Large-scale production of oligosaccharides using engineered bacteria

Utilization of glycosyltransferases as a seamless tool for synthesis and modification of the oligosaccharides-A review

Glycosyltransferases (GTs) catalyze the transfer of active monosaccharide donors to carbohydrates to create a wide range of oligosaccharide structures. GTs display strong regioselectivity and stereoselectivity in producing glycosidic bonds, making them extremely valuable in the in vitro synthesis of oligosaccharides. The synthesis of oligosaccharides by GTs often gives high yields; however, the enzyme activity may experience product inhibition. Additionally, the higher cost of nucleotide sugars limits the usage of GTs for oligosaccharide synthesis. In this review, we comprehensively discussed the structure and mechanism of GTs based on recent literature and the CAZY website data. To provide innovative ideas for the functional studies of GTs, we summarized several remarkable characteristics of GTs, including folding, substrate specificity, regioselectivity, donor sugar nucleotides, catalytic reversibility, and differences between GTs and GHs. In particular, we highlighted the recent advancements in multi-enzyme cascade reactions and co-immobilization of GTs, focusing on overcoming problems with product inhibition and cost issues. Finally, we presented various types of GT that have been successfully used for oligosaccharide synthesis. We concluded that there is still an opportunity for improvement in enzymatically produced oligosaccharide yield, and future research should focus on improving the yield and reducing the production cost.

An in vitro synthetic biology platform for emerging industrial biomanufacturing: Bottom-up pathway design

Although most in vitro (cell-free) synthetic biology projects are usually used for the purposes of fundamental research or the formation of high-value products, in vitro synthetic biology platform, which can implement complicated biochemical reactions by the in vitro assembly of numerous enzymes and coenzymes, has been proposed for low-cost biomanufacturing of bioenergy, food, biochemicals, and nutraceuticals. In addition to the most important advantage-high product yield, in vitro synthetic biology platform features several other biomanufacturing advantages, such as fast reaction rate, easy product separation, open process control, broad reaction condition, tolerance to toxic substrates or products, and so on. In this article, we present the basic bottom-up design principles of in vitro synthetic pathway from basic building blocks-BioBricks (thermoenzymes and/or immobilized enzymes) to building modules (e.g., enzyme complexes or multiple enzymes as a module) with specific functions. With development in thermostable building blocks-BioBricks and modules, the in vitro synthetic biology platform would open a new biomanufacturing age for the cost-competitive production of biocommodities.

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.

Purification of caprine oligosaccharides at pilot-scale

The purification of caprine milk oligosaccharides (COS) by membrane filtration has been hampered by the low concentration of target COS and high concentration of lactose. In addition, their molecular weight proximity hinders the recovery of a COS fraction with high degree of purity and recovery yield. In this work, the recovery of a high purity COS concentrate was obtained by the optimization of an integrated approach including complete lactose hydrolysis, fermentation of the resulting monosaccharides and nanofiltration. All carbohydrates were quantified using High Performance Anion Exchange Chromatography with Pulsed Amperometric Detection (HPAEC PAD). Defatted goat whey was ultrafiltered with discontinuous diafiltrations to increase the recovery of COS in the whey permeate which was then subsequently concentrated by nanofiltration. COS recovery yields of 75% with negligible amounts of monosaccharides (0.3% of the initial amount of lactose in the whey permeate) were achieved. A final retentate containing 67.6 and 34.4% of acidic and neutral oligosaccharides respectively was obtained from caprine milk.

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.

Enzymatic Synthesis of Oligosaccharides and Neoglycoconjugates

Oligosaccharides involved in glycoconjugates play important roles in a number of biological events. To elucidate the biological functions of oligosaccharides, sufficient quantities of structurally defined oligosaccharides, are of limited availability by traditional purification methods, are required. Hence, chemical and enzymatic syntheses of oligosaccharides are becoming increasingly important in glycobiology and glycotechnology. In addition, oligosaccharides often occur as glycoconjugates attached to proteins or lipids. Hence, the development of simple and effective methods for synthesizing neoglycoconjugates such as neoglycoprotein and neoglycolipids is essential for an understanding of the biological function of these molecules. Here we review the most recent developments in the enzymatic synthesis of oligosaccharides and neoglycoconjugates.

Targeting the bacteria-host interface: strategies in anti-adhesion therapy

Bacterial infections are a major cause of morbidity and mortality worldwide and are increasingly problematic to treat due to the rise in antibiotic-resistant strains. It becomes more and more challenging to develop new antimicrobials that are able to withstand the ever-increasing repertoire of bacterial resistance mechanisms. This necessitates the development of alternative approaches to prevent and treat bacterial infections. One of the first steps during bacterial infection is adhesion of the pathogen to host cells. A pathogen’s ability to colonize and invade host tissues strictly depends on this process. Thus, interference with adhesion (anti-adhesion therapy) is an efficient way to prevent or treat bacterial infections. As a basis to present different strategies to interfere with pathogen adhesion, this review briefly introduces general concepts of bacterial attachment to host cells. We further discuss advantages and disadvantages of anti-adhesion treatments and issues that are in need of improvement so as to make anti-adhesion compounds a more broadly applicable alternative to conventional antimicrobials.

Synthesis of nucleotide sugars and α-galacto-oligosaccharides by recombinant Escherichia coli cells with trehalose substrate

Useful nucleoside diphosphate (NDP)-sugars and α-galacto-oligosaccharides were synthesized by recombinant Escherichia coli whole cells and compared to those produced by enzyme-coupling. Production yields of NDP-glucoses (Glcs) by whole cells harboring trehalose synthase (TS) were 60% for ADP-Glc, 82% for GDP-Glc, and 27% for UDP-Glc, based on NDP used. Yield of UDP-galactose (Gal) by the whole-cell harboring a UDP-Gal 4-epimerase (pGALE) was 26% of the quantity of UDP-Glc. α-Galacto-oligosaccharides, α-Gal epitope (Galα-3Galβ-4Glu) and globotriose (Galα-4Galβ-4Glu), were produced by the combination of three recombinant whole cells harboring TS, pGALE, and α-galactosyltransferase, with production yields of 48% and 54%, based on UDP, respectively. Production yields of NDP-sugars and α-galacto-oligosaccharides by recombinant whole-cell reactions were approximately 1.5 times greater than those of enzyme-coupled reactions. These results suggest that a recombinant whole-cell system using cells harboring TS with trehalose as a substrate may be used as an alternative and practical method for the production of NDP-sugars and α-galacto-oligosaccharides.

Coupling reactions of trehalose synthase from Pyrococcus horikoshii: cost-effective synthesis and anti-adhesive activity of α-galactosyl oligosaccharides using a one-pot three-enzyme system with trehalose

A new sugar nucleotide cycling (SNC) process was established in a one-pot three enzyme-coupled reaction using disaccharide trehalose. Trehalose synthase from Pyrococcus horikoshii could be applied to the SNC process for the synthesis of functional α-galactosyl oligosaccharides, α-galactose (Gal) epitopes and globotriose, using the effective regeneration of UDP-Gal. The α-Gal epitopes and globotriose were found to attach to the cell-surface of enteropathogenic Escherichia coli O127 (EPEC) which were bound to human Caco-2 cells. These α-galactosyl oligosaccharides were able to prohibit the attachment of EPEC, which could have resulted in colonization and disease. The α-Gal epitope III with a lactulose acceptor showed the most inhibitory activity of anti-adhesion. The results suggest that the α-galactosyl oligosaccharides may be alternative anti-adhesion molecules that overcome antibiotic resistance.

Cell-free biosystems for biomanufacturing

Although cell-free biosystems have been used as a tool for investigating fundamental aspects of biological systems for more than 100 years, they are becoming an emerging biomanufacturing platform in the production of low-value biocommodities (e.g., H(2), ethanol, and isobutanol), fine chemicals, and high-value protein and carbohydrate drugs and their precursors. Here we would like to define the cell-free biosystems containing more than three catalytic components in a single reaction vessel, which although different from one-, two-, or three-enzyme biocatalysis can be regarded as a straightforward extension of multienzymatic biocatalysis. In this chapter, we compare the advantages and disadvantages of cell-free biosystems versus living organisms, briefly review the history of cell-free biosystems, highlight a few examples, analyze any remaining obstacles to the scale-up of cell-free biosystems, and suggest potential solutions. Cell-free biosystems could become a disruptive technology to microbial fermentation, especially in the production of high-impact low-value biocommodities mainly due to the very high product yields and potentially low production costs.

Toward automated oligosaccharide synthesis

Carbohydrates have been shown to play important roles in biological processes. The pace of development in carbohydrate research is, however, relatively slow due to the problems associated with the complexity of carbohydrate structures and the lack of general synthetic methods and tools available for the study of this class of biomolecules. Recent advances in synthesis have demonstrated that many of these problems can be circumvented. In this Review, we describe the methods developed to tackle the problems of carbohydrate-mediated biological processes, with particular focus on the issue related to the development of the automated synthesis of oligosaccharides. Further applications of carbohydrate microarrays and vaccines to human diseases are also highlighted.

Chitin research revisited

Two centuries after the discovery of chitin, it is widely accepted that this biopolymer is an important biomaterial in many aspects. Numerous studies on chitin have focused on its biomedical applications. In this review, various aspects of chitin research including sources, structure, biosynthesis, chitinolytic enzyme, chitin binding protein, genetic engineering approach to produce chitin, chitin and evolution, and a wide range of applications in bio- and nanotechnology will be dealt with.

Microwave effect for glycosylation promoted by solid super acid in supercritical carbon dioxide

The effects of microwave irradiation (2.45 GHz, 200 W) on glycosylation promoted by a solid super acid in supercritical carbon dioxide was investigated with particular attention paid to the structure of the acceptor substrate. Because of the symmetrical structure and high diffusive property of supercritical carbon dioxide, microwave irradiation did not alter the temperature of the reaction solution, but enhanced reaction yield when aliphatic acceptors are employed. Interestingly, the use of a phenolic acceptor under the same reaction conditions did not show these promoting effects due to microwave irradiation. In the case of aliphatic diol acceptors, the yield seemed to be dependent on the symmetrical properties of the acceptors. The results suggest that microwave irradiation do not affect the reactivity of the donor nor promoter independently. We conclude that the effect of acceptor structure on glycosylation yield is due to electric delocalization of hydroxyl group and dielectrically symmetric structure of whole molecule.

Biochemical characterization of two xylanases from yeast Pseudozyma hubeiensis producing only xylooligosaccharides

Two distinct xylanases from Pseudozyma hubeiensis NCIM 3574 were purified to homogeneity. The molecular masses of two native xylanases were 33.3 kDa (PhX33) and 20.1 kDa (PhX20). PhX33 is predominant with alpha-helix and PhX20 contained predominantly beta-sheets. Xylanase, PhX33, possesses three tryptophan and one carboxyl residues at the active site. The active site of PhX20 comprises one residue each of tryptophan, carboxyl and histidine. Carboxyl residue is mainly involved in catalysis and tryptophane residues are solely involved in substrate binding. Histidine residue present at the active site of PhX20 appeared to have a role in substrate binding. Both the xylanases produced only xylooligosaccharides (XOS) with degree of polymerization (DP) 3-7 without formation of xylose and xylobiose. These XOS could be used in functional foods or as prebiotics. Lc ms-ms ion search of tryptic digestion of these xylanases revealed that there is no significant homology of peptides with known fungal xylanase sequences which indicate that these xylanases appear to be new.

Chemical synthesis of UDP-Glc-2,3-diNAcA, a key intermediate in cell surface polysaccharide biosynthesis in the human respiratory pathogens B. pertussis and P. aeruginosa

In connection with studies on lipopolysaccharide biosynthesis in respiratory pathogens we had a need to access potential biosynthetic intermediate sugar nucleotides. Herein we report the chemical synthesis of uridine 5'-diphospho 2,3-diacetamido-2,3-dideoxy-alpha-D-glucuronic acid (UDP-Glc-2,3-diNAcA) (1) from N-acetyl-D-glucosamine in 17 steps and approximately 9% overall yield. This compound has proved invaluable in the elucidation of biosynthetic pathways leading to the formation of 2,3-diacetamido-2,3-dideoxy-D-mannuronic acid-containing polysaccharides.

Assessment of the Two Helicobacter pylori α-1,3-Fucosyltransferase Ortholog Genes for the Large-Scale Synthesis of LewisX Human Milk Oligosaccharides by Metabolically Engineered Escherichia coli

We previously described a bacterial fermentation process for the in vivo conversion of lactose into fucosylated derivatives of lacto-N-neotetraose Gal(beta1-4)GlcNAc(beta1-3)Gal(beta1-4)Glc (LNnT). The major product obtained was lacto-N-neofucopentaose-V Gal(beta1-4)GlcNAc(beta1-3)Gal(beta1-4)[Fuc(alpha1-3)]Glc, carrying fucose on the glucosyl residue of LNnT. Only a small amount of oligosaccharides fucosylated on N-acetylglucosaminyl residues and thus carrying the LewisX group (Le(X)) was also produced. We report here a fermentation process for the large-scale production of Le(X) oligosaccharides. The two fucosyltransferase genes futA and futB of Helicobacter pylori (strain 26695) were compared in order to optimize fucosylation in vivo. futA was found to provide the best activity on the LNnT acceptor, whereas futB expressed a better Le(X) activity in vitro. Both genes were expressed to produce oligosaccharides in engineered Escherichia coli (E. coli) cells. The fucosylation pattern of the recombinant oligosaccharides was closely correlated with the specificity observed in vitro, FutB favoring the formation of Le(X) carrying oligosaccharides. Lacto-N-neodifucohexaose-II Gal(beta1-4)[Fuc(alpha1-3)]GlcNAc(beta1-3)Gal(beta1-4)[Fuc(alpha1-3)]Glc represented 70% of the total oligosaccharide amount of futA-on-driven fermentation and was produced at a concentration of 1.7 g/L. Fermentation driven by futB led to equal amounts of both lacto-N-neofucopentaose-V and lacto-N-neofucopentaose-II Gal(beta1-4)[Fuc(alpha1-3)]GlcNAc(beta1-3)Gal(beta1-4)Glc, produced at 280 and 260 mg/L, respectively. Unexpectedly, a noticeable proportion (0.5 g/L) of the human milk oligosaccharide 3-fucosyllactose Gal(beta1-4)[Fuc(alpha1-3)]Glc was produced in futA-on-driven fermentation, underlining the activity of fucosyltransferase FutA in E. coli and leading to a reassessment of its activity on lactose. All oligosaccharides produced by the products of both fut genes were natural compounds of human milk.

S-layer nanoglycobiology of bacteria

Cell surface layers (S-layers) are common structures of the bacterial cell envelope with a lattice-like appearance that are formed by a self-assembly process. Frequently, the constituting S-layer proteins are modified with covalently linked glycan chains facing the extracellular environment. S-layer glycoproteins from organisms of the Bacillaceae family possess long, O-glycosidically linked glycans that are composed of a great variety of sugar constituents. The observed variations already exceed the display found in eukaryotic glycoproteins. Recent investigations of the S-layer protein glycosylation process at the molecular level, which has lagged behind the structural studies due to the lack of suitable molecular tools, indicated that the S-layer glycoprotein glycan biosynthesis pathway utilizes different modules of the well-known biosynthesis routes of lipopolysaccharide O-antigens. The genetic information for S-layer glycan biosynthesis is usually present in S-layer glycosylation (slg) gene clusters acting in concert with housekeeping genes. To account for the nanometer-scale cell surface display feature of bacterial S-layer glycosylation, we have coined the neologism 'nanoglycobiology'. It includes structural and biochemical aspects of S-layer glycans as well as molecular data on the machinery underlying the glycosylation event. A key aspect for the full potency of S-layer nanoglycobiology is the unique self-assembly feature of the S-layer protein matrix. Being aware that in many cases the glycan structures associated with a protein are the key to protein function, S-layer protein glycosylation will add a new and valuable component to an 'S-layer based molecular construction kit'. In our long-term research strategy, S-layer nanoglycobiology shall converge with other functional glycosylation systems to produce 'functional' S-layer neoglycoproteins for diverse applications in the fields of nanobiotechnology and vaccine technology. Recent advances in the field of S-layer nanoglycobiology have made our overall strategy a tangible aim of the near future.

Bioengineering novel in vitro metabolic pathways using synthetic biology

Huge numbers of enzymes have evolved in nature to function in aqueous environments at moderate temperatures and neutral pH. This gives us, in principle, the unique opportunity to construct multistep reaction systems of considerable catalytic complexity in vitro. However, this opportunity is rarely exploited beyond research scale, because such systems are difficult to assemble and to operate productively. Recent advances in DNA synthesis, genome engineering, high-throughput analytics, model-based analysis of biochemical systems and (semi-)rational protein engineering suggest that we have all the tools available to rationally design and efficiently operate such systems of enzymes, and finally harvest their potential for preparative syntheses.

Production of GDP-L-fucose, L-fucose donor for fucosyloligosaccharide synthesis, in recombinant Escherichia coli

A recombinant Escherichia coli strain was developed to produce guanosine 5'-diphosphate (GDP)-L-fucose, donor of L-fucose, which is an essential substrate for the synthesis of fucosyloligosaccharides. GDP-D: -mannose-4, 6-dehydratase (GMD) and GDP-4-keto-6-deoxymannose 3, 5-epimerase 4-reductase (WcaG), the two crucial enzymes for the de novo GDP-L-fucose biosynthesis, were overexpressed in recombinant E. coli by constructing inducible overexpression vectors. Optimum expression conditions for GMD and WcaG in recombinant E. coli BL21(DE3) were 25 degrees C and 0.1 mM isopropyl-beta-D-thioglucopyranoside. Maximum GDP-L-fucose concentration of 38.9 +/- 0.6 mg l(-1) was obtained in a glucose-limited fed-batch cultivation, and it was enhanced further by co-expression of NADPH-regenerating glucose-6-phosphate dehydrogenase encoded by the zwf gene to achieve 55.2 +/- 0.5 mg l(-1) GDP-L-fucose under the same cultivation condition.

Fucosylation in prokaryotes and eukaryotes

Fucosylated carbohydrate structures are involved in a variety of biological and pathological processes in eukaryotic organisms including tissue development, angiogenesis, fertilization, cell adhesion, inflammation, and tumor metastasis. In contrast, fucosylation appears less common in prokaryotic organisms and has been suggested to be involved in molecular mimicry, adhesion, colonization, and modulating the host immune response. Fucosyltransferases (FucTs), present in both eukaryotic and prokaryotic organisms, are the enzymes responsible for the catalysis of fucose transfer from donor guanosine-diphosphate fucose to various acceptor molecules including oligosaccharides, glycoproteins, and glycolipids. To date, several subfamilies of mammalian FucTs have been well characterized; these enzymes are therefore delineated and used as models. Non-mammalian FucTs that possess different domain construction or display distinctive acceptor substrate specificity are highlighted. It is noteworthy that the glycoconjugates from plants and schistosomes contain some unusual fucose linkages, suggesting the presence of novel FucT subfamilies as yet to be characterized. Despite the very low sequence homology, striking functional similarity is exhibited between mammalian and Helicobacter pylori alpha1,3/4 FucTs, implying that these enzymes likely share a conserved mechanistic and structural basis for fucose transfer; such conserved functional features might also exist when comparing other FucT subfamilies from different origins. Fucosyltranferases are promising tools used in synthesis of fucosylated oligosaccharides and glycoconjugates, which show great potential in the treatment of infectious and inflammatory diseases and tumor metastasis.

Metabolic engineering of Agrobacterium sp. for UDP-galactose regeneration and oligosaccharide synthesis

Curdlan-producing Agrobacterium sp. is unique in possessing a highly efficient UDP-glucose regeneration system. A broad-host-range expression strategy was successfully developed to exploit the unique metabolic capability for UDP-galactose regeneration during oligosaccharide synthesis. The engineered Agrobacterium cells functioned as a UDP-galactose regeneration system, allowing galactose-containing disaccharides to be synthesized from glucose or other simple sugars. Unexpectedly, a lag period of 24h preceded the active synthesis, which could be eliminated with rifampicin. An intracellular nucleotide profiling revealed that the UMP level was elevated by 3.8 fold in the presence of rifampicin, suggesting that rifampicin simulated a nitrogen-limitation condition that triggered the metabolic change. Product selectivity was improved nearly 40-fold by using high acceptor concentration and restricting glucose supply. N-acetyllactosamine concentration near 20 mM (7.5 g/l) was obtained, demonstrating the effectiveness of the engineered strain in UDP-galactose regeneration. This organism could be engineered to regenerate other UDP-sugar nucleotides using the same strategy as illustrated here.

Carbohydrates as future anti-adhesion drugs for infectious diseases

Adhesion of pathogenic organisms to host tissues is the prerequisite for the initiation of the majority of infectious diseases. In many systems, it is mediated by lectins present on the surface of the infectious organism that bind to complementary carbohydrates on the surface of the host tissues. Lectin-deficient mutants often lack the ability to initiate infection. The bacterial lectins are typically in the form of elongated submicroscopic multi-subunit protein appendages, known as fimbriae (or pili). The best characterized of these are the mannose-specific type 1 fimbriae, the galabiose-specific P fimbriae and the N-acetylglucosamine-specific fimbriae of Escherichia coli. Soluble carbohydrates recognized by the bacterial surface lectins block the adhesion of the bacteria to animal cells in vitro. Aromatic alpha-mannosides are potent inhibitors of type 1 fimbriated E. coli, being up to 1000 times more active than MealphaMan, with affinities in the nanomolar range. This is due to the presence of a hydrophobic region next to the monosaccharide-binding site of the fimbriae, as recently demonstrated by X-ray studies. Polyvalent saccharides (e.g., neoglycoproteins or dendrimers) are also powerful inhibitors of bacterial adhesion in vitro. Very significantly, lectin-inhibitory saccharides have been shown to protect mice, rabbits, calves and monkeys against experimental infection by lectin-carrying bacteria. Since anti-adhesive agents do not act by killing or arresting the growth of the pathogens, it is very likely that strains resistant to such agents will emerge at a markedly lower rate than of strains that are resistant to antibiotics. Suitable sugars also inhibit the binding to cells of carbohydrate-specific toxins, among them those of Shigella dysenteriae Type 1, and of the homologous Verotoxins of E. coli, specific for galabiose. Appropriately designed polyvalent ligands are up to six orders of magnitude stronger inhibitors of toxin binding in vitro than the monovalent ones, and they protect mice against the Shigella toxin. The above data provide clear proof for the feasibility of anti-adhesion therapy of infectious diseases, although this has not yet been successful in humans. All in all, however, there is little doubt that inhibitors of microbial lectins will in the near future join the arsenal of drugs for therapy of infectious diseases.

Purification, Kinetic Characterization, and Mapping of the Minimal Catalytic Domain and the Key Polar Groups of Helicobacter pylori α-(1,3/1,4)-Fucosyltransferases

The minimal catalytic domain of alpha-(1,3/1,4)-fucosyltransferases (FucTs) from Helicobacter pylori strains NCTC11639 and UA948 was mapped by N- and C-terminal truncations. Only the C terminus could be truncated without significant loss of activity. 11639FucT and UA948FucT contain 10 and 8 heptad repeats, respectively, which connect the catalytic domain with the C-terminal putative amphipathic alpha-helices. Deletion of all heptad repeats almost completely abolished enzyme activity. Nevertheless, with only one heptad repeat 11639FucT is fully active, whereas UA948FucT is partially active. Removal of the two putative amphipathic alpha-helices dramatically increased protein expression and solubility, enabling purification with yields of milligrams/liter. Steady-state kinetic analysis of the purified FucTs showed that 11639FucTs possessed slightly tighter binding affinity for both Type II acceptor and GDP-fucose donor than UA948FucT, and its kcat of 2.3 s(-1) was double that of UA948FucT, which had a kcat value of 1.1 s(-1) for both Type II and Type I acceptors. UA948FucT strongly favors Type II over the Type I acceptor with a 20-fold difference in acceptor Km. Sixteen modified Type I and Type II series acceptors were employed to map the molecular determinants of acceptors required for recognition by H. pylori alpha-(1,3/1,4)-FucTs. Deoxygenation at 6-C of the galactose in Type II acceptor caused a 5000-fold decrease in alpha1,3 activity, whereas in Type I acceptor this completely abolished alpha1,4 activity, indicating that this hydroxyl group is a key polar group.

Plant Glycosyltransferases: Their Potential as Novel Biocatalysts

The potential application of glycosyltransferases in glycoconjugate synthesis has attracted considerable interest from the biotechnology community in recent years. This concept article focuses on the current understanding of the chemistry of a family of plant enzymes capable of glycosylating small lipophilic molecules. These enzymes are discussed in terms of their regio- and enantioselective substrate recognition, sugar-donor selectivity and their utility as biocatalysts in whole-cell systems.

A single aromatic amino acid at the carboxyl terminus of Helicobacter pylori {alpha}1,3/4 fucosyltransferase determines substrate specificity

Fucosyltransferases (FucT) from different Helicobacter pylori strains display distinct Type I (Galbeta1,3GlcNAc) or Type II (Galbeta1,4GlcNAc) substrate specificity. FucT from strain UA948 can transfer fucose to the OH-3 of Type II acceptors as well as to the OH-4 of Type I acceptors on the GlcNAc moiety, so it has both alpha1,3 and alpha1,4 activities. In contrast, FucT from strain NCTC11639 has exclusive alpha1,3 activity. Our domain swapping study (Ma, B., Wang, G., Palcic, M. M., Hazes, B., and Taylor, D. E. (2003) J. Biol. Chem. 278, 21893-21900) demonstrated that exchange of the hypervariable loops, (347)DNPFIFC(353) in 11639FucT and (345)CNDAHYSALH(354) in UA948FucT, were sufficient to either confer or abolish alpha1,4 activity. Here we performed alanine scanning site-directed mutagenesis to identify which amino acids within (345)CNDAHYSALH(354) of UA948FucT confer Type I substrate specificity. The Tyr(350) --> Ala mutation dramatically reduced alpha1,4 activity without lowering alpha1,3 activity. None of the other alanine substitutions selectively eliminated alpha1,4 activity. To elucidate how Tyr(350) determines alpha1,4 specificity, mutants Tyr(350) --> Phe, Tyr(350) --> Trp, and Tyr(350) --> Gly were constructed in UA948FucT. These mutations did not decrease alpha1,3 activity but reduced the alpha1,4 activity to 66.9, 55.6, and 3.1% [corrected] of wild type level, respectively. Apparently the aromatic nature, but not the hydroxyl group of Tyr(350), is essential for alpha1,4 activity. Our data demonstrate that a single amino acid (Tyr(350)) in the C-terminal hypervariable region of UA948FucT determines Type I acceptor specificity. Notably, a single aromatic residue (Trp) has also been implicated in controlling Type I acceptor preference for human FucT III, but it is located in an N-terminal hypervariable stem domain.

Genetic engineering of Escherichia coli for the production of NI,NII-diacetylchitobiose (chitinbiose) and its utilization as a primer for the synthesis of complex carbohydrates

Chitinbiose was produced at more than 4 g L-1 by a high cell density culture of an Escherichia coli strain that co-expressed the rhizobial chitinoligosaccharide synthase gene nodC and a truncated form of the chitinase gene chiA which has been designed to be functionally produced in the E. coli cytoplasm. Chitinpentaose, which has previously been shown to be produced by the nodC protein in growing E. coli, was formed as an intermediate that was subsequently hydrolyzed into chitinbiose by the chitinase encoded by chiA. Chitinbiose was mainly recovered in the extracellular medium and to prevent its catabolism, the genes for the chitinbiose PTS permease had to be disrupted. When the additional gene lgtB for beta1,4-galactosyltransferase was expressed, intracellular chitinbiose was converted into the trisaccharide Galbeta-4GlcNAcbeta-4GlcNAc which could serve as acceptor for glycosyltransferase that recognize the terminal N-acetyllactosamine structure.

Enzymes in the synthesis of bioactive compounds: the prodigious decades

The growing demand for enantiomerically pure pharmaceuticals has impelled research on enzymes as catalysts for asymmetric synthetic transformations. However, the use of enzymes for this purpose was rather limited until the discovery that enzymes can work in organic solvents. Since the advent of the PCR the number of available enzymes has been growing rapidly and the tailor-made biocatalysts are becoming a reality. Thus, it has been possible the use of enzymes for the synthesis of new innovative medicines such as carbohydrates and their incorporation to modern methods for drug development, such as combinatorial chemistry. Finally, the genomic research is allowing the manipulation of whole genomes opening the door to the combinatorial biosynthesis of compounds. In this review, our intention is to highlight the main landmarks that have led to transfer the chemical efficiency shown by the enzymes in the cell to the synthesis of bioactive molecules in the lab during the last 20 years.

Large-scale synthesis of globotriose derivatives through recombinant E. coli

The carbohydrate chains decorating cell membranes and secreted proteins participate in a range of important biological processes. However, their ultimate significance and possible therapeutic potential have not been fully explored due to the lack of economical methods for their production. This study is an example of the use of a genetically engineered bacterial strain in the preparation of diverse oligosaccharides. Based on an ex vivo biosynthetic pathway, an artificial gene cluster was constructed by linking the genes of five associated enzymes on a plasmid vector. This plasmid was inserted into the E. coli NM522 strain to form globotriose-producing cells ('superbug' pLDR20-CKTUF). The specific strain was conveniently applied to the synthesis of globotriose trisaccharide and its derivatives, as potential neutralizers for Shiga toxin. This work demonstrates a novel and economical method for generating ligand diversity for carbohydrate drug development.

Large-scale in vivo synthesis of the carbohydrate moieties of gangliosides GM1 and GM2 by metabolically engineered Escherichia coli

Two metabolically engineered Escherichia coli strains have been constructed to produce the carbohydrate moieties of gangliosides GM2 (GalNAcbeta-4(NeuAcalpha-3)Galbeta-4Glc; Gal = galactose, Glc = glucose, Ac = acetyl) and GM1 (Galbeta-3GalNAcbeta-4(NeuAcalpha-3)Galbeta-4Glc. The GM2 oligosaccharide-producing strain TA02 was devoid of both beta-galactosidase and sialic acid aldolase activities and overexpressed the genes for CMP-NeuAc synthase (CMP = cytidine monophosphate), alpha-2,3-sialyltransferase, UDP-GlcNAc (UDP = uridine diphosphate) C4 epimerase, and beta-1,4-GalNAc transferase. When this strain was cultivated on glycerol, exogenously added lactose and sialic acid were shown to be actively internalized into the cytoplasm and converted into GM2 oligosaccharide. The in vivo synthesis of GM1 oligosaccharide was achieved by taking a similar approach but using strain TA05, which additionally overexpressed the gene for beta-1,3-galactosyltransferase. In high-cell-density cultures, the production yields for the GM2 and GM1 oligosaccharides were 1.25 g L(-1) and 0.89 g L(-1), respectively.

Simplifying oligosaccharide synthesis: efficient synthesis of lactosamine and siaylated lactosamine oligosaccharide donors

A practical sequence is described for converting d-glucosamine into peracetylated Gal(beta-1,4)GlcNTroc(beta1-S)Ph and Neu5Ac(alpha-2,3)Gal(beta-1,4)GlcNTroc(beta1-S)Ph building blocks using a synthetic strategy based on chemoenzymatic oligosaccharide synthesis. The known trichloroethoxycarbonyl, N-Troc, protecting group was selected as a suitable protecting group for both enzymatic and chemical reaction conditions. These oligosaccharide building blocks proved effective donors for the beta-selective glycosylation of the unreactive OH-3 of a polymeric PEG-bound acceptor and for the axial OH-2 of a mannose acceptor in good yields. The resulting complex oligosaccharides are useful for vaccine and pharmaceutical applications.

"Multivalent" saccharides: development of new approaches for inhibiting the effects of glycosphingolipid-binding pathogens

A number of diseases are initiated by the adherence of viruses, bacteria, or bacterial toxins to cell surface carbohydrates, a number of which are components of glycosphingolipids (GSLs). Studies of the binding of lectins indicated that many adhered weakly to monomeric carbohydrate ligands. The seminal observation that lectins adhered more strongly to a ligand with multiple carbohydrate binding sites initiated a plethora of studies designed to identify effective "multivalent" carbohydrate ligands for pathogens expressing multiple carbohydrate-binding sites. In addition to more completely defining ligand specificity of the carbohydrate-binding pathogen, identification of "multivalent" carbohydrate ligands has led to studies of their efficacy as pathogen inhibitors. This commentary focuses on pathogens that recognize the carbohydrate portion of GSLs. Because many GSL-binding pathogens have been shown to bind "multivalent" saccharides, approaches for identifying and preparing them as well as methods for characterizing their effectiveness as ligands are reviewed. Also discussed are areas of promise that should be investigated and pitfalls that might be encountered in the development of "multivalent" saccharides as pharmacologic agents.

Sialyloligosaccharides in human and bovine milk and in infant formulas: variations with the progression of lactation

Several lines of research support a role for human milk oligosaccharides in the defense of breast-fed infants against pathogens. Some ofthese oligosaccharides contain at least one moiety of sialic acid and are, thus, termed sialyloligosaccharides. These constitute a significant component (>1 g/L) of human milk. It is well established that milk composition varies among species, and previous reports have indicated that one ofthe differences between human and bovine milk is precisely their contents of sialyloligosaccharides. Because most infant formulas are manufactured with bovine milk components, it follows that formula-fed and breast-fed infants ingest dissimilar quantities of these carbohydrate structures. To ascertain these differences and their impact along lactation, the contents of oligosaccharide-bound sialic acids and major sialyloligosaccharides in samples of human and bovine milk (obtained at different lactation stages) were determined. In addition, infant formulas were assayed for their sialyloligosaccharide contents. Seven sialyloligosaccharides were identified in human milk; namely, 3'-sialyl-3-fucosyllactose and sialyllacto-N-tetraoses (a and b+c), the predominant structures at all lactation stages. Five sialyloligosaccharides were identified in bovine milk, of which 6'-sialyllactosamine and 3'-sialyllactose were the most abundant. In addition, sialyloligosaccharides in human and bovine milk decreased along lactation, and infant formulas did not contain significant amounts of sialyloligosaccharides. The results point to the general conclusion that regarding both qualitative and quantitative aspects, milk from humans and cows and infant formulas have different oligosaccharide contents. In this sense, bottle-fed infants are subject to reduced sialyloligosaccharide intake as compared to breast-fed infants.

The sialylated fraction of milk oligosaccharides is partially responsible for binding to enterotoxigenic and uropathogenic Escherichia coli human strains

Milk oligosaccharides can act as soluble receptors that block bacterial adhesion to the different epithelia. Colonization factor antigens (CFA)/I- and CFA/II-expressing enterotoxigenic Escherichia coli (ETEC) strains constitute one of the main causes of diarrhea in infants. Here, the inhibition of hemagglutination mediated by these strains by milk oligosaccharides was tested. Human milk oligosaccharides showed a strong inhibitory capacity, which decreased when the oligosaccharides were desialylated. Because milk oligosaccharides also are present in the urine of neonates receiving mothers' milk, their ability to bind two uropathogenic Escherichia coli (UPEC) strains was also examined. UPEC strains expressing P (Pap) and P-like (Prs) fimbriae are responsible for infections of the urinary tract such as pyelonephritis and cystitis. The hemagglutination mediated by these strains was inhibited by human milk oligosaccharides. The sialylated fraction was partially responsible for this inhibition in the case of the UPEC expressing the P-like fimbria because differences were found after desialylation. Although bovine milk oligosaccharides were less efficient at inhibiting the hemagglutination of ETEC strains, they were still quite good inhibitors of UPEC strains.

A new fermentation process allows large-scale production of human milk oligosaccharides by metabolically engineered bacteria

When fed to a beta-galactosidase-negative (lacZ(-)) Escherichia coli strain that was grown on an alternative carbon source (such as glycerol), lactose accumulated intracellularly on induction of the lactose permease. We showed that intracellular lactose was efficiently glycosylated when genes of glycosyltransferase that use lactose as acceptor were expressed. High-cell-density cultivation of lacZ(-) strains that overexpressed the beta 1,3 N acetyl glucosaminyltransferase lgtA gene of Neisseria meningitidis resulted in the synthesis of 6 g x L(-1) of the expected trisaccharide (GlcNAc beta 1-3Gal beta 1-4Glc). When the beta 1,4 galactosyltransferase lgtB gene of N. meningitidis was coexpressed with lgtA, the trisaccharide was further converted to lacto-N-neotetraose (Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4Glc) and lacto-N-neoheaxose with a yield higher than 5 g x L(-1). In a similar way, the nanA(-) E. coli strain that was devoid of NeuAc aldolase activity accumulated NeuAc on induction of the NanT permease and the lacZ(-) nanA(-) strain that overexpressed the N. meningitidis genes of the alpha2,3 sialyltransferase and of the CMP-NeuAc synthase efficiently produced sialyllactose (NeuAc alpha 2-3Gal beta 1-4Glc) from exogenous NeuAc and lactose.

Reassembled Biosynthetic Pathway for Large-Scale Carbohydrate Synthesis:α-Gal Epitope Producing “Superbug”

A metabolic pathway engineered Escherichia coli strain (superbug) containing one plasmid harboring an artificial gene cluster encoding all the five enzymes in the biosynthetic pathway of Galalpha l,3Lac through galactose metabolism has been developed. The plasmid contains a lambda promoter, a c1857 repressor gene, an ampicillin resistance gene, and a T7 terminator. Each gene was preceded by a Shine - Dalgarno sequence for ribosome binding. In a reaction catalyzed by the recombinant E. coli strain, Galalpha 1,3Lac trisaccharide accumulated at concentrations of 14.2 mM (7.2 gL(-1)) in a reaction mixture containing galactose, glucose, lactose, and a catalytic amount of uridine 5'-diphosphoglucose. This work demonstrates that large-scale synthesis of complex oligosaccharides can be achieved economically and efficiently through a single, biosynthetic pathway engineered microorganism.

Chemoenzymatic iterative synthesis of difficult linkages of oligosaccharides on soluble polymeric supports

[reaction: see text]. A trisaccharide donor containing a cis-Galpalpha(1-->4)Galp linkage was prepared using a synthetic strategy based on chemoenzymatic oligosaccharide synthesis on a soluble polymeric support. Significantly, only retaining glycosyltransferases gave complete reactions, whereas inverting enzymes showed little or no activity with poly(ethylene glycol) (MPEG)-bound lactose as an acceptor. The MPEG-attached trisaccharide was shown to bind to Verotoxin-1 by transfer NOE studies through the Galpalpha(1-->4)Galp portion of the molecule.

Glycan mimicry as a basis for novel anti-infective drugs

The idea of using carbohydrate-based drugs to prevent attachment of microbial pathogens to host tissues has been around for about three decades. This concept evolved from the observation that many pathogenic microbes bind to complex carbohydrate sequences on the surface of host cells. It stands to reason, therefore, that analogs of the carbohydrate sequences pathogens bind to could be used to competitively inhibit these interactions, thereby preventing microbial damage to the host. This article will summarize some of the recent advances in developing such carbohydrate-based anti-infective drugs.

Biocatalytic modification of natural products

Natural products are ideal training compounds for enzymatic catalysis. New transformations have become possible on a preparative scale thanks to molecular biology, which has made many new enzymes available. Additionally, new synthetic pathways have been developed to regenerate expensive cofactors in situ and to improve enzyme selectivity.

Safe as mother's milk: carbohydrates as future anti-adhesion drugs for bacterial diseases

The majority of infectious diseases are initiated by adhesion of pathogenic organisms to the tissues of the host. In many cases, this adhesion is mediated by lectins present on the surface of the infectious organism that bind to complementary carbohydrates on the surface of the host tissues. Lectin-deficient mutants often lack ability to initiate infection. Soluble carbohydrates recognized by the bacterial lectins block the adhesion of the bacteria to animal cells in vitro. Moreover, they have also been shown to protect against experimental infection by lectin-carrying bacteria in different organs of mammals such as mice, rabbits, calves and monkeys. In a phase II clinical trial, a pentasaccharide shown to have anti-adhesive activity against Streptococcus pneumoniae and Hemophilus influenzae in vitro failed to protect young children from nasopharyngeal colonization with these organisms and from developing otitis media. This could be because insufficient drug was delivered via nasal spray, because bacteria express multiple specificities, the inhibition of which may require a cocktail of oligosaccharides, or because children have different carbohydrate receptors from those of adults. The results of a clinical trial in which N-acetylneuraminyl(alpha2-3)lactose was administered orally to Helicobacter pylori positive patients in an effort to reduce or eradicate bacterial colonization, are awaited with interest. Although the high cost of production of the required oligosaccharides is falling with the recent introduction of enzymatic methods of synthesis, new technologies, in particular the use of engineered bacteria, promise to lower it even further. Attachment of the oligosaccharides to soluble polymeric carriers will increase greatly their effectiveness as antiadhesion agents. There is no doubt that anti-adhesive oligosaccharides will in the near future join the arsenal of drugs for the therapy of bacterial diseases.