Chemoenzymatic synthesis of α-dystroglycan core M1 O-mannose glycans

Chemical Evolution of Enzyme-Catalyzed Glycosylation

ConspectusThe limited availability of structurally well-defined diverse glycans remains a major obstacle for deciphering biological functions as well as biomedical applications of carbohydrates. Despite tremendous progress that has been made in past decades, the synthesis of structurally well-defined complex glycans still represents one of the most challenging topics in synthetic chemistry. Chemical synthesis of glycans is a time-consuming and labor-intensive process that requires elaborate planning and skilled personnel. In contrast, glycosyltransferase-catalyzed enzymatic synthesis provides a more efficient, convenient, low-cost, and sustainable alternative to affording diverse and complex glycans. However, the existing methods are still insufficient to fulfill the increasing demand for specific synthetic glycan libraries necessary for functional glycomics research. This is mainly attributed to the inherent character of the glycan biosynthetic pathway. In nature, there are too many glycosyltransferases involved in the in vivo glycan synthesis, but only a small number of them are available for in vitro enzymatic synthesis. For instance, humans have over 200 glycosyltransferases, but only a few of them could be produced from the conventional bacterial expression system, and most of these membrane-associated enzymes could be overexpressed only in eukaryotic cells. Moreover, the glycan biosynthetic pathway is a nontemplate-driven process, which eventually ends up with heterogeneous glycan product mixtures. Therefore, it is not a practical solution for the in vitro enzymatic synthesis of complex glycans by simply copying the glycan biosynthetic pathway.In the past decade, we have tried to develop a simplified and transformable approach to the enzymatic modular assembly of a human glycan library. Despite the structural complexity of human glycans, the glycoinformatic analysis based on the known glycan structure database and the human glycosyltransferase database indicates that there are approximately 56 disaccharide patterns present in the human glycome and only 16 disaccharide linkages are required to account for over 80% of the total disaccharide fragments, while 35 disaccharide linkages are sufficient to cover over 95% of all disaccharide fragments of human glycome. Regardless of the substrate specificity, if one glycosyltransferase could be used for the synthesis of all of the same glycosidic linkages in human glycome, it will require only a few dozen glycosyltransferases for the assembly of entire human glycans. According to the glycobioinformatics analysis results, we rationally designed about two dozen enzyme modules for the synthesis of over 20 common glycosidic linkages in human glycome, in which each enzyme module contains a glycosyltransferase and a group of enzymes for the in situ generation of a nucleotide-activated sugar donor. By sequential glycosylation using orchestrated enzyme modules, we have completed the synthesis of over 200 structurally well-defined complex human glycans including blood group antigens, O-mannosyl glycans, human milk oligosaccharides, and others. To overcome the product microheterogeneity problem of enzymatic synthesis in the nontemplate-driven glycan biosynthetic pathway, we developed several substrate engineering strategies to control or manipulate the outcome of glycosyltransferase-catalyzed reactions for the precise synthesis of structurally well-defined isomeric complex glycans.

Recombinant snail sialic acid aldolase is promiscuous towards aliphatic aldehydes

Aldolases are enzymes that reversibly catalyze the cleavage of carbon-carbon bonds. Here we describe a recombinant sialic acid aldolase originating from the freshwater snail Biomphalaria glabrata (sNPL), and compare its substrate spectrum with a sialic acid aldolase originating from chicken (chNPL). In contrast to vertebrate animals which can synthesize, degrade, and incorporate sialic acids on glycoconjugate ubiquitously, snails (as all mollusks) cannot synthesize sialic acids endogenously, and therefore the biological function and substrate scope of sNPL ought to differ significantly from vertebrate sialic aldolases such as chNPL. sNPL was active towards a series of sialic acid derivatives but was in contrast to chNPL unable to catalyze the cleavage of N-acetylneuraminic acid into N-acetylmannosamine and pyruvate. Interestingly, chNPL and sNPL showed contrasting C4 (R) / (S) diastereoselectivity towards the substrates d-mannose and d-galactose in the presence of pyruvate. In addition, sNPL was also able to synthesize a series of 4-hydroxy-2-oxoates using the corresponding aliphatic aldehyde substrates in the presence of pyruvate, which could be not achieved by chNPL.

Enzymatic modular assembly of hybrid Lewis antigens

The enzymatic synthesis of hybrid Lewis antigens including KH-1 (Lewis y-Lewis x-Lactose, Ley-Lex-Lac), Lewis a-Lewis x-Lactose (Lea-Lex-Lac), and Lewis b-Lewis x-Lactose (Leb-Lex-Lac) has been achieved using a facile enzymatic modular assembly strategy. Starting from a readily available tetrasaccharide, 3 complex hybrid Lewis antigens were achieved in over 40% total yields in less than 5 linear steps of sequential enzymatic glycosylation using 6 enzyme modules. The regio-selective fucosylation was achieved by simply controlling the donor-acceptor ratio. This strategy provides an easy access to these biologically important complex hybrid Lewis antigens at preparative scales.

High degree of conservation of the enzymes synthesizing the laminin-binding glycoepitope of α-dystroglycan

The dystroglycan (DG) complex plays a pivotal role for the stabilization of muscles in Metazoa. It is formed by two subunits, extracellular α-DG and transmembrane β-DG, originating from a unique precursor via a complex post-translational maturation process. The α-DG subunit is extensively glycosylated in sequential steps by several specific enzymes and employs such glycan scaffold to tightly bind basement membrane molecules. Mutations of several of these enzymes cause an alteration of the carbohydrate structure of α-DG, resulting in severe neuromuscular disorders collectively named dystroglycanopathies. Given the fundamental role played by DG in muscle stability, it is biochemically and clinically relevant to investigate these post-translational modifying enzymes from an evolutionary perspective. A first phylogenetic history of the thirteen enzymes involved in the fabrication of the so-called 'M3 core' laminin-binding epitope has been traced by an overall sequence comparison approach, and interesting details on the primordial enzyme set have emerged, as well as substantial conservation in Metazoa. The optimization along with the evolution of a well-conserved enzymatic set responsible for the glycosylation of α-DG indicate the importance of the glycosylation shell in modulating the connection between sarcolemma and surrounding basement membranes to increase skeletal muscle stability, and eventually support movement and locomotion.

Biological strategies for oligo/polysaccharide synthesis: biocatalyst and microbial cell factory

Oligosaccharides and polysaccharides constitute the principal components of carbohydrates, which are important biomacromolecules that demonstrate considerable bioactivities. However, the variety and structural complexity of oligo/polysaccharides represent a major challenge for biological and structural explorations. To access structurally defined oligo/polysaccharides, biological strategies using glycoenzyme biocatalysts have shown remarkable synthetic potential attributed to their regioselectivity and stereoselectivity that allow mild, structurally controlled reaction without addition of protecting groups necessary in chemical strategies. This review summarizes recent biotechnological approaches of oligo/polysaccharide synthesis, which mainly includes in vitro enzymatic synthesis and cell factory synthesis. We have discussed the important factors involved in the production of nucleotide sugars. Furthermore, the strategies established in the cell factory and enzymatic syntheses are summarized, and we have highlighted concepts like metabolic flux rebuilding and regulation, enzyme engineering, and route design as important strategies. The research challenges and prospects are also outlined and discussed.

Liquid-Phase and Ultrahigh-Frequency-Acoustofluidics-Based Solid-Phase Synthesis of Biotin-Tagged 6′/3′-Sialyl-N-Acetylglucosamine by Sequential One-Pot Multienzyme System

6′/3′-Sialylated N-acetyllactosamine (6′/3′-SLN) is important for discrimination of the source (human or avian) of influenza virus strains. Biotinylated oligosaccharides have been widely used for analysis and quick detection. The development of efficient strategies to synthesize biotin-tagged 6′/3′-SLN have become necessary. Effective mixing is essential for enzymatic solid-phase oligosaccharide synthesis (SPOS). In the current study, newly developed technology ultrahigh-frequency-acoustofluidics (UHFA), which can provide a powerful source for efficient microfluidic mixing, solid-phase oligosaccharide synthesis and one-pot multienzyme (OPME) system, were used to develop a new strategy for oligosaccharide synthesis. Firstly, biotinylated N-acetylglucosamine was designed and chemically synthesized through traditional approaches. Secondly, biotinylated 6′- and 3′-sialyl-N-acetylglucosamines were prepared in solution through two sequential OPME modules in with a yield of ~95%. Thirdly, 6′-SLN was also prepared through UHFA-based enzymatic solid-phase synthesis on magnetic beads with a yield of 64.4%. The current strategy would be potentially used for synthesis of functional oligosaccharides.

Chemoenzymatic Synthesis of O-Mannose Glycans Containing Sulfated or Nonsulfated HNK-1 Epitope

The human natural killer-1 (HNK-1) epitope is a unique sulfated trisaccharide sequence presented on O- and N-glycans of various glycoproteins and on glycolipids. It is overexpressed in the nervous system and plays crucial roles in nerve regeneration, synaptic plasticity, and neuronal diseases. However, the investigation of functional roles of HNK-1 in a more complex glycan context at the molecular level remains a big challenge due to lack of access to related structurally well-defined complex glycans. Herein, we describe a highly efficient chemoenzymatic approach for the first collective synthesis of HNK-1-bearing O-mannose glycans with different branching patterns, and for their nonsulfated counterparts. The successful strategy relies on both chemical glycosylation of a trisaccharide lactone donor for the introduction of sulfated HNK-1 branch and substrate promiscuities of bacterial glycosyltransferases that can tolerate sulfated substrates for enzymatic diversification. Glycan microarray analysis with the resulting complex synthetic glycans demonstrated their recognition by two HNK-1-specific antibodies including anti-HNK-1/N-CAM (CD57) and Cat-315, which provided further evidence for the recognition epitopes of these antibodies and the essential roles of the sulfate group for HNK-1 glycan-antibody recognition.

Synthesis of N-Glycolylneuraminic Acid (Neu5Gc) and Its Glycosides

Sialic acids constitute a family of negatively charged structurally diverse monosaccharides that are commonly presented on the termini of glycans in higher animals and some microorganisms. In addition to N-acetylneuraminic acid (Neu5Ac), N-glycolyl neuraminic acid (Neu5Gc) is among the most common sialic acid forms in nature. Nevertheless, unlike most animals, human cells loss the ability to synthesize Neu5Gc although Neu5Gc-containing glycoconjugates have been found on human cancer cells and in various human tissues due to dietary incorporation of Neu5Gc. Some pathogenic bacteria also produce Neu5Ac and the corresponding glycoconjugates but Neu5Gc-producing bacteria have yet to be found. In addition to Neu5Gc, more than 20 Neu5Gc derivatives have been found in non-human vertebrates. To explore the biological roles of Neu5Gc and its naturally occurring derivatives as well as the corresponding glycans and glycoconjugates, various chemical and enzymatic synthetic methods have been developed to obtain a vast array of glycosides containing Neu5Gc and/or its derivatives. Here we provide an overview on various synthetic methods that have been developed. Among these, the application of highly efficient one-pot multienzyme (OPME) sialylation systems in synthesizing compounds containing Neu5Gc and derivatives has been proven as a powerful strategy.

Chemoenzymatic Assembly of Mammalian O-Mannose Glycans

O-Mannose glycans account up to 30 % of total O-glycans in the brain. Previous synthesis and functional studies have only focused on the core M3 O-mannose glycans of α-dystroglycan, which are a causative factor for various muscular diseases. In this study, a highly efficient chemoenzymatic strategy was developed that enabled the first collective synthesis of 63 core M1 and core M2 O-mannose glycans. This chemoenzymatic strategy features the gram-scale chemical synthesis of five judiciously designed core structures, and the diversity-oriented modification of the core structures with three enzyme modules to provide 58 complex O-mannose glycans in a linear sequence that does not exceed four steps. The binding profiles of synthetic O-mannose glycans with a panel of lectins, antibodies, and brain proteins were also explored by using a printed O-mannose glycan array.

Facile Chemoenzymatic Synthesis of O-Mannosyl Glycans

O Mannosylation is a vital protein modification involved in brain and muscle development whereas the biological relevance of O-mannosyl glycans has remained largely unknown owing to the lack of structurally defined glycoforms. An efficient scaffold synthesis/enzymatic extension (SSEE) strategy was developed to prepare such structures by combining gram-scale convergent chemical syntheses of three scaffolds and strictly controlled sequential enzymatic extension catalyzed by glycosyltransferases. In total, 45 O-mannosyl glycans were obtained, covering the majority of identified mammalian structures. Subsequent glycan microarray analysis revealed fine specificities of glycan-binding proteins and specific antisera.

Induction of Antibodies Directed Against Branched Core O-Mannosyl Glycopeptides-Selectivity Complimentary to the ConA Lectin

Mammalian protein O-mannosylation, initiated by attachment of α-mannopyranose to Ser or Thr residues, comprise a group of post-translational modifications (PTMs) involved in muscle and brain development. Recent advances in glycoproteomics methodology and the "SimpleCell" strategy have enabled rapid identification of glycoproteins and specific glycosylation sites. Despite the enormous progress made, the biological impact of the mammalian O-mannosyl glycoproteome remains largely unknown to date. Tools are still needed to investigate the structure, role, and abundance of O-mannosyl glycans. Although O-mannosyl branching has been shown to be of relevance in integrin-dependent cell migration, and also plays a role in demyelinating diseases, such as multiple sclerosis, a broader understanding of the biological roles of branched O-mannosyl glycans is lacking in part due to the paucity of detection tools. In this work, a glycopeptide vaccine construct was synthesized and used to generate antibodies against branched O-mannosyl glycans. Glycopeptide microarray screening revealed high selectivity of the induced antibodies for branched glycan core structures presented on different peptide backbones, with no cross-reactivity observed with related linear glycans. For comparison, microarray screening of the mannose-binding lectin concanavalin A (ConA), which is commonly used in glycoproteomics workflows to enrich tryptic O-mannosyl peptides, showed that the ConA lectin did not recognize branched O-mannosyl glycans. The binding preference of ConA for short linear O-mannosyl glycans was rationalized in terms of molecular structure using crystallographic data augmented by molecular modeling. The contrast between the ConA binding specificity and that of the new antibodies indicates a novel role for the antibodies in studies of protein O-mannosylation.

One-pot multienzyme (OPME) systems for chemoenzymatic synthesis of carbohydrates

Glycosyltransferase-catalyzed enzymatic and chemoenzymatic syntheses are powerful approaches for the production of oligosaccharides, polysaccharides, glycoconjugates, and their derivatives. Enzymes involved in the biosynthesis of sugar nucleotide donors can be combined with glycosyltransferases in one pot for efficient production of the target glycans from simple monosaccharides and acceptors. The identification of enzymes involved in the salvage pathway of sugar nucleotide generation has greatly facilitated the development of simplified and efficient one-pot multienzyme (OPME) systems for synthesizing major glycan epitopes in mammalian glycomes. The applications of OPME methods are steadily gaining popularity mainly due to the increasing availability of wild-type and engineered enzymes. Substrate promiscuity of these enzymes and their mutants allows OPME synthesis of carbohydrates with naturally occurring post-glycosylational modifications (PGMs) and their non-natural derivatives using modified monosaccharides as precursors. The OPME systems can be applied in sequence for synthesizing complex carbohydrates. The sequence of the sequential OPME processes, the glycosyltransferase used, and the substrate specificities of the glycosyltransferases define the structures of the products. The OPME and sequential OPME strategies can be extended to diverse glycans in other glycomes when suitable enzymes with substrate promiscuity become available. This Perspective summarizes the work of the authors and collaborators on the development of glycosyltransferase-based OPME systems for carbohydrate synthesis. Future directions are also discussed.

Investigation of benzoyloximes as benzoylating reagents: benzoyl-Oxyma as a selective benzoylating reagent

Benzoyl-Oxyma is a highly crystalline, readily prepared, safer alternative to benzoyloxybenzotriazole, useful in the selective benzoylation of carbohydrate polyols.