the effective information content of heparan sulfate

The linear, sulfated polysaccharide heparan sulfate occupies a pivotal position in intercellular signalling events, interacting with numerous proteins on the cell surface and in the extracellular matrix. Its complex sequences suggest high potential information content but, despite extensive efforts, a clear relationship between its substitution pattern and biological activity remains elusive. This results from technical limitations, compounded by attempts to correlate substitution pattern directly with activity without considering other conformational factors. For a series of systematically modified analogues of heparan sulfate, the relationship between substitution pattern and experimental (13)C NMR chemical shifts, which act as reporters of the presence of conformational change, particularly around the glycosidic linkages, was explored through chemometric analysis. From analysis of the experimental data it was evident that wide linkage variation arose from O-sulfation in iduronate and N-sulfation in glucosamine residues but, their effects were distinct, while 6-O-sulfation had much less impact. Models of saccharide sequences showed that the maximum spread of variation in glycosidic linkages occurred before maximum sequence diversity and revealed a highly degenerate system: a fraction of possible sequences is sufficient to provide diverse backbone conformations to satisfy particular protein binding requirements. The unique information content potentially available in HS sequences, defined ultimately by conformation, is vastly inferior to the potential sequence diversity.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2003-2004

This review is the third update of the original review, published in 1999, on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings the topic to the end of 2004. Both fundamental studies and applications are covered. The main topics include methodological developments, matrices, fragmentation of carbohydrates and applications to large polymeric carbohydrates from plants, glycans from glycoproteins and those from various glycolipids. Other topics include the use of MALDI MS to study enzymes related to carbohydrate biosynthesis and degradation, its use in industrial processes, particularly biopharmaceuticals and its use to monitor products of chemical synthesis where glycodendrimers and carbohydrate-protein complexes are highlighted.

A model ternary heparin conjugate by direct covalent bond strategy applied to drug delivery system

A model ternary heparin conjugate by direct covalent bond strategy has been developed, in which modified heparin using active mix anhydride as intermediate conjugates with model drug molecule and model specific ligand, respectively. Designed ester bonds between model drug and heparin facilitate hydrolysis kinetics research. The strategy can be extended to design and synthesize a targeted drug delivery system. The key point is to use mixed anhydride groups as activating intermediates to mediate the synthesis of the ternary heparin conjugate. Formation of mixed anhydride is detected by the conductimetry experiment. The ternary heparin conjugate is characterized by (13)C NMR, FT-IR and GPC, respectively. The decreased trend on degree of substitution (DS) is consistent with that of introduced anticancer drug and specific ligand in drug delivery system. Moreover, their anticoagulant activity is evaluated by measuring activated partial thromboplastin time (APTT) and anti-factor Xa activity. The results show that model ternary heparin conjugate with reduced anticoagulant activity may avoid the risk of severe hemorrhagic complication during the administration and is potential to develop a safe and effective drug delivery system on anticancer research.

Chemical Approaches to Define the Structure-Activity Relationship of Heparin-like Glycosaminoglycans

Heparin, the drug of choice for the prevention and treatment of thromboembolic disorders, has been shown to interact with many proteins. Despite its widespread medical use, little is known about the precise sequences that interact with specific proteins. The minimum heparin binding sequence for FGF1 and FGF2 necessary to promote signaling was investigated. A characteristic pentasaccharide sequence, DEFGH, is required to accelerate the inhibition of thrombin and factor Xa in the blood-coagulation cascade. The first synthetic heparin pentasaccharide drug has been approved in Europe and the US and is sold under the trade name Arixtra. Other oligosaccharides with different composition are under clinical investigation. The enormous interest in the assembly of heparin oligosaccharides will stimulate the development of new synthetic approaches. Heparin-oligosaccharide-synthesis automation similar to that of DNA or peptide synthesis will play an important role.

Dynamic properties of biologically active synthetic heparin-like hexasaccharides

A complete study of the dynamics of two synthetic heparin-like hexasaccharides, D-GlcNHSO3-6-SO4-alpha-(1-->4)-L-IdoA-2-SO4-alpha-(1-->4)-D-GlcNHSO3-6-SO4-alpha-(1-->4)-L-IdoA-2-SO4-alpha-(1-->4)-D-GlcNHSO3-6-SO4-alpha-(1-->4)-L-IdoA-2-SO4-alpha-1-->iPr (1) and -->4)-L-IdoA-2-SO4-alpha-(1-->4)-D-GlcNHAc-6-SO4-alpha-(1-->4)-L-IdoA-alpha-(1-->4)-D-GlcNHSO3-alpha-(1-->4)-L-IdoA-2-SO4-alpha-1-->iPr (2), has been performed using 13C-nuclear magnetic resonance (NMR) relaxation parameters, T1, T2, and heteronuclear nuclear Overhauser effect (NOEs). Compound 1 is constituted from sequences corresponding to the major polysaccharide heparin region, while compound 2 contains a sequence never found in natural heparin. They differ from each other only in sulphation patterns, and are capable of stimulating fibroblast growth factors (FGFs)-1 induced mitogenesis. Both oligosaccharides exhibit a remarkable anisotropic overall motion in solution as revealed by their anisotropic ratios (tau /tau||), 4.0 and 3.0 respectively. This is a characteristic behaviour of natural glycosaminoglycans (GAG) which has also been observed for the antithrombin (AT) binding pentasaccharide D-GlcNHSO3-6-SO4-alpha-(1-->4)-D-GlcA-beta-(1-->4)-D-GlcNHSO3-(3,6-SO4)-alpha-(1-->4)-L-IdoA-2-SO4-alpha-(1-->4)-D-GlcNHSO3-6-SO4-alpha-1-->Me (3) (Hricovíni, M., Guerrini, M., Torri, G., Piani, S., and Ungarelli, F. (1995) Conformational analysis of heparin epoxide in aqueous solution. An NMR relaxation study. Carbohydr. Res., 277, 11-23). The motional properties observed for 1 and 2 provide additional support to the suitability of these compounds as heparin models in agreement with previous structural (de Paz, J.L., Angulo, J., Lassaletta, J.M., Nieto, P.M., Redondo-Horcajo, M., Lozano, R.M., Jiménez-Gallego, G., and Martín-Lomas, M. (2001) The activation of fibroblast growth factors by heparin: synthesis, structure and biological activity of heparin-like oligosaccharides. Chembiochem, 2, 673-685; Ojeda, R., Angulo, J., Nieto, P.M., and Martin-Lomas. M. (2002) The activation of fibroblast growth factors by heparin: synthesis and structural study of rationally modified heparin-like oligosaccharides. Can. J. Chem,. 80, 917-936; Lucas, R., Angulo, J., Nieto, P.M., and Martin-Lomas, M. (2003) Synthesis and structural studies of two new heparin-like hexasaccharides. Org. Biomol. Chem., 1, 2253-2266) and biological data (Angulo, J., Ojeda, R., de Paz, J.L., Lucas, R., Nieto, P.M., Lozano, R.M., Redondo-Horcajo, M., Giménez-Gallego, G., and Martín-Lomas, M. (2004) The activation of fibroblast growth factors (FGFs) by glycosaminoglycans: influence of the sulphation pattern on the biological activity of FGF-1. Chembiochem, 5, 55-61). Fast internal motions observed for the less sulphated compound 2, as compared with 1, may be related to their different behavior in stimulating FGF1-induced mitogenic activity.

Synthesis of heparin-like oligosaccharides on polymer supports

The biological functions of a variety of proteins are regulated by heparan sulfate glycosaminoglycans. In order to facilitate the elucidation of the molecular basis of glycosaminoglycan-protein interactions we have developed syntheses of heparin-like oligosaccharides on polymer supports. A completely stereoselective strategy previously developed by us for the synthesis of these oligosaccharides in solution has been extended to the solid phase using an acceptor-bound approach. Both a soluble polymer support and a polyethylene glycol-grafted polystyrene resin have been used and different strategies for the attachment of the acceptor to the support have been explored. The attachment of fully protected disaccharide building blocks to a soluble support through the carboxylic group of the uronic acid unit by a succinic ester linkage, the use of trichloroacetimidates as glycosylating agents and of a functionalized Merryfield type resin for the capping process allowed for the construction of hexasaccharide and octasaccharide fragments containing the structural motif of the regular region of heparin. This strategy may facilitate the synthesis of glycosaminoglycan oligosaccharides by using the required building blocks in the glycosylation sequence.

The activation of fibroblast growth factors (FGFs) by glycosaminoglycans: influence of the sulfation pattern on the biological activity of FGF-1

Six synthetic heparin-like oligosaccharides have been used to investigate the effect of the oligosaccharide sulfation pattern on the stimulation of acidic fibroblast growth factor (FGF-1) induced mitogenesis signaling and the biological significance of FGF-1 trans dimerization in the FGF-1 activation process. It has been found that some molecules with a sulfation pattern that does not contain the internal trisaccharide motif, which has been proposed for high affinity for FGF-1, stimulate FGF-1 more efficiently than those with the structure of the regular region of heparin. In contrast to regular region oligosaccharides, in which the sulfate groups are distributed on both sides of their helical three-dimensional structures, the molecules containing this particular sulfation pattern display the sulfate groups only on one side of the helix. These results and the fact that these oligosaccharides do not promote FGF-1 dimerization according to sedimentation-equilibrium analysis, confirm the importance of negative-charge distribution in the activation process and strongly suggest that FGF dimerization is not a general and absolute requirement for biological activity.

The heparin–Ca2+ interaction: the influence of the O-sulfation pattern on binding

The specific binding of Ca(2+) to synthetic hexasaccharide models of modified heparin has been investigated by NMR and molecular modeling and compared with previous results on a model of regular heparin. These two models represent the regular region of heparin lacking one type of O-sulfate group, either at C-6 of glucosamine or at C-2 of iduronate. The NMR experiments show different responses to the presence of Ca(2+). In the case of the compound lacking O-sulfate groups at C-2, the results are indicative of specific binding similar to that observed for the regular heparin, while the model lacking sulfate groups in position 6 interacts more weakly with Ca(2+). In order to understand the basis of this difference, a molecular modeling study based on a rigid body docking approach of the interaction of these carbohydrates with Ca(2+) and Na(+) was performed. We have found that the results are strongly dependent on the starting orientation of the lateral side chains of the charged groups of the carbohydrate, and that the best agreement with the experimental results is obtained when the starting conformations are taken from previous simulations in the presence of Ca(2+).

Synthesis of heparin-like oligosaccharides on a soluble polymer support

Based on previously developed solution phase chemistry, an effective general approach to the synthesis of heparin-like oligosaccharides on a soluble polymer support is reported.