The synthesis of sialylated oligosaccharides using a CMP-Neu5Ac synthetase/sialyltransferase fusion

Highly efficient biosynthesis of 3'-sialyllactose in engineered Escherichia coli

3'-Sialyllactose (3'-SL), one of the abundant and important sialylated human milk oligosaccharides, is an emerging food ingredient used in infant formula milk. We previously developed an efficient route for 3'-SL biosynthesis in metabolically engineered Escherichia coli BL21(DE3). Here, several promising α2,3-sialyltransferases were re-evaluated from the byproduct synthesis perspective. The α2,3-sialyltransferase from Neisseria meningitidis MC58 (NST) with great potential and the least byproducts was selected for subsequent molecular modification. Computer-assisted mutation sites combined with a semi-rational modification were designed and performed. A combination of two mutation sites (P120H/N113D) of NST was finally confirmed as the best one, which significantly improved 3'-SL biosynthesis, with extracellular titers of 24.5 g/L at 5-L fed-batch cultivations. When NST-P120H/N113D was additionally integrated into the genome of host EZAK (E. coli BL21(DE3)ΔlacZΔnanAΔnanT), the final strain generated 32.1 g/L of extracellular 3'-SL in a 5-L fed-batch fermentation. Overall, we underscored the existence of by-products and improved 3'-SL production by engineering N. meningitidis α2,3-sialyltransferase.

Recent progress on health effects and biosynthesis of two key sialylated human milk oligosaccharides, 3'-sialyllactose and 6'-sialyllactose

Human milk oligosaccharides (HMOs), the third major solid component in breast milk, are recognized as the first prebiotics for health benefits in infants. Sialylated HMOs are an important type of HMOs, accounting for approximately 13% of total HMOs. 3'-Sialyllactose (3'-SL) and 6'-sialyllactose (6'-SL) are two simplest sialylated HMOs. Both SLs display promising prebiotic effects, especially in promoting the proliferation of bifidobacteria and shaping the gut microbiota. SLs exhibit several health effects, including antiadhesive antimicrobial ability, antiviral activity, prevention of necrotizing enterocolitis, immunomodulatory activity, regulation of intestinal epithelial cell response, promotion of brain development, and cognition improvement. Both SLs have been approved as "Generally Recognized as Safe" by the American Food and Drug Administration and are commercially added to infant formula. The biosynthesis of SLs using enzymatic or microbial approaches has been widely studied. The enzymatic synthesis of SLs can be realized by two types of enzymes, sialidases with trans-sialidase activity and sialyltransferases. Microbial synthesis can be achieved by the multiple recombinant bacteria in one-pot reaction, which express the enzymes involved in SL synthesis pathways separately or in combination, or by metabolically engineered strains in a fermentation process. In this article, the physiological properties of 3'-SL and 6'-SL are summarized in detail and the biosynthesis of these SLs via enzymatic and microbial synthesis is comprehensively reviewed.

A Water Soluble Pd2 L4 Cage for Selective Binding of Neu5Ac

The sialic acid N‐acetylneuraminic acid (Neu5Ac) and its derivatives are involved in many biological processes including cell‐cell recognition and infection by influenza. Molecules that can recognize Neu5Ac might thus be exploited to intervene in or monitor such events. A key obstacle in this development is the sparse availability of easily prepared molecules that bind to this carbohydrate in its natural solvent; water. Here, we report that the carbohydrate binding pocket of an organic soluble [Pd₂L₄]⁴⁺ cage could be equipped with guanidinium‐terminating dendrons to give the water soluble [Pd₂L₄][NO₃]₁₆ cage 7. It was shown by means of NMR spectroscopy that 7 binds selectively to anionic monosaccharides and strongest to Neu5Ac with K_a=24 M⁻¹. The cage had low to no affinity for the thirteen neutral saccharides studied. Aided by molecular modeling, the selectivity for anionic carbohydrates such as Neu5Ac could be rationalized by the presence of charge assisted hydrogen bonds and/or the presence of a salt bridge with a guanidinium solubilizing arm of 7. Establishing that a simple coordination cage such as 7 can already selectively bind to Neu5Ac in water paves the way to improve the stability, affinity and/or selectivity properties of M₂L₄ cages for carbohydrates and other small molecules.

Efficient Production of 3′-Sialyllactose by Single Whole-Cell in One-Pot Biosynthesis

Sialyllactose (SL) is one of the most important acidic oligosaccharides in human milk, which plays an important role in the health of infants. In this work, an efficient multi-enzyme cascade was developed in a single whole cell to produce 3′-SL. We constructed two compatible plasmids with double cloning sites to co-express four genes. Different combinations were assessed to verify the optimal catalytic ability. Then, the conversion temperature, pH, and stability under the optimal temperature and pH were investigated. Moreover, the optimal conversion conditions and surfactant concentration were determined. By using the optimal conditions (35 °C, pH 7.0, 20 mM polyphosphate, 10 mM cytidine monophosphate (CMP), 20 mM MgCl2), 25 mL and 4 L conversion systems were carried out to produce 3′-SL. Similar results were obtained between different volume conversion reactions, which led the maximum production of 3′-SL to reach 53 mM from 54.2 mM of sialic acid (SA) in the 25 mL system and 52.8 mM of 3′-SL from 53.8 mM of SA in the 4 L system. These encouraging results demonstrate that the developed single whole-cell multi-enzyme system exhibits great potential and economic competitiveness for the manufacture of 3′-SL.

Nature Inspired Multienzyme Immobilization: Strategies and Concepts

In a biological system, the spatiotemporal arrangement of enzymes in a dense cellular milieu, subcellular compartments, membrane-associated enzyme complexes on cell surfaces, scaffold-organized proteins, protein clusters, and modular enzymes have presented many paradigms for possible multienzyme immobilization designs that were adapted artificially. In metabolic channeling, the catalytic sites of participating enzymes are close enough to channelize the transient compound, creating a high local concentration of the metabolite and minimizing the interference of a competing pathway for the same precursor. Over the years, these phenomena had motivated researchers to make their immobilization approach naturally realistic by generating multienzyme fusion, cluster formation via affinity domain-ligand binding, cross-linking, conjugation on/in the biomolecular scaffold of the protein and nucleic acids, and self-assembly of amphiphilic molecules. This review begins with the discussion of substrate channeling strategies and recent empirical efforts to build it synthetically. After that, an elaborate discussion covering prevalent concepts related to the enhancement of immobilized enzymes' catalytic performance is presented. Further, the central part of the review summarizes the progress in nature motivated multienzyme assembly over the past decade. In this section, special attention has been rendered by classifying the nature-inspired strategies into three main categories: (i) multienzyme/domain complex mimic (scaffold-free), (ii) immobilization on the biomolecular scaffold, and (iii) compartmentalization. In particular, a detailed overview is correlated to the natural counterpart with advances made in the field. We have then discussed the beneficial account of coassembly of multienzymes and provided a synopsis of the essential parameters in the rational coimmobilization design.

Multi-Enzymatic Cascade One-Pot Biosynthesis of 3'-Sialyllactose Using Engineered Escherichia coli

Among the human milk oligosaccharides (HMOs), one of the most abundant oligosaccharides and has great benefits for human health is 3′-sialyllactose (3′-SL). Given its important physiological functions and the lack of cost-effective production processes, we constructed an in vitro multi-enzymatic cofactor recycling system for the biosynthesis of 3′-SL from a low-cost substrate. First, we constructed the biosynthetic pathway and increased the solubility of cytidine monophosphate kinase (CMK) with chaperones. We subsequently identified that β-galactosidase (lacZ) affects the yield of 3′-SL, and hence with the lacZ gene knocked out, a 3.3-fold increase in the production of 3′-SL was observed. Further, temperature, pH, polyphosphate concentration, and concentration of divalent metal ions for 3′-SL production were optimized. Finally, an efficient biotransformation system was established under the optimized conditions. The maximum production of 3′-SL reached 38.7 mM, and a molar yield of 97.1% from N-acetylneuraminic acid (NeuAc, sialic acid, SA) was obtained. The results demonstrate that the multi-enzymatic cascade biosynthetic pathway with cofactor regeneration holds promise as an industrial strategy for producing 3′-SL.

Total Chemical Syntheses of the GM3 and F-GM3 Ganglioside Epitopes and Comparative Pre-Clinical Evaluation for Non-Invasive Imaging of Oligodendrocyte Differentiation

Gangliosides are intimately involved in a plenum of (neuro)inflammatory processes, yet progress in establishing structure-function interplay is frequently hindered by the availability of well-defined glycostructures. Motivated by the ubiquity of the ganglioside GM₃ in chemical neurology, and in particular by its conspicuous presence in myelin, the GM₃ epitope was examined with a view to preclinical validation as a tracer. The suitability of this scaffold for the noninvasive imaging of oligodendrocyte differentiation in Multiple sclerosis is disclosed. The stereocontrolled synthesis of a site-selectively fluorinated analogue (F-GM₃) is also disclosed to enable a comparative analysis in oligodendrocyte (OL) differentiation. Whereas the native epitope caused a decrease in the viability in a dose-dependent manner, the addition of distinct F-GM₃ concentrations over 48 h had no impact on the OL viability. This is likely a consequence of the enhanced hydrolytic stability imparted by the fluorination and highlights the potential of fluorinated glycostructures in the field of molecular imaging. Given the predominant expression of GM₃ in oligodendrocytes and the capacity of GM₃ to interact with myelin-associated proteins, this preclinical evaluation has revealed F-GM₃ to be an intriguing candidate for neurological imaging.

Comparison of α2,6-sialyltransferases for sialylation of therapeutic proteins

The development of therapeutic proteins for the treatment of numerous diseases is one of the fastest growing areas of biotechnology. Therapeutic efficacy and serum half-life are particularly important, and these properties rely heavily on the glycosylation state of the protein. Expression systems to produce authentically fully glycosylated therapeutic proteins with appropriate terminal sialic acids are not yet perfected. The in vitro modification of therapeutic proteins by recombinant sialyltransferases offers a promising and elegant strategy to overcome this problem. Thus, the detailed expression and characterization of sialyltransferases for completion of the glycan chains is of great interest to the community. We identified a novel α2,6-sialyltransferase from Helicobacter cetorum and compared it to the human ST6Gal1 and a Photobacterium sp. sialyltransferase using glycoprotein substrates in a 96-well microtiter-plate-based assay. We demonstrated that the recombinant α2,6-sialyltransferase from H. cetorum is an excellent catalyst for modification of N-linked glycans of different therapeutic proteins.

Production of human milk oligosaccharides by enzymatic and whole-cell microbial biotransformations

Human milk oligosaccharides (HMO) are almost unique constituents of breast milk and are not found in appreciable amounts in cow milk. Due to several positive aspects of HMO for the development, health, and wellbeing of infants, production of HMO would be desirable. As a result, scientists from different disciplines have developed methods for the preparation of single HMO compounds. Here, we review approaches to HMO preparation by (chemo-)enzymatic syntheses or by whole-cell biotransformation with recombinant bacterial cells. With lactose as acceptor (in vitro or in vivo), fucosyltransferases can be used for the production of 2'-fucosyllactose, 3-fucosyllactose, or more complex fucosylated core structures. Sialylated HMO can be produced by sialyltransferases and trans-sialidases. Core structures as lacto-N-tetraose can be obtained by glycosyltransferases from chemical donor compounds or by multi-enzyme cascades; recent publications also show production of lacto-N-tetraose by recombinant Escherichia coli bacteria and approaches to obtain fucosylated core structures. In view of an industrial production of HMOs, the whole cell biotransformation is at this stage the most promising option to provide human milk oligosaccharides as food additive.

Construction and evaluation of a novel bifunctional phenylalanine–formate dehydrogenase fusion protein for bienzyme system with cofactor regeneration

Phenylalanine dehydrogenase (PheDH) plays an important role in enzymatic synthesis of l-phenylalanine for aspartame (sweetener) and detection of phenylketonuria (PKU), suggesting that it is important to obtain a PheDH with excellent characteristics. Gene fusion of PheDH and formate dehydrogenase (FDH) was constructed to form bifunctional multi-enzymes for bioconversion of l-phenylalanine coupled with coenzyme regeneration. Comparing with the PheDH monomer from Microbacterium sp., the bifunctional PheDH–FDH showed noteworthy stability under weakly acidic and alkaline conditions (pH 6.5–9.0). The bifunctional enzyme can produce 153.9 mM l-phenylalanine with remarkable performance of enantiomers choice by enzymatic conversion with high molecular conversion rate (99.87 %) in catalyzing phenylpyruvic acid to l-phenylalanine being 1.50-fold higher than that of the separate expression system. The results indicated the potential application of the PheDH and PheDH–FDH with coenzyme regeneration for phenylpyruvic acid analysis and l-phenylalanine biosynthesis in medical diagnosis and pharmaceutical field.

Sucrose synthase: A unique glycosyltransferase for biocatalytic glycosylation process development

Sucrose synthase (SuSy, EC 2.4.1.13) is a glycosyltransferase (GT) long known from plants and more recently discovered in bacteria. The enzyme catalyzes the reversible transfer of a glucosyl moiety between fructose and a nucleoside diphosphate (NDP) (sucrose+NDP↔NDP-glucose+fructose). The equilibrium for sucrose conversion is pH dependent, and pH values between 5.5 and 7.5 promote NDP-glucose formation. The conversion of a bulk chemical to high-priced NDP-glucose in a one-step reaction provides the key aspect for industrial interest. NDP-sugars are important as such and as key intermediates for glycosylation reactions by highly selective Leloir GTs. SuSy has gained renewed interest as industrially attractive biocatalyst, due to substantial scientific progresses achieved in the last few years. These include biochemical characterization of bacterial SuSys, overproduction of recombinant SuSys, structural information useful for design of tailor-made catalysts, and development of one-pot SuSy-GT cascade reactions for production of several relevant glycosides. These advances could pave the way for the application of Leloir GTs to be used in cost-effective processes. This review provides a framework for application requirements, focusing on catalytic properties, heterologous enzyme production and reaction engineering. The potential of SuSy biocatalysis will be presented based on various biotechnological applications: NDP-sugar synthesis; sucrose analog synthesis; glycoside synthesis by SuSy-GT cascade reactions.

CMP-Sialic Acid Synthetase: The Point of Constriction in the Sialylation Pathway

Sialoglycoconjugates form the outermost layer of animal cells and play a crucial role in cellular communication processes. An essential step in the biosynthesis of sialylated glycoconjugates is the activation of sialic acid to the monophosphate diester CMP-sialic acid. Only the activated sugar is transported into the Golgi apparatus and serves as a substrate for the linkage-specific sialyltransferases. Interference with sugar activation abolishes sialylation and is embryonic lethal in mammals. In this chapter we focus on the enzyme catalyzing the activation of sialic acid, the CMP-sialic acid synthetase (CMAS), and compare the enzymatic properties of CMASs isolated from different species. Information concerning the reaction mechanism and active site architecture is included. Moreover, the unusual nuclear localization of vertebrate CMASs as well as the biotechnological application of bacterial CMAS enzymes is addressed.

All-in-one assay for β-d-galactoside sialyltransferases: Quantification of productive turnover, error hydrolysis, and site selectivity

Sialyltransferases are important enzymes of glycobiology and the related biotechnologies. The development of sialyltransferases calls for access to quick, inexpensive, and robust analytical tools. We have established an assay for simultaneous characterization of sialyltransferase activity, error hydrolysis, and site selectivity. The described assay does not require expensive substrates, is very sensitive (limit of detection=0.3 μU), and is easy to perform. It is based on sialylation of nitrophenyl galactosides; the products thereof are separated and quantified by ion pair reversed phase high-performance liquid chromatography with ultraviolet detection.

Galactose-limited fed-batch cultivation of Escherichia coli for the production of lacto-N-tetraose

Lacto-N-tetraose (Gal(β1-3)GlcNAc(β1-3)Gal(β1-4)Glc) is one of the most abundant oligosaccharide structures in human milk. We recently described the synthesis of lacto-N-tetraose by a whole-cell biotransformation with recombinant Escherichia coli cells. However, only about 5% of the lactose was converted into lacto-N-tetraose by this approach. The major product obtained was the intermediate lacto-N-triose II (GlcNAc(β1-3)Gal(β1-4)Glc). In order to improve the bioconversion of lactose to lacto-N-tetraose, we have investigated the influence of the carbon source on the formation of lacto-N-tetraose and on the intracellular availability of the glycosyltransferase substrates, UDP-N-acetylglucosamine and UDP-galactose. By growth of the recombinant E. coli cells on D-galactose, the yield of lacto-N-tetraose (810.8 mg L(-1) culture) was 3.6-times higher compared to cultivation on D-glucose. Using fed-batch cultivation with galactose as sole energy and carbon source, a large-scale synthesis of lacto-N-tetraose was demonstrated. During the 26 h feeding phase the growth rate (μ = 0.05) was maintained by an exponential galactose feed. In total, 16 g L(-1) lactose were fed and resulted in final yields of 12.72 ± 0.21 g L(-1) lacto-N-tetraose and 13.70 ± 0.10 g L(-1) lacto-N-triose II. In total, 173 g of lacto-N-tetraose were produced with a space-time yield of 0.37 g L(-1) h(-1).

Complete switch from α-2,3- to α-2,6-regioselectivity in Pasteurella dagmatis β-d-galactoside sialyltransferase by active-site redesign

Incorporation of Pro7His and Met117Ala substitutions resulted in a completely regioselective and highly efficient α-2,6-sialyltransferase.

Human Milk Oligosaccharides (HMOS): Structure, Function, and Enzyme-Catalyzed Synthesis

The important roles played by human milk oligosaccharides (HMOS), the third major component of human milk, in the health of breast-fed infants have been increasingly recognized, as the structures of more than 100 different HMOS have now been elucidated. Despite the recognition of the various functions of HMOS as prebiotics, antiadhesive antimicrobials, and immunomodulators, the roles and the applications of individual HMOS species are less clear. This is mainly due to the limited accessibility to large amounts of individual HMOS in their pure forms. Current advances in the development of enzymatic, chemoenzymatic, whole-cell, and living-cell systems allow for the production of a growing number of HMOS in increasing amounts. This effort will greatly facilitate the elucidation of the important roles of HMOS and allow exploration into the applications of HMOS both as individual compounds and as mixtures of defined structures with desired functions. The structures, functions, and enzyme-catalyzed synthesis of HMOS are briefly surveyed to provide a general picture about the current progress on these aspects. Future efforts should be devoted to elucidating the structures of more complex HMOS, synthesizing more complex HMOS including those with branched structures, and developing HMOS-based or HMOS-inspired prebiotics, additives, and therapeutics.

An integrated membrane system for the biocatalytic production of 3'-sialyllactose from dairy by-products

An integrated membrane system was investigated for the production of 3'-sialyllactose by an engineered sialidase using casein glycomacropeptide (CGMP) and lactose as substrates. CGMP was purified by ultrafiltration (UF) to remove any small molecules present and then an enzymatic membrane reactor (EMR) was used to separate the product and reuse the enzyme. A PLCC regenerated cellulose membrane was found to be the most suitable for both the UF purification and EMR. Subsequently, nanofiltration (NF) was conducted to increase the purity of the 3'-sialyllactose by removing the excess lactose present. The NTR7450 membrane outperformed others in NF due to its high retention of 3'-sialyllactose (98%) and relatively low rejection of lactose (40%). The lactose in the permeate could be concentrated by the NF45 membrane and recycled into the EMR. The described integrated membrane system enables a more economic and efficient enzymatic production of 3'-sialyllactose.

A sialyltransferase mutant with decreased donor hydrolysis and reduced sialidase activities for directly sialylating LewisX

Glycosyltransferases are important catalysts for enzymatic and chemoenzymatic synthesis of complex carbohydrates and glycoconjugates. The glycosylation efficiencies of wild-type glycosyltransferases vary considerably when different acceptor substrates are used. Using a multifunctional Pasteurella multocida sialyltransferase 1 (PmST1) as an example, we show here that the sugar nucleotide donor hydrolysis activity of glycosyltransferases contributes significantly to the low yield of glycosylation when a poor acceptor substrate is used. With a protein crystal structure-based rational design, we generated a single mutant (PmST1 M144D) with decreased donor hydrolysis activity without significantly affecting its α2-3-sialylation activity when a poor fucose-containing acceptor substrate was used. The single mutant also has a drastically decreased α2-3-sialidase activity. X-ray and NMR structural studies revealed that unlike the wild-type PmST1, which changes to a closed conformation once a donor binds, the M144D mutant structure adopts an open conformation even in the presence of the donor substrate. The PmST1 M144D mutant with decreased donor hydrolysis and reduced sialidase activity has been used as a powerful catalyst for efficient chemoenzymatic synthesis of complex sialyl Lewis(x) antigens containing different sialic acid forms. This work sheds new light on the effect of donor hydrolysis activity of glycosyltransferases on glycosyltransferase-catalyzed reactions and provides a novel strategy to improve glycosyltransferase substrate promiscuity by decreasing its donor hydrolysis activity.

Sialic acid metabolism and sialyltransferases: natural functions and applications

Sialic acids are a family of negatively charged monosaccharides which are commonly presented as the terminal residues in glycans of the glycoconjugates on eukaryotic cell surface or as components of capsular polysaccharides or lipooligosaccharides of some pathogenic bacteria. Due to their important biological and pathological functions, the biosynthesis, activation, transfer, breaking down, and recycle of sialic acids are attracting increasing attention. The understanding of the sialic acid metabolism in eukaryotes and bacteria leads to the development of metabolic engineering approaches for elucidating the important functions of sialic acid in mammalian systems and for large-scale production of sialosides using engineered bacterial cells. As the key enzymes in biosynthesis of sialylated structures, sialyltransferases have been continuously identified from various sources and characterized. Protein crystal structures of seven sialyltransferases have been reported. Wild-type sialyltransferases and their mutants have been applied with or without other sialoside biosynthetic enzymes for producing complex sialic acid-containing oligosaccharides and glycoconjugates. This mini-review focuses on current understanding and applications of sialic acid metabolism and sialyltransferases.

Engineering bifunctional laccase-xylanase chimeras for improved catalytic performance

Two bifunctional enzymes exhibiting combined xylanase and laccase activities were designed, constructed, and characterized by biochemical and biophysical methods. The Bacillus subtilis cotA and xynA genes were used as templates for gene fusion, and the xynA coding sequence was inserted into a surface loop of the cotA. A second chimera was built replacing the wild-type xynA gene by a thermostable variant (xynAG3) previously obtained by in vitro molecular evolution. Kinetic measurements demonstrated that the pH and temperature optima of the catalytic domains in the chimeras were altered by less than 0.5 pH units and 5 °C, respectively, when compared with the parental enzymes. In contrast, the catalytic efficiency (k(cat)/K(m)) of the laccase activity in both chimeras was 2-fold higher than for the parental laccase. Molecular dynamics simulations of the CotA-XynA chimera indicated that the two domains are in close contact, which was confirmed by the low resolution structure obtained by small angle x-ray scattering. The simulation also indicates that the formation of the inter-domain interface causes the dislocation of the loop comprising residues Leu-558 to Lys-573 in the laccase domain, resulting in a more accessible active site and exposing the type I Cu(2+) ion to the solvent. These structural changes are consistent with the results from UV-visible electronic and EPR spectroscopy experiments of the type I copper between the native and chimeric enzymes and are likely to contribute to the observed increase in catalytic turnover number.

Advances in the biology and chemistry of sialic acids

Sialic acids are a subset of nonulosonic acids, which are nine-carbon alpha-keto aldonic acids. Natural existing sialic acid-containing structures are presented in different sialic acid forms, various sialyl linkages, and on diverse underlying glycans. They play important roles in biological, pathological, and immunological processes. Sialobiology has been a challenging and yet attractive research area. Recent advances in chemical and chemoenzymatic synthesis, as well as large-scale E. coli cell-based production, have provided a large library of sialoside standards and derivatives in amounts sufficient for structure-activity relationship studies. Sialoglycan microarrays provide an efficient platform for quick identification of preferred ligands for sialic acid-binding proteins. Future research on sialic acid will continue to be at the interface of chemistry and biology. Research efforts not only will lead to a better understanding of the biological and pathological importance of sialic acids and their diversity but also could lead to the development of therapeutics.

Combined approaches to the synthesis and study of glycoproteins

Carbohydrates are the basis for many therapeutic and diagnostic strategies, yet the full potential of glycans in medicine has not been realized. The study of the precise role of different carbohydrates, bound to either proteins or lipids, is hampered by difficulties in accessing pure, well-defined glycoconjugates. This Review highlights recent advances in glycobiology with a particular emphasis on oligosaccharide synthesis and conjugation techniques for the construction of homogeneous glycoconjugates. New methods for the study of protein-glycan interactions such as carbohydrate arrays and in vivo visualization of glycosylation pattern changes will also be addressed. The development of glycotherapeutics is just beginning, and much remains to be understood about the relationship between glycoconjugate structure and function. The emergence of novel tools will certainly facilitate and expand the use of carbohydrates in therapeutics and diagnostics.

Overproduction of geranylgeraniol by metabolically engineered Saccharomyces cerevisiae

(E, E, E)-Geranylgeraniol (GGOH) is a valuable starting material for perfumes and pharmaceutical products. In the yeast Saccharomyces cerevisiae, GGOH is synthesized from the end products of the mevalonate pathway through the sequential reactions of farnesyl diphosphate synthetase (encoded by the ERG20 gene), geranylgeranyl diphosphate synthase (the BTS1 gene), and some endogenous phosphatases. We demonstrated that overexpression of the diacylglycerol diphosphate phosphatase (DPP1) gene could promote GGOH production. We also found that overexpression of a BTS1-DPP1 fusion gene was more efficient for producing GGOH than coexpression of these genes separately. Overexpression of the hydroxymethylglutaryl-coenzyme A reductase (HMG1) gene, which encodes the major rate-limiting enzyme of the mevalonate pathway, resulted in overproduction of squalene (191.9 mg liter(-1)) rather than GGOH (0.2 mg liter(-1)) in test tube cultures. Coexpression of the BTS1-DPP1 fusion gene along with the HMG1 gene partially redirected the metabolic flux from squalene to GGOH. Additional expression of a BTS1-ERG20 fusion gene resulted in an almost complete shift of the flux to GGOH production (228.8 mg liter(-1) GGOH and 6.5 mg liter(-1) squalene). Finally, we constructed a diploid prototrophic strain coexpressing the HMG1, BTS1-DPP1, and BTS1-ERG20 genes from multicopy integration vectors. This strain attained 3.31 g liter(-1) GGOH production in a 10-liter jar fermentor with gradual feeding of a mixed glucose and ethanol solution. The use of bifunctional fusion genes such as the BTS1-DPP1 and ERG20-BTS1 genes that code sequential enzymes in the metabolic pathway was an effective method for metabolic engineering.

Bacterial CMP-sialic acid synthetases: production, properties, and applications

Sialic acids are abundant nine-carbon sugars expressed terminally on glycoconjugates of eukaryotic cells and are crucial for a variety of cell biological functions such as cell-cell adhesion, intracellular signaling, and in regulation of glycoproteins stability. In bacteria, N-acetylneuraminic acid (Neu5Ac) polymers are important virulence factors. Cytidine 5'-monophosphate (CMP)-N-acetylneuraminic acid synthetase (CSS; EC 2.7.7.43), the key enzyme that synthesizes CMP-N-acetylneuraminic acid, the donor molecule for numerous sialyltransferase reactions, is present in both prokaryotes and eukaryotic systems. Herein, we emphasize the source, function, and biotechnological applications of CSS enzymes from bacterial sources. To date, only a few CSS from pathogenic bacterial species such as Neisseria meningitidis, Escherichia coli, group B streptococci, Haemophilus ducreyi, and Pasteurella hemolytica and an enzyme from nonpathogenic bacterium, Clostridium thermocellum, have been described. Overall, the enzymes from both Gram-positive and Gram-negative bacteria share common catalytic properties such as their dependency on divalent cation, temperature and pH profiles, and catalytic mechanisms. The enzymes, however, can be categorized as smaller and larger enzymes depending on their molecular weight. The larger enzymes in some cases are bifunctional; they have exhibited acetylhydrolase activity in addition to their sugar nucleotidyltransferase activity. The CSSs are important enzymes for the chemoenzymatic synthesis of various sialooligosaccharides of significance in biotechnology.

Cloning and characterization of a heat-stable CMP-N-acylneuraminic acid synthetase from Clostridium thermocellum

In this study, we report the cloning, recombinant expression, and biochemical characterization of a heat-stable CMP-N-acylneuraminic acid (NeuAc) synthetase from Clostridium thermocellum ATCC 27405. A high throughput electrospray ionization mass spectrometry (ESI-MS)-based assay demonstrates that the enzyme has an absolute requirement for a divalent cation for activity and reaches maximum activity in the presence of 10 mM Mn(2+). The enzyme is active at pH 8-13 in Tris-HCl buffer and at 37-60 degrees C, and maximum activity is observed at pH 9.5 and 50 degrees C in the presence of 0.2 mM dithiothreitol. In addition to NeuAc, the enzyme also accepts the analog N-glycolylneuraminic acid (NeuGc) as a substrate. The apparent Michaelis constants for cytidine triphosphate and NeuAc or NeuGc are 240 +/- 20, 130 +/- 10, and 160 +/- 10 microM, respectively, with corresponding turnover numbers of 3.33, 2.25, and 1.66 s(-1), respectively. An initial velocity study of the enzymatic reaction indicates an ordered bi-bi catalytic mechanism. In addition to demonstration of a thermostable and substrate-tolerant enzyme, confirmation of the biochemical function of a gene for CMP-NeuAc synthetase in C. thermocellum also opens the question of the biological function of CMP-NeuAc in such nonpathogenic microorganisms.

Alternative donor substrates for inverting and retaining glycosyltransferases

By the use of glycosyl donors containing aromatic leaving groups linked with opposite anomeric configurations compared to those of the natural donor substrates, an inverting (Cst II) and a retaining (LgtC) glycosyltransferase were found to catalyse glycosylation reactions of natural acceptor substrates in the presence of the corresponding nucleotide.

Assembling a novel bifunctional cellulase–xylanase from Thermotoga maritima by end-to-end fusion

An artificial, bifunctional, thermostable cellulase-xylanase enzyme from Thermotoga maritima by gene fusion. The fusion protein exhibited both cellulase and xylanase activity when xynA was fused downstream of cel5C but no activities were shown when xynA was fused upstream of cel5C. The enzyme was optimally active at pH 5.0 and 80 degrees C over 30 min. E. coli expressed the fusion enzyme, with an apparent molecular mass of approximately 152 kDa by carboxymethyl cellulose- and xylan-SDS-PAGE.

Large-scale chemoenzymatic synthesis of blood group and tumor-associated poly-N-acetyllactosamine antigens

Poly-N-acetyllactosamines (pLNs) are common terminal sugars of many N- and O-linked glycan structures present in glycoproteins and glycolipids. Utilizing various glycosyltransferases, we developed new and efficient chemoenzymatic methods for the synthesis of pLNs in gram-scale. Specifically, the use of sialyltransferases and fucosyltransferases enabled us to synthesize and purify 24 blood group and tumor-associated pLN derivatives with alpha-(2-->3)- and alpha-(2-->6)-linked sialic acid, as well as with alpha-(1-->2)- and alpha-(1-->3)-linked fucose. All synthesized derivatives were linked to a short 2-azidoethyl spacer for further modification.

Advances in glycoprotein synthesis

The development of chemical and enzymatic methods for the synthesis of homogeneous glycoproteins is a fascinating challenge at the interface between chemistry and biology. Discussed here are the currently available methods for preparation of homogeneous glycoproteins. These methods include (1) glycopeptide ligation; (2) glycoprotein remodeling; and (3) in vivo suppressor tRNA technology.

Chemoenzymatic synthesis of GM3and GM2gangliosides containing a truncated ceramide functionalized for glycoconjugate synthesis and solid phase applications

Analogues of GM3 and GM2 gangliosides were chemoenzymatically synthesized on a multifunctional ceramide-type tether designed to facilitate diverse strategies for glycoconjugate synthesis. The truncated ceramide aglycon maintains the stereogenic centres of natural ceramide while avoiding extensive hydrophobicity that can hamper synthesis and purification of the glycolipids. Tetanus toxoid and BSA glycoconjugates of these two gangliosides were prepared for immunization of mice, and for solid phase assays to screen for ganglioside-specific antibodies. Inhibition experiments showed that antibodies generated by tetanus toxoid conjugates of GM3 and GM2 exhibited specificity for the carbohydrate epitope and the stereogenic centres of the ceramide.

A Multifunctional Pasteurella multocida Sialyltransferase: A Powerful Tool for the Synthesis of Sialoside Libraries

A multifunctional sialyltransferase has been cloned from Pasteurella multocida strain P-1059 and expressed in E. coli as a truncated C-terminal His6-tagged recombinant protein (tPm0188Ph). Biochemical studies indicate that the obtained protein is (1) an alpha2,3-sialyltransferase (main function), (2) an alpha2,6-sialyltransferase, (3) an alpha2,3-sialidase, and (4) an alpha2,3-trans-sialidase. The recombinant tPm0188Ph is a powerful tool in the synthesis of structurally diverse sialoside libraries due to its relaxed substrate specificity, high solubility, high expression level, and multifunctionality.