Efficient Synthesis of Sugar Oxazolines from Unprotected N-Acetyl-2-amino Sugars by Using Chloroformamidinium Reagent in Water

Carbohydrate-active enzyme (CAZyme) discovery and engineering via (Ultra)high-throughput screening

Carbohydrate-active enzymes (CAZymes) constitute a diverse set of enzymes that catalyze the assembly, degradation, and modification of carbohydrates. These enzymes have been fashioned into potent, selective catalysts by millennia of evolution, and yet are also highly adaptable and readily evolved in the laboratory. To identify and engineer CAZymes for different purposes, (ultra)high-throughput screening campaigns have been frequently utilized with great success. This review provides an overview of the different approaches taken in screening for CAZymes and how mechanistic understandings of CAZymes can enable new approaches to screening. Within, we also cover how cutting-edge techniques such as microfluidics, advances in computational approaches and synthetic biology, as well as novel assay designs are leading the field towards more informative and effective screening approaches.

Chemical synthesis and functional evaluation of glycopeptides and glycoproteins containing rare glycosyl amino acid linkages

Covering: 1987 to 2023Naturally existing glycoproteins through post-translational protein glycosylation are highly heterogeneous, which not only impedes the structure-function studies, but also hinders the development of their potential medical usage. Chemical synthesis represents one of the most powerful tools to provide the structurally well-defined glycoforms. Being the key step of glycoprotein synthesis, glycosylation usually takes place at serine, threonine, and asparagine residues, leading to the predominant formation of the O- and N-glycans, respectively. However, other amino acid residues containing oxygen, nitrogen, sulfur, and nucleophilic carbon atoms have also been found to be glycosylated. These diverse glycoprotein linkages, occurring from microorganisms to plants and animals, play also pivotal biological roles, such as in cell-cell recognition and communication. The availability of these homogenous rare glycopeptides and glycoproteins can help decipher the glyco-code for developing therapeutic agents. This review highlights the chemical approaches for assembly of the functional glycopeptides and glycoproteins bearing these "rare" carbohydrate-amino acid linkages between saccharide and canonical amino acid residues and their derivatives.

Synthesis of N-Glycosylated Soluble Fas Ligand

Controlled cell death is essential for the regulation of the immune system and plays a role in pathogen defense. It is often altered in pathogenic conditions such as cancer, viral infections and autoimmune diseases. The Fas receptor and its corresponding membrane-bound ligand (FasL) are part of the extrinsic apoptosis pathway activated in these cases. A soluble form of FasL (sFasL), produced by ectodomain shedding, displays a diverse but still elusive set of non-apoptotic functions and sometimes even serves as a pro-survival factor. To gather more knowledge about the characteristics of this protein and the impact N-glycosylations may have, access to homogeneous posttranslationally modified variants of sFasL is needed. Therefore, we developed a flexible strategy to obtain such homogeneously N-glycosylated variants of sFasL by applying chemical protein synthesis. This strategy can be flexibly combined with enzymatic methods to introduce more complex, site selective glycosylations.

Synthesis and Evaluation of Glycosyl Luciferins

Measuring glycosidase activity is important to monitor any aberrations in carbohydrate hydrolase activity, but also for the screening of potential glycosidase inhibitors. To this end, synthetic substrates are needed which provide an enzyme-dependent read-out upon hydrolysis by the glycosidase. Herein, we present two new routes for the synthesis of caged luminescent carbohydrates, which can be used for determining glycosidase activity with a luminescent reporter molecule. The substrates were validated with glycosidase and revealed a clear linear range and enzyme-dependent signal upon the in situ generation of the luciferin moiety from the corresponding nitrile precursors. Besides, we showed that these compounds could directly be synthesized from unprotected glycosyl-α-fluorides in a two-step procedure with yields up to 75 %. The intermediate methyl imidate appeared a key intermediate which also reacted with d-cysteine to give the corresponding d-luciferin substrate rendering this a highly attractive method for synthesizing glycosyl luciferins in good yields.

Stereoselective protecting-group-free synthesis of alkyl glycosides using dibenzyloxy triazine type glycosyl donors

Chemical O-glycosylation is a key step for the synthesis of sugar-containing molecules such as glycolipids. However, traditional carbohydrate chemistry is characterized by extensive use of protective groups, resulting in laborious manipulations and poor atom economy. Here, we present a protecting-group-free glycosylation strategy employing dibenzyloxy-1,3,5-triazin-2-yl glycosides (DBT-glycosides) as glycosyl donors. The DBT-glycosyl donors could be prepared directly through an alkaline nucleophilic substitution from unprotected sugars in aqueous media. The O-glycosylation of alcohols by using DBT-glycosyl donors has been carried out under mild hydrogenolytic conditions, affording the corresponding alkyl glycosides stereo-selectively in good yields.

Green-Chemical Strategies for Production of Tailor-Made Chitooligosaccharides with Enhanced Biological Activities

Chitooligosaccharides (COSs) are b-1,4-linked homo-oligosaccharides of N-acetylglucosamine (GlcNAc) or glucosamine (GlcN), and also include hetero-oligosaccharides composed of GlcNAc and GlcN. These sugars are of practical importance because of their various biological activities, such as antimicrobial, anti-inflammatory, antioxidant and antitumor activities, as well as triggering the innate immunity in plants. The reported data on bioactivities of COSs used to contain some uncertainties or contradictions, because the experiments were conducted with poorly characterized COS mixtures. Recently, COSs have been satisfactorily characterized with respect to their structures, especially the degree of polymerization (DP) and degree of N-acetylation (DA); thus, the structure-bioactivity relationship of COSs has become more unambiguous. To date, various green-chemical strategies involving enzymatic synthesis of COSs with designed sequences and desired biological activities have been developed. The enzymatic strategies could involve transglycosylation or glycosynthase reactions using reducing end-activated sugars as the donor substrates and chitinase/chitosanase and their mutants as the biocatalysts. Site-specific chitin deacetylases were also proposed to be applicable for this purpose. Furthermore, to improve the yields of the COS products, metabolic engineering techniques could be applied. The above-mentioned approaches will provide the opportunity to produce tailor-made COSs, leading to the enhanced utilization of chitin biomass.

Synthesis of pyranobenzopyrans from D-glucal using ionic liquids

Imidazolium room temperature ionic liquids (RTILs) were used as solvents in the Lewis acid-catalyzed cycloaddition reactions of D-glucal with substituted salicylaldehydes. These reactions selectively led to various, novel, cis-pyrano [4,3-b]benzopyrans in modest yields, different products than those isolated from analogous solution studies. Furan diol was isolated as the major byproduct from all the reactions. The use of RTILs facilitated the use of unprotected sugars in these reactions.

LC-MS/MS-PRM Quantification of IgG Glycoforms Using Stable Isotope Labeled IgG1 Fc Glycopeptide Standard

Targeted quantification of proteins is a standard methodology with broad utility, but targeted quantification of glycoproteins has not reached its full potential. The lack of optimized workflows and isotopically labeled standards limits the acceptance of glycoproteomics quantification. In this work, we introduce an efficient and streamlined chemoenzymatic synthesis of a library of isotopically labeled glycopeptides of IgG1 which we use for quantification in an energy optimized LC-MS/MS-PRM workflow. Incorporation of the stable isotope labeled N-acetylglucosamine enables an efficient monitoring of all major fragment ions of the glycopeptides generated under the soft higher-energy C-trap dissociation (HCD) conditions, which reduces the coefficients of variability (CVs) of the quantification to 0.7-2.8%. Our results document, for the first time, that the workflow using a combination of stable isotope labeled standards with intrascan normalization enables quantification of the glycopeptides by an electron transfer dissociation (ETD) workflow, as well as the HCD workflow, with the highest sensitivity compared to traditional workflows. This was exemplified by a rapid quantification (13 min) of IgG1 Fc glycoforms from COVID-19 patients.

Selective Anomeric Acetylation of Unprotected Sugars with Acetic Anhydride in Water

Unprotected sugars are selectively acetylated simply by stirring in aqueous solution in the presence of acetic anhydride and a weak base such as sodium carbonate. The reaction is selective for acetylation of the anomeric hydroxyl group of mannose, 2-acetamido, and 2-deoxy sugars and can be performed on a large scale. Competitive intramolecular migration of the 1-O-acetate to the 2-hydroxyl group when these two substituents are cis causes over-reaction and the formation of product mixtures.

Protecting-group-free glycosylation of phosphatidic acid in aqueous media

The glycosylation of unprotected carbohydrates has emerged as an area of significant interest because it obviates the need for long reaction sequences involving protecting-group manipulations. Herein, we report the one-pot synthesis of anomeric glycosyl phosphates through the condensation of unprotected carbohydrates with phospholipid derivatives while retaining high stereo- and regioselective control. The anomeric center was activated using 2-chloro-1,3-dimethylimidazolinium chloride to facilitate condensation with glycerol-3-phosphate derivatives in an aqueous solution. A water/propionitrile mixture provided superior stereoselectivity while maintaining good yields. Under these optimized conditions, the condensation of stable isotope-labeled glucose with phosphatidic acid provided efficient access to labeled glycophospholipids as an internal standard for mass spectrometry.

ez-ADiCon: A novel glyco-remodeling based strategy that enables preparation of homogenous antibody-drug conjugates via one-step enzymatic transglycosylation with payload-preloaded bi-antennary glycan complexes

The conserved N-linked glycan at the Fc domain of recombinant monoclonal antibodies is an attractive target for site-specific payload conjugation for preparation of homogenous antibody-drug conjugates (ADCs). Here, we report a novel ADC constructing strategy, named "ez-ADiCon", that is achieved by one-step enzymatic transglycosylation of a payload-preloaded bi-antennary glycan oxazoline onto a deglycosylated antibody. In this method, a mixture of different glycoforms of the Fc-glycan is replaced with a pre-defined payload-linked glycan. Since two payloads are linked on each donor glycan substrate, efficient conjugation results in a highly homogenous ADC with mostly-four drug molecules per antibody, facilitating hydrophobic interaction chromatography analysis and purification. We validated this conjugation strategy using Monomethyl auristatin E (MMAE) and an anti-Human epidermal growth factor receptor 2 (anti-Her2) antibody as the model ADC components and demonstrated its target-specific in vitro cytotoxicity. Our novel conjugation strategy, ez-ADiCon, provides a new approach for the preparation of next generation ADCs.

N-Glycan Engineering: Constructing the N-GlcNAc Stump

N-Glycosylation is often essential for the structure and function of proteins. However, N-glycosylated proteins from natural sources exhibit considerable heterogeneity in the appended oligosaccharides, bringing daunting challenges to corresponding basic research and therapeutic applications. To address this issue, various synthetic, enzymatic, and chemoenzymatic approaches have been elegantly designed. Utilizing the endoglycosidase-catalyzed transglycosylation method, a single N-acetylglucosamine (N-GlcNAc, analogous to a tree stump) on proteins can be converted to various homogeneous N-glycosylated forms, thereby becoming the focus of research efforts. In this concept article, we briefly introduce the methods that allow the generation of N-GlcNAc and its close analogues on proteins and peptides and highlight the current challenges and opportunities the scientific community is facing.

Site-Specific Chemoenzymatic Conjugation of High-Affinity M6P Glycan Ligands to Antibodies for Targeted Protein Degradation

Lysosome-targeting chimeras (LYTACs) offer an opportunity for the degradation of extracellular and membrane-associated proteins of interest. Here, we report an efficient chemoenzymatic method that enables a single-step and site-specific conjugation of high-affinity mannose-6-phosphate (M6P) glycan ligands to antibodies without the need of protein engineering and conventional click reactions that would introduce "unnatural" moieties, yielding homogeneous antibody-M6P glycan conjugates for targeted degradation of membrane-associated proteins. Using trastuzumab and cetuximab as model antibodies, we showed that the wild-type endoglycosidase S (Endo-S) could efficiently perform the antibody deglycosylation and simultaneous transfer of an M6P-glycan from a synthetic M6P-glycan oxazoline to the deglycosylated antibody in a one-pot manner, giving structurally well-defined antibody-M6P glycan conjugates. A two-step procedure, using wild-type Endo-S2 for deglycosylation followed by transglycosylation with an Endo-S2 mutant (D184M), was also efficient to provide M6P glycan-antibody conjugates. The chemoenzymatic approach was highly specific for Fc glycan remodeling when both Fc and Fab domains were glycosylated, as exemplified by the selective Fc-glycan remodeling of cetuximab. SPR binding analysis indicated that the M6P conjugates possessed a nanomolar range of binding affinities for the cation-independent mannose-6-phosphate receptor (CI-MPR). Preliminary cell-based assays showed that the M6P-trastuzumab and M6P-cetuximab conjugates were able to selectively degrade the membrane-associated HER2 and EGFR, respectively. This modular glycan-remodeling strategy is expected to find wide applications for antibody-based lysosome-targeted degradation of extracellular and membrane proteins.

Plant Chitinase Mutants as the Catalysts for Chitooligosaccharide Synthesis Using the Sugar Oxazoline Derivatives

Sugar oxazolines, (GlcNAc)_n-oxa (n = 2, 3, 4, and 5), were synthesized from a mixture of chitooligosaccharides, (GlcNAc)_n (n = 2, 3, 4, and 5), and utilized for synthesis of (GlcNAc)₇ with higher elicitor activity using plant chitinase mutants as the catalysts. From isothermal titration calorimetry, the binding affinity of (GlcNAc)₂-oxa toward an inactive mutant obtained from Arabidopsis thaliana GH18 chitinase was found to be higher than those of the other (GlcNAc)_n-oxa (n = 3, 4, and 5). To synthesize (GlcNAc)₇, the donor/acceptor substrates with different size combinations, (GlcNAc)₂-oxa/(GlcNAc)₅ (1), (GlcNAc)₃-oxa/(GlcNAc)₄ (2), (GlcNAc)₄-oxa/(GlcNAc)₃ (3), and (GlcNAc)₅-oxa/(GlcNAc)₂ (4), were incubated with hypertransglycosylating mutants of GH18 chitinases from A. thaliana and Cycas revoluta. The synthetic activities of these plant chitinase mutants were lower than that of a mutant of Bacillus circulans chitinase A1. Nevertheless, in the plant chitinase mutants, the synthetic efficiency of combination (1) was higher than those of the other combinations (2), (3), and (4), suggesting that the synthetic reaction is mostly dominated by the binding affinities of (GlcNAc)_n-oxa. In contrast, the Bacillus enzyme mutant with a different subsite arrangement synthesized (GlcNAc)₇ from combination (1) in the lowest efficiency. Donor/acceptor-size dependency of the enzymatic synthesis appeared to be strongly related to the subsite arrangement of the enzyme used as the catalyst. The A. thaliana chitinase mutant was found to be useful when combination (1) is employed for the substrates.

LC-MS/MS-PRM Quantification of IgG glycoforms using stable isotope labeled IgG1 Fc glycopeptide standard

Targeted quantification of proteins is a standard methodology with broad utility, but targeted quantification of glycoproteins has not reached its full potential. The lack of optimized workflows and isotopically labeled standards limits the acceptance of glycoproteomics quantification. In this paper, we introduce an efficient and streamlined chemoenzymatic synthesis of a library of isotopically labeled glycopeptides of IgG1 which we use for quantification in an energy optimized LC-MS/MS-PRM workflow. Incorporation of the stable isotope labeled N-acetylglucosamine enables an efficient monitoring of all major fragment ions of the glycopeptides generated under the soft collision induced dissociation (CID) conditions which reduces the CVs of the quantification to 0.7-2.8%. Our results document, for the first time, that the workflow using a combination of stable isotope labeled standards with intra-scan normalization enables quantification of the glycopeptides by an electron transfer dissociation (ETD) workflow as well as the CID workflow with the highest sensitivity compared to traditional workflows., This was exemplified by a rapid quantification (13-minute) of IgG1 Fc glycoforms from COVID-19 patients.

Graphic Abstract

Protecting-group- and microwave-free synthesis of β-glycosyl esters and aryl β-glycosides of N-acetyl-d-glucosamine

A protecting-group-free method for synthesis of β-glycosyl esters and aryl β-glycosides was developed by using latent chemical reactivity of N-acetyl-d-glucosamine (GlcNAc) oxazoline. The GlcNAc oxazoline was spontaneously reacted with carboxylic acids and phenol derivatives via the oxazoline ring opening without the use of a catalyst or heating conditions (i.e., microwave irradiation), affording the desired products in moderate to excellent yields with β-selectivity. This simple protecting-group-free method exhibits a wide substrate scope and good functional group tolerance, and it allows the efficient production of a novel class of GlcNAc-conjugated biomaterials and prodrug candidates.

Selective reaction monitoring approach using structure-defined synthetic glycopeptides for validating glycopeptide biomarkers pre-determined by bottom-up glycoproteomics

Clusterin is a heavily glycosylated protein that is upregulated in various cancer and neurological diseases. The findings by the Hancock and Iliopoulos group that levels of the tryptic glycopeptide derived from plasma clusterin, 372Leu-Ala-Asn-Leu-Thr-Gln-Gly-Glu-Asp-Gln-Tyr-Tyr-Leu-Arg385 with a biantennary disialyl N-glycan (A2G2S2 or FA2G2S2) at Asn374 differed significantly prior to and after curative nephrectomy for clear cell renal cell carcinoma (RCC) patients motivated us to verify the feasibility of this glycopeptide as a novel biomarker of RCC. To determine the precise N-glycan structure attached to Asn374, whether A2G2S2 is composed of the Neu5Acα2,3Gal or/and the Neu5Acα2,6Gal moiety, we synthesized key glycopeptides having one of the two putative isomers. Selective reaction monitoring assay using synthetic glycopeptides as calibration standards allowed "top-down glycopeptidomics" for the absolute quantitation of targeted label-free glycopeptides in a range from 313.3 to 697.5 nM in the complex tryptic digests derived from serum samples of RCC patients and healthy controls. Our results provided evidence that the Asn374 residue of human clusterin is modified dominantly with the Neu5Acα2,6Gal structure and the levels of clusterin bearing an A2G2S2 with homo Neu5Acα2,6Gal terminals at Asn374 decrease significantly in RCC patients as compared with healthy controls. The present study elicits that a new strategy integrating the bottom-up glycoproteomics with top-down glycopeptidomics using structure-defined synthetic glycopeptides enables the confident identification and quantitation of the glycopeptide targets pre-determined by the existing methods for intact glycopeptide profiling.

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.

Carbohydrate-active enzymes (CAZymes) in the gut microbiome

The 10¹³-10¹⁴ microorganisms present in the human gut (collectively known as the human gut microbiota) dedicate substantial percentages of their genomes to the degradation and uptake of carbohydrates, indicating the importance of this class of molecules. Carbohydrates function not only as a carbon source for these bacteria but also as a means of attachment to the host, and a barrier to infection of the host. In this Review, we focus on the diversity of carbohydrate-active enzymes (CAZymes), how gut microorganisms use them for carbohydrate degradation, the different chemical mechanisms of these CAZymes and the roles that these microorganisms and their CAZymes have in human health and disease. We also highlight examples of how enzymes from this treasure trove have been used in manipulation of the microbiota for improved health and treatment of disease, in remodelling the glycans on biopharmaceuticals and in the potential production of universal O-type donor blood.

Synthetic Antibody-Rhamnose Cluster Conjugates Show Potent Complement-Dependent Cell Killing by Recruiting Natural Antibodies

Monoclonal antibodies (mAbs) are one of the most rapidly growing drug classes used for the treatment of cancer, infectious and autoimmune diseases. Complement-dependent cytotoxicity (CDC) is one of the effector functions for antibodies to deplete target cells. We report here an efficient chemoenzymatic synthesis of structurally well-defined conjugates of a monoclonal antibody with a rhamnose- and an αGal trisaccharide-cluster to recruit natural anti-rhamnose and anti-αGal antibodies, respectively, to enhance the CDC-dependent targeted cell killing. The synthesis was achieved by using a modular antibody Fc-glycan remodeling method that includes site-specific chemoenzymatic Fc-glycan functionalization and subsequent click conjugation of synthetic rhamnose- and αGal trisaccharide-cluster to provide the respective homogeneous antibody conjugates. Cell-based assays indicated that the antibody-rhamnose cluster conjugates could mediate potent CDC activity for targeted cancer cell killing and showed much more potent efficacy than the antibody-αGal trisaccharide cluster conjugates for CDC effects.

Anomeric alkylations and acylations of unprotected mono- and disaccharides mediated by pyridoneimine in aqueous solutions

A site-specific deprotonation followed by alkylations and acylations of sugar hemiacetals to the corresponding alkyl glycosides and acylated sugars in aqueous solutions is disclosed herein. Pyridoneimine as a new base is developed to mediate the deprotonation of readily available sugar hemiacetals and further reactions with alkylation and acylation agents.

Streamlined Iterative Assembly of Thio-Oligosaccharides by Aqueous S-Glycosylation of Diverse Deoxythio Sugars

A streamlined iterative assembly of thio-oligosaccharides was developed by aqueous glycosylation. Facile syntheses of various deoxythio sugars with the sulfur on different positions from commercially available starting materials were described. These syntheses featured efficient chemical methods including our recently reported BTM-catalyzed site-selective acylation. The resulting deoxythio sugars could then be used for the Ca(OH)₂ -promoted protecting group-free S-glycosylation in water at room temperature. The aqueous glycosylation reaction proceeded smoothly to afford the corresponding 1,2-trans S-glycosides in good yields with high chemo- and stereoselectivity. An appropriate choice of protecting groups for the thiol in the glycosyl donor was necessary for the development of iterative synthesis of thio-oligosaccharides. The aqueous glycosylation was then applied to the synthesis of a trimannoside moiety of N-linked glycans core region.

Improvement of the Transglycosylation Efficiency of a Lacto-N-Biosidase from Bifidobacterium bifidum by Protein Engineering

The lacto-N-biosidase LnbB from Bifidobacterium bifidum JCM 1254 was engineered to improve its negligible transglycosylation efficiency with the purpose of enzymatically synthesizing lacto-N-tetraose (LNT; Gal-β1,3-GlcNAc-β1,3-Gal-β1,4-Glc) in one enzymatic step. LNT is a prebiotic human milk oligosaccharide in itself and constitutes the structural core of a range of more complex human milk oligosaccharides as well. Thirteen different LnbB variants were expressed and screened for transglycosylation activity by monitoring transglycosylation product formation using lacto-N-biose 1,2-oxazoline as donor substrate and lactose as acceptor substrate. LNT was the major reaction product, yet careful reaction analysis revealed the formation of three additional LNT isomers, which we identified to have a β1,2-linkage, a β1,6-linkage, and a 1,1-linkage, respectively, between lacto-N-biose (Gal-β1,3-GlcNAc) and lactose. Considering both maximal transglycosylation yield and regioselectivity as well as minimal product hydrolysis, the best variant was LnbB W394H, closely followed by W465H and Y419N. A high transglycosylation yield was also obtained with W394F, yet the substitution of W394 and W465 of the subsite −1 hydrophobic platform in the enzyme with His dramatically impaired the undesirable product hydrolysis as compared to substitution with Phe; the effect was most pronounced for W465. Using p-nitrophenyl-β-lacto-N-bioside as donor substrate manifested W394 as an important target position. The optimization of the substrate concentrations confirmed that high initial substrate concentration and high acceptor-to-donor ratio both favor transglycosylation.

General and Robust Chemoenzymatic Method for Glycan-Mediated Site-Specific Labeling and Conjugation of Antibodies: Facile Synthesis of Homogeneous Antibody-Drug Conjugates

Site-specific labeling and conjugation of antibodies are highly desirable for fundamental research and for developing more efficient diagnostic and therapeutic methods. We report here a general and robust chemoenzymatic method that permits a one-pot site-specific functionalization of antibodies. A series of selectively modified disaccharide oxazoline derivatives were designed, synthesized, and evaluated as donor substrates of different endoglycosidases for antibody Fc glycan remodeling. We found that among several endoglycosidases tested, wild-type endoglycosidase from Streptococcus pyogenes of serotype M49 (Endo-S2) exhibited remarkable activity in transferring the functionalized disaccharides carrying site-selectively modified azide, biotin, or fluorescent tags to antibodies without hydrolyzing the resulting transglycosylation products. This discovery, together with the excellent Fc deglycosylation activity of Endo-S2 on recombinant antibodies, allowed direct labeling and functionalization of antibodies in a one-pot manner without the need of intermediate and enzyme separation. The site-specific introduction of varied numbers of azide groups enabled a highly efficient synthesis of homogeneous antibody-drug conjugates (ADCs) with a precise control of the drug-to-antibody ratio (DAR) ranging from 2 to 12 via a copper-free strain-promoted click reaction. Cell viability assays showed that ADCs with higher DARs were more potent in killing antigen-overexpressed cells than the ADCs with lower DARs. This new method is expected to find applications not only for antibody-drug conjugation but also for cell labeling, imaging, and diagnosis.

Reprint of: Advanced glycosidases as ingenious biosynthetic instruments

Until recently, glycosidases, naturally hydrolyzing carbohydrate-active enzymes, have found few synthetic applications in industry, being primarily used for cleaving unwanted carbohydrates. With the establishment of glycosynthase and transglycosidase technology by genetic engineering, the view of glycosidases as industrial biotechnology tools has started to change. Their easy production, affordability, robustness, and substrate versatility, added to the possibility of controlling undesired side hydrolysis by enzyme engineering, have made glycosidases competitive synthetic tools. Current promising applications of engineered glycosidases include the production of well-defined chitooligomers, precious galactooligosaccharides or specialty chemicals such as glycosylated flavonoids. Other synthetic pathways leading to human milk oligosaccharides or remodeled antibodies are on the horizon. This work provides an overview of the synthetic achievements to date for glycosidases, emphasizing the latest trends and outlining possible developments in the field.

One-Pot Conversion of Free Sialoglycans to Functionalized Glycan Oxazolines and Efficient Synthesis of Homogeneous Antibody-Drug Conjugates through Site-Specific Chemoenzymatic Glycan Remodeling

Antibody-drug conjugates (ADCs) are an important class of therapeutic agents that harness the highly specific antigen targeting property of antibodies to deliver toxic drugs for targeted cell killing. Site-specific conjugation methods are highly desirable for constructing homogeneous ADCs that possess a well-defined antibody-to-drug ratio, stability, ideal pharmacological profile, and optimal therapeutic index. We report here a facile synthesis of functionalized glycan oxazolines from free sialoglycans that are key donor substrates for enzymatic Fc glycan remodeling and the application of an efficient endoglycosidase mutant (Endo-S2 D184M) for site-specific glycan transfer to construct homogeneous ADCs. We found that by a sequential use of two coupling reagents under optimized conditions, free sialoglycans could be efficiently converted to selectively functionalized glycan oxazolines carrying azide-, cyclopropene-, and norbornene-tags, respectively, in excellent yield and in a simple one-pot manner. We further demonstrated that the recently reported Endo-S2 D184 M mutant was highly efficient for Fc glycan remodeling with the selectively modified glycan oxazolines to introduce tags into an antibody, which required a significantly smaller amount of glycan oxazolines and a much shorter reaction time than that of the Endo-S D233Q-catalyzed reaction, thus minimizing the side reactions. Finally homogeneous ADCs were constructed with three different click reactions. The resulting ADCs showed excellent serum stability, and in vitro cytotoxicity assays indicated that all the three ADCs generated from the distinct click reactions possessed potent and comparable cytotoxicity for targeted cancer cell killing.

GH18 endo-β-N-acetylglucosaminidases use distinct mechanisms to process hybrid-type N-linked glycans

N-glycosylation is one of the most abundant posttranslational modifications of proteins, essential for many physiological processes, including protein folding, protein stability, oligomerization and aggregation, and molecular recognition events. Defects in the N-glycosylation pathway cause diseases that are classified as congenital disorders of glycosylation. The ability to manipulate protein N-glycosylation is critical not only to our fundamental understanding of biology but also for the development of new drugs for a wide range of human diseases. Chemoenzymatic synthesis using engineered endo-β-N-acetylglucosaminidases (ENGases) has been used extensively to modulate the chemistry of N-glycosylated proteins. However, defining the molecular mechanisms by which ENGases specifically recognize and process N-glycans remains a major challenge. Here we present the X-ray crystal structure of the ENGase EndoBT-3987 from Bacteroides thetaiotaomicron in complex with a hybrid-type glycan product. In combination with alanine scanning mutagenesis, molecular docking calculations and enzymatic activity measurements conducted on a chemically engineered monoclonal antibody substrate unveil two mechanisms for hybrid-type recognition and processing by paradigmatic ENGases. Altogether, the experimental data provide pivotal insight into the molecular mechanism of substrate recognition and specificity for GH18 ENGases and further advance our understanding of chemoenzymatic synthesis and remodeling of homogeneous N-glycan glycoproteins.

Chemical (neo)glycosylation of biological drugs

Biological drugs, specifically proteins and peptides, are a privileged class of medicinal agents and are characterized with high specificity and high potency of therapeutic activity. However, biologics are fragile and require special care during storage, and are often modified to optimize their pharmacokinetics in terms of proteolytic stability and blood residence half-life. In this review, we showcase glycosylation as a method to optimize biologics for storage and application. Specifically, we focus on chemical glycosylation as an approach to modify biological drugs. We present case studies that illustrate the success of this methodology and specifically address the highly important question: does connectivity within the glycoconjugate have to be native or not? We then present the innovative methods of chemical glycosylation of biologics and specifically highlight the emerging and established protecting group-free methodologies of glycosylation. We discuss thermodynamic origins of protein stabilization via glycosylation, and analyze in detail stabilization in terms of proteolytic stability, aggregation upon storage and/or heat treatment. Finally, we present a case study of protein modification using sialic acid-containing glycans to avoid hepatic clearance of biological drugs. This review aims to spur interest in chemical glycosylation as a facile, powerful tool to optimize proteins and peptides as medicinal agents.

Advanced glycosidases as ingenious biosynthetic instruments

Until recently, glycosidases, naturally hydrolyzing carbohydrate-active enzymes, have found few synthetic applications in industry, being primarily used for cleaving unwanted carbohydrates. With the establishment of glycosynthase and transglycosidase technology by genetic engineering, the view of glycosidases as industrial biotechnology tools has started to change. Their easy production, affordability, robustness, and substrate versatility, added to the possibility of controlling undesired side hydrolysis by enzyme engineering, have made glycosidases competitive synthetic tools. Current promising applications of engineered glycosidases include the production of well-defined chitooligomers, precious galactooligosaccharides or specialty chemicals such as glycosylated flavonoids. Other synthetic pathways leading to human milk oligosaccharides or remodeled antibodies are on the horizon. This work provides an overview of the synthetic achievements to date for glycosidases, emphasizing the latest trends and outlining possible developments in the field.

Importance and Monitoring of Therapeutic Immunoglobulin G Glycosylation

The complex diantennary-type oligosaccharides at Asn297 residues of the IgG heavy chains have a profound impact on the safety and efficacy of therapeutic IgG monoclonal antibodies (mAbs). Fc glycosylation of a mAb is an established critical quality attribute (CQA), and its oligosaccharide profile is required to be thoroughly characterized by state-of-the-art analytical methods. The Fc oligosaccharides are highly heterogeneous, and the differentially glycosylated species (glycoforms) of IgG express unique biological activities. Glycoengineering is a promising approach for the production of selected mAb glycoforms with improved effector functions, and non- and low-fucosylated mAbs exhibiting enhanced antibody-dependent cellular cytotoxicity activity have been approved or are under clinical evaluation for treatment of cancers, autoimmune/chronic inflammatory diseases, and infection. Recently, the chemoenzymatic glycoengineering method that allows for the transfer of structurally defined oligosaccharides to Asn-linked GlcNAc residues with glycosynthase has been developed for remodeling of IgG-Fc oligosaccharides with high efficiency and flexibility. Additionally, various glycoengineering methods have been developed that utilize the Fc oligosaccharides of IgG as reaction handles to conjugate cytotoxic agents by "click chemistry", providing new routes to the design of antibody-drug conjugates (ADCs) with tightly controlled drug-antibody ratios (DARs) and homogeneity. This review focuses on current understanding of the biological relevance of individual IgG glycoforms and advances in the development of next-generation antibody therapeutics with improved efficacy and safety through glycoengineering.

Structural insights into the mechanisms and specificities of IgG-active endoglycosidases

The conserved N-glycan on Asn297 of immunoglobulin G (IgG) has significant impacts on antibody effector functions, and is a frequent target for antibody engineering. Chemoenzymatic synthesis has emerged as a strategy for producing antibodies with homogenous glycosylation and improved effector functions. Central to this strategy is the use of enzymes with activity on the Asn297 glycan. EndoS and EndoS2, produced by Streptococcus pyogenes, are endoglycosidases with remarkable specificity for Asn297 glycosylation, making them ideal tools for chemoenzymatic synthesis. Although both enzymes are specific for IgG, EndoS2 recognizes a wider range of glycans than EndoS. Recent progress has been made in understanding the structural basis for their activities on antibodies. In this review, we examine the molecular mechanism of glycosidic bond cleavage by these enzymes and how specific point mutations convert them into glycosynthases. We also discuss the structural basis for differences in the glycan repertoire that IgG-active endoglycosidases recognize, which focuses on the structure of the loops within the glycoside hydrolase (GH) domain. Finally, we discuss the important contributions of carbohydrate binding modules (CBMs) to endoglycosidase activity, and how CBMs work in concert with GH domains to produce optimal activity on IgG.

Applications of Shoda's reagent (DMC) and analogues for activation of the anomeric centre of unprotected carbohydrates

2-Chloro-1,3-dimethylimidazolinium chloride (DMC, herein also referred to as Shoda's reagent) and its derivatives are useful for numerous synthetic transformations in which the anomeric centre of unprotected reducing sugars is selectively activated in aqueous solution. As such unprotected sugars can undergo anomeric substitution with a range of added nucleophiles, providing highly efficient routes to a range of glycosides and glycoconjugates without the need for traditional protecting group manipulations. This mini-review summarizes the development of DMC and some of its derivatives/analogues, and highlights recent applications for protecting group-free synthesis.

One-Step Synthesis of Sugar Nucleotides

The chemical synthesis of sugar nucleotides requires a multistep procedure to ensure a selective reaction. Herein, sugar nucleotides were synthesized in one step using 2-chloro-1,3-dimethylimidazolinium chloride as the condensation reagent. The products were obtained in yields of 12-30%, and the yields were increased to 35-47% by the addition of a tuning reagent. NMR identification of the sugar nucleotides showed that mainly 1,2-trans-glycosides were present. The reported method represents a one-step route to sugar nucleotides from commercially available materials.

One-pot chemoenzymatic synthesis of glycopolymers from unprotected sugars via glycosidase-catalysed glycosylation using triazinyl glycosides

Glycopolymers were successfully synthesised from unprotected sugars in aqueous media via a one-pot chemoenzymatic process of three reactions; the direct synthesis of 4,6-dimethoxy-1,3,5-triazin-2-yl glycosides from unprotected sugars, a glycosidase-catalysed glycosylation using the triazinyl glycoside to afford glycomonomers and a radical polymerisation. The resulting glycopolymers exhibited specific interactions with the corresponding lectin as glycoclusters.

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.

Scope of the DMC mediated glycosylation of unprotected sugars with phenols in aqueous solution

Activation of reducing sugars in aqueous solution using DMC and triethylamine in the presence of phenols allows direct stereoselective conversion to the corresponding 1,2-trans aryl glycosides without the need for any protecting groups.

Guanidinylation of Chitooligosaccharides Involving Internal Cyclization of the Guanidino Group on the Reducing End and Effect of Guanidinylation on Protein Binding Ability

In order to synthesize a promising material for developing a novel peptide/protein delivery system, guanidinylation of chitooligosaccharides with 1-amidinopyrazole hydrochloride was investigated herein. The production of guanidinylated chitooligosaccharides was demonstrated by infrared spectroscopy (IR), nuclear magnetic resonance (NMR), and elemental analyses. Interestingly, we found that the reducing end in the guanidinylated chitooligosaccharides was converted to a cyclic guanidine structure (2-[(aminoiminomethyl)amino]-2-deoxy-d-glucose structure). This reaction was carefully proven by the guanidinylation of d-glucosamine. Although this is not the first report on the synthesis of the 2-[(aminoiminomethyl)amino]-2-deoxy-d-glucose, it has provided a rational synthetic route using the high reactivity of the reducing end. Furthermore, we found that the interaction between chitooligosaccharides and bovine serum albumin is weak when in a neutral pH environment; however, it is significantly improved by guanidinylation. The guanidinylated chitooligosaccharides are useful not only for the development of a novel drug delivery system but also as a chitinase/chitosanase inhibitor and an antibacterial agent.

Effects of terminal galactose residues in mannose α1-6 arm of Fc-glycan on the effector functions of therapeutic monoclonal antibodies

Typical crystallizable fragment (Fc) glycans attached to the CH2 domain in therapeutic monoclonal antibodies (mAbs) are core-fucosylated and asialo-biantennary complex-type glycans, e.g., G2F (full galactosylation), G1aF (terminal galactosylation on the Man α1-6 arm), G1bF (terminal galactosylation on the Man α1-3 arm), and G0F (non-galactosylation). Terminal galactose (Gal) residues of Fc-glycans are known to influence effector functions such as antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity (CDC), but the impact of the G1F isomers (G1aF and G1bF) on the effector functions has not been reported. Here, we prepared four types of glycoengineered anti-CD20 mAbs bearing homogeneous G2F, G1aF, G1bF, or G0F (G2F mAb, G1aF mAb, G1bF mAb, or G0F mAb, respectively), and evaluated their biological activities. Interestingly, G1aF mAb showed higher C1q- and FcγR-binding activities, CDC activity, and FcγR-activation property than G1bF mAb. The activities of G1aF mAb and G1bF mAb were at the same level as G2F mAb and G0F mAb, respectively. Hydrogen–deuterium exchange/mass spectrometry analysis of dynamic structures of mAbs revealed the greater involvement of the terminal Gal residue on the Man α1-6 arm in the structural stability of the CH2 domain. Considering that mAbs interact with FcγR and C1q via their hinge proximal region in the CH2 domain, the structural stabilization of the CH2 domain by the terminal Gal residue on the Man α1-6 arm of Fc-glycans may be important for the effector functions of mAbs. To our knowledge, this is the first report showing the impact of G1F isomers on the effector functions and dynamic structure of mAbs.

Abbreviations: ABC, ammonium bicarbonate solution; ACN, acetonitrile; ADCC, antibody-dependent cell-mediated cytotoxicity; C1q, complement component 1q; CDC, complement-dependent cytotoxicity; CQA, critical quality attribute; Endo, endo-β-N-acetylglucosaminidase; FA, formic acid; Fc, crystallizable fragment; FcγR, Fcγ receptors; Fuc, fucose; Gal, galactose; GlcNAc, N-acetylglucosamine; GST, glutathione S-transferase; HER2, human epidermal growth factor receptor 2; HDX, hydrogen–deuterium exchange; HILIC, hydrophilic interaction liquid chromatography; HLB-SPE, hydrophilic-lipophilic balance–solid-phase extraction; HPLC, high-performance liquid chromatography; mAb, monoclonal antibody; Man, mannose; MS, mass spectrometry; PBS, phosphate-buffered saline; SGP, hen egg yolk sialylglycopeptides.

Meet the Board of ChemistryOpen: Antony J. Fairbanks

Antony J. Fairbanks is a Professor in the Department of Chemistry at the University of Canterbury in New Zealand. The research of his group focuses on the broad areas of organic synthesis, particularly applied to carbohydrates. He currently serves as an active Editorial Board member for ChemistryOpen.

A chemo-enzymatic approach for the synthesis of human milk oligosaccharide backbone structures

The ability of an engineered β-N-acetylhexosaminidase to utilize a reactive oxazoline as donor molecule for transglycosylation reaction to synthesize human milk oligosaccharide backbone structures was studied. The human milk oligosaccharide precursor lacto-N-triose II and three regioisomers could be synthesized using the oxazoline, which was either in situ-generated resulting in a chemo-enzymatic sequential cascade or was used as a purified compound. The highest observed concentration of overall transglycosylation products in a cascade reaction was 13.7 mM after 18.5 h, whereas the use of purified oxazoline resulted in 25.0 mM of transglycosylation products after 6.5 h. Remarkably, the in situ-generated oxazoline could be used without any further purification and it was shown that the used enzyme tolerated significant amounts of reagents such as triethylamine, which is reported for the first time for an enzyme from the glycoside hydrolase family 20.

Site-selective chemoenzymatic glycoengineering of Fab and Fc glycans of a therapeutic antibody

The N-glycans attached to the Fab and Fc domains play distinct roles in modulating the functions of antibodies. However, posttranslational site-selective modifications of glycans in antibodies and other multiply glycosylated proteins remain a challenging task. Here, we report a chemoenzymatic method that permits independent manipulation of the Fab and Fc N-glycans, using cetuximab as a model therapeutic monoclonal antibody. Taking advantage of the substrate specificity of three endoglycosidases (Endo-S, Endo-S2, and Endo-F3) and their glycosynthase mutants, together with an unexpected substrate site-selectivity of a bacterial α1,6-fucosidase from Lactobacillus casei (AlfC), we were able to synthesize an optimal homogeneous glycoform of cetuximab in which the heterogeneous and immunogenic Fab N-glycans were replaced with a single sialylated N-glycan, and the core-fucosylated Fc N-glycans were remodeled with a nonfucosylated and fully galactosylated N-glycan. The glycoengineered cetuximab demonstrated increased affinity for the FcγIIIa receptor and significantly enhanced antibody-dependent cell-mediated cytotoxicity (ADCC) activity.

V. RK, Miyoshi R, Yokoi Y, Tanaka M, Hinou H, Nishimura SI. Synthetic Glycopeptides Allow for the Quantitation of Scarce Nonfucosylated IgG Fc N-Glycans of Therapeutic Antibody

Glycans attached to the IgG Fc domain affect strongly biological activities such as antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) of therapeutic antibodies. However, molecular mechanism in the glycoform-dependent functional modulation of the IgGs remains elusive. The present study communicates that selected reaction monitoring (SRM)-based assay of tryptic IgG Fc glycopeptides is a promising approach for the characterization of antibodies when combined with structure-defined synthetic Fc peptides having a focused N-glycoform as a calibration standard. We describe a novel synthetic approach to the human IgG1 Fc peptide having a bisected decasaccharide and its nonbisected counterpart compound, the signatures of antibodies involving Fc domain with rare N-glycans expected to show much higher ADCC/CDC than abundant IgG N-glycans, and their application to the SRM-based quantitative glycoproteomics. Use of a key intermediate, phenyl (2-O-benzyl-4,6-O-benzylidine-β-d-mannopyranosyl)-(1 → 4)-3,6-di-O-benzyl-2-azido-2-deoxy-1-thio-β-d-glucopyranoside, derived from locust bean gum galactomannan, facilitated greatly the synthesis of a bisected nonasaccharide as a stable precursor of oxazoline derivative needed for the enzymatic trans-glycosylation with Fc nonapeptide carrying a GlcNAc at Asn297 residue, while the coupling reaction catalyzed by mutant endo-M-N175Q proceeded very slowly. Strikingly, SRM assay using the synthetic Fc glycopeptides as calibration standards uncovered the occurrence of the targeted IgG1 Fc fragment carrying a nonfucosylated and bisected (315 fmol, 0.20%) and its nonbisected counterpart (1154 fmol, 0.73%) in the tryptic digests from 158 pmol of anticancer antibody Herceptin (trastuzumab). The results suggest that aberrantly glycosylated IgG Fc variants may contribute to the total biological activities of the therapeutic antibodies.

Chemoenzymatic Methods for the Synthesis of Glycoproteins

Glycosylation is one of the most prevalent posttranslational modifications that profoundly affects the structure and functions of proteins in a wide variety of biological recognition events. However, the structural complexity and heterogeneity of glycoproteins, usually resulting from the variations of glycan components and/or the sites of glycosylation, often complicates detailed structure-function relationship studies and hampers the therapeutic applications of glycoproteins. To address these challenges, various chemical and biological strategies have been developed for producing glycan-defined homogeneous glycoproteins. This review highlights recent advances in the development of chemoenzymatic methods for synthesizing homogeneous glycoproteins, including the generation of various glycosynthases for synthetic purposes, endoglycosidase-catalyzed glycoprotein synthesis and glycan remodeling, and direct enzymatic glycosylation of polypeptides and proteins. The scope, limitation, and future directions of each method are discussed.