Donor substrate promiscuity of the N-acetylglucosaminyltransferase activities of Pasteurella multocida heparosan synthase 2 (PmHS2) and Escherichia coli K5 KfiA

Enzyme-mediated green synthesis of glycosaminoglycans and catalytic process intensification

Glycosaminoglycans (GAGs) are a family of structurally complex heteropolysaccharides that play pivotal roles in biological functions, including the regulation of cell proliferation, enzyme inhibition, and activation of growth factor receptors. Therefore, the synthesis of GAGs is a hot research topic in drug development. The enzymatic synthesis of GAGs has received widespread attention due to their eco-friendly nature, high regioselectivity, and stereoselectivity. The enhancement of the enzymatic synthesis process is the key to its industrial applications. In this review, we overviewed the construction of more efficient in vitro biomimetic synthesis systems of glycosaminoglycans and presented the different strategies to improve enzyme catalysis, including the combination of chemical and enzymatic methods, solid-phase synthesis, and protein engineering to solve the problems of enzyme stability, separation and purification of the product, preparation of structurally defined sugar chains, etc., and discussed the challenges and opportunities in large-scale green synthesis of GAGs.

Biosynthetic production of anticoagulant heparin polysaccharides through metabolic and sulfotransferases engineering strategies

Heparin is an important anticoagulant drug, and microbial heparin biosynthesis is a potential alternative to animal-derived heparin production. However, effectively using heparin synthesis enzymes faces challenges, especially with microbial recombinant expression of active heparan sulfate N-deacetylase/N-sulfotransferase. Here, we introduce the monosaccharide N-trifluoroacetylglucosamine into Escherichia coli K5 to facilitate sulfation modification. The Protein Repair One-Stop Service-Focused Rational Iterative Site-specific Mutagenesis (PROSS-FRISM) platform is used to enhance sulfotransferase efficiency, resulting in the engineered NST-M8 enzyme with significantly improved stability (11.32-fold) and activity (2.53-fold) compared to the wild-type N-sulfotransferase. This approach can be applied to engineering various sulfotransferases. The multienzyme cascade reaction enables the production of active heparin from bioengineered heparosan, demonstrating anti-FXa (246.09 IU/mg) and anti-FIIa (48.62 IU/mg) activities. This study offers insights into overcoming challenges in heparin synthesis and modification, paving the way for the future development of animal-free heparins using a cellular system-based semisynthetic strategy.

Advancements in heparosan production through metabolic engineering and improved fermentation

Heparin is one of the most widely used natural drugs, and has been the preferred anticoagulant and antithrombotic agent in the clinical setting for nearly a century. Heparin also shows increasing therapeutic potential for treating inflammation, cancer, and microbial and viral diseases, including COVID-19. With advancements in synthetic biology, heparin production through microbial engineering of heparosan offers a cost-effective and scalable alternative to traditional extraction from animal tissues. Heparosan serves as the starting carbon backbone for the chemoenzymatic synthesis of bioengineered heparin, possessing a chain length that is critically important for the production of heparin-based therapeutics with specific molecular weight (MW) distributions. Recent advancements in metabolic engineering of microbial cell factories have resulted in high-yield heparosan production. This review systematically analyzes the key modules involved in microbial heparosan biosynthesis and the latest metabolic engineering strategies for enhancing production, regulating MW, and optimizing the fermentation scale-up of heparosan. It also discusses future studies, remaining challenges, and prospects in the field.

Production of different molecular weight glycosaminoglycans with microbial cell factories

Glycosaminoglycans (GAGs) are naturally occurring acidic polysaccharides with wide applications in pharmaceuticals, cosmetics, and health foods. The diverse biological activities and physiological functions of GAGs are closely associated with their molecular weights and sulfation patterns. Except for the non-sulfated hyaluronan which can be synthesized naturally by group A Streptococcus, all the other GAGs such as heparin and chondroitin sulfate are mainly acquired from animal tissues. Microbial cell factories provide a more effective platform for the production of structurally homogeneous GAGs. Enhancing the production efficiency of polysaccharides, accurately regulating the GAGs molecular weight, and effectively controlling the sulfation degree of GAGs represent the major challenges of developing GAGs microbial cell factories. Several enzymatic, metabolic engineering, and synthetic biology strategies have been developed to tackle these obstacles and push forward the industrialization of biotechnologically produced GAGs. This review summarizes the recent advances in the construction of GAGs synthesis cell factories, regulation of GAG molecular weight, and modification of GAGs chains. Furthermore, the challenges and prospects for future research in this field are also discussed.

Novel β1,4 N-acetylglucosaminyltransferase in de novo enzymatic synthesis of hyaluronic acid oligosaccharides

The efficiency of de novo synthesis of hyaluronic acid (HA) using Pasteurella multocida hyaluronate synthase (PmHAS) is limited by its low catalytic activity during the initial reaction steps when monosaccharides are the acceptor substrates. In this study, we identified and characterized a β-1,4-N-acetylglucosaminyl-transferase (EcGnT) derived from the O-antigen gene synthesis cluster of Escherichia coli O8:K48:H9. Recombinant β1,4 EcGnT effectively catalyzed the production of HA disaccharides when the glucuronic acid monosaccharide derivative 4-nitrophenyl-β-D-glucuronide (GlcA-pNP) was used as the acceptor. Compared with PmHAS, β1,4 EcGnT exhibited superior N-acetylglucosamine transfer activity (~ 12-fold) with GlcA-pNP as the acceptor, making it a better option for the initial step of de novo HA oligosaccharide synthesis. We then developed a biocatalytic approach for size-controlled HA oligosaccharide synthesis using the disaccharide produced by β1,4 EcGnT as a starting material, followed by stepwise PmHAS-catalyzed synthesis of longer oligosaccharides. Using this approach, we produced a series of HA chains of up to 10 sugar monomers. Overall, our study identifies a novel bacterial β1,4 N-acetylglucosaminyltransferase and establishes a more efficient process for HA oligosaccharide synthesis that enables size-controlled production of HA oligosaccharides. KEY POINTS: • A novel β-1,4-N-acetylglucosaminyl-transferase (EcGnT) from E. coli O8:K48:H9. • EcGnT is superior to PmHAS for enabling de novo HA oligosaccharide synthesis. • Size-controlled HA oligosaccharide synthesis relay using EcGnT and PmHAS.

Imaging of Escherichia coli K5 and glycosaminoglycan precursors via targeted metabolic labeling of capsular polysaccharides in bacteria

The introduction of unnatural chemical moieties into glycosaminoglycans (GAGs) has enormous potential to facilitate studies of the mechanism and application of these critical, widespread molecules. Unnatural N-acetylhexosamine analogs were metabolically incorporated into the capsule polysaccharides of Escherichia coli and Bacillus subtilis via bacterial metabolism. Targeted metabolic labeled hyaluronan and the precursors of heparin and chondroitin sulfate were obtained. The azido-labeled polysaccharides (purified or in capsules) were reacted with dyes, via bioorthogonal chemistry, to enable detection and imaging. Site-specific introduction of fluorophores directly onto cell surfaces affords another choice for observing and quantifying bacteria in vivo and in vitro. Furthermore, azido-polysaccharides retain similar biological properties to their natural analogs, and reliable and predictable introduction of functionalities, such as fluorophores, onto azido-N-hexosamines in the disaccharide repeat units provides chemical tools for imaging and metabolic analysis of GAGs in vivo and in vitro.

Heparosan oligosaccharide synthesis using engineered single-function glycosyltransferases

A 24-fold increase in GlcNAc-transferase ability through KfiA screening and engineering. An approach for heparosan oligosaccharide synthesis relying on single-function glycosyltransferases.

Enzymatic glycosylation involving fluorinated carbohydrates

Fluorinated carbohydrates, where one (or more) fluorine atom(s) have been introduced into a carbohydrate structure, typically through deoxyfluorination chemistry, have a wide range of applications in the glycosciences. Fluorinated derivatives of galactose, glucose, N-acetylgalactosamine, N-acetylglucosamine, talose, fucose and sialic acid have been employed as either donor or acceptor substrates in glycosylation reactions. Fluorinated donors can be synthesised by synthetic methods or produced enzymatically from chemically fluorinated sugars. The latter process is mediated by enzymes such as kinases, phosphorylases and nucleotidyltransferases. Fluorinated donors produced by either method can subsequently be used in glycosylation reactions mediated by glycosyltransferases, or phosphorylases yielding fluorinated oligosaccharide or glycoconjugate products. Fluorinated acceptor substrates are typically synthesised chemically. Glycosyltransferases are most commonly used in conjunction with natural donors to further elaborate fluorinated acceptor substrates. Glycoside hydrolases are used with either fluorinated donors or acceptors. The activity of enzymes towards fluorinated sugars is often lower than towards the natural sugar substrates irrespective of donor or acceptor. This may be in part attributed to elimination of the contribution of the hydroxyl group to the binding of the substrate to enzymes. However, in many cases, enzymes still maintain a significant activity, and reactions may be optimised where necessary, enabling enzymes to be used more successfully in the production of fluorinated carbohydrates. This review describes the current state of the art regarding chemoenzymatic production of fluorinated carbohydrates, focusing specifically on examples of the enzymatic production of activated fluorinated donors and enzymatic glycosylation involving fluorinated sugars as either glycosyl donors or acceptors.

Enzymatic Synthesis of Glycans and Glycoconjugates

Glycoconjugates have great potential to improve human health in a multitude of different ways and fields. Prominent examples are human milk oligosaccharides and glycosaminoglycans. The typical choice for the production of homogeneous glycoconjugates is enzymatic synthesis. Through the availability of expression and purification protocols, recombinant Leloir glycosyltransferases are widely applied as catalysts for the synthesis of a wide range of glycoconjugates. Extensive utilization of these enzymes also depends on the availability of activated sugars as building blocks. Multi-enzyme cascades have proven a versatile technique to synthesize and in situ regenerate nucleotide sugar.In this chapter, the functions and mechanisms of Leloir glycosyltransferases are revisited, and the advantage of prokaryotic sources and production systems is discussed. Moreover, in vivo and in vitro pathways for the synthesis of nucleotide sugar are reviewed. In the second part, recent and prominent examples of the application of Leloir glycosyltransferase are given, i.e., the synthesis of glycosaminoglycans, glycoconjugate vaccines, and human milk oligosaccharides as well as the re-glycosylation of biopharmaceuticals, and the status of automated glycan assembly is revisited.

Glycosyltransferase-Induced Morphology Transition of Glycopeptide Self-Assemblies with Proteoglycan Residues

We previously proposed the deprotection-induced block copolymer self-assembly (DISA), that is, the deprotection of hydroxyl groups of saccharides resulted in self-assembly of glycopolymers (Qi et al. J. Am. Chem. Soc. 2018, 140 (28), 8851-8857 and Su et al. ACS Macro Lett. 2014, 3 (6), 534-539). In this study, we further combined glycochemistry and self-assembly strategy by introducing glycosyltransferase as the trigger, which constructs another glycosidic bonds and another carbohydrate building blocks in situ. Herein, we propose to utilize glycosyltransferase to induce the morphology transition of glycopeptide assemblies in the process of glycosidic bonds construction, which has never been reported in literature. This strategy provides us an alternative tool to construct proteoglycan-mimicking polymeric materials and deepens our understanding on the natural process of proteoglycan construction better in the future.

Engineer P. multocida Heparosan Synthase 2 (PmHS2) for Size-Controlled Synthesis of Longer Heparosan Oligosaccharides

Pasteurella multocida heparosan synthase 2 (PmHS2) is a dual-function polysaccharide synthase having both α1-4-N-acetylglucosaminyltransferase (α1-4-GlcNAcT) and β1-4-glucuronyltransferase (β1-4-GlcAT) activities located in two separate catalytic domains. We found that removing PmHS2 N-terminal 80-amino acid residues improved enzyme stability and expression level while retaining its substrate promiscuity. We also identified the reverse glycosylation activities of PmHS2 which complicated its application in size-controlled synthesis of oligosaccharides longer than hexasaccharide. Engineered Δ80PmHS2 single-function-glycosyltransferase mutants Δ80PmHS2_D291N (α1-4-GlcNAcT lacking both forward and reverse β1-4-GlcAT activities) and Δ80PmHS2_D569N (β1-4-GlcAT lacking both forward and reverse α1-4-GlcNAcT activities) were designed and showed to minimize side product formation. They were successfully used in a sequential one-pot multienzyme (OPME) platform for size-controlled high-yield production of oligosaccharides up to decasaccharide. The study draws attention to the consideration of reverse glycosylation activities of glycosyltransferases, including polysaccharide synthases, when applying them in the synthesis of oligosaccharides and polysaccharides. The mutagenesis strategy has the potential to be extended to other multifunctional polysaccharide synthases with reverse glycosylation activities to generate catalysts with improved synthetic efficiency.

Chemoenzymatic Synthesis of Glycosaminoglycans

Glycosaminoglycans (GAGs) are a family of structurally complex heteropolysaccharides composed of alternating hexosamine and uronic acid or galatose residue that include hyaluronan, chondroitin sulfate and dermatan sulfate, heparin and heparan sulfate, and keratan sulfate. GAGs display a range of critical biological functions, including regulating cell-cell interactions and cell proliferation, inhibiting enzymes, and activating growth factor receptors during various metabolic processes. Indeed, heparin is a widely used GAG-based anticoagulant drug. Unfortunately, naturally derived GAGs are highly heterogeneous, limiting studies of their structure-activity relationships and even resulting in safety concerns. For example, the heparin contamination crisis in 2007 reportedly killed more than a hundred people in the United States. Unfortunately, the chemical synthesis of GAGs, or their oligosaccharides, based on repetitive steps of protection, activation, coupling, and deprotection, is incredibly challenging. Recent advances in chemoenzymatic synthesis integrate the flexibility of chemical derivatization with enzyme-catalyzed reactions, mimicking the biosynthetic pathway of GAGs, and represent a promising strategy to solve many of these synthetic challenges. In this critical Account, we examine the recent progress made, in our laboratory and by others, in the chemoenzymatic synthesis of GAGs, focusing on heparan sulfate and heparin, a class of GAGs with profound physiological and pharmacological importance. A major challenge for the penetration of the heparin market by homogeneous heparin products is their cost-effective large-scale synthesis. In the past decade, we and our collaborators have systematically explored the key factors that impact this process, including better enzyme expression, improved biocatalysts using protein engineering and immobilization, low cost production of enzyme cofactors, optimization of the order of enzymatic transformations, as well as development of efficient technologies, such as using ultraviolet absorbing or fluorous tags, to detect and purify synthetic intermediates. These improvements have successfully resulted in multigram-scale synthesis of low-molecular-weight heparins (LMWHs), with some showing excellent anticoagulant activity and even resulting in more effective protamine reversal than commercial, animal-sourced LMWH drugs. Sophisticated structural analysis is another challenge for marketing heparins, since impurities and contaminants can be present that are difficult to distinguish from heparin drug products. The availability of the diverse library of structurally defined heparin oligosaccharides has facilitated the systematic analytical studies undertaken by our group, resulting in important information for characterizing diverse heparin products, safeguarding their quality. Recently, a series of chemically modified nucleotide sugars have been investigated in our laboratory and have been accepted by synthases to obtain novel GAGs and GAG oligosaccharides. These include fluoride and azido regioselectively functionalized sugars and stable isotope-enriched GAGs and GAG oligosaccharides, critical for better understanding the biological roles of these important biopolymers. We speculate that the repertoire of unnatural acceptors and nucleotide sugar donors will soon be expanded to afford many new GAG analogues with new biological and pharmacological properties including improved specificity and metabolic stability.

The second member of the bacterial UDP-N-acetyl-d-glucosamine:heparosan alpha-1, 4-N-acetyl-d-glucosaminyltransferase superfamily: GaKfiA from Gallibacterium anatis

Bacterial UDP-N-acetyl-d-glucosamine:heparosan alpha-1, 4-N-acetyl-d-glucosaminyltransferases (KfiAs) are in high demand for the development of animal-free heparin (HP) production. Until now, EcKfiA from Escherichia coli O10:K5:H4 was the sole identified member of this family. The lack of known members has limited research into molecular structure and catalytic mechanism of the KfiA superfamily, and restricted its application in enzymatic glycan synthesis. Herein, we report the identification and characterization of Gallibacterium anatis GaKfiA, doubling the number of known members of the KfiA family. GaKfiA is a monofunctional enzyme that transfers N-acetyl-d-glucosamine (GlcNAc) residues from their nucleotide forms to the nonreducing ends of saccharide chains structurally equivalent to the backbone of HP. The catalytic efficiency of GaKfiA is lower than that of EcKfiA. However, a single mutation of GaKfiA, N56D, resulted in a drastic increase in k_cat/K_m compared with wild-type GaKfiA. These data once again indicate the key role of a complete DXD motif for the catalytic efficiency of glycosyltransferases. This study deepens understanding of the mechanism of KfiA, and will assist in research into animal-free HP production.

Tools for Studying Glycans: Recent Advances in Chemoenzymatic Glycan Labeling

The study of cellular glycosylation presents many challenges due, in large part, to the nontemplated nature of glycan biosynthesis and glycans' structural complexity. Chemoenzymatic glycan labeling (CeGL) has emerged as a new technique to address the limitations of existing methods for glycan detection. CeGL combines glycosyltransferases and unnatural nucleotide sugar donors equipped with a bioorthogonal chemical tag to directly label specific glycan acceptor substrates in situ within biological samples. This article reviews the current CeGL strategies that are available to characterize cell-surface and intracellular glycans. Applications include imaging glycan expression status in live cells and tissue samples, proteomic analysis of glycoproteins, and target validation. Combined with genetic and biochemical tools, CeGL provides new opportunities to elucidate the functional roles of glycans in human health and disease.

High-throughput assays of leloir-glycosyltransferase reactions: The applications of rYND1 in glycotechnology

Glycosyltransferases (GTs) play a critical role in the enzymatic and chemoenzymatic synthesis of oligosaccharides and glycoconjugates. However, the development of these synthetic approaches has been limited by a lack of sensitive screening methods for the isolation of novel natural GTs or their active variants. Herein, we describe the results of our investigation towards the soluble expression and potential application of the Saccharomyces cerevisiae apyrase YND1. By replacing the hydrophobic transmembrane domain of YND1 with three glycine-serine repeats, this protein was successfully expressed in a soluble form in Escherichia coli. This new protein was then used to develop a two-step nucleoside diphosphate (NDP)-based Leloir-GT high-throughput assay. Purified rYND1 was initially added to a GT reaction to hydrolyze NDP to nucleoside phosphate plus inorganic phosphate, which was determined using a phosphorus molybdenum blue chromogenic reaction. Purified rYND1 was shown to have a positive effect on saccharide synthesis by eliminating the potential by-product inhibition from NDP. Most of the mono-sugar donors used for Leloir-GTs are activated by uridine diphosphate and guanosine diphosphate, which can be catalyzed by rYND1. The rYND1 is amenable to screening methods and could be applied to a wide range of Leloir-GT-catalyzed reactions, therefore representing a remarkable step forward in glycotechnology.

Improved one-pot multienzyme (OPME) systems for synthesizing UDP-uronic acids and glucuronides

Arabidopsis thaliana glucuronokinase (AtGlcAK) was cloned and shown to be able to use various uronic acids as substrates to produce the corresponding uronic acid-1-phosphates. AtGlcAK or Bifidobacterium infantis galactokinase (BiGalK) was used with a UDP-sugar pyrophosphorylase, an inorganic pyrophosphatase, with or without a glycosyltransferase for highly efficient synthesis of UDP-uronic acids and glucuronides. These improved cost-effective one-pot multienzyme (OPME) systems avoid the use of nicotinamide adenine dinucleotide (NAD(+))-cofactor in dehydrogenase-dependent UDP-glucuronic acid production processes and can be broadly applied for synthesizing various glucuronic acid-containing molecules.

Uncovering the Catalytic Direction of Chondroitin AC Exolyase: FROM THE REDUCING END TOWARDS THE NON-REDUCING END

Glycosaminoglycans (GAGs) are polysaccharides that play vital functional roles in numerous biological processes, and compounds belonging to this class have been implicated in a wide variety of diseases. Chondroitin AC lyase (ChnAC) (EC 4.2.2.5) catalyzes the degradation of various GAGs, including chondroitin sulfate and hyaluronic acid, to give the corresponding disaccharides containing an Δ(4)-unsaturated uronic acid at their non-reducing terminus. ChnAC has been isolated from various bacteria and utilized as an enzymatic tool for study and evaluating the sequencing of GAGs. Despite its substrate specificity and the fact that its crystal structure has been determined to a high resolution, the direction in which ChnAC catalyzes the cleavage of oligosaccharides remain unclear. Herein, we have determined the structural cues of substrate depolymerization and the cleavage direction of ChnAC using model substrates and recombinant ChnAC protein. Several structurally defined oligosaccharides were synthesized using a chemoenzymatic approach and subsequently cleaved using ChnAC. The degradation products resulting from this process were determined by mass spectrometry. The results revealed that ChnAC cleaved the β1,4-glycosidic linkages between glucuronic acid and glucosamine units when these bonds were located on the reducing end of the oligosaccharide. In contrast, the presence of a GlcNAc-α-1,4-GlcA unit at the reducing end of the oligosaccharide prevented ChnAC from cleaving the GalNAc-β1,4-GlcA moiety located in the middle or at the non-reducing end of the chain. These interesting results therefore provide direct proof that ChnAC cleaves oligosaccharide substrates from their reducing end toward their non-reducing end. This conclusion will therefore enhance our collective understanding of the mode of action of ChnAC.

Masquerading microbial pathogens: capsular polysaccharides mimic host-tissue molecules

The increasing prevalence of antibiotic-resistant bacteria portends an impending postantibiotic age, characterized by diminishing efficacy of common antibiotics and routine application of multifaceted, complementary therapeutic approaches to treat bacterial infections, particularly multidrug-resistant organisms. The first line of defense for most bacterial pathogens consists of a physical and immunologic barrier known as the capsule, commonly composed of a viscous layer of carbohydrates that are covalently bound to the cell wall in Gram-positive bacteria or often to lipids of the outer membrane in many Gram-negative bacteria. Bacterial capsular polysaccharides are a diverse class of high molecular weight polysaccharides contributing to virulence of many human pathogens in the gut, respiratory tree, urinary tract, and other host tissues, by hiding cell surface components that might otherwise elicit host immune response. This review highlights capsular polysaccharides that are structurally identical or similar to polysaccharides found in mammalian tissues, including polysialic acid and glycosaminoglycan capsules hyaluronan, heparosan, and chondroitin. Such nonimmunogenic coatings render pathogens insensitive to certain immune responses, effectively increasing residence time in host tissues and enabling pathologically relevant population densities to be reached. Biosynthetic pathways and capsular involvement in immune system evasion are described, providing a basis for potential therapies aimed at supplementing or replacing antibiotic treatment.