The synthesis of heterosaccharides related to the fucoidan from Chordaria flagelliformis bearing an α-L-fucofuranosyl unit

Automated Synthesis of Algal Fucoidan Oligosaccharides

Fucoidan, a sulfated polysaccharide found in algae, plays a central role in marine carbon sequestration and exhibits a wide array of bioactivities. However, the molecular diversity and structural complexity of fucoidan hinder precise structure-function studies. To address this, we present an automated method for generating well-defined linear and branched α-fucan oligosaccharides. Our syntheses include oligosaccharides with up to 20 cis-glycosidic linkages, diverse branching patterns, and 11 sulfate monoesters. In this study, we demonstrate the utility of these oligosaccharides by (i) characterizing two endo-acting fucoidan glycoside hydrolases (GH107), (ii) utilizing them as standards for NMR studies to confirm suggested structures of algal fucoidans, and (iii) developing a fucoidan microarray. This microarray enabled the screening of the molecular specificity of four monoclonal antibodies (mAb) targeting fucoidan. It was found that mAb BAM4 has cross-reactivity to β-glucans, while mAb BAM2 has reactivity to fucoidans with 4-O-sulfate esters. Knowledge of the mAb BAM2 epitope specificity provided evidence that a globally abundant marine diatom, Thalassiosira weissflogii, synthesizes a fucoidan with structural homology to those found in brown algae. Automated glycan assembly provides access to fucoidan oligosaccharides. These oligosaccharides provide the basis for molecular level investigations into fucoidan's roles in medicine and carbon sequestration.

Synthesis of the Reducing-end Hexasaccharide Fragment of Marine Lipopolysaccharide Axinelloside A

Chemical synthesis of an orthogonally protected hexasaccharide relevant to the reducing-end half of axinelloside A, a highly sulfated marine lipopolysaccharide, is disclosed. The synthesis features preparation of the scyllo-inositol unit via a Ferrier-type-II rearrangement, construction of the 1,2-cis-glycosidic bonds via remote participation, and concise [2+2+2] assembly via Au(I)-catalyzed glycosylation.

Recent progress in the synthesis of glycosphingolipids

To accelerate the biological study and application of the diverse functions of glycosphingolipids (GSLs), the production of structurally defined GSLs has been greatly demanded. In this review, we focus on the recent developments in the chemical and chemoenzymatic synthesis of GSLs. In the chemical synthesis section, the syntheses based on glucosyl ceramide cassette, late-stage sialylation, and diversity-oriented strategies for GSLs or ganglioside synthesis are highlighted, which delivered terpioside B, fluorescent sialyl lactotetraosyl ceramide, and analogs of lacto-ganglio-series GSLs, respectively. In the chemoenzymatic synthesis section, the synthesis of ganglioside GM1 by multistep one-pot multienzyme method and the total synthesis of highly complex ganglioside LLG-5 using a water-soluble lactosyl ceramide as a key substrate for enzymatic sialylation are described.

Stereocontrolled Synthesis and Conformational Analysis of a Series of Disaccharides α,β-d-GlcA-(1→3)-α-L-Fuc

D-Glucuronic acid is a fundamental building block of many biologically important polysaccharides, either in its non-substituted form or bearing a variety of substituents, among them sulfates. We have previously performed a study of the effects of exhaustive sulfation on the conformational behavior of β-gluronopyranosides. Herein, we report an investigation comparing α- and β-derivatives of this monosaccharide within the title disaccharides using NMR and quantum chemistry approaches. It was found that for α-linked disaccharides, the introduction of sulfates did not greatly affect their conformational behavior. However, for β-derivatives, considerable conformational changes were observed. In general, they resemble those that took place for the monosaccharides, except that NOESY experiments and calculations of intra-ring spin-spin coupling constants suggest the presence of a 1S5 conformer along with 3S1 in the fully sulfated disaccharide. During the synthesis of model compounds, hydrogen bond-mediated aglycone delivery was used as an α-directing stereocontrol approach in the glucuronidation reaction.

Advances in glycoside and oligosaccharide synthesis

The structural complexity of glycans poses a serious challenge in the chemical synthesis of glycosides, oligosaccharides and glycoconjugates. Glycan complexity, determined by composition, connectivity, and configuration far exceeds what nature achieves with nucleic acids and proteins. Consequently, glycoside synthesis ranks among the most complex tasks in organic synthesis, despite involving only a simple type of bond-forming reaction. Here, we introduce the fundamental principles of glycoside bond formation and summarize recent advances in glycoside bond formation and oligosaccharide synthesis.

Fucoidans of Brown Algae: Comparison of Sulfated Polysaccharides from Fucus vesiculosus and Ascophyllum nodosum

Preparations of sulfated polysaccharides obtained from brown algae are known as fucoidans. These biopolymers have attracted considerable attention due to many biological activities which may find practical applications. Two Atlantic representatives of Phaeophyceae, namely, Fucus vesiculosus and Ascophyllum nodosum, belonging to the same order Fucales, are popular sources of commercial fucoidans, which often regarded as very similar in chemical composition and biological actions. Nevertheless, these two fucoidan preparations are polysaccharide mixtures which differ considerably in amount and chemical nature of components, and hence, this circumstance should be taken into account in the investigation of their biological properties and structure-activity relationships. In spite of these differences, fractions with carefully characterized structures prepared from both fucoidans may have valuable applications in drug development.

Perspectives for the Use of Fucoidans in Clinical Oncology

Fucoidans are natural sulfated polysaccharides that have a wide range of biological functions and are regarded as promising antitumor agents. The activity of various fucoidans and their derivatives has been demonstrated in vitro on tumor cells of different histogenesis and in experiments on mice with grafted tumors. However, these experimental models showed low levels of antitumor activity and clinical trials did not prove that this class of compounds could serve as antitumor drugs. Nevertheless, the anti-inflammatory, antiangiogenic, immunostimulating, and anticoagulant properties of fucoidans, as well as their ability to stimulate hematopoiesis during cytostatic-based antitumor therapy, suggest that effective fucoidan-based drugs could be designed for the supportive care and symptomatic therapy of cancer patients. The use of fucoidans in cancer patients after chemotherapy and radiation therapy might promote the rapid improvement of hematopoiesis, while their anti-inflammatory, immunomodulatory, and anticoagulant effects have the potential to improve the quality of life of patients with advanced cancer.

Novel and Promising Strategies for Therapy of Post-Transplant Chronic GVHD

Despite the achievements that have increased viability after the transplantation of allogeneic hematopoietic stem cells (aHSCT), chronic graft-versus-host disease (cGVHD) remains the main cause of late complications and post-transplant deaths. At the moment, therapy alternatives demonstrate limited effectiveness in steroid-refractory illness; in addition, we have no reliable data on the mechanism of this condition. The lack of drugs of choice for the treatment of GVHD underscores the significance of the design of new therapies. Improved understanding of the mechanism of chronic GVHD has secured new therapy goals, and organized diagnostic recommendations and the development of medical tests have ensured a general language and routes for studies in this field. These factors, combined with the rapid development of pharmacology, have helped speed up the search of medicines and medical studies regarding chronic GVHD. At present, we can hope for success in curing this formidable complication. This review summarizes the latest clinical developments in new treatments for chronic GVHD.

Comparative study on the mechanisms of anti-lung cancer activities of three sulfated galactofucans

Sulfated galactofucans, as the active compositions of fucoidan, were reported to exhibit antitumor activity. In the current study, a sulfated galactofucan (SGF) from Sargassum thunbergii and its three derivatives (SGF-H, SGF-L, and SGF-S) were prepared for structural analysis. Structural analysis showed that SGF-H was a high molecular weight sulfated galactofucan (51.5/17.8 kDa) with a high molar ratio of galactose (Gal) to fucose (Fuc) (0.66 : 1), SGF-L was a low molecular weight sulfated galactofucan (17.7 kDa) with a low molar ratio of Gal to Fuc (0.20 : 1), and SGF-S was a mixture (1.7 kDa) of sulfated galacto-fuco-oligomers or fuco-oligomers. It was noteworthy that the linkage of Gal residues in SGF-H was a β-linkage while SGF-L was an α-linkage. A comparative study on the anti-lung cancer activity in vitro and in vivo, antimetastatic effects, the metastasis-associated protein expression, and binding abilities to fibroblast growth factors (FGFs) of SGF, SGF-H, and SGF-L was performed to understand the structure-activity relationship. To some extent, SGF-L showed the strongest activity in the inhibition of human lung cancer cells A549 cell proliferation, while SGF-H exhibited the strongest activity in the inhibition of human bronchial epithelial cells BEAS-2B cell proliferation. SGF-L showed the strongest antimetastatic activity, followed by SGF-H and SGF. The expression of metastasis-associated proteins showed only a small difference. The in vivo tumor inhibition of SGF, SGF-H, and SGF-L was 45%, 41%, and 31%, respectively. SPR analysis showed SGF-H binds preferentially to FGF1 and FGF2, while SGF-L preferentially binds to FGF7 and FGF10, suggesting that the anti-lung cancer activity from sulfated galactofucan could involve the FGF-FAK/mTOR pathway.

The structure-activity relationship of the interactions of SARS-CoV-2 spike glycoproteins with glucuronomannan and sulfated galactofucan from Saccharina japonica

The SARS-CoV-2 spike glycoproteins (SGPs) and human angiotensin converting enzyme 2 (ACE2) are the two key targets for the prevention and treatment of COVID-19. Host cell surface heparan sulfate (HS) is believed to interact with SARS-CoV-2 SGPs to facilitate host cell entry. In the current study, a series of polysaccharides from Saccharina japonica were prepared to investigate the structure-activity relationship on the binding abilities of polysaccharides (oligosaccharides) to pseudotype particles, including SARS-CoV-2 SGPs, and ACE2 using surface plasmon resonance. Sulfated galactofucan (SJ-D-S-H) and glucuronomannan (Gn) displayed strongly inhibited interaction between SARS-CoV-2 SGPs and heparin while showing negligible inhibition of the interaction between SARS-CoV-2 SGPs and ACE2. The IC50 values of SJ-D-S-H and Gn in blocking heparin SGP binding were 27 and 231 nM, respectively. NMR analysis showed that the structure of SJ-D-S-H featured with a backbone of 1, 3-linked α-L-Fucp residues sulfated at C4 and C2/C4 and 1, 3-linked α-L-Fucp residues sulfated at C4 and branched with 1, 6-linked β-D-galacto-biose; Gn had a backbone of alternating 1, 4-linked β-D-GlcAp residues and 1, 2-linked α-D-Manp residues. The sulfated galactofucan and glucuronomannan showed strong binding ability to SARS-CoV-2 SGPs, suggesting that these polysaccharides might be good candidates for preventing and/or treating SARS-CoV-2.

Interactions of fibroblast growth factors with sulfated galactofucan from Saccharina japonica

A total 68 types of marine algae oligosaccharides and polysaccharides were prepared and used to study the structure-activity relationship of oligosaccharides and polysaccharides in their interactions with fibroblast growth factors (FGF) 1 and 2. Factors considered include different types of algae, extraction methods, molecular weight, sulfate content and fractions. In the case of low molecular weight polysaccharide (SJ-D) from Saccharina japonica and its fractions eluting from anion exchange column, both 1.0 M NaCl fraction (SJ-D-I) and 2.0 M NaCl fraction (SJ-D-S) had stronger binding affinity than the parent SJ-D, suggesting that sulfated galactofucans represented the major tight binding component. Nuclear magnetic resonance showed that SJ-D-I was a typical sulfated galactofucan, composed of four units: 1, 3-linked 4-sulfated α-L-fucose (Fuc); 1, 3-linked 2, 4-disulfated α-L-Fuc; 1, 6-linked 4-sulfated β-D-Gal and/or 1, 6-linked 3, 4-sulfated β-D-Gal. Modification by autohydrolysis to oligosaccharides and desulfation decreased the FGF binding affinity while oversulfation increased the affinity. The solution-based affinities of SJ-D-I to FGF1 and FGF2 were 69 nM and 3.9 nM, suggesting that SJ-D-I showed better preferentially binding to FGF1 than a natural ligand, heparin, suggesting that sulfated galactofucan might represent a good regulator of FGF1.

Synthesis of an unusual hexasaccharide repeating unit from the cell wall polysaccharide of Eubacterium saburreum strain T19

Eubacterium saburreum is one of the human oral pathogens and has been proved to play a significant role in the development of periodontal diseases.

Influence of per-O-sulfation upon the conformational behaviour of common furanosides

The studies on the recently discovered pyranoside-into-furanoside rearrangement have led us to conformational investigations of furanosides upon their total sulfation. Experimental NMR data showed that in some cases drastic changes of the ring conformation occurred while sometimes only the conformation of the exocyclic C4-C5 linkage changed. Herein we describe a combined quantum chemical and NMR conformational investigation of three common monosaccharide furanosides as their propyl glycosides: α-mannose, β-glucose and β-galactose. Full exploration of the furanoside ring by means of ab initio calculations was performed and coupling constants were calculated for each of the low-energy conformers. The results demonstrated preferred trans-orientation of H4-H5 protons in the non-sulfated molecules which changed to gauche-orientation upon sulfation. The effect is less pronounced in the galactosides. For all the studied structures changes in the conformational distribution were revealed by quantum mechanical calculations, that explained the observed changes in intraring coupling constants occurring upon introduction of sulfates.

Driving Force of the Pyranoside-into-Furanoside Rearrangement

Ab initio calculations of fully O-sulfated model monosaccharides, including common hexoses (glucose, galactose, fucose, and mannose) and pentoses (arabinose and xylose), were performed to study the energetic properties of the recently discovered pyranoside-into-furanoside (PIF) rearrangement. It was shown that the per-O-sulfated derivatives of furanoside isomers generally had lower energies than the corresponding per-O-sulfated pyranosides, while nonsulfated furanosides were always less favored than nonsulfated pyranosides. Mannose, which is known to be unreactive in PIF rearrangement, was the only exception. The results of the theoretical calculations were confirmed by experimental studies of monosaccharide models and explained the driving force of such unusual ring contraction process as PIF rearrangement. The conclusions of performed investigation can be used for prediction of new substrates applicability for PIF rearrangement.

Enzymatic Synthesis of GDP-α-l-fucofuranose by MtdL and Hyg20

Two mutases, MtdL and Hyg20, are reported. Both are able to functionally drive the biosynthesis of GDP-α-l-fucofuranose. Both enzymes catalyze similar functions, catalytically enabling the bidirectional reaction between GDP-β-l-fucopyranose and GDP-α-l-fucofuranose using only divalent cations as cofactors. This realization is but one of a number of important insights into fucofuranose biosynthesis presented herein.

Recent advances in the synthesis of fungal antigenic oligosaccharides

The driving force for the constant improvement and development of new synthetic methodologies in carbohydrate chemistry is a growing demand for biologically important oligosaccharide ligands and neoglycoconjugates thereof for numerous biochemical investigations such as cell-to-pathogen interactions, immune response, cell adhesion, etc. Here we report our syntheses of the spacer-armed antigenic oligosaccharides related to three groups of the polysaccharides of the fungal cell-wall including α- and β-mannan, α- and β-glucan and galactomannan chains, which include new rationally designed synthetic blocks, efficient solutions for the stereoselective construction of glycoside bonds, and novel strategy for preparation of furanoside-containing oligosaccharides based on recently discovered pyranoside-into-furanoside (PIF) rearrangement.

The Use of Pyranoside-into-Furanoside Rearrangement and Controlled O(5) → O(6) Benzoyl Migration as the Basis of a Synthetic Strategy To Assemble (1→5)- and (1→6)-Linked Galactofuranosyl Chains

A new pyranoside-into-furanoside (PIF) rearrangement of selectively protected galactopyranosides, followed by controlled O(5) → O(6) benzoate migration, gives either 5-OH or 6-OH products. It has been applied for the synthesis of four oligosaccharides related to the galactomannan from Aspergillus fumigatus. The assembly of target oligosaccharides containing both (1→5) and (1→6) linkages between galactofuranosyl residues was performed by applying terminal mannoside and digalactofuranoside blocks, forming a versatile approach toward fungal and bacterial carbohydrate antigens containing both 5-O- and 6-O-substituted galactofuranoside residues.

Design of α-Selective Glycopyranosyl Donors Relying on Remote Anchimeric Assistance

Oligosaccharides have a variety of versatile biological effects, but unlike peptides and oligonucleotides, investigation of the roles of oligosaccharides is not easy. Since biosynthesis of oligosaccharides does not comply with direct genetic control, their isolation from natural sources and biotechnological preparation are complicated, due to the heterogeneous composition of raw carbohydrates. Oligosaccharide synthesis is needed for the establishment or confirmation of the structure of natural glycocompounds. Also, synthetically prepared, defined oligosaccharides and their derivatives are becoming increasingly important tools for many biological and immunological research projects. The key step of oligosaccharide synthesis involves glycosylation, a reaction that builds glycosidic bonds. Usually, construction of 1,2-trans-bonds is easy, and therefore, this reaction can even be included into automated syntheses. At this time, installation of the 1,2-cis-bond remains simultaneously frustrating and compelling. In our and other laboratories, a strategy of α-selective (1,2-cis) glycosylation, relying on remote anchimeric assistance with acyl groups, is studied. The state of the art in this work is summarized in this review.