A Photobacterium sp. α2-6-sialyltransferase (Psp2,6ST) mutant with an increased expression level and improved activities in sialylating Tn antigens

Characterization and application of active human α2,6-sialyltransferases ST6GalNAc V and ST6GalNAc VI recombined in Escherichia coli

Eukaryotic sialyltransferases play key roles in many physiological and pathological events. The expression of active human recombinant sialyltransferases in bacteria is still challenging. In the current study, the genes encoding human N-acetylgalactosaminide α2,6-sialyltransferase V (hST6GalNAc V) and N-acetylgalactosaminide α2,6-sialyltransferase VI (hST6GalNAc VI) lacking the N-terminal transmembrane domains were cloned into the expression vectors, pET-32a and pET-22b, respectively. Soluble and active forms of recombinant hST6GalNAc V and hST6GalNAc VI when coexpressed with the chaperone plasmid pGro7 were successfully achieved in Escherichia coli. Further, lactose (Lac), Lacto-N-triose II (LNT II), lacto-N-tetraose (LNT), and sialyllacto-N-tetraose a (LSTa) were used as acceptor substrates to investigate their activities and substrate specificities. Unexpectedly, both can transfer sialic acid onto all those substrates. Compared with hST6GalNAc V expressed in the mammalian cells, the recombinant two α2,6-sialyltransferases in bacteria displayed flexible substrate specificities and lower enzymatic efficiency. In addition, an important human milk oligosaccharide disialyllacto-N-tetraose (DSLNT) can be synthesized by both human α2,6-sialyltransferases expressed in E. coli using LSTa as an acceptor substrate. To the best of our knowledge, these two active human α2,6-sialyltransferases enzymes were expressed in bacteria for the first time. They showed a high potential to be applied in biotechnology and investigating the molecular mechanisms of biological and pathological interactions related to sialylated glycoconjugates.

Enabling Chemoenzymatic Strategies and Enzymes for Synthesizing Sialyl Glycans and Sialyl Glycoconjugates

Sialic acids are fascinating negatively charged nine-carbon monosaccharides. Sialic acid-containing glycans and glycoconjugates are structurally diverse, functionally important, and synthetically challenging molecules. We have developed highly efficient chemoenzymatic strategies that combine the power of chemical synthesis and enzyme catalysis to make sialic acids, sialyl glycans, sialyl glycoconjugates, and their derivatives more accessible, enabling the efforts to explore their functions and applications. The Account starts with a brief description of the structural diversity and the functional importance of naturally occurring sialic acids and sialosides. The development of one-pot multienzyme (OPME) chemoenzymatic sialylation strategies is then introduced, highlighting its advantages in synthesizing structurally diverse sialosides with a sialyltransferase donor substrate engineering tactic. With the strategy, systematic access to sialosides containing different sialic acid forms with modifications at C3/4/5/7/8/9, various internal glycans, and diverse sialyl linkages is now possible. Also briefly described is the combination of the OPME sialylation strategy with bacterial sialidases for synthesizing sialidase inhibitors. With the goal of simplifying the product purification process for enzymatic glycosylation reactions, glycosphingolipids that contain a naturally existing hydrophobic tag are attractive targets for chemoenzymatic total synthesis. A user-friendly highly efficient chemoenzymatic strategy is developed which involves three main processes, including chemical synthesis of lactosyl sphingosine as a water-soluble hydrophobic tag-containing intermediate, OPME enzymatic extension of its glycan component with a single C18-cartridge purification of the product, followed by a facile chemical acylation reaction. The strategy allows the introduction of different sialic acid forms and diverse fatty acyl chains into the products. Gram-scale synthesis has been demonstrated. OPME sialylation has also been demonstrated for the chemoenzymatic synthesis of sialyl glycopeptides and in vitro enzymatic N-glycan processing for the formation of glycoproteins with disialylated biantennary complex-type N-glycans. For synthesizing human milk oligosaccharides (HMOs) which are glycans with a free reducing end, acceptor substrate engineering and process engineering strategies are developed, which involve the design of a hydrophobic tag that can be easily installed into the acceptor substrate to allow facile purification of the product from enzymatic reactions and can be conveniently removed in the final step to produce target molecules. The process engineering involves heat-inactivation of enzymes in the intermediate steps in multistep OPME reactions for the production of long-chain sialoside targets in a single reaction pot and with a single C18-cartridge purification process. In addition, a chemoenzymatic synthon strategy has been developed. It involves the design of a derivative of the sialyltransferase donor substrate precursor, which is tolerated by enzymes in OPME reactions, introduced to enzymatic products, and then chemically converted to the desired target structures in the final step. The chemoenzymatic synthon approach has been used together with the acceptor substrate engineering method in the synthesis of complex bacterial glycans containing sialic acids, legionaminic acids, and derivatives. The biocatalysts characterized and their engineered mutants developed by the Chen group are described, with highlights on synthetically useful enzymes. We anticipate further development of chemoenzymatic strategies and biocatalysts to enable exploration of the sialic acid space.

Glycoprotein In Vitro N-Glycan Processing Using Enzymes Expressed in E. coli

Protein N-glycosylation is a common post-translational modification that plays significant roles on the structure, property, and function of glycoproteins. Due to N-glycan heterogeneity of naturally occurring glycoproteins, the functions of specific N-glycans on a particular glycoprotein are not always clear. Glycoprotein in vitro N-glycan engineering using purified recombinant enzymes is an attractive strategy to produce glycoproteins with homogeneous N-glycoforms to elucidate the specific functions of N-glycans and develop better glycoprotein therapeutics. Toward this goal, we have successfully expressed in E. coli glycoside hydrolases and glycosyltransferases from bacterial and human origins and developed a robust enzymatic platform for in vitro processing glycoprotein N-glycans from high-mannose-type to α2-6- or α2-3-disialylated biantennary complex type. The recombinant enzymes are highly efficient in step-wise or one-pot reactions. The platform can find broad applications in N-glycan engineering of therapeutic glycoproteins.

Cellular and Molecular Engineering of Glycan Sialylation in Heterologous Systems

Glycans have been shown to play a key role in many biological processes, such as signal transduction, immunogenicity, and disease progression. Among the various glycosylation modifications found on cell surfaces and in biomolecules, sialylation is especially important, because sialic acids are typically found at the terminus of glycans and have unique negatively charged moieties associated with cellular and molecular interactions. Sialic acids are also crucial for glycosylated biopharmaceutics, where they promote stability and activity. In this regard, heterogenous sialylation may produce variability in efficacy and limit therapeutic applications. Homogenous sialylation may be achieved through cellular and molecular engineering, both of which have gained traction in recent years. In this paper, we describe the engineering of intracellular glycosylation pathways through targeted disruption and the introduction of carbohydrate active enzyme genes. The focus of this review is on sialic acid-related genes and efforts to achieve homogenous, humanlike sialylation in model hosts. We also discuss the molecular engineering of sialyltransferases and their application in chemoenzymatic sialylation and sialic acid visualization on cell surfaces. The integration of these complementary engineering strategies will be useful for glycoscience to explore the biological significance of sialic acids on cell surfaces as well as the future development of advanced biopharmaceuticals.

Engineering a bacterial sialyltransferase for di-sialylation of a therapeutic antibody

Terminal α-2,6-sialylation of N-glycans is a humanized glycosylation that affects the properties and efficacy of therapeutic glycoproteins. Fc di-sialylation (a biantennary N-glycan with two α-2,6-linked sialic acids) of IgG antibodies imparts them with enhanced anti-inflammatory activity and other roles. However, the microheterogeneity of N-glycoforms presents a challenge for therapeutic development. Therefore, controlled sialylation has drawn considerable attention, but direct access to well-defined di-sialylated antibodies remains limited. Herein, a one-pot three-enzyme protocol was developed by engineering a bacterial sialyltransferase to facilitate the modification of therapeutic antibodies with N-acetylneuraminic acid or its derivatives towards optimized glycosylation. To overcome the low proficiency of bacterial sialyltransferase in antibody remodeling, the Photobacterium sp. JT-ISH-224 α-2,6-sialyltransferase (Psp2,6ST) was genetically engineered by terminal truncation and site-directed mutagenesis based on its protein crystal structure. With the optimized reaction conditions and using activity-based screening of various Psp2,6ST variants, a truncated mutant Psp2,6ST (111-511)-His6 A235M/A366G was shown to effectively improve the catalytic efficiency of antibody di-sialylation. Herceptin and the donor substrate promiscuity allow the introduction of bioorthogonal modifications of N-acetylneuraminic acid into antibodies for site-specific conjugation. 2-AB hydrophilic interaction chromatography analysis of the released N-glycans and intact mass characterization confirmed the high di-sialylation of Herceptin via the optimized one-pot three-enzyme reaction. This study established a versatile enzymatic approach for producing highly di-sialylated IgG antibodies. It provides new insights into engineering bacterial sialyltransferase for adaptation to the enzymatic glycoengineering of therapeutic antibodies and the glycosite-specific conjugation of antibodies.

The role of 9-O-acetylated glycan receptor moieties in the typhoid toxin binding and intoxication

Typhoid toxin is an A2B5 toxin secreted from Salmonella Typhi-infected cells during human infection and is suggested to contribute to typhoid disease progression and the establishment of chronic infection. To deliver the enzymatic 'A' subunits of the toxin to the site of action in host cells, the receptor-binding 'B' subunit PltB binds to the trisaccharide glycan receptor moieties terminated in N-acetylneuraminic acid (Neu5Ac) that is α2-3 or α2-6 linked to the underlying disaccharide, galactose (Gal) and N-acetylglucosamine (GlcNAc). Neu5Ac is present in both unmodified and modified forms, with 9-O-acetylated Neu5Ac being the most common modification in humans. Here we show that host cells associated with typhoid toxin-mediated clinical signs express both unmodified and 9-O-acetylated glycan receptor moieties. We found that PltB binds to 9-O-acetylated α2-3 glycan receptor moieties with a markedly increased affinity, while the binding affinity to 9-O-acetylated α2-6 glycans is only slightly higher, as compared to the affinities of PltB to the unmodified counterparts, respectively. We also present X-ray co-crystal structures of PltB bound to related glycan moieties, which supports the different effects of 9-O-acetylated α2-3 and α2-6 glycan receptor moieties on the toxin binding. Lastly, we demonstrate that the cells exclusively expressing unmodified glycan receptor moieties are less susceptible to typhoid toxin than the cells expressing 9-O-acetylated counterparts, although typhoid toxin intoxicates both cells. These results reveal a fine-tuning mechanism of a bacterial toxin that exploits specific chemical modifications of its glycan receptor moieties for virulence and provide useful insights into the development of therapeutics against typhoid fever.

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.

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.

Harnessing glycoenzyme engineering for synthesis of bioactive oligosaccharides

Combined with chemical synthesis, the use of glycoenzyme biocatalysts has shown great synthetic potential over recent decades owing to their remarkable versatility in terms of substrates and regio- and stereoselectivity that allow structurally controlled synthesis of carbohydrates and glycoconjugates. Nonetheless, the lack of appropriate enzymatic tools with requisite properties in the natural diversity has hampered extensive exploration of enzyme-based synthetic routes to access relevant bioactive oligosaccharides, such as cell-surface glycans or prebiotics. With the remarkable progress in enzyme engineering, it has become possible to improve catalytic efficiency and physico-chemical properties of enzymes but also considerably extend the repertoire of accessible catalytic reactions and tailor novel substrate specificities. In this review, we intend to give a brief overview of the advantageous use of engineered glycoenzymes, sometimes in combination with chemical steps, for the synthesis of natural bioactive oligosaccharides or their precursors. The focus will be on examples resulting from the three main classes of glycoenzymes specialized in carbohydrate synthesis: glycosyltransferases, glycoside hydrolases and glycoside phosphorylases.

Strategies for chemoenzymatic synthesis of carbohydrates

Carbohydrates are structurally complex but functionally important biomolecules. Therefore, they have been challenging but attractive synthetic targets. While substantial progress has been made on advancing chemical glycosylation methods, incorporating enzymes into carbohydrate synthetic schemes has become increasingly practical as more carbohydrate biosynthetic and metabolic enzymes as well as their mutants with synthetic application are identified and expressed for preparative and large-scale synthesis. Chemoenzymatic strategies that integrate the flexibility of chemical derivatization with enzyme-catalyzed reactions have been extremely powerful. Briefly summarized here are our experiences on developing one-pot multienzyme (OPME) systems and representative chemoenzymatic strategies from others using glycosyltransferase-catalyzed reactions for synthesizing diverse structures of oligosaccharides, polysaccharides, and glycoconjugates. These strategies allow the synthesis of complex carbohydrates including those containing naturally occurring carbohydrate postglycosylational modifications (PGMs) and non-natural functional groups. By combining these srategies with facile purification schemes, synthetic access to the diverse space of carbohydrate structures can be automated and will not be limited to specialists.

A Diazido Mannose Analogue as a Chemoenzymatic Synthon for Synthesizing Di-N-acetyllegionaminic Acid-Containing Glycosides

A chemoenzymatic synthon was designed to expand the scope of the chemoenzymatic synthesis of carbohydrates. The synthon was enzymatically converted into carbohydrate analogues, which were readily derivatized chemically to produce the desired targets. The strategy is demonstrated for the synthesis of glycosides containing 7,9-di-N-acetyllegionaminic acid (Leg5,7Ac₂ ), a bacterial nonulosonic acid (NulO) analogue of sialic acid. A versatile library of α2-3/6-linked Leg5,7Ac₂ -glycosides was built by using chemically synthesized 2,4-diazido-2,4,6-trideoxymannose as a chemoenzymatic synthon for highly efficient one-pot multienzyme (OPME) sialylation followed by downstream chemical conversion of the azido groups into acetamido groups. The syntheses required 10 steps from commercially available d-fucose and had an overall yield of 34-52 %, thus representing a significant improvement over previous methods. Free Leg5,7Ac₂ monosaccharide was also synthesized by a sialic acid aldolase-catalyzed reaction.

Chemoenzymatic synthesis of Neu5Ac9NAc-containing α2-3- and α2-6-linked sialosides and their use for sialidase substrate specificity studies

O-Acetylation of sialic acid (Sia) modulates its recognition by sialic acid-binding proteins and plays an important role in biological and pathological processes. 9-O-Acetylation is the most common modification of sialic acid in human. However, study of O-acetylated sialoglycans is hampered due to the instability of O-acetyl group towards pH changes and sensitivity to esterases. Our previous studies demonstrated a chemical biology method to this problem by replacing the oxygen atom in the C9 ester group of sialic acid by a nitrogen to form an amide. Here, we synthesized a library of sixteen new 9-acetamido-9-deoxy-N-acetylneuraminic acid (Neu5Ac9NAc)-containing α2-3- and α2-6-linked sialosides with various underlying glycans using efficient one-pot three-enzyme (OP3E) sialylation systems. Neu5Ac9NAc-containing compounds with a para-nitrophenol aglycon have been used together with their 9-O-acetyl analogs in microtiter plate-based high-throughput substrate specificity studies of nine different sialidases including those from humans and bacteria. In general, similar to 9-O-acetylation, 9-N-acetyl modification of sialic acid in the substrates lowers sialic acid-cleavage activity of most sialidases. In most cases, Neu5Ac9NAc is a good analog of 9-O-acetyl sialic acid. However, exceptions do exist. For example, 9-N- and 9-O-acetyl modifications have different effects on the sialosides cleave efficiencies of a commercially available C. perfringens sialidase as well as recombinant Streptococcus pneumoniae sialidase SpNanC and Bifidobacterium infantis sialidase BiNanH2. The mechanism for the difference awaits further investigation.

Active-Site His85 of Pasteurella dagmatis Sialyltransferase Facilitates Productive Sialyl Transfer and So Prevents Futile Hydrolysis of CMP-Neu5Ac

Sialyltransferases of the GT-80 glycosyltransferase family are considered multifunctional because of the array of activities detected. They exhibit glycosyl transfer, trans-sialylation, and hydrolysis activities. How these enzymes utilize their active-site residues in balancing the different enzymatic activities is not well understood. In this study of Pasteurella dagmatis α2,3sialyltransferase, we show that the conserved His85 controls efficiency and selectivity of the sialyl transfer. A His85→Asn variant was 200 times less efficient than wild-type for sialylation of lactose, and exhibited relaxed site selectivity to form not only the α2,3- but also the α2,6-sialylated product (21 %). The H85N variant was virtually inactive in trans-sialylation but showed almost the same CMP-Neu5Ac hydrolase activity as wild-type. The competition between sialyl transfer and hydrolysis in the conversion of CMP-Neu5Ac was dependent on the lactose concentration; this was characterized by a kinetic partition ratio of 85 m^-1 for the H85N variant, compared to 17 000 m^-1 for the wild-type enzyme. His85 promotes the productive sialyl transfer to lactose and so prevents hydrolysis of CMP-Neu5Ac in the reaction.

Converting Pasteurella multocidaα2-3-sialyltransferase 1 (PmST1) to a regioselective α2-6-sialyltransferase by saturation mutagenesis and regioselective screening

A microtiter plate-based screening assay capable of determining the activity and regioselectivity of sialyltransferases was developed. This assay was used to screen two single-site saturation libraries of Pasteurella multocidaα2-3-sialyltransferase 1 (PmST1) for α2-6-sialyltransferase activity and total sialyltransferase activity. PmST1 double mutant P34H/M144L was found to be the most effective α2-6-sialyltransferase and displayed 50% reduced donor hydrolysis and 50-fold reduced sialidase activity compared to the wild-type PmST1. It retained the donor substrate promiscuity of the wild-type enzyme and was used in an efficient one-pot multienzyme (OPME) system to selectively catalyze the sialylation of the terminal galactose residue in a multigalactose-containing tetrasaccharide lacto-N-neotetraoside.

Glycosyltransferase engineering for carbohydrate synthesis

Glycosyltransferases (GTs) are powerful tools for the synthesis of complex and biologically-important carbohydrates. Wild-type GTs may not have all the properties and functions that are desired for large-scale production of carbohydrates that exist in nature and those with non-natural modifications. With the increasing availability of crystal structures of GTs, especially those in the presence of donor and acceptor analogues, crystal structure-guided rational design has been quite successful in obtaining mutants with desired functionalities. With current limited understanding of the structure-activity relationship of GTs, directed evolution continues to be a useful approach for generating additional mutants with functionality that can be screened for in a high-throughput format. Mutating the amino acid residues constituting or close to the substrate-binding sites of GTs by structure-guided directed evolution (SGDE) further explores the biotechnological potential of GTs that can only be realized through enzyme engineering. This mini-review discusses the progress made towards GT engineering and the lessons learned for future engineering efforts and assay development.

Effective one-pot multienzyme (OPME) synthesis of monotreme milk oligosaccharides and other sialosides containing 4-O-acetyl sialic acid

A facile one-pot two-enzyme chemoenzymatic approach has been established for the gram (Neu4,5Ac2α3Lac, 1.33 g) and preparative scale (Neu4,5Ac2α3LNnT) synthesis of monotreme milk oligosaccharides. Other O-acetyl-5-N-acetylneuraminic acid (Neu4,5Ac2)- or 4-O-acetyl-5-N-glycolylneuraminic acid (Neu4Ac5Gc) -containing α2-3-sialosides have also been synthesized in the preparative scale. Used as an effective probe, Neu4,5Ac2α3GalβpNP was found to be a suitable substrate by human influenza A viruses but not bacterial sialidases.

Two N-terminally truncated variants of human β-galactoside α2,6 sialyltransferase I with distinct properties for in vitro protein glycosylation

Sialic acid groups of protein N-glycans are important determinants of biological activity. Exposed at the end of the glycan chain, they are potential targets for glycan remodeling. Sialyltransferases (STs; EC 2.4.99) are the enzymes that catalyze the sialic acid transfer from a CMP-activated donor on to a carbohydrate acceptor in vivo. Recombinant expression of the full-length human β-galactoside α2,6 sialyltransferase I (ST6Gal-I) was hampered and therefore variants with truncated N-termini were investigated. We report on the distinct properties of two N-terminally truncated versions of ST6Gal-I, namely Δ89ST6Gal-I and Δ108ST6Gal-I, which were successfully expressed in human embryonic kidney cells. The different properties of these enzymes result most probably from the loss of interactions from helix α1 in the Δ108ST6Gal-I variant, which plays a role in acceptor substrate binding. The K_m for N-acetyl-d-lactosamine was 10-fold increased for Δ108ST6Gal-I (84 mM) as compared to Δ89ST6Gal-I (8.3 mM). The two enzyme variants constitute a suitable tool box for the terminal modification of N-glycans. While the enzyme Δ89ST6Gal-I exhibited both ST (di-sialylation) and sialidase activity on a monoclonal antibody, the enzyme Δ108ST6Gal-I showed only ST activity with specificity for mono-sialylation.

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.