Glycosyl azide—a novel substrate for enzymatic transglycosylations

Advanced high-affinity glycoconjugate ligands of galectins

Galectins are proteins of the family of human lectins. By binding terminal galactose units of cell surface glycans, they moderate biological and pathological processes such as cell signaling, cell adhesion, apoptosis, fibrosis, carcinogenesis, and metabolic disorders. The binding of monovalent glycans to galectins is usually relatively weak. Therefore, the presentation of carbohydrate ligands on multivalent scaffolds can efficiently increase and/or discriminate the affinity of the glycoconjugate to different galectins. A library of glycoclusters and glycodendrimers with various structural presentations of the common functionalized N-acetyllactosamine ligand was prepared to evaluate how the mode of presentation affects the affinity and selectivity to the two most abundant galectins, galectin-1 (Gal-1) and galectin-3 (Gal-3). In addition, the effect of a one- to two-unit carbohydrate spacer on the affinity of the glycoconjugates was determined. A new design of the biolayer interferometry (BLI) method with specific AVI-tagged constructs was used to determine the affinity to galectins, and compared with the gold-standard method of isothermal titration calorimetry (ITC). This study reveals new routes to low nanomolar glycoconjugate inhibitors of galectins of interest for biomedical research.

Mechanism-based inhibition of GH127/146 cysteine glycosidases by stereospecifically functionalized l-arabinofuranosides

To understand the precise mechanism of the glycoside hydrolase (GH) family 127, a cysteine β-l-arabinofuranosidase (Arafase) - HypBA1 - has been isolated from Bifidobacterium longum in the human Gut microbiota, and the design and synthesis of the mechanism-based inhibitors such as l-Araf-haloacetamides have been carried out. The α-l-Araf-azide derivative was used as the monoglycosylamine equivalent to afford the l-Araf-chloroacetamides (α/β-1-Cl) as well as bromoacetamides (α/β-1-Br) in highly stereoselective manner through Staudinger reaction followed by amide formation with/without anomerization. Against HypBA1, the probes 1, especially in the case of α/β-1-Br inhibited the hydrolysis. Conformational implications of these observations are discussed in this manuscript. Additional examinations using l-Araf-azides (α/β-5) resulted in further mechanistic observations of the GH127/146 cysteine glycosidases, including the hydrolysis of β-5 as the substrate and oxidative inhibition by α-5 using the GH127 homologue.

Mutation Hotspot for Changing the Substrate Specificity of β-N-Acetylhexosaminidase: A Library of GlcNAcases

β-N-Acetylhexosaminidase from Talaromyces flavus (TfHex; EC 3.2.1.52) is an exo-glycosidase with dual activity for cleaving N-acetylglucosamine (GlcNAc) and N-acetylgalactosamine (GalNAc) units from carbohydrates. By targeting a mutation hotspot of the active site residue Glu332, we prepared a library of ten mutant variants with their substrate specificity significantly shifted towards GlcNAcase activity. Suitable mutations were identified by in silico methods. We optimized a microtiter plate screening method in the yeast Pichia pastoris expression system, which is required for the correct folding of tetrameric fungal β-N-acetylhexosaminidases. While the wild-type TfHex is promiscuous with its GalNAcase/GlcNAcase activity ratio of 1.2, the best single mutant variant Glu332His featured an 8-fold increase in selectivity toward GlcNAc compared with the wild-type. Several prepared variants, in particular Glu332Thr TfHex, had significantly stronger transglycosylation capabilities than the wild-type, affording longer chitooligomers - they behaved like transglycosidases. This study demonstrates the potential of mutagenesis to alter the substrate specificity of glycosidases.

Protecting group free glycosylation: one-pot stereocontrolled access to 1,2-trans glycosides and (1→6)-linked disaccharides of 2-acetamido sugars

Unprotected 2-acetamido sugars may be directly converted into their oxazolines using 2-chloro-1,3-dimethylimidazolinium chloride (DMC), and a suitable base, in aqueous solution. Freeze drying and acid catalysed reaction with an alcohol as solvent produces the corresponding 1,2-trans-glycosides in good yield. Alternatively, dissolution in an aprotic solvent system and acidic activation in the presence of an excess of an unprotected glycoside as a glycosyl acceptor, results in the stereoselective formation of the corresponding 1,2-trans linked disaccharides without any protecting group manipulations. Reactions using aryl glycosides as acceptors are completely regioselective, producing only the (1→6)-linked disaccharides.

Synthesis of N-Acetyllactosamine and N-Acetyllactosamine-Based Bioactives

N-Acetyllactosamine (LacNAc) or more specifically β-d-galactopyranosyl-1,4-N-acetyl-d-glucosamine is a unique acyl-amino sugar and a key structural unit in human milk oligosaccharides, an antigen component of many glycoproteins, and an antiviral active component for the development of effective drugs against viruses. LacNAc is useful itself and as a basic building block for producing various bioactive oligosaccharides, notably because this synthesis may be used to add value to dairy lactose. Despite a significant amount of information in the literature on the benefits, structures, and types of different LacNAc-derived oligosaccharides, knowledge about their effective synthesis for large-scale production is still in its infancy. This work provides a comprehensive analysis of existing production strategies for LacNAc and important LacNAc-based structures, including sialylated LacNAc as well as poly- and oligo-LacNAc. We conclude that direct extraction from milk is too complex, while chemical synthesis is also impractical at an industrial scale. Microbial routes have application when multiple step reactions are needed, but the major route to large-scale biochemical production will likely lie with enzymatic routes, particularly those using β-galactosidases (for LacNAc synthesis), sialidases (for sialylated LacNAc synthesis), and β-N-acetylhexosaminidases (for oligo-LacNAc synthesis). Glycosyltransferases, especially for the biosynthesis of extended complex LacNAc structures, could also play a major role in the future. In these cases, immobilization of the enzyme can increase stability and reduce cost. Processing parameters, such as substrate concentration and purity, acceptor/donor ratio, water activity, and temperature, can affect product selectivity and yield. More work is needed to optimize these reaction parameters and in the development of robust, thermally stable enzymes to facilitate commercial production of these important bioactive substances.

Engineered Regulon to Enable Autonomous Azide Ion Biosensing, Recombinant Protein Production, and in Vivo Glycoengineering

Detection of azide-tagged biomolecules (e.g., azido sugars) inside living cells using "click" chemistry has been revolutionary to the field of chemical biology. However, we currently still lack suitable synthetic biology tools to autonomously and rapidly detect azide ions. Here, we have developed an engineered synthetic promoter system called cyn regulon, and complementary Escherichia coli engineered strains, to selectively detect azide ions and autonomously induce downstream expression of reporter genes. The engineered cyn azide operon allowed highly tunable reporter green fluorescent protein (GFP) expression over three orders of inducer azide ion concentrations (0.01-5 mM) and rapidly induced GFP expression by over 600-fold compared to the uninduced control. Next, we showcase the superior performance of this engineered cyn-operon over the classical lac-operon for recombinant protein production. Finally, we highlight how this synthetic biology toolkit can enable glycoengineering-based applications by facilitating in vivo activity screening of mutant carbohydrate-active enzymes (CAZymes), called glycosynthases, using azido sugars as donor substrates.

β-N-Acetylhexosaminidases for Carbohydrate Synthesis via Trans-Glycosylation

β-N-acetylhexosaminidases (EC 3.2.1.52) are retaining hydrolases of glycoside hydrolase family 20 (GH20). These enzymes catalyze hydrolysis of terminal, non-reducing N-acetylhexosamine residues, notably N-acetylglucosamine or N-acetylgalactosamine, in N-acetyl-β-D-hexosaminides. In nature, bacterial β-N-acetylhexosaminidases are mainly involved in cell wall peptidoglycan synthesis, analogously, fungal β-N-acetylhexosaminidases act on cell wall chitin. The enzymes work via a distinct substrate-assisted mechanism that utilizes the 2-acetamido group as nucleophile. Curiously, the β-N-acetylhexosaminidases possess an inherent trans-glycosylation ability which is potentially useful for biocatalytic synthesis of functional carbohydrates, including biomimetic synthesis of human milk oligosaccharides and other glycan-functionalized compounds. In this review, we summarize the reaction engineering approaches (donor substrate activation, additives, and reaction conditions) that have proven useful for enhancing trans-glycosylation activity of GH20 β-N-acetylhexosaminidases. We provide comprehensive overviews of reported synthesis reactions with GH20 enzymes, including tables that list the specific enzyme used, donor and acceptor substrates, reaction conditions, and details of the products and yields obtained. We also describe the active site traits and mutations that appear to favor trans-glycosylation activity of GH20 β-N-acetylhexosaminidases. Finally, we discuss novel protein engineering strategies and suggest potential “hotspots” for mutations to promote trans-glycosylation activity in GH20 for efficient synthesis of specific functional carbohydrates and other glyco-engineered products.

High-Affinity N-(2-Hydroxypropyl)methacrylamide Copolymers with Tailored N-Acetyllactosamine Presentation Discriminate between Galectins

N-Acetyllactosamine (LacNAc; Galβ4GlcNAc) is a typical disaccharide ligand of galectins. The most abundant members of these human lectins, galectin-1 (Gal-1) and galectin-3 (Gal-3), participate in a number of pathologies including cancerogenesis and metastatic formation. In this study, we synthesized a series of fifteen N-(2-hydroxypropyl)methacrylamide (HPMA)-based glycopolymers with varying LacNAc amounts and presentations and evaluated the impact of their architecture on the binding affinity to Gal-1 and Gal-3. The controlled radical reversible addition-fragmentation chain transfer copolymerization technique afforded linear polymer precursors with comparable molecular weight (M_n ≈ 22,000 g mol^-1) and narrow dispersity (D̵ ≈ 1.1). The precursors were conjugated with the functionalized LacNAc disaccharide (4-22 mol % content in glycopolymer) prepared by enzymatic synthesis under catalysis by β-galactosidase from Bacillus circulans. The structure-affinity relationship study based on the enzyme-linked immunosorbent assay revealed that the type of LacNAc presentation, individual or clustered on bi- or trivalent linkers, brings a clear discrimination (almost 300-fold) between Gal-1 and Gal-3, reaching avidity to Gal-1 in the nanomolar range. Whereas Gal-1 strongly preferred a dense presentation of individually distributed LacNAc epitopes, Gal-3 preferred a clustered LacNAc presentation. Such a strong galectin preference based just on the structure of a multivalent glycopolymer type is exceptional. The prepared nontoxic, nonimmunogenic, and biocompatible glycopolymers are prospective for therapeutic applications requiring selectivity for one particular galectin.

Converting a β-N-acetylhexosaminidase into two trans-β-N-acetylhexosaminidases by domain-targeted mutagenesis

We have recently derived a β-N-acetylhexosaminidase, BbhI, from Bifidobacterium bifidum JCM 1254, which could regioselectively synthesize GlcNAcβ1-3Galβ1-4Glc with a yield of 44.9%. Here, directed evolution of BbhI by domain-targeted mutagenesis was carried out. Firstly, the GH20 domain was selected for random mutagenesis using MEGAWHOP method and a small library of 1300 clones was created. A total of 734 colonies with reduced hydrolytic activity were isolated, and three mutants with elevated transglycosylation yields, GlcNAcβ1-3Galβ1-4Glc yields of 68.5%, 74.7%, and 81.1%, respectively, were obtained. Subsequently, nineteen independent mutants were constructed according to all the mutation sites in these three mutants. After transglycosylation analysis, Asp714 and Trp773 were identified as key residues for improvement in transglycosylation ability and were chosen for the second round of directed evolution by site-saturation mutagenesis. Two most efficient mutants D714T and W773R that acted as trans-β-N-acetylhexosaminidase were finally achieved. D714T with the substitution at the putative nucleophile assistant residue Asp714 by threonine showed high yield of 84.7% with unobserved hydrolysis towards transglycosylation product. W773R with arginine substitution at Trp773 residue locating at the entrance of catalytic cavity led to the yield up to 81.8%. The k_cat/K_m values of D714T and W773R for hydrolysis of pNP-β-GlcNAc displayed drastic decreases. NMR investigation of protein-substrate interaction revealed an invariable mode of the catalytic cavity of D714T, W773R, and WT BbhI. The collective motions of protein model showed the mutations Thr714 and Arg773 exerted little effect on the dynamics of the inside but a large effect on the dynamics of the outside of catalytic cavity.

Leloir Glycosyltransferases in Applied Biocatalysis: A Multidisciplinary Approach

Enzymes are nature’s catalyst of choice for the highly selective and efficient coupling of carbohydrates. Enzymatic sugar coupling is a competitive technology for industrial glycosylation reactions, since chemical synthetic routes require extensive use of laborious protection group manipulations and often lack regio- and stereoselectivity. The application of Leloir glycosyltransferases has received considerable attention in recent years and offers excellent control over the reactivity and selectivity of glycosylation reactions with unprotected carbohydrates, paving the way for previously inaccessible synthetic routes. The development of nucleotide recycling cascades has allowed for the efficient production and reuse of nucleotide sugar donors in robust one-pot multi-enzyme glycosylation cascades. In this way, large glycans and glycoconjugates with complex stereochemistry can be constructed. With recent advances, LeLoir glycosyltransferases are close to being applied industrially in multi-enzyme, programmable cascade glycosylations.

β-N-Acetylhexosaminidases-the wizards of glycosylation

β-N-Acetylhexosaminidases (EC 3.2.1.52) are a unique family of glycoside hydrolases with dual substrate specificity and a particular reaction mechanism. Though hydrolytic enzymes per se, their good stability, easy recombinant production, absolute stereoselectivity, and a broad substrate specificity predestine these enzymes for challenging applications in carbohydrate synthesis. This mini-review aims to demonstrate the catalytic potential of β-N-acetylhexosaminidases in a range of unusual reactions, processing of unnatural substrates, formation of unexpected products, and demanding reaction designs. The use of unconventional media can considerably alter the progress of transglycosylation reactions. By means of site-directed mutagenesis, novel catalytic machineries can be constructed. Glycosylation of difficult substrates such as sugar nucleotides was accomplished, and the range of afforded glycosidic bonds comprises unique non-reducing sugars. Specific functional groups may be tolerated in the substrate molecule, which makes β-N-acetylhexosaminidases invaluable allies in difficult synthetic problems.

The β-N-Acetylhexosaminidase in the Synthesis of Bioactive Glycans: Protein and Reaction Engineering

N-Acetylhexosamine oligosaccharides terminated with GalNAc act as selective ligands of galectin-3, a biomedically important human lectin. Their synthesis can be accomplished by β-N-acetylhexosaminidases (EC 3.2.1.52). Advantageously, these enzymes tolerate the presence of functional groups in the substrate molecule, such as the thiourea linker useful for covalent conjugation of glycans to a multivalent carrier, affording glyconjugates. β-N-Acetylhexosaminidases exhibit activity towards both N-acetylglucosamine (GlcNAc) and N-acetylgalactosamine (GalNAc) moieties. A point mutation of active-site amino acid Tyr into other amino acid residues, especially Phe, His, and Asn, has previously been shown to strongly suppress the hydrolytic activity of β-N-acetylhexosaminidases, creating enzymatic synthetic engines. In the present work, we demonstrate that Tyr470 is an important mutation hotspot for altering the ratio of GlcNAcase/GalNAcase activity, resulting in mutant enzymes with varying affinity to GlcNAc/GalNAc substrates. The enzyme selectivity may additionally be manipulated by altering the reaction medium upon changing pH or adding selected organic co-solvents. As a result, we are able to fine-tune the β-N-acetylhexosaminidase affinity and selectivity, resulting in a high-yield production of the functionalized GalNAcβ4GlcNAc disaccharide, a selective ligand of galectin-3.

Biocompatible glyconanomaterials based on HPMA-copolymer for specific targeting of galectin-3

Background

Galectin-3 (Gal-3) is a promising target in cancer therapy with a high therapeutic potential due to its abundant localization within the tumor tissue and its involvement in tumor development and proliferation. Potential clinical application of Gal-3-targeted inhibitors is often complicated by their insufficient selectivity or low biocompatibility. Nanomaterials based on N-(2-hydroxypropyl)methacrylamide (HPMA) nanocarrier are attractive for in vivo application due to their good water solubility and lack of toxicity and immunogenicity. Their conjugation with tailored carbohydrate ligands can yield specific glyconanomaterials applicable for targeting biomedicinally relevant lectins like Gal-3.

Results

In the present study we describe the synthesis and the structure-affinity relationship study of novel Gal-3-targeted glyconanomaterials, based on hydrophilic HPMA nanocarriers. HPMA nanocarriers decorated with varying amounts of Gal-3 specific epitope GalNAcβ1,4GlcNAc (LacdiNAc) were analyzed in a competitive ELISA-type assay and their binding kinetics was described by surface plasmon resonance. We showed the impact of various linker types and epitope distribution on the binding affinity to Gal-3. The synthesis of specific functionalized LacdiNAc epitopes was accomplished under the catalysis by mutant β-N-acetylhexosaminidases. The glycans were conjugated to statistic HPMA copolymer precursors through diverse linkers in a defined pattern and density using Cu(I)-catalyzed azide–alkyne cycloaddition. The resulting water-soluble and structurally flexible synthetic glyconanomaterials exhibited affinity to Gal-3 in low μM range.

Conclusions

The results of this study reveal the relation between the linker structure, glycan distribution and the affinity of the glycopolymer nanomaterial to Gal-3. They pave the way to specific biomedicinal glyconanomaterials that target Gal-3 as a therapeutic goal in cancerogenesis and other disorders.

Electronic supplementary material

The online version of this article (10.1186/s12951-018-0399-1) contains supplementary material, which is available to authorized users.

Glycan-decorated HPMA copolymers as high-affinity lectin ligands

New conjugates of N-(2-hydroxypropyl) methacrylamide (HPMA) copolymers tethered with chitooligosaccharidic epitopes of varying lengths are potent ligands of wheat germ agglutinin, reaching subnanomolar binding affinities.

Enzymatic glycosylation of small molecules: challenging substrates require tailored catalysts

Glycosylation can significantly improve the physicochemical and biological properties of small molecules like vitamins, antibiotics, flavors, and fragrances. The chemical synthesis of glycosides is, however, far from trivial and involves multistep routes that generate lots of waste. In this review, biocatalytic alternatives are presented that offer both stricter specificities and higher yields. The advantages and disadvantages of different enzyme classes are discussed and illustrated with a number of recent examples. Progress in the field of enzyme engineering and screening are expected to result in new applications of biocatalytic glycosylation reactions in various industrial sectors.

Iron(III) chloride as an efficient catalyst for stereoselective synthesis of glycosyl azides and a cocatalyst with Cu(0) for the subsequent click chemistry

A highly efficient and mild method for azido glycosylation of glycosyl β-peracetates to 1,2-trans glycosyl azides was developed by using inexpensive FeCl(3) as the catalyst. In addition, we demonstrated, for the first time, that FeCl(3) in combination with copper powder can promote 1,3-dipolar cycloaddition (click chemistry) of azido glycosides with terminal alkynes. Good to excellent yields were obtained with exclusive formation of a single isomer in both glycosyl azidation and subsequent cycloaddition processes.