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

Biosynthetic production of anticoagulant heparin polysaccharides through metabolic and sulfotransferases engineering strategies

Heparin is an important anticoagulant drug, and microbial heparin biosynthesis is a potential alternative to animal-derived heparin production. However, effectively using heparin synthesis enzymes faces challenges, especially with microbial recombinant expression of active heparan sulfate N-deacetylase/N-sulfotransferase. Here, we introduce the monosaccharide N-trifluoroacetylglucosamine into Escherichia coli K5 to facilitate sulfation modification. The Protein Repair One-Stop Service-Focused Rational Iterative Site-specific Mutagenesis (PROSS-FRISM) platform is used to enhance sulfotransferase efficiency, resulting in the engineered NST-M8 enzyme with significantly improved stability (11.32-fold) and activity (2.53-fold) compared to the wild-type N-sulfotransferase. This approach can be applied to engineering various sulfotransferases. The multienzyme cascade reaction enables the production of active heparin from bioengineered heparosan, demonstrating anti-FXa (246.09 IU/mg) and anti-FIIa (48.62 IU/mg) activities. This study offers insights into overcoming challenges in heparin synthesis and modification, paving the way for the future development of animal-free heparins using a cellular system-based semisynthetic strategy.

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.

Structural and mechanistic characterization of bifunctional heparan sulfate N-deacetylase-N-sulfotransferase 1

Heparan sulfate (HS) polysaccharides are major constituents of the extracellular matrix, which are involved in myriad structural and signaling processes. Mature HS polysaccharides contain complex, non-templated patterns of sulfation and epimerization, which mediate interactions with diverse protein partners. Complex HS modifications form around initial clusters of glucosamine-N-sulfate (GlcNS) on nascent polysaccharide chains, but the mechanistic basis underpinning incorporation of GlcNS itself into HS remains unclear. Here, we determine cryo-electron microscopy structures of human N-deacetylase-N-sulfotransferase (NDST)1, the bifunctional enzyme primarily responsible for initial GlcNS modification of HS. Our structures reveal the architecture of both NDST1 deacetylase and sulfotransferase catalytic domains, alongside a non-catalytic N-terminal domain. The two catalytic domains of NDST1 adopt a distinct back-to-back topology that limits direct cooperativity. Binding analyses, aided by activity-modulating nanobodies, suggest that anchoring of the substrate at the sulfotransferase domain initiates the NDST1 catalytic cycle, providing a plausible mechanism for cooperativity despite spatial domain separation. Our data shed light on key determinants of NDST1 activity, and describe tools to probe NDST1 function in vitro and in vivo.

Novel β1,4 N-acetylglucosaminyltransferase in de novo enzymatic synthesis of hyaluronic acid oligosaccharides

The efficiency of de novo synthesis of hyaluronic acid (HA) using Pasteurella multocida hyaluronate synthase (PmHAS) is limited by its low catalytic activity during the initial reaction steps when monosaccharides are the acceptor substrates. In this study, we identified and characterized a β-1,4-N-acetylglucosaminyl-transferase (EcGnT) derived from the O-antigen gene synthesis cluster of Escherichia coli O8:K48:H9. Recombinant β1,4 EcGnT effectively catalyzed the production of HA disaccharides when the glucuronic acid monosaccharide derivative 4-nitrophenyl-β-D-glucuronide (GlcA-pNP) was used as the acceptor. Compared with PmHAS, β1,4 EcGnT exhibited superior N-acetylglucosamine transfer activity (~ 12-fold) with GlcA-pNP as the acceptor, making it a better option for the initial step of de novo HA oligosaccharide synthesis. We then developed a biocatalytic approach for size-controlled HA oligosaccharide synthesis using the disaccharide produced by β1,4 EcGnT as a starting material, followed by stepwise PmHAS-catalyzed synthesis of longer oligosaccharides. Using this approach, we produced a series of HA chains of up to 10 sugar monomers. Overall, our study identifies a novel bacterial β1,4 N-acetylglucosaminyltransferase and establishes a more efficient process for HA oligosaccharide synthesis that enables size-controlled production of HA oligosaccharides. KEY POINTS: • A novel β-1,4-N-acetylglucosaminyl-transferase (EcGnT) from E. coli O8:K48:H9. • EcGnT is superior to PmHAS for enabling de novo HA oligosaccharide synthesis. • Size-controlled HA oligosaccharide synthesis relay using EcGnT and PmHAS.

Microbial synthesis of glycosaminoglycans and their oligosaccharides

Compared with chemical synthesis and tissue extraction methods, microbial synthesis of glycosaminoglycans (GAGs) is attractive because of the advantages of eco-friendly processes, production safety, and sustainable development. However, boosting the efficiency of microbial cell factories, precisely regulating GAG molecular weights, and rationally controlling the sulfation degree of GAGs remain challenging. To address these issues, various strategies, including genetic, enzymatic, metabolic, and fermentation engineering, have been developed. In this review, we summarize the recent progress in the construction of efficient GAG-producing microbial cell factories, regulation of the molecular weight of GAGs, and modification of GAG chains. Moreover, future studies, remaining challenges, and potential solutions in this field are discussed.

Chemical, Molecular, and Single-nucleus Analysis Reveal Chondroitin Sulfate Proteoglycan Aberrancy in Fibrolamellar Carcinoma

Fibrolamellar carcinoma (FLC) is an aggressive liver cancer with no effective therapeutic options. The extracellular environment of FLC tumors is poorly characterized and may contribute to cancer growth and/or metastasis. To bridge this knowledge gap, we assessed pathways relevant to proteoglycans, a major component of the extracellular matrix. We first analyzed gene expression data from FLC and nonmalignant liver tissue (n = 27) to identify changes in glycosaminoglycan (GAG) biosynthesis pathways and found that genes associated with production of chondroitin sulfate, but not other GAGs, are significantly increased by 8-fold. We then implemented a novel LC/MS-MS based method to quantify the abundance of different types of GAGs in patient tumors (n = 16) and found that chondroitin sulfate is significantly more abundant in FLC tumors by 6-fold. Upon further analysis of GAG-associated proteins, we found that versican (VCAN) expression is significantly upregulated at the mRNA and protein levels, the latter of which was validated by IHC. Finally, we performed single-cell assay for transposase-accessible chromatin sequencing on FLC tumors (n = 3), which revealed for the first time the different cell types in FLC tumors and also showed that VCAN is likely produced not only from FLC tumor epithelial cells but also activated stellate cells. Our results reveal a pathologic aberrancy in chondroitin (but not heparan) sulfate proteoglycans in FLC and highlight a potential role for activated stellate cells.

Significance

This study leverages a multi-disciplinary approach, including state-of-the-art chemical analyses and cutting-edge single-cell genomic technologies, to identify for the first time a marked chondroitin sulfate aberrancy in FLC that could open novel therapeutic avenues in the future.

Expression of glycosaminoglycans in cirrhotic liver and hepatocellular carcinoma-a pilot study including etiology

Chronic liver diseases have both high incidence and mortality rates; therefore, a deeper understanding of the underlying molecular mechanisms is essential. We have determined the content and sulfation pattern of chondroitin sulfate (CS) and heparan sulfate (HS) in human hepatocellular carcinoma and cirrhotic liver tissues, considering the etiology of the diseases. A variety of pathological conditions such as alcoholic liver disease, hepatitis B and C virus infections, and primary sclerosing cholangitis were studied. Major differences were observed in the total abundance and sulfation pattern of CS and HS chains. For example, the 6-O-sulfation of CS is fundamentally different regarding etiologies of cirrhosis, and a 2-threefold increase in HS N-sulfation/O-sulfation ratio was observed in hepatocellular carcinoma compared to cirrhotic tissues.

Preparation and characterization of 2-deacetyl-3-O-sulfo-heparosan and its antitumor effects via the fibroblast growth factor receptor pathway

Heparosan, with a linear chain of disaccharide repeating units of → 4) β-D-glucuronic acid (GlcA) (1 → 4)-α-D-N-acetylglucosamine (GlcNAc) (1→, is a potential starting chemical for heparin synthesis. However, the chemoenzymatic synthesis of single-site sulfated heparosan and its antitumor activity have not been studied. In this study, 2-deacetyl-3-O-sulfo-heparosan (DSH) was prepared successively by the N-deacetylation chemical reaction and enzymatic modification of human 3-O-sulfotransferase-1 (3-OST-1). Structural characterization of DSH was shown the success of the sulfation with the sulfation degree of 0.87. High performance gel permeation chromatography (HPGPC) analysis revealed that DSH had only one symmetrical sharp peak with a molecular weight of 9.6334 × 10⁴ Da. Biological function studies showed that DSH could inhibit tumor cell (A549, HepG2 and HCT116) viability and induce the apoptosis of A549 cells. Further in vitro mechanistic studies showed that DSH may induce apoptosis via the JNK signaling pathway, and the upstream signal of this process may be fibroblast growth factor receptors. These results indicated that DSH could be developed as one of a potential chemical for tumor treatment.

Heparosan oligosaccharide synthesis using engineered single-function glycosyltransferases

A 24-fold increase in GlcNAc-transferase ability through KfiA screening and engineering. An approach for heparosan oligosaccharide synthesis relying on single-function glycosyltransferases.

Alterations in the Expression of the Genes Responsible for the Synthesis of Heparan Sulfate in Brains With Alzheimer Disease

The saccharide chains of heparan sulfate appear to be involved in several aspects Alzheimer disease (AD) pathogenesis. Their structural complexity is due to the expression of different isoenzymes. We studied the differential transcription of heparan sulfate chain biosynthesis in AD brains, analyzing different brain regions in patients with different extents of AD pathology. The transcriptomic study was performed by RT-PCR using samples of amygdala, anterior hippocampus, posterior hippocampus, claustrum, calcarine fissure, globus pallidus and cerebellum from patients with mild, moderate, or severe AD, as well as healthy individuals. Certain heparan sulfate epitopes were also detected by immunohistochemistry. Several genes, across all stages of heparan sulfate synthesis, showed altered transcription in different brain regions of AD patients. The numbers of alterations were greater in in moderate versus mild AD patients. In severe patients, there were fewer alterations in genes related to early stages of biosynthesis, and overexpression of genes involved in late stages. The alterations correlated with progressive brain atrophy, although alterations were more common in the cerebellum. Detection of some heparan sulfate epitopes by immunohistochemistry was consistent with previous studies. In conclusion, transcriptional alterations in the biosynthetic genes of heparan sulfate depend on the brain region and the degree of AD pathology.

Synthesis and Screening of α-Xylosides in Human Glioblastoma Cells

Glycosaminoglycans (GAGs) such as heparan sulfate and chondroitin sulfate decorate all mammalian cell surfaces. These mucopolysaccharides act as coreceptors for extracellular ligands, regulating cell signaling, growth, proliferation, and adhesion. In glioblastoma, the most common type of primary malignant brain tumor, dysregulated GAG biosynthesis results in altered chain length, sulfation patterns, and the ratio of contributing monosaccharides. These events contribute to the loss of normal cellular function, initiating and sustaining malignant growth. Disruption of the aberrant cell surface GAGs with small molecule inhibitors of GAG biosynthetic enzymes is a potential therapeutic approach to blocking the rogue signaling and proliferation in glioma, including glioblastoma. Previously, 4-azido-xylose-α-UDP sugar inhibited both xylosyltransferase (XYLT-1) and β-1,4-galactosyltransferase-7 (β-GALT-7)-the first and second enzymes of GAG biosynthesis-when microinjected into a cell. In another study, 4-deoxy-4-fluoro-β-xylosides inhibited β-GALT-7 at 1 mM concentration in vitro. In this work, we seek to solve the enduring problem of drug delivery to human glioma cells at low concentrations. We developed a library of hydrophobic, presumed prodrugs 4-deoxy-4-fluoro-2,3-dibenzoyl-(α- or β-) xylosides and their corresponding hydrophilic inhibitors of XYLT-1 and β-GALT-7 enzymes. The prodrugs were designed to be activatable by carboxylesterase enzymes overexpressed in glioblastoma. Using a colorimetric MTT assay in human glioblastoma cell lines, we identified a prodrug-drug pair (4-nitrophenyl-α-xylosides) as lead drug candidates. The candidates arrest U251 cell growth at an IC50 = 380 nM (prodrug), 122 μM (drug), and U87 cells at IC50 = 10.57 μM (prodrug). Molecular docking studies were consistent with preferred binding of the α- versus β-nitro xyloside conformer to XYLT-1 and β-GALT-7 enzymes.

Heparan Sulfate Proteoglycans: Key Mediators of Stem Cell Function

Heparan sulfate proteoglycans (HSPGs) are an evolutionarily ancient subclass of glycoproteins with exquisite structural complexity. They are ubiquitously expressed across tissues and have been found to exert a multitude of effects on cell behavior and the surrounding microenvironment. Evidence has shown that heterogeneity in HSPG composition is crucial to its functions as an essential scaffolding component in the extracellular matrix as well as a vital cell surface signaling co-receptor. Here, we provide an overview of the significance of HSPGs as essential regulators of stem cell function. We discuss the various roles of HSPGs in distinct stem cell types during key physiological events, from development through to tissue homeostasis and regeneration. The contribution of aberrant HSPG production to altered stem cell properties and dysregulated cellular homeostasis characteristic of cancer is also reviewed. Finally, we consider approaches to better understand and exploit the multifaceted functions of HSPGs in influencing stem cell characteristics for cell therapy and associated culture expansion strategies.

Heparan Sulfate Proteoglycans Biosynthesis and Post Synthesis Mechanisms Combine Few Enzymes and Few Core Proteins to Generate Extensive Structural and Functional Diversity

Glycosylation is a common and widespread post-translational modification that affects a large majority of proteins. Of these, a small minority, about 20, are specifically modified by the addition of heparan sulfate, a linear polysaccharide from the glycosaminoglycan family. The resulting molecules, heparan sulfate proteoglycans, nevertheless play a fundamental role in most biological functions by interacting with a myriad of proteins. This large functional repertoire stems from the ubiquitous presence of these molecules within the tissue and a tremendous structural variety of the heparan sulfate chains, generated through both biosynthesis and post synthesis mechanisms. The present review focusses on how proteoglycans are “gagosylated” and acquire structural complexity through the concerted action of Golgi-localized biosynthesis enzymes and extracellular modifying enzymes. It examines, in particular, the possibility that these enzymes form complexes of different modes of organization, leading to the synthesis of various oligosaccharide sequences.

Multi-target approaches to CNS repair: olfactory mucosa-derived cells and heparan sulfates

Spinal cord injury (SCI) remains one of the biggest challenges in the development of neuroregenerative therapeutics. Cell transplantation is one of numerous experimental strategies that have been identified and tested for efficacy at both preclinical and clinical levels in recent years. In this Review, we briefly discuss the state of human olfactory cell transplantation as a therapy, considering both its current clinical status and its limitations. Furthermore, we introduce a mesenchymal stromal cell derived from human olfactory tissue, which has the potential to induce multifaceted reparative effects in the environment within and surrounding the lesion. We argue that no single therapy will be sufficient to treat SCI effectively and that a combination of cell-based, rehabilitation and pharmaceutical interventions is the most promising approach to aid repair. For this reason, we also introduce a novel pharmaceutical strategy based on modifying the activity of heparan sulfate, an important regulator of a wide range of biological cell functions. The multi-target approach that is exemplified by these types of strategies will probably be necessary to optimize SCI treatment.

The second member of the bacterial UDP-N-acetyl-d-glucosamine:heparosan alpha-1, 4-N-acetyl-d-glucosaminyltransferase superfamily: GaKfiA from Gallibacterium anatis

Bacterial UDP-N-acetyl-d-glucosamine:heparosan alpha-1, 4-N-acetyl-d-glucosaminyltransferases (KfiAs) are in high demand for the development of animal-free heparin (HP) production. Until now, EcKfiA from Escherichia coli O10:K5:H4 was the sole identified member of this family. The lack of known members has limited research into molecular structure and catalytic mechanism of the KfiA superfamily, and restricted its application in enzymatic glycan synthesis. Herein, we report the identification and characterization of Gallibacterium anatis GaKfiA, doubling the number of known members of the KfiA family. GaKfiA is a monofunctional enzyme that transfers N-acetyl-d-glucosamine (GlcNAc) residues from their nucleotide forms to the nonreducing ends of saccharide chains structurally equivalent to the backbone of HP. The catalytic efficiency of GaKfiA is lower than that of EcKfiA. However, a single mutation of GaKfiA, N56D, resulted in a drastic increase in k_cat/K_m compared with wild-type GaKfiA. These data once again indicate the key role of a complete DXD motif for the catalytic efficiency of glycosyltransferases. This study deepens understanding of the mechanism of KfiA, and will assist in research into animal-free HP production.

Structural basis of oligosaccharide processing by glycosaminoglycan sulfotransferases

Heparan sulfate (HS) is a sulfated polysaccharide that plays a key role in morphogenesis, physiology and pathogenesis. The biosynthesis of HS takes place in the Golgi apparatus by a group of enzymes that polymerize, epimerize and sulfate the sugar chain. This biosynthetic process introduces varying degrees of sulfate substitution, which are tightly regulated and directly dictate binding specificity to different cytokines, morphogens and growth factors. Here, we report the use of molecular dynamics simulations to investigate the dynamics of substrate recognition of two glycosaminoglycan (GAG) sulfotransferases, N-deacetylase-N-sulfotransferase and 2-O-sulfotransferase to the HS chain during the biosynthetic process. We performed multiple simulations of the binding of the sulfotransferase domains to both the HS oligosaccharide substrate and sulfate donor, 3'-phosphoadenosine-5'-phosphosulfate. Analysis of extended simulations provide detailed and useful insights into the atomic interactions that are at work during oligosaccharide processing. The fast information matching method was used to detect the enzyme global dynamics and to predict the pairwise contact of residues responsible for GAG-enzyme binding and unbinding. The correlation between HS displacement and the location of the modified GAG chain were calculated, indicating a possible route for HS and heparin during sulfotransferase processing. Our data also show sulfotransferases contain a conserved interspaced positively charged amino acid residues that form a patch which controls the protein-GAG binding equilibrium. Together, our findings provide further understanding on the fine-tuned complex mechanism of GAG biosynthesis. Our findings can also be extrapolated to other systems for calculating rates of protein-GAG binding.

Design and synthesis of structurally defined heparan sulfate (HS)-FK506 conjugates as an exogenous approach to investigate biological functions of nucleus HS

Although heparan sulfate (HS) is widely implicated in numerous physiological and pathological processes, the biological function of nucleus HS remains underexplored, largely due to its complex structure and high hydrophilic property. To supplement these efforts, ideal vehicles are drawing attention as they combine attractive features including lipid solubility for penetrating cell membrane, high affinity binding to its target receptor, metabolic stability, and no cellular actions resulting in toxicity. Herein, we develop a convenient and promising strategy to prepare HS-FK506 conjugates for membrane transport and entry into nucleus, where click chemistry takes easily place between the exocyclic allyl group of a clinic drug FK506 and thiol as a handle incorporated into HS analogues. HS derivatives for constructing the conjugates were synthesized using a cutting-edge chemoenzymatic method. Meantime, [³⁵S] labeled 3'-phosphoadenosine 5'-phosphosulfate (PAP³⁵S) and [¹⁴C] glucuronic acid (Glc A) were adopted to label HS-FK506 conjugates, respectively, to evaluate their efficiency of nucleus entry, as a result, ¹⁴C Glc A was sensitive, effective and reliable whereas PAP³⁵S gave rise to a mixture of labeled compounds, hampering the understanding of structure-function relationship of nucleus HS. Compared with the corresponding HS, the amount of HS-FK506 conjugates to translocate into nucleus from radioactive assay experiments sharply increased, e.g. tridecasaccharide-FK506 1d increased by approximate 10 folds, offering a simple and robust platform for enabling hydrophilic compounds including carbohydrates to translocate into nucleus and shedding light on their biological functions.

Epigenetic Regulation of the Biosynthesis & Enzymatic Modification of Heparan Sulfate Proteoglycans: Implications for Tumorigenesis and Cancer Biomarkers

Emerging evidence suggests that the enzymes in the biosynthetic pathway for the synthesis of heparan sulfate moieties of heparan sulfate proteoglycans (HSPGs) are epigenetically regulated at many levels. As the exact composition of the heparan sulfate portion of the resulting HSPG molecules is critical to the broad spectrum of biological processes involved in oncogenesis, the epigenetic regulation of heparan sulfate biosynthesis has far-reaching effects on many cellular activities related to cancer progression. Given the current focus on developing new anti-cancer therapeutics focused on epigenetic targets, it is important to understand the effects that these emerging therapeutics may have on the synthesis of HSPGs as alterations in HSPG composition may have profound and unanticipated effects. As an introduction, this review will briefly summarize the variety of important roles which HSPGs play in a wide-spectrum of cancer-related cellular and physiological functions and then describe the biosynthesis of the heparan sulfate chains of HSPGs, including how alterations observed in cancer cells serve as potential biomarkers. This review will then focus on detailing the multiple levels of epigenetic regulation of the enzymes in the heparan sulfate synthesis pathway with a particular focus on regulation by miRNA and effects of epigenetic therapies on HSPGs. We will also explore the use of lectins to detect differences in heparan sulfate composition and preview their potential diagnostic and prognostic use in the clinic.

Pharmacology of Heparin and Related Drugs

Heparin has been recognized as a valuable anticoagulant and antithrombotic for several decades and is still widely used in clinical practice for a variety of indications. The anticoagulant activity of heparin is mainly attributable to the action of a specific pentasaccharide sequence that acts in concert with antithrombin, a plasma coagulation factor inhibitor. This observation has led to the development of synthetic heparin mimetics for clinical use. However, it is increasingly recognized that heparin has many other pharmacological properties, including but not limited to antiviral, anti-inflammatory, and antimetastatic actions. Many of these activities are independent of its anticoagulant activity, although the mechanisms of these other activities are currently less well defined. Nonetheless, heparin is being exploited for clinical uses beyond anticoagulation and developed for a wide range of clinical disorders. This article provides a "state of the art" review of our current understanding of the pharmacology of heparin and related drugs and an overview of the status of development of such drugs.

NDST2 (N-Deacetylase/N-Sulfotransferase-2) Enzyme Regulates Heparan Sulfate Chain Length

Analysis of heparan sulfate synthesized by HEK 293 cells overexpressing murine NDST1 and/or NDST2 demonstrated that the amount of heparan sulfate was increased in NDST2- but not in NDST1-overexpressing cells. Altered transcript expression of genes encoding other biosynthetic enzymes or proteoglycan core proteins could not account for the observed changes. However, the role of NDST2 in regulating the amount of heparan sulfate synthesized was confirmed by analyzing heparan sulfate content in tissues isolated from Ndst2(-/-) mice, which contained reduced levels of the polysaccharide. Detailed disaccharide composition analysis showed no major structural difference between heparan sulfate from control and Ndst2(-/-) tissues, with the exception of heparan sulfate from spleen where the relative amount of trisulfated disaccharides was lowered in the absence of NDST2. In vivo transcript expression levels of the heparan sulfate-polymerizing enzymes Ext1 and Ext2 were also largely unaffected by NDST2 levels, pointing to a mode of regulation other than increased gene transcription. Size estimation of heparan sulfate polysaccharide chains indicated that increased chain lengths in NDST2-overexpressing cells alone could explain the increased heparan sulfate content. A model is discussed where NDST2-specific substrate modification stimulates elongation resulting in increased heparan sulfate chain length.

The design and synthesis of new synthetic low-molecular-weight heparins

Low-molecular-weight heparin (LMWH) has remained the most favorable form of heparin in clinics since the 1990s owing to its predictable pharmacokinetic properties. However, LMWH is mainly eliminated through the kidney, which limits its use in renal-impaired patients. In addition, the anticoagulant activity of LMWH is only partially neutralized by protamine. LMWH is obtained from a full-length, highly sulfated polysaccharide harvested from porcine mucosal tissue. The depolymerization involved in LMWH production generates a broad distribution of LMWH fragments (6-22 sugar residues). This, combined with the various methods used to produce commercial LMWHs, results in variable pharmacological and pharmacokinetic properties. An alternative chemoenzymatic approach offers a method for the synthesis of LMWH that has the potential to overcome the limitations of current LMWHs. This review summarizes the application of a chemoenzymatic approach to generate LMWH and the rationale for development of a synthetic LMWH.

Heparan Sulfate: Biosynthesis, Structure, and Function

Heparan sulfate (HS) proteoglycans (PGs) are ubiquitously expressed on cell surfaces and in the extracellular matrix of most animal tissues, having essential functions in development and homeostasis, as well as playing various roles in disease processes. The functions of HSPGs are mainly dependent on interactions between the HS-side chains with a variety of proteins including cytokines, growth factors, and their receptors. In a given HS polysaccharide, negatively charged sulfate and carboxylate groups are arranged in various types of domains, generated through strictly regulated biosynthetic reactions and with enormous potential for structural variability. The mode of HS-protein interactions is assessed through binding experiments using saccharides of defined composition in vitro, signaling assays in cell models where HS structures are manipulated, and targeted disruption of genes for biosynthetic enzymes in animals (mouse, zebrafish, Drosophila, and Caenorhabditis elegans) followed by phenotype analysis. Whereas some protein ligands appear to require strictly defined HS structure, others bind to variable saccharide domains without apparent dependence on distinct saccharide sequence. These findings raise intriguing questions concerning the functional significance of regulation in HS biosynthesis and the potential for development of therapeutics targeting HS-protein interactions.

"Coding" and "Decoding": hypothesis for the regulatory mechanism involved in heparan sulfate biosynthesis

Heparan sulfate (HS) is widely distributed in mammalian tissues in the form of HS proteoglycans, which play essential roles in various physiological and pathological processes. In contrast to the template-guided processes involved in the synthesis of DNA and proteins, HS biosynthesis is not believed to involve a template. However, it appears that the final structure of HS chains was strictly regulated. Herein, we report research based hypothesis that two major steps, namely "coding" and "decoding" steps, are involved in the biosynthesis of HS, which strictly regulate its chemical structure and biological activity. The "coding" process in this context is based on the distribution of sulfate moieties on the amino groups of the glucosamine residues in the HS chains. The sulfation of these amine groups is catalyzed by N-deacetylase/N-sulfotransferase, which has four isozymes. The composition and distribution of sulfate groups and iduronic acid residues on the glycan chains of HS are determined by several other modification enzymes, which can recognize these coding sequences (i.e., the "decoding" process). The degree and pattern of the sulfation and epimerization in the HS chains determines the extent of their interactions with several different protein factors, which further influences their biological activity.

High-throughput assays of leloir-glycosyltransferase reactions: The applications of rYND1 in glycotechnology

Glycosyltransferases (GTs) play a critical role in the enzymatic and chemoenzymatic synthesis of oligosaccharides and glycoconjugates. However, the development of these synthetic approaches has been limited by a lack of sensitive screening methods for the isolation of novel natural GTs or their active variants. Herein, we describe the results of our investigation towards the soluble expression and potential application of the Saccharomyces cerevisiae apyrase YND1. By replacing the hydrophobic transmembrane domain of YND1 with three glycine-serine repeats, this protein was successfully expressed in a soluble form in Escherichia coli. This new protein was then used to develop a two-step nucleoside diphosphate (NDP)-based Leloir-GT high-throughput assay. Purified rYND1 was initially added to a GT reaction to hydrolyze NDP to nucleoside phosphate plus inorganic phosphate, which was determined using a phosphorus molybdenum blue chromogenic reaction. Purified rYND1 was shown to have a positive effect on saccharide synthesis by eliminating the potential by-product inhibition from NDP. Most of the mono-sugar donors used for Leloir-GTs are activated by uridine diphosphate and guanosine diphosphate, which can be catalyzed by rYND1. The rYND1 is amenable to screening methods and could be applied to a wide range of Leloir-GT-catalyzed reactions, therefore representing a remarkable step forward in glycotechnology.

Bioengineered heparins and heparan sulfates

Heparin and heparan sulfates are closely related linear anionic polysaccharides, called glycosaminoglycans, which exhibit a number of important biological and pharmacological activities. These polysaccharides, having complex structures and polydispersity, are biosynthesized in the Golgi of animal cells. While heparan sulfate is a widely distributed membrane and extracellular glycosaminoglycan, heparin is found primarily intracellularly in the granules of mast cells. While heparin has historically received most of the scientific attention for its anticoagulant activity, interest has steadily grown in the multi-faceted role heparan sulfate plays in normal and pathophysiology. The chemical synthesis of these glycosaminoglycans is largely precluded by their structural complexity. Today, we depend on livestock animal tissues for the isolation and the annual commercial production of hundred ton quantities of heparin used in the manufacture of anticoagulant drugs and medical device coatings. The variability of animal-sourced heparin and heparan sulfates, their inherent impurities, the limited availability of source tissues, the poor control of these source materials and their manufacturing processes, suggest a need for new approaches for their production. Over the past decade there have been major efforts in the biotechnological production of these glycosaminoglycans, driven by both therapeutic applications and as probes to study their natural functions. This review focuses on the complex biology of these glycosaminoglycans in human health and disease, and the use of recombinant technology in the chemoenzymatic synthesis and metabolic engineering of heparin and heparan sulfates.

Uncovering the Catalytic Direction of Chondroitin AC Exolyase: FROM THE REDUCING END TOWARDS THE NON-REDUCING END

Glycosaminoglycans (GAGs) are polysaccharides that play vital functional roles in numerous biological processes, and compounds belonging to this class have been implicated in a wide variety of diseases. Chondroitin AC lyase (ChnAC) (EC 4.2.2.5) catalyzes the degradation of various GAGs, including chondroitin sulfate and hyaluronic acid, to give the corresponding disaccharides containing an Δ(4)-unsaturated uronic acid at their non-reducing terminus. ChnAC has been isolated from various bacteria and utilized as an enzymatic tool for study and evaluating the sequencing of GAGs. Despite its substrate specificity and the fact that its crystal structure has been determined to a high resolution, the direction in which ChnAC catalyzes the cleavage of oligosaccharides remain unclear. Herein, we have determined the structural cues of substrate depolymerization and the cleavage direction of ChnAC using model substrates and recombinant ChnAC protein. Several structurally defined oligosaccharides were synthesized using a chemoenzymatic approach and subsequently cleaved using ChnAC. The degradation products resulting from this process were determined by mass spectrometry. The results revealed that ChnAC cleaved the β1,4-glycosidic linkages between glucuronic acid and glucosamine units when these bonds were located on the reducing end of the oligosaccharide. In contrast, the presence of a GlcNAc-α-1,4-GlcA unit at the reducing end of the oligosaccharide prevented ChnAC from cleaving the GalNAc-β1,4-GlcA moiety located in the middle or at the non-reducing end of the chain. These interesting results therefore provide direct proof that ChnAC cleaves oligosaccharide substrates from their reducing end toward their non-reducing end. This conclusion will therefore enhance our collective understanding of the mode of action of ChnAC.

Deciphering the Mode of Action of the Processive Polysaccharide Modifying Enzyme Dermatan Sulfate Epimerase 1 by Hydrogen-Deuterium Exchange Mass Spectrometry

Distinct from template-directed biosynthesis of nucleic acids and proteins, the enzymatic synthesis of heterogeneous polysaccharides is a complex process that is difficult to study using common analytical tools. Therefore, the mode of action and processivity of those enzymes are largely unknown. Dermatan sulfate epimerase 1 (DS-epi1) is the predominant enzyme during the formation of iduronic acid residues in the glycosaminoglycan dermatan sulfate. Using recombinant DS-epi1 as a model enzyme, we describe a tandem mass spectrometry-based method to study the mode of action of polysaccharide processing enzymes. The enzyme action on the substrate was monitored by hydrogen-deuterium exchange mass spectrometry and the sequence information was then fed into mathematical models with two different assumptions of the mode of action for the enzyme: processive reducing end to non-reducing end, and processive non-reducing end to reducing end. Model data was scored by correlation to experimental data and it was found that DS-epi1 attacks its substrate on a random position, followed by a processive mode of modification towards the non-reducing end and that the substrate affinity of the enzyme is negatively affected by each additional epimerization event. It could also be shown that the smallest active substrate was the reducing end uronic acid in a tetrasaccharide and that octasaccharides and longer oligosaccharides were optimal substrates. The method of using tandem mass spectrometry to generate sequence information of the complex enzymatic products in combination with in silico modeling can be potentially applied to study the mode of action of other enzymes involved in polysaccharide biosynthesis.

Heparin and related polysaccharides: synthesis using recombinant enzymes and metabolic engineering

Glycosaminoglycans are linear anionic polysaccharides that exhibit a number of important biological and pharmacological activities. The two most prominent members of this class of polysaccharides are heparin/heparan sulfate and the chondroitin sulfates (including dermatan sulfate). These polysaccharides, having complex structures and polydispersity, are biosynthesized in the Golgi of most animal cells. The chemical synthesis of these glycosaminoglycans is precluded by their structural complexity. Today, we depend on food animal tissues for their isolation and commercial production. Ton quantities of these glycosaminoglycans are used annually as pharmaceuticals and nutraceuticals. The variability of animal-sourced glycosaminoglycans, their inherent impurities, the limited availability of source tissues, the poor control of these source materials, and their manufacturing processes suggest a need for new approaches for their production. Over the past decade, there have been major efforts in the biotechnological production of these glycosaminoglycans. This mini-review focuses on the use of recombinant enzymes and metabolic engineering for the production of heparin and chondroitin sulfates.

Role of Deacetylase Activity of N-Deacetylase/N-Sulfotransferase 1 in Forming N-Sulfated Domain in Heparan Sulfate

Heparan sulfate (HS) is a highly sulfated polysaccharide that plays important physiological roles. The biosynthesis of HS involves a series of enzymes, including glycosyltransferases (or HS polymerase), epimerase, and sulfotransferases. N-Deacetylase/N-Sulfotransferase isoform 1 (NDST-1) is a critical enzyme in this pathway. NDST-1, a bifunctional enzyme, displays N-deacetylase and N-sulfotransferase activities to convert an N-acetylated glucosamine residue to an N-sulfo glucosamine residue. Here, we report the cooperative effects between N-deacetylase and N-sulfotransferase activities. Using baculovirus expression in insect cells, we obtained three recombinant proteins: full-length NDST-1 and the individual N-deacetylase and N-sulfotransferase domains. Structurally defined oligosaccharide substrates were synthesized to test the substrate specificities of the enzymes. We discovered that N-deacetylation is the limiting step and that interplay between the N-sulfotransferase and N-deacetylase accelerates the reaction. Furthermore, combining the individually expressed N-deacetylase and N-sulfotransferase domains produced different sulfation patterns when compared with that made by the NDST-1 enzyme. Our data demonstrate the essential role of domain cooperation within NDST-1 in producing HS with specific domain structures.

C5-epimerase and 2-O-sulfotransferase associate in vitro to generate contiguous epimerized and 2-O-sulfated heparan sulfate domains

Heparan sulfate (HS), a complex polysaccharide of the cell surface, is endowed with the remarkable ability to bind numerous proteins and, as such, regulates a large variety of biological processes. Protein binding depends on HS structure; however, in the absence of a template driving its biosynthesis, the mechanism by which protein binding sequences are assembled remains poorly known. Here, we developed a chemically defined 13C-labeled substrate and NMR based experiments to simultaneously follow in real time the activity of HS biosynthetic enzymes and characterize the reaction products. Using this new approach, we report that the association of C5-epimerase and 2-O-sulfotransferase, which catalyze the production of iduronic acid and its 2-O-sulfation, respectively, is necessary to processively generate extended sequences of contiguous IdoA2S-containing disaccharides, whereas modifications are randomly introduced when the enzymes are uncoupled. These data shed light on the mechanisms by which HS motifs are generated during biosynthesis. They support the view that HS structure assembly is controlled not only by the availability of the biosynthetic enzymes but also by their physical association, which in the case of the C5-epimerase and 2-O-sulfotransferase was characterized by an affinity of 80 nM as demonstrated by surface plasmon resonance experiments.

Chemoenzymatic synthesis of heparan sulfate and heparin

Heparan sulfate is a polysaccharide that plays essential physiological functions in the animal kingdom. Heparin, a highly sulfated form of heparan sulfate, is a widely prescribed anticoagulant drug worldwide. The heparan sulfate and heparin isolated from natural sources are highly heterogeneous mixtures differing in their polysaccharide chain lengths and sulfation patterns. The access to structurally defined heparan sulfate and heparin is critical to probe the contribution of specific sulfated saccharide structures to the biological functions as well as for the development of the next generation of heparin-based anticoagulant drugs. The synthesis of heparan sulfate and heparin, using a purely chemical approach, has proven extremely difficult, especially for targets larger than octasaccharides having a high degree of site-specific sulfation. A new chemoenzymatic method has emerged as an effective alternative approach. This method uses recombinant heparan sulfate biosynthetic enzymes combined with unnatural uridine diphosphate-monosaccharide donors. Recent examples demonstrate the successful synthesis of ultra-low molecular weight heparin, low-molecular weight heparin and bioengineered heparin with unprecedented efficiency. The new method provides an opportunity to develop improved heparin-based therapeutics.

Heparan sulfate differences in rheumatoid arthritis versus healthy sera

Heparan sulfate (HS) is a complex and highly variable polysaccharide, expressed ubiquitously on the cell surface as HS proteoglycans (HSPGs), and found in the extracellular matrix as free HS fragments. Its heterogeneity due to various acetylation and sulfation patterns endows a multitude of functions. In animal tissues, HS interacts with a wide range of proteins to mediate numerous biological activities; given its multiple roles in inflammation processes, characterization of HS in human serum has significant potential for elucidating disease mechanisms. Historically, investigation of HS was limited by its low concentration in human serum, together with the complexity of the serum matrix. In this study, we used a modified mass spectrometry method to examine HS disaccharide profiles in the serum of 50 women with rheumatoid arthritis (RA), and compared our results to 51 sera from healthy women. Using various purification methods and online LC-MS/MS, we discovered statistically significant differences in the sulfation and acetylation patterns between populations. Since early diagnosis of RA is considered important in decelerating the disease's progression, identification of specific biomolecule characterizations may provide crucial information towards developing new therapies for suppressing the disease in its early stages. This is the first report of potential glycosaminoglycan biomarkers for RA found in human sera, while acknowledging the obvious fact that a larger population set, and more stringent collection parameters, will need to be investigated in the future.

PAPST1 regulates sulfation of heparan sulfate proteoglycans in epithelial MDCK II cells

Proteoglycan (PG) sulfation depends on activated nucleotide sulfate, 3'-phosphoadenosine-5'-phosphosulfate (PAPS). Transporters in the Golgi membrane translocate PAPS from the cytoplasm into the organelle lumen where PG sulfation occurs. Silencing of PAPS transporter (PAPST) 1 in epithelial MDCK cells reduced PAPS uptake into Golgi vesicles. Surprisingly, at the same time sulfation of heparan sulfate (HS) was stimulated. The effect was pathway specific in polarized epithelial cells. Basolaterally secreted proteoglycans (PGs) displayed an altered HS sulfation pattern and increased growth factor binding capacity. In contrast, the sulfation pattern of apically secreted PGs was unchanged while the secretion was reduced. Regulation of PAPST1 allows epithelial cells to prioritize between PG sulfation in the apical and basolateral secretory routes at the level of the Golgi apparatus. This provides sulfation patterns that ensure PG functions at the extracellular level, such as growth factor binding.

Role of heparan sulfate proteoglycans in optic disc and stalk morphogenesis

BACKGROUND

Heparan sulfate proteoglycans (HSPG) are important for embryonic development by means of the regulation of gradient formation and signaling of multiple growth factors and morphogens. Previous studies have shown that Bmp/Shh/Fgf signaling are required for the regionalization of the optic vesicle (OV) and for the closure of the optic fissure (OF), the disturbance of which underlie ocular anomalies such as microphthalmia, coloboma, and optic nerve hypoplasia.

RESULTS

To study HSPG-dependent coordination of these signaling pathways during mammalian visual system development, we have generated a series of OV-specific mutations in the heparan sulfate (HS) N-sulfotransferase genes (Ndst1 and Ndst2) and HS O-sulfotransferase genes (Hs2st, Hs6st1, and Hs6st2) in mice. Of interest, the resulting HS undersulfation still allowed for normal retinal neurogenesis and optic fissure closure, but led to defective optic disc and stalk development. The adult mutant animals further developed optic nerve aplasia/hypoplasia and displayed retinal degeneration. We observed that MAPK/ERK signaling was down-regulated in Ndst mutants, and consistent with this, HS-related optic nerve morphogenesis defects in mutant mice could partially be rescued by constitutive Kras activation.

CONCLUSIONS

These results suggest that HSPGs, depending on their HS sulfation pattern, regulate multiple signaling pathways in optic disc and stalk morphogenesis.

miR-191: an emerging player in disease biology

Specific microRNAs have emerged as key players in disease biology by playing crucial role in disease development and progression. This review draws attention to one such microRNA, miR-191 that has been recently reported to be abnormally expressed in several cancers (>20) and various other diseases like diabetes-type 2, Crohn' s, pulmonary hypertension, and Alzheimer' s. It regulates important cellular processes such as cell proliferation, differentiation, apoptosis, and migration by targeting important transcription factors, chromatin remodelers, and cell cycle associated genes. Several studies have demonstrated it to be an excellent biomarker for cancer diagnosis and prognosis leading to two patents already in its kitty. In this first review we summarize the current knowledge of the regulation, functions and targets of miR-191 and discuss its potential as a promising disease biomarker and therapeutic target.

Vascular biomechanical properties in mice with smooth muscle specific deletion of Ndst1

Heparan sulfate proteoglycans act as co-receptors for many chemokines and growth factors. The sulfation pattern of the heparan sulfate chains is a critical regulatory step affecting the binding of chemokines and growth factors. N-deacetylase-N-sulfotransferase1 (Ndst1) is one of the first enzymes to catalyze sulfation. Previously published work has shown that HSPGs alter tangent moduli and stiffness of tissues and cells. We hypothesized that loss of Ndst1 in smooth muscle would lead to significant changes in heparan sulfate modification and the elastic properties of arteries. In line with this hypothesis, the axial tangent modulus was significantly decreased in aorta from mice lacking Ndst1 in smooth muscle (SM22αcre(+)Ndst1(-/-), p < 0.05, n = 5). The decrease in axial tangent modulus was associated with a significant switch in myosin and actin types and isoforms expressed in aorta and isolated aortic vascular smooth muscle cells. In contrast, no changes were found in the compliance of smaller thoracodorsal arteries of SM22αcre(+)Ndst1(-/-) mice. In summary, the major findings of this study were that targeted ablation of Ndst1 in smooth muscle cells results in altered biomechanical properties of aorta and differential expression of myosin and actin types and isoforms.

Podocyte-specific deletion of NDST1, a key enzyme in the sulfation of heparan sulfate glycosaminoglycans, leads to abnormalities in podocyte organization in vivo

Heparan sulfate proteoglycans have been shown to modulate podocyte adhesion to- and pedicel organization on- the glomerular basement membrane. Recent studies showed that foot process effacement developed in a mutant mouse model whose podocytes were unable to assemble heparan sulfate glycosaminoglycan chains. This study, a further refinement, explored the role of heparan N-sulfation on podocyte behavior. A novel mutant mouse (Ndst1^-/-) was developed, having podocyte-specific deletion of NDST1, the enzyme responsible for N-sulfation of heparan sulfate chains. Podocytes having this mutation had foot process effacement and abnormal adhesion to Bowman's capsule. Although glomerular hypertrophy did develop in the kidneys of mutant animals, mesangial expansion was not seen. The lack of heparan N-sulfation did not affect the expression of agrin or perlecan proteoglycan core proteins. Loss of N-sulfation did not result in significant proteinuria, but the increase in the albumin/creatinine ratio was coincident with the development of the enlarged lysosomes in the proximal tubules. Thus, although the renal phenotype of the Ndst1^-/- mouse is mild, the data show that heparan chain N-sulfation plays a key role in podocyte organization.

NDST4 is a novel candidate tumor suppressor gene at chromosome 4q26 and its genetic loss predicts adverse prognosis in colorectal cancer

Background

Genomic deletion at tumor suppressor loci is a common genetic aberration in human cancers. The study aimed to explore candidate tumor suppressor genes at chromosome 4q25-q28.2 and to delineate novel prognostic biomarkers associated with colorectal cancer (CRC).

Methods

Deletion mapping of chromosome 4q25-q28.2 was conducted in 114 sporadic CRC by loss of heterozygosity study with 11 microsatellite markers. A novel candidate tumor suppressor gene, namely NDST4, was identified at 4q26. Gene expression of NDST4 was investigated in 52 pairs of primary CRC tissues by quantitative reverse transcription-polymerase chain reaction. Allelic loss of NDST4 gene was further determined in 174 colorectal carcinomas by loss of heterozygosity analysis, and then was assessed for clinical relevance.

Results

One minimal deletion region was delineated between D4S2297 and D4S2303 loci at 4q26, where NDST4 was the only gene that had markedly been downregulated in CRC tumors. By laser capture microdissection, NDST4 RNA expression was demonstrated in colonic epithelial cells, but was undetectable in tumor cells. In total, 30 (57.7%) of 52 colorectal carcinomas showed a dramatic reduction in NDST4 gene expression compared with matched normal mucosae. The genetic loss of NDST4 was significantly associated with advanced pathological stage (P = 0.039) and poorer overall survival of patients (P = 0.036).

Conclusions

NDST4 gene is a novel candidate tumor suppressor gene in human cancer, and the loss of its function might be involved in CRC progression. In addition, the loss of heterozygosity assay, which was established to determine the allelic loss of NDST4 gene, could be a cost-effective tool for providing a useful biomarker of adverse prognosis in CRC.

Method development and analysis of free HS and HS in proteoglycans from pre- and postmenopausal women: evidence for biosynthetic pathway changes in sulfotransferase and sulfatase enzymes

Heparan sulfate (HS) is one of the most complex and informative biopolymers found on the cell surface or in the extracellular matrix as either free HS fragments or constituents of HS proteoglycans (HSPGs). Analysis of free HS and HSPG sugar chains in human serum at the disaccharide level has great potential for early disease diagnosis and prognosis; however, the low concentration of HS in human serum, together with the complexity of the serum matrix, limits the information on HS. In this study, we present and validate the development of a new sensitive method for in-depth compositional analysis of free HS and HSPG sugar chains. This protocol involved several steps including weak anion exchange chromatography, ultrafiltration, and solid-phase extraction for enhanced detection prior to LC-MS/MS analysis. Using this protocol, a total of 51 serum samples from 26 premenopausal and 25 postmenopausal women were analyzed. Statistically significant differences in heparin/HS disaccharide profiles were observed. The proportion of N-acetylation and N-sulfation in both free HS and HSPG sugar chains were significantly different between pre- and postmenopausal women, indicating changes in N-deacetylase/N-sulfotransferases (NDSTs), the enzymes involved in the initial step of the biosynthetic pathway. Differences in the proportion of 6-O-sulfation suggest that 6-O-sulfotransferase and/or 6-O-sulfatase enzymes may also be implicated.

Use of biosynthetic enzymes in heparin and heparan sulfate synthesis

Heparan sulfate and heparin are highly sulfated polysaccharides consisting of repeating disaccharide units of glucuronic acid or iduronic acid that is linked to glucosamine. Heparan sulfate displays a range of biological functions, and heparin is a widely used anticoagulant drug in hospitals. It has been known to organic chemists that the chemical synthesis of heparan sulfate and heparin oligosaccharides is extremely difficult. Recent advances in the study of the biosynthesis of heparan sulfate/heparin offer a chemoenzymatic approach to synthesize heparan sulfate and heparin. Compared to chemical synthesis, the chemoenzymatic method shortens the synthesis and improves the product yields significantly, providing an excellent opportunity to advance the understanding of the structure and function relationships of heparan sulfate. In this review, we attempt to summarize the progress of the chemoenzymatic synthetic method and its application in heparan sulfate and heparin research.

Heparan sulfate biosynthesis: regulation and variability

Nearly all vertebrate cells have been shown to express heparan sulfate proteoglycans (HSPGs) at the cell surface. The HSPGs bind to many secreted signaling proteins, including numerous growth factors, cytokines, and morphogens, to affect their tissue distribution and signaling. The heparan sulfate (HS) chains may have variable length and may differ with regard to both degree and pattern of sulfation. As the sulfation pattern of HS chains in most cases will determine if an interaction with a potential ligand will take place, as well as the affinity of the interaction, a key to understanding the function of HSPGs is to clarify how HS biosynthesis is regulated in different biological contexts. This review provides an introduction to the current understanding of HS biosynthesis and its regulation, and identifies research areas where more knowledge is needed to better understand how the HS biosynthetic machinery works.

Understanding the substrate specificity of the heparan sulfate sulfotransferases by an integrated biosynthetic and crystallographic approach

Heparan sulfates (HSs) have potential therapeutic value as anti-inflammatory and antimetastasis drugs, in addition to their current use as anticoagulants. Recent advances in chemoenzymatic synthesis of HS provide a way to conveniently produce homogenous HS with different biological properties. Crystal structures of sulfotransferases involved in this process are providing atomic detail of their substrate binding clefts and interactions with their HS substrates. In theory, the flexibility of this method can be increased by modifying the specificities of the sulfotransferases based on the structures, thereby producing a new array of products.

Uncovering biphasic catalytic mode of C5-epimerase in heparan sulfate biosynthesis

Heparan sulfate (HS), a highly sulfated polysaccharide, is biosynthesized through a pathway involving several enzymes. C(5)-epimerase (C(5)-epi) is a key enzyme in this pathway. C(5)-epi is known for being a two-way catalytic enzyme, displaying a "reversible" catalytic mode by converting a glucuronic acid to an iduronic acid residue, and vice versa. Here, we discovered that C(5)-epi can also serve as a one-way catalyst to convert a glucuronic acid to an iduronic acid residue, displaying an "irreversible" catalytic mode. Our data indicated that the reversible or irreversible catalytic mode strictly depends on the saccharide substrate structures. The biphasic mode of C(5)-epi offers a novel mechanism to regulate the biosynthesis of HS with the desired biological functions.