Preparation and characterization of (15)N-enriched, size-defined heparan sulfate precursor oligosaccharides

Enzymatic construction of a library of even- and odd-numbered heparosan oligosaccharides and their N-sulfonated derivatives

Low-molecular-weight heparins (LMWHs), especially the specific-sized heparin oligosaccharides, are attractive for the therapeutic applications, while their synthesis remains challenging. In the present study, unsaturated even-numbered heparosan oligosaccharides were firstly prepared by cleaving high-molecular-weight heparosan using recombinant heparinase III (HepIII). The conversion rates of the unsaturated disaccharides, tetrasaccharides, hexasaccharides, octasaccharides, and decasaccharides were 33.9 %, 47.9 %, 78.7 %, 71.8 %, and 53.4 %, respectively. After processing the aforementioned heparosan oligosaccharides with the Δ4,5 unsaturated glycuronidase, saturated odd-numbered heparosan trisaccharides, pentasaccharides, heptasaccharides, and nonasaccharides were produced. It was observed that among them, the pentasaccharides were the smallest units of saturated odd-numbered oligosaccharides recognized by HepIII. These oligosaccharides were further catalyzed with bifunctional heparan sulfate N-deacetylase/N-sulfotransferase (NDST) under optimized reaction conditions. It was found that the tetrasaccharide was defined as the smallest recognition unit for NDST, obtaining the N-sulfonated heparosan tetrasaccharides, pentasaccharides, and hexasaccharides with a single sulfonate group, as well as N-sulfonated heparosan heptasaccharides, octasaccharides, and nonasaccharides with multiple sulfonate groups. These results provide an easy pathway for constructing a library of specific-sized N-sulfonated heparosan oligosaccharides that can be used as the substrates for the enzymatic synthesis of LMWHs and heparin oligosaccharides, shedding new light on the substrate preference of NDST.

Metabolic labeling of hyaluronan: Biosynthesis and quantitative analysis of 13C,15N-enriched hyaluronan by NMR and MS-based methods

Hyaluronan (HA), a member of the GAG family of glycans, has many diverse biological functions that vary a lot depending on the length of the HA chain and its concentration. A better understanding of the structure of different-sized HA at the atomic level is therefore crucial to decipher these biological functions. NMR is a method of choice for conformational studies of biomolecules, but there are limitations due to the low natural abundance of the NMR active nuclei ¹³C and ¹⁵N. We describe here the metabolic labeling of HA using the bacterium Streptococcus equi subsp. Zooepidemicus and the subsequent analysis by NMR and mass spectrometry. The level of ¹³C and ¹⁵N isotope enrichment at each position was determined quantitatively by NMR spectroscopy and was further confirmed by high-resolution mass spectrometry analysis. This study provides a valid methodological approach that can be applied to the quantitative assessment of isotopically labeled glycans and will help improve detection capabilities and facilitate future structure-function relationship analysis of complex glycans.

Preparation of Isotope-Enriched Heparan Sulfate Precursors for Structural Biology Studies

Heparan sulfate (HS) plays numerous important roles in biological systems through their interactions with a wide array of proteins. Structural biology studies of heparan sulfate are often challenging due to the heterogeneity and complexity of the HS molecules. Radioisotope metabolic labeling of HS in cellular systems has enabled the elucidation of HS structures as well as the interactions between HS and proteins. However, radiolabeled structures are not amenable for advanced structural glycobiology studies using sophisticated instruments such as nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). The utilization of stable isotope-enriched HS precursors is an appealing approach to overcome these challenges. The application of stable isotope-enriched HS precursors has facilitated the HS structural analysis by NMR spectroscopy and mass spectrometry. Herein we describe two simple methods to prepare isotopically enriched HS precursors and HS.

Preparation of isotope-enriched heparan sulfate precursors for structural biology studies

Heparan sulfate (HS) plays numerous important roles in biological systems through their interactions with a wide array of proteins. Structural biology studies of heparan sulfate are often challenging due to the heterogeneity and complexity of the HS molecules. Radioisotope metabolic labeling of HS in cellular systems has enabled the elucidation of HS structures as well as the interactions between HS and proteins. However, radiolabeled structures are not amenable for advanced structural glycobiology studies using sophisticated instruments such as nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). The utilization of stable isotope-enriched HS precursors is an appealing approach to overcome these challenges. The application of stable isotope-enriched HS precursors has facilitated the HS structural analysis by NMR spectroscopy and mass spectrometry. Herein we describe a simple method to prepare isotopically enriched HS precursors.

A closer look at the nitrogen next door: 1H-15N NMR methods for glycosaminoglycan structural characterization

Recently, experimental conditions were presented for the detection of the N-sulfoglucosamine (GlcNS) NHSO(3)(-) or sulfamate (1)H and (15)N NMR resonances of the pharmaceutically and biologically important glycosaminoglycan (GAG) heparin in aqueous solution. In the present work, we explore further the applicability of nitrogen-bound proton detection to provide structural information for GAGs. Compared to the detection of (15)N chemical shifts of aminosugars through long-range couplings using the IMPACT-HNMBC pulse sequence, the more sensitive two-dimensional (1)H-(15)N HSQC-TOCSY experiments provided additional structural data. The IMPACT-HNMBC experiment remains a powerful tool as demonstrated by the spectrum measured for the unsubstituted amine of 3-O-sulfoglucosamine (GlcN(3S)), which cannot be observed with the (1)H-(15)N HSQC-TOCSY experiment due to the fast exchange of the amino group protons with solvent. The (1)H-(15)N HSQC-TOCSY NMR spectrum reported for the mixture of model compounds GlcNS and N-acetylglucosamine (GlcNAc) demonstrate the broad utility of this approach. Measurements for the synthetic pentasaccharide drug Arixtra® (Fondaparinux sodium) in aqueous solution illustrate the power of this NMR pulse sequence for structural characterization of highly similar N-sulfoglucosamine residues in GAG-derived oligosaccharides.

The dominating role of N-deacetylase/N-sulfotransferase 1 in forming domain structures in heparan sulfate

Heparan sulfate (HS) is a highly sulfated polysaccharide participated in essential physiological functions from regulating cell growth to blood coagulation. HS contains sulfated domains known as N-S domains and low sulfate domains known as N-Ac domains. The distribution of the domain structures is likely governed by the action of glucosaminyl N-deacetylase/N-sulfotransferase (NDST). Here, we sought to determine the substrate specificity of NDST using model substrates and recombinant NDST protein. We discovered that NDST-1 carries out the modification in a highly ordered fashion. The enzyme sulfates the substrate from the nonreducing end toward the reducing end consecutively, leading to the product with a cluster of N-sulfo glucosamine residues. Furthermore, a preexisting N-sulfo glucosamine residue prevents the action of NDST-1 at the residues immediately located at the nonreducing end, allowing the formation of an N-Ac domain. Our results provide the long sought evidence for understanding the formation of sulfated versus nonsulfated domains in the HS isolated from cells and tissues. The study demonstrates the regulating role of NDST-1 in mapping the sulfation patterns of HS.

Hydrogen/deuterium exchange-LC-MS approach to characterize the action of heparan sulfate C5-epimerase

Heparan sulfate (HS) proteoglycans regulate a number of biological functions in many systems. Most of the functions of HS are attributed to its unique structure, consisting of sulfated and non-sulfated domains, arising from the differential presence of iduronyl and glucuronyl residues along the polysaccharide chain. A single glucuronyl C5-epimerase enzyme acts on HS precursors, converts glucuronyl residues into iduronyl residues, and modulates subsequent biosynthetic steps in vivo. Previously, the ratios of non-sulfated epimers within the polysaccharide chain have been calculated by resolving radiolabeled GlcA-(A)Man(R) and IdoA-(A)Man(R) disaccharides using a tedious paper chromatography technique. This radioactive assay, based on measuring either the release or incorporation of (3)H at C5 carbon of uronyl residues of (3)H-labeled HS precursor substrate, has been in use over three decades to characterize the action of HS C5-epimerase. We have developed a non-radioactive assay to estimate the epimerase activity through resolving GlcA-(A)Man(R) and IdoA-(A)Man(R) disaccharides on high-performance liquid chromatography in conjunction with hydrogen/deuterium exchange upon epimerization protocol-liquid chromatography mass spectrometry (DEEP-LC-MS). Utilizing this new, non-radioactive-based assay, DEEP-LC-MS, we were able to determine the extent of both forward and reverse reactions on the same substrate catalyzed by C5-epimerase. The results from this study also provide insights into the action of C5-epimerase and provide an opportunity to delineate snapshots of biosynthetic events that occur during the HSPG assembly in the Golgi.

Getting to know the nitrogen next door: HNMBC measurements of amino sugars

Long-range ¹H-¹⁵N correlations detected by the heteronuclear multiple-bond correlation (HMBC) experiment are explored for the characterization of amino sugars. The gradient-enhanced HMBC, IMPACT-HMBC, and a modified pulse sequence with the ¹J-filters removed, IMPACT-HNMBC, are compared for sensitivity and resolution. ¹⁵N chemical shifts and long-range proton correlations are reported using the IMPACT-HNMBC experiment for N-acetyl-glucosamine, N-acetyl-galactosamine, and for a series of glucosamine analogs with an N-sulfo substitution, unmodified amino group, and 6-O-sulfonation. As is common with sugars, for all the compounds examined both anomeric forms are present in solution. For each compound studied, the ¹⁵N chemical shifts of the α anomer are downfield of the β form. For the N-acetylated sugars, the β anomer has a unique long-range ¹⁵N correlation to the anomeric proton not observed for the α anomer. Though N-sulfonation results in a significant change in the ¹⁵N chemical shift of the glucosamine analogs, 6-O sulfo substitution has no significant effect on the local environment of the amino nitrogen. For N-acetylated sugars in D₂O solution, peaks in the ¹⁵N projection of the HMBC spectrum appear as triplets as a result of J-modulation due to ²H-¹⁵N coupling.

Applications of isotopes in advancing structural and functional heparanomics

Heparanomics is the study of all the biologically active oligosaccharide domain structures in the entire heparanome and the nature of the interactions among these domains and their protein ligands. Structural elucidation of heparan sulfate and heparin oligosaccharides is a major obstacle in advancing structure-function relationships and heparanomics. There are several factors that exacerbate the challenges involved in the structural elucidation of heparin and heparan sulfate; therefore, there is great interest in developing novel strategies and analytical tools to overcome the barriers in decoding the enigmatic heparanome. This review focuses on the applications of isotopes, both radioisotopes and stable isotopes, in the structural elucidation of the complex heparanome at the disaccharide or oligosaccharide level using liquid chromatography, nuclear magnetic resonance spectroscopy, and mass spectrometry. This review also outlines the utility of isotopes in determining the substrate specificity of biosynthetic enzymes that eventually dictate the emergence of biologically active oligosaccharides.

Characterization of uniformly and atom-specifically (13)C-labeled heparin and heparan sulfate polysaccharide precursors using (13)C NMR spectroscopy and ESI mass spectrometry

The biological actions of heparin and heparan sulfate, two structurally related glycosaminoglycans, depend on the organization of the complex heparanome. Due to the structural complexity of the heparanome, the sequence of variably sulfonated uronic acid and glucosamine residues is usually characterized by the analysis of smaller oligosaccharide and disaccharide fragments. Even characterization of smaller heparin and heparan sulfate oligosaccharide or disaccharide fragments using simple 1D (1)H NMR spectroscopy is often complicated by the extensive signal overlap. (13)C NMR signals, on the other hand, overlap less and therefore, (13)C NMR spectroscopy can greatly facilitate the structural elucidation of the complex heparanome and provide finer insights into the structural basis for biological functions. This is the first report of the preparation of anomeric carbon-specific (13)C-labeled heparin and heparan sulfate precursors from the Escherichia coli K5 strain. Uniformly (13)C- and (15)N-labeled precursors were also produced and characterized by (13)C NMR spectroscopy. Mass spectrometric analysis of enzymatically fragmented disaccharides revealed that anomeric carbon-specific labeling efforts resulted in a minor loss/scrambling of (13)C in the precursor backbone, whereas uniform labeling efforts resulted in greater than 95% (13)C isotope enrichment in the precursor backbone. These labeled precursors provided high-resolution NMR signals with great sensitivity and set the stage for studying the heparanome-proteome interactions.

Chemoenzymatic design of heparan sulfate oligosaccharides

Heparan sulfate is a sulfated glycan that exhibits essential physiological functions. Interrogation of the specificity of heparan sulfate-mediated activities demands a library of structurally defined oligosaccharides. Chemical synthesis of large heparan sulfate oligosaccharides remains challenging. We report the synthesis of oligosaccharides with different sulfation patterns and sizes from a disaccharide building block using glycosyltransferases, heparan sulfate C(5)-epimerase, and sulfotransferases. This method offers a generic approach to prepare heparan sulfate oligosaccharides possessing predictable structures.

Analysis of E. coli K5 capsular polysaccharide heparosan

Heparosan is the key precursor for the preparation of bioengineered heparin, a potential replacement for porcine intestinal heparin, an important anticoagulant drug. The molecular weight (MW) distribution of heparosan produced by the fermentation of E. coli K5 was investigated. Large-slab isocratic and mini-slab gradient polyacrylamide gel electrophoresis (PAGE) were used to analyze the MW and polydispersity of heparosan. A preparative method that allowed fractionation by continuous-elution PAGE was used to obtain heparosan MW standards. The MWs of the heparosan standards were determined by electrospray ionization Fourier-transform mass spectrometry (ESI-FT-MS). A ladder of the standards was then used to determine the MW properties of polydisperse heparosan samples. Unbleached and bleached heparosan produced by fermentation of E. coli K5 had similar number-averaged MWs (M(N)), weight-averaged MWs (M(W)), and MW ranges of 3,000 to 150,000 Da.