Highly Efficient Synthesis of β(1 → 4)-Oligo- and -Polysaccharides Using a Mutant Cellulase

Bottom-Up Synthesized Glucan Materials: Opportunities from Applied Biocatalysis

Linear d-glucans are natural polysaccharides of simple chemical structure. They are comprised of d-glucosyl units linked by a single type of glycosidic bond. Noncovalent interactions within, and between, the d-glucan chains give rise to a broad variety of macromolecular nanostructures that can assemble into crystalline-organized materials of tunable morphology. Structure design and functionalization of d-glucans for diverse material applications largely relies on top-down processing and chemical derivatization of naturally derived starting materials. The top-down approach encounters critical limitations in efficiency, selectivity, and flexibility. Bottom-up approaches of d-glucan synthesis offer different, and often more precise, ways of polymer structure control and provide means of functional diversification widely inaccessible to top-down routes of polysaccharide material processing. Here the natural and engineered enzymes (glycosyltransferases, glycoside hydrolases and phosphorylases, glycosynthases) for d-glucan polymerization are described and the use of applied biocatalysis for the bottom-up assembly of specific d-glucan structures is shown. Advanced material applications of the resulting polymeric products are further shown and their important role in the development of sustainable macromolecular materials in a bio-based circular economy is discussed.

Engineering cascade biocatalysis in whole cells for bottom-up synthesis of cello-oligosaccharides: flux control over three enzymatic steps enables soluble production

BACKGROUND

Soluble cello-oligosaccharides (COS, β-1,4-D-gluco-oligosaccharides with degree of polymerization DP 2-6) have been receiving increased attention in different industrial sectors, from food and feed to cosmetics. Development of large-scale COS applications requires cost-effective technologies for their production. Cascade biocatalysis by the three enzymes sucrose-, cellobiose- and cellodextrin phosphorylase is promising because it enables bottom-up synthesis of COS from expedient substrates such as sucrose and glucose. A whole-cell-derived catalyst that incorporates the required enzyme activities from suitable co-expression would represent an important step towards making the cascade reaction fit for production. Multi-enzyme co-expression to reach distinct activity ratios is challenging in general, but it requires special emphasis for the synthesis of COS. Only a finely tuned balance between formation and elongation of the oligosaccharide precursor cellobiose results in the desired COS.

RESULTS

Here, we show the integration of cellodextrin phosphorylase into a cellobiose-producing whole-cell catalyst. We arranged the co-expression cassettes such that their expression levels were upregulated. The most effective strategy involved a custom vector design that placed the coding sequences for cellobiose phosphorylase (CbP), cellodextrin phosphorylase (CdP) and sucrose phosphorylase (ScP) in a tricistron in the given order. The expression of the tricistron was controlled by the strong T7_lacO promoter and strong ribosome binding sites (RBS) for each open reading frame. The resulting whole-cell catalyst achieved a recombinant protein yield of 46% of total intracellular protein in an optimal ScP:CbP:CdP activity ratio of 10:2.9:0.6, yielding an overall activity of 315 U/g dry cell mass. We demonstrated that bioconversion catalyzed by a semi-permeabilized whole-cell catalyst achieved an industrial relevant COS product titer of 125 g/L and a space-time yield of 20 g/L/h. With CbP as the cellobiose providing enzyme, flux into higher oligosaccharides (DP ≥ 6) was prevented and no insoluble products were formed after 6 h of conversion.

CONCLUSIONS

A whole-cell catalyst for COS biosynthesis was developed. The coordinated co-expression of the three biosynthesis enzymes balanced the activities of the individual enzymes such that COS production was maximized. With the flux control set to minimize the share of insolubles in the product, the whole-cell synthesis shows a performance with respect to yield, productivity, product concentration and quality that is promising for industrial production.

Recent advances in enzymatic synthesis of β-glucan and cellulose

Bottom-up synthesis of β-glucans such as callose, fungal β-(1,3)(1,6)-glucan and cellulose, can create the defined compounds that are needed to perform fundamental studies on glucan properties and develop applications. With the importance of β-glucans and cellulose in high-profile fields such as nutrition, renewables-based biotechnology and materials science, the enzymatic synthesis of such relevant carbohydrates and their derivatives has attracted much attention. Here we review recent developments in enzymatic synthesis of β-glucans and cellulose, with a focus on progress made over the last five years. We cover the different types of biocatalysts employed, their incorporation in cascades, the exploitation of enzyme promiscuity and their engineering, and reaction conditions affecting the production as well as in situ self-assembly of (non)functionalised glucans. The recent achievements in the application of glycosyl transferases and β-1,4- and β-1,3-glucan phosphorylases demonstrate the high potential and versatility of these biocatalysts in glucan synthesis in both industrial and academic contexts.

A promiscuous glycosyltransferase generates poly-β-1,4-glucan derivatives that facilitate mass spectrometry-based detection of cellulolytic enzymes

Promiscuous activity of a glycosyltransferase was exploited to polymerise glucose from UDP-glucose via the generation of β-1,4-glycosidic linkages. The biocatalyst was incorporated into biocatalytic cascades and chemo-enzymatic strategies to synthesise cello-oligosaccharides with tailored functionalities on a scale suitable for employment in mass spectrometry-based assays. The resulting glycan structures enabled reporting of the activity and selectivity of celluloltic enzymes.

Reducing end thiol-modified nanocellulose: Bottom-up enzymatic synthesis and use for templated assembly of silver nanoparticles into biocidal composite material

Nanoparticle-polymer composites are important functional materials but structural control of their assembly is challenging. Owing to its crystalline internal structure and tunable nanoscale morphology, cellulose is promising polymer scaffold for templating such composite materials. Here, we show bottom-up synthesis of reducing end thiol-modified cellulose chains by iterative bi-enzymatic β-1,4-glycosylation of 1-thio-β-d-glucose (10 mM), to a degree of polymerization of ∼8 and in a yield of ∼41% on the donor substrate (α-d-glucose 1-phosphate, 100 mM). Synthetic cellulose oligomers self-assemble into highly ordered crystalline (cellulose allomorph II) material showing long (micrometers) and thin nanosheet-like morphologies, with thickness of 5-7 nm. Silver nanoparticles were attached selectively and well dispersed on the surface of the thiol-modified cellulose, in excellent yield (≥ 95%) and high loading efficiency (∼2.2 g silver/g thiol-cellulose). Examined against Escherichia coli and Staphylococcus aureus, surface-patterned nanoparticles show excellent biocidal activity. Bottom-up approach by chemical design to a functional cellulose nanocomposite is presented. Synthetic thiol-containing nanocellulose can expand the scope of top-down produced cellulose materials.

Phosphorylase-catalyzed bottom-up synthesis of short-chain soluble cello-oligosaccharides and property-tunable cellulosic materials

Cellulose-based materials are produced industrially in countless varieties via top-down processing of natural lignocellulose substrates. By contrast, cellulosic materials are only rarely prepared via bottom up synthesis and oligomerization-induced self-assembly of cellulose chains. Building up a cellulose chain via precision polymerization is promising, however, for it offers tunability and control of the final chemical structure. Synthetic cellulose derivatives with programmable material properties might thus be obtained. Cellodextrin phosphorylase (CdP; EC 2.4.1.49) catalyzes iterative β-1,4-glycosylation from α-d-glucose 1-phosphate, with the ability to elongate a diversity of acceptor substrates, including cellobiose, d-glucose and a range of synthetic glycosides having non-sugar aglycons. Depending on the reaction conditions leading to different degrees of polymerization (DP), short-chain soluble cello-oligosaccharides (COS) or insoluble cellulosic materials are formed. Here, we review the characteristics of CdP as bio-catalyst for synthetic applications and show advances in the enzymatic production of COS and reducing end-modified, tailored cellulose materials. Recent studies reveal COS as interesting dietary fibers that could provide a selective prebiotic effect. The bottom-up synthesized celluloses involve chains of DP ≥ 9, as precipitated in solution, and they form ~5 nm thick sheet-like crystalline structures of cellulose allomorph II. Solvent conditions and aglycon structures can direct the cellulose chain self-assembly towards a range of material architectures, including hierarchically organized networks of nanoribbons, or nanorods as well as distorted nanosheets. Composite materials are also formed. The resulting materials can be useful as property-tunable hydrogels and feature site-specific introduction of functional and chemically reactive groups. Therefore, COS and cellulose obtained via bottom-up synthesis can expand cellulose applications towards product classes that are difficult to access via top-down processing of natural materials.

Chemoenzymatic Methods for the Synthesis of Glycoproteins

Glycosylation is one of the most prevalent posttranslational modifications that profoundly affects the structure and functions of proteins in a wide variety of biological recognition events. However, the structural complexity and heterogeneity of glycoproteins, usually resulting from the variations of glycan components and/or the sites of glycosylation, often complicates detailed structure-function relationship studies and hampers the therapeutic applications of glycoproteins. To address these challenges, various chemical and biological strategies have been developed for producing glycan-defined homogeneous glycoproteins. This review highlights recent advances in the development of chemoenzymatic methods for synthesizing homogeneous glycoproteins, including the generation of various glycosynthases for synthetic purposes, endoglycosidase-catalyzed glycoprotein synthesis and glycan remodeling, and direct enzymatic glycosylation of polypeptides and proteins. The scope, limitation, and future directions of each method are discussed.

Endoglycosidases for the Synthesis of Polysaccharides and Glycoconjugates

Recent advances in glycobiology have implicated essential roles of oligosaccharides and glycoconjugates in many important biological recognition processes, including intracellular signaling, cell adhesion, cell differentiation, cancer progression, host-pathogen interactions, and immune responses. A detailed understanding of the biological functions, as well as the development of carbohydrate-based therapeutics, often requires structurally well-defined oligosaccharides and glycoconjugates, which are usually difficult to isolate in pure form from natural sources. To meet with this urgent need, chemical and chemoenzymatic synthesis has become increasingly important as the major means to provide homogeneous compounds for functional glycocomics studies and for drug/vaccine development. Chemoenzymatic synthesis, an approach that combines chemical synthesis and enzymatic manipulations, is often the method of choice for constructing complex oligosaccharides and glycoconjugates that are otherwise difficult to achieve by purely chemical synthesis. Among these, endoglycosidases, a class of glycosidases that hydrolyze internal glycosidic bonds in glycoconjugates and polysaccharides, are emerging as a very attractive class of enzymes for synthetic purposes, due to their transglycosylation activity and their capability of transferring oligosaccharide units en bloc in a single step, in contrast to the limitation of monosaccharide transfers by common glycosyltransferases. In this chapter, we provide an overview on the application of endoglycosidases for the synthesis of complex carbohydrates, including oligosaccharides, polysaccharides, glycoproteins, glycolipids, proteoglycans, and other biologically relevant polysaccharides. The scope, limitation, and future directions of endoglycosidase-catalyzed synthesis are discussed.

Advances in Enzymatic Glycoside Synthesis

A robust platform for facile defined glycan synthesis does not exist. Yet the need for such technology has never been greater as researchers seek to understand the full scope of carbohydrate function, stretching beyond the classical roles of structure and energy storage to encompass highly nuanced cell signaling events. To comprehensively explore and exploit the full diversity of carbohydrate functions, we must first be able to synthesize them in a controlled manner. Toward this goal, traditional chemical syntheses are inefficient while nature's own synthetic enzymes, the glycosyl transferases, can be challenging to express and expensive to employ on scale. Glycoside hydrolases represent a pool of glycan processing enzymes that can be either used in a transglycosylation mode or, better, engineered to function as "glycosynthases," mutant enzymes capable of assembling glycosides. Glycosynthases grant access to valuable glycans that act as functional and structural probes or indeed as inhibitors and therapeutics in their own right. The remodelling of glycosylation patterns in therapeutic proteins via glycoside hydrolases and their mutants is an exciting frontier in both basic research and industrial scale processes.

Enzymes as Green Catalysts for Precision Macromolecular Synthesis

The present article comprehensively reviews the macromolecular synthesis using enzymes as catalysts. Among the six main classes of enzymes, the three classes, oxidoreductases, transferases, and hydrolases, have been employed as catalysts for the in vitro macromolecular synthesis and modification reactions. Appropriate design of reaction including monomer and enzyme catalyst produces macromolecules with precisely controlled structure, similarly as in vivo enzymatic reactions. The reaction controls the product structure with respect to substrate selectivity, chemo-selectivity, regio-selectivity, stereoselectivity, and choro-selectivity. Oxidoreductases catalyze various oxidation polymerizations of aromatic compounds as well as vinyl polymerizations. Transferases are effective catalysts for producing polysaccharide having a variety of structure and polyesters. Hydrolases catalyzing the bond-cleaving of macromolecules in vivo, catalyze the reverse reaction for bond forming in vitro to give various polysaccharides and functionalized polyesters. The enzymatic polymerizations allowed the first in vitro synthesis of natural polysaccharides having complicated structures like cellulose, amylose, xylan, chitin, hyaluronan, and chondroitin. These polymerizations are "green" with several respects; nontoxicity of enzyme, high catalyst efficiency, selective reactions under mild conditions using green solvents and renewable starting materials, and producing minimal byproducts. Thus, the enzymatic polymerization is desirable for the environment and contributes to "green polymer chemistry" for maintaining sustainable society.

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.

Characterization of Glycosynthase Mutants Derived from Glycoside Hydrolase Family 10 Xylanases

Four xylanases belonging to glycoside hydrolase family 10-Thermotoga maritima XylB (TM), Clostridium stercorarium XynB (CS), Bacillus halodurans XynA (BH), and Cellulomonas fimi Cex (CF)-were converted to glycosynthases by substituting the nucleophilic glutamic acid residues with glycine, alanine, and serine. The glycine mutants exhibited the highest levels of glycosynthase activity with all four enzymes. All the glycine mutants formed polymeric beta-1,4-linked xylopyranose as a precipitate during reaction with alpha-xylobiosyl fluoride. Two glycine mutants (TM and CF) recognized X(2) as an effective acceptor molecule to prohibit the formation of the polymer, while the other two (CS and BH) did not. The difference in acceptor specificity is considered to reflect the difference in substrate affinity at their +2 subsites. The results agreed with the structural predictions of the subsite, where TM and CF exhibit high affinity at subsite 2, suggesting that the glycosynthase technique is useful for investigating the affinity of +subsites.

CGTase-catalysed cis-glucosylation of L-rhamnosides for the preparation of Shigella flexneri 2a and 3a haptens

We report the enzymatic synthesis of α-D-glucopyranosyl-(1→4)-α-L-rhamnopyranoside and α-D-glucopyranosyl-(1→3)-α-L-rhamnopyranoside by using a wild-type transglucosidase in combination with glucoamylase and glucose oxidase. It was shown that Bacillus circulans 251 cyclodextrin glucanotransferase (CGTase, EC 2.1.4.19) can efficiently couple an α-L-rhamnosyl acceptor to a maltodextrin molecule with an α-(1→4) linkage, albeit in mixture with the α-(1→3) regioisomer, thus giving two glucosylated acceptors in a single reaction. Optimisation of the CGTase coupling reaction with β-cyclodextrin as the donor substrate and methyl or allyl α-L-rhamnopyranoside as acceptors resulted in good conversion yields (42-70%) with adjustable glycosylation regioselectivity. Moreover, the efficient chemical conversion of the products of CGTase-mediated cis-glucosylation into protected building blocks (previously used in the synthesis of O-antigen fragments of several Shigella flexneri serotypes) was substantiated. These novel chemoenzymatic strategies towards useful, convenient intermediates in the synthesis of S. flexneri serotypes 2a and 3a oligosaccharides might find applications in developments towards synthetic carbohydrate-based vaccine candidates against bacillary dysentery.

Degradation and synthesis of β-glucans by a Magnaporthe oryzae endotransglucosylase, a member of the glycoside hydrolase 7 family

BACKGROUND

Plant pathogens secrete enzymes that degrade plant cell walls to enhance infection and nutrient acquisition.

RESULTS

A novel endotransglucosylase catalyzes cleavage and transfer of β-glucans and decreases the physical strength of plant cell walls.

CONCLUSION

Endotransglucosylation causes depolymerization and polymerization of β-glucans, depending on substrate molecular size.

SIGNIFICANCE

Enzymatic degradation of plant cell walls is required for wall loosening, which enhances pathogen invasion. A Magnaporthe oryzae enzyme, which was encoded by the Mocel7B gene, was predicted to act on 1,3-1,4-β-glucan degradation and transglycosylation reaction of cellotriose after partial purification from a culture filtrate of M. oryzae cells, followed by liquid chromatography-tandem mass spectrometry. A recombinant MoCel7B prepared by overexpression in M. oryzae exhibited endo-typical depolymerization of polysaccharides containing β-1,4-linkages, in which 1,3-1,4-β-glucan was the best substrate. When cellooligosaccharides were used as the substrate, the recombinant enzyme generated reaction products with both shorter and longer chain lengths than the substrate. In addition, incorporation of glucose and various oligosaccharides including sulforhodamine-conjugated cellobiose, laminarioligosaccharides, gentiobiose, xylobiose, mannobiose, and xyloglucan nonasaccharide into β-1,4-linked glucans were observed after incubation with the enzyme. These results indicate that the recombinant enzyme acts as an endotransglucosylase (ETG) that cleaves the glycosidic bond of β-1,4-glucan as a donor substrate and transfers the cleaved glucan chain to another molecule functioning as an acceptor substrate. Furthermore, ETG treatment caused greater extension of heat-treated wheat coleoptiles. The result suggests that ETG functions to induce wall loosening by cleaving the 1,3-1,4-β-glucan tethers of plant cell walls. On the other hand, use of cellohexaose as a substrate for ETG resulted in the production of cellulose II with a maximum length (degree of polymerization) of 26 glucose units. Thus, ETG functions to depolymerize and polymerize β-glucans, depending on the size of the acceptor substrate.

Enzymatic synthesis of cellulose II-like substance via cellulolytic enzyme-mediated transglycosylation in an aqueous medium

The enzymatic synthesis of cellulose-like substance via a non-biosynthetic pathway has been achieved by transglycosylation in an aqueous system of the corresponding substrate, cellotriose for cellulolytic enzyme endo-acting endoglucanase I (EG I) from Hypocrea jecorina. A significant amount of water-insoluble product precipitated out from the reaction system. MALDI-TOF mass analysis showed that the resulting precipitate had a degree of polymerization (DP) of up to 16 from cellotriose. Solid-state (13)C NMR spectrum of the resulting water-insoluble product revealed that all carbon resonance lines were assigned to two kinds of anhydroglucose residues in the corresponding structure of cellulose II. X-ray diffraction (XRD) measurement as well as (13)C NMR analysis showed that the crystal structure corresponds to cellulose II with a high degree of crystallinity. We propose the multiple oligomers form highly crystalline cellulose II as a result of self-assembly via oligomer-oligomer interaction when they precipitate.

The role of the oligosaccharide binding cleft of rice BGlu1 in hydrolysis of cellooligosaccharides and in their synthesis by rice BGlu1 glycosynthase

Rice BGlu1 β-glucosidase nucleophile mutant E386G is a glycosynthase that can synthesize p-nitrophenyl (pNP)-cellooligosaccharides of up to 11 residues. The X-ray crystal structures of the E386G glycosynthase with and without α-glucosyl fluoride were solved and the α-glucosyl fluoride complex was found to contain an ordered water molecule near the position of the nucleophile of the BGlu1 native structure, which is likely to stabilize the departing fluoride. The structures of E386G glycosynthase in complexes with cellotetraose and cellopentaose confirmed that the side chains of N245, S334, and Y341 interact with glucosyl residues in cellooligosaccharide binding subsites +2, +3, and +4. Mutants in which these residues were replaced in BGlu1 β-glucosidase hydrolyzed cellotetraose and cellopentaose with k(cat) /K(m) values similar to those of the wild type enzyme. However, the Y341A, Y341L, and N245V mutants of the E386G glycosynthase synthesize shorter pNP-cellooligosaccharides than do the E386G glycosynthase and its S334A mutant, suggesting that Y341 and N245 play important roles in the synthesis of long oligosaccharides. X-ray structural studies revealed that cellotetraose binds to the Y341A mutant of the glycosynthase in a very different, alternative mode not seen in complexes with the E386G glycosynthase, possibly explaining the similar hydrolysis, but poorer synthesis of longer oligosaccharides by Y341 mutants.

Cellulases from thermophilic fungi: recent insights and biotechnological potential

Thermophilic fungal cellulases are promising enzymes in protein engineering efforts aimed at optimizing industrial processes, such as biomass degradation and biofuel production. The cloning and expression in recent years of new cellulase genes from thermophilic fungi have led to a better understanding of cellulose degradation in these species. Moreover, crystal structures of thermophilic fungal cellulases are now available, providing insights into their function and stability. The present paper is focused on recent progress in cloning, expression, regulation, and structure of thermophilic fungal cellulases and the current research efforts to improve their properties for better use in biotechnological applications.

Extraction of cellulose-synthesizing activity of Gluconacetobacter xylinus by alkylmaltoside

This study reinvestigated the synthesis of cellulose in vitro with a well-known cellulose-producing bacterium, Gluconacetobacter xylinus. Alkylmaltoside detergents, which are more frequently used in recent structural biological researches, are uniquely used in this study to solubilize cellulose-synthesizing activity from the cell membrane of G. xylinus. Activity comparable to that previously reported is obtained, while the synthesized cellulose is crystallized into a non-native polymorph of cellulose (cellulose II) as well as the previous studies. In spite of this failure to recover the native activity to synthesize cellulose I microfibril in vitro, the product is a polymer with a degree of polymerization greater than 45 as determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). It was thus concluded that the established protocol can solubilize cellulose-synthesizing activity of G. xylinus with polymerizing activity.

Building custom polysaccharides in vitro with an efficient, broad-specificity xyloglucan glycosynthase and a fucosyltransferase

The current drive for applications of biomass-derived compounds, for energy and advanced materials, has led to a resurgence of interest in the manipulation of plant polymers. The xyloglucans, a family of structurally complex plant polysaccharides, have attracted significant interest due to their intrinsic high affinity for cellulose, both in muro and in technical applications. Moreover, current cell wall models are limited by the lack of detailed structure-property relationships of xyloglucans, due to a lack of molecules with well-defined branching patterns. Here, we have developed a new, broad-specificity "xyloglucan glycosynthase", selected from active-site mutants of a bacterial endoxyloglucanase, which catalyzed the synthesis of high molar mass polysaccharides, with complex side-chain structures, from suitable glycosyl fluoride donor substrates. The product range was further extended by combination with an Arabidopsis thaliana α(1→2)-fucosyltransferase to achieve the in vitro synthesis of fucosylated xyloglucans typical of dicot primary cell walls. These enzymes thus comprise a toolkit for the controlled enzymatic synthesis of xyloglucans that are otherwise impossible to obtain from native sources. Moreover, this study demonstrates the validity of a chemo-enzymatic approach to polysaccharide synthesis, in which the simplicity and economy of glycosynthase technology is harnessed together with the exquisite specificity of glycosyltransferases to control molecular complexity.

Artificial mixed-linked β-glucans produced by glycosynthase-catalyzed polymerization: tuning morphology and degree of polymerization

The glycosynthase derived from Bacillus licheniformis 1,3-1,4-β-glucanase was able to polymerize glycosyl fluoride donors (G4)(m)G3GαF (m = 0-2, G = Glcβ) leading to artificial mixed-linked β-glucans with regular sequences and variable β1,3 to β1,4 linkage ratios. With the E134A glycosynthase mutant, polymers had average molecular masses (M(w)) of 10-15 kDa. Whereas polymer 2 ([4G4G3G](n)) was an amorphous precipitate, the water-insoluble polymers 1 ([4G3G](n)) and 3 ([4G4G4G3G](n)) formed spherulites of 10-20 μm diameter. With the more active E134S glycosynthase mutant, polymerization led to high molecular mass polysaccharides, where M(w) was linearly dependent on enzyme concentration. Remarkably, a homo-polysaccharide [4G4G4G3G](n) with M(w) as high as 30.5 kDa (n ≈ 47) was obtained, which contained a small fraction of products up to 70 kDa, a value that is in the range of the molecular masses of low viscosity cereal 1,3-1,4-β-glucans, and among the largest products produced by a glycosynthase. Access to a range of novel tailor-made β-glucans through the glycosynthase technology will allow to evaluate the implications of polysaccharide fine structures in their physicochemical properties and their applications as biomaterials, as well as to provide valuable tools for biochemical characterization of β-glucan degrading enzymes and binding modules.

Recent Developments in Glycoside Synthesis with Glycosynthases and Thioglycoligases

Glycosynthases are hydrolytically incompetent engineered glycosidases that catalyze the high-yielding synthesis of glycoconjugates from glycosyl fluoride donor substrates and appropriate acceptors. Glycosynthases from more than 10 glycoside hydrolase families have now been generated, allowing the synthesis of a wide range of oligosaccharides. Recent examples include glycosynthase-mediated syntheses of xylo-oligosaccharides, xyloglucans, glycolipids, and aryl glycosides. Glycosynthases have also now been generated from inverting glycosidases, increasing the range of enzyme scaffolds. Improvement of glycosynthase activity and broadening of specificity has been achieved through directed evolution approaches, and several novel high-throughput screens have been developed to allow this. Finally, metabolically stable glycoside analogues have been generated using another class of mutant glycosidases: thioglycoligases. Recent developments in all these aspects are discussed.

A Chemoenzymatic Route to Conjugatable β(1→3)-Glucan Oligosaccharides

3II-O-Allyl-α-laminaribiosyl fluoride was prepared as a key synthon for the enzymatic synthesis of β(1→3)-glucan oligosaccharides, catalyzed by a mutated β(1→3)-glucanase (E231G) from barley (Hordeum vulgare L.). A strategy was developed for enzymatic elongation of the β(1→3)-glucan chain from the reducing end, using a single glucoside acceptor. When β-glucoside phenyl disulfide was used as the acceptor, this methodology generated laminari-oligosaccharides conjugatable at both their reducing and non-reducing ends.

Teaching old enzymes new tricks: engineering and evolution of glycosidases and glycosyl transferases for improved glycoside synthesis

The therapeutic potential of glycosides has made them an attractive target for drug development. The biological extraction and chemical synthesis of these molecules is often challenging and low yielding, thus alternative methods for the synthesis of polysaccharides are being pursued. A new class of enzymes, glycosynthases, which are nucleophile mutants of glycosidases, can perform the transglycosylation reaction without hydrolyzing the product, and thus provide a valuable resource for polysaccharide and glycan synthesis. Directed evolution of glycosynthases has expanded the repertoire of glycosidic linkages formed and the donors and acceptors (both sugar and nonsugar) that can be used by the glycosynthase. The application of new screening methods, such as FACS, to the directed evolution of glycosynthases will aid in the development of enzymes that are able to efficiently synthesize new, and therapeutically relevant glycosidic linkages.

Glycosynthase activity of Geobacillus stearothermophilus GH52 beta-xylosidase: efficient synthesis of xylooligosaccharides from alpha-D-xylopyranosyl fluoride through a conjugated reaction

Glycosynthases are mutant glycosidases in which the acidic nucleophile is replaced by a small inert residue. In the presence of glycosyl fluorides of the opposite anomeric configuration (to that of their natural substrates), these enzymes can catalyze glycosidic bond formation with various acceptors. In this study we demonstrate that XynB2E335G, a nucleophile-deficient mutant of a glycoside hydrolase family 52 beta-xylosidase from G. stearothermophilus, can function as an efficient glycosynthase, using alpha-D-xylopyranosyl fluoride as a donor and various aryl sugars as acceptors. The mutant enzyme can also catalyze the self-condensation reaction of alpha-D-xylopyranosyl fluoride, providing mainly alpha-D-xylobiosyl fluoride. The self-condensation kinetics exhibited apparent classical Michaelis-Menten behavior, with kinetic constants of 1.3 s(-1) and 2.2 mM for k(cat) and K(M(acceptor)), respectively, and a k(cat)/K(M(acceptor)) value of 0.59 s(-1) mM(-1). When the beta-xylosidase E335G mutant was combined with a glycoside hydrolase family 10 glycosynthase, high-molecular-weight xylooligomers were readily obtained from the affordable alpha-D-xylopyranosyl fluoride as the sole substrate.

Glycosynthase activity of hybrid aspen xyloglucan endo-transglycosylase PttXET16-34 nucleophile mutants

Glycosynthases are active-site mutants of glycoside hydrolases that catalyse glycosyl transfer using suitable activated donor substrates without competing product hydrolysis (S. M. Hancock, M. D. Vaughan and S. G. Withers, Curr. Opin. Chem. Biol., 2006, 10, 509-519). Site-directed mutagenesis of the catalytic nucleophile, Glu-85, of a Populus tremula x tremuloides xyloglucan endo-transglycosylase (PttXET16-34, EC 2.4.1.207) into alanine, glycine, and serine yielded enzymes with glycosynthase activity. Product analysis indicated that PttXET16-34 E85A in particular was able to catalyse regio- and stereospecific homo- and hetero-condensations of alpha-xylogluco-oligosaccharyl fluoride donors XXXGalphaF and XLLGalphaF to produce xyloglucans with regular sidechain substitution patterns. This substrate promiscuity contrasts that of the Humicola insolens Cel7B E197A glycosynthase, which was not able to polymerise the di-galactosylated substrate XLLGalphaF. The production of the PttXET16-34 E85A xyloglucosynthase thus expands the repertoire of glycosynthases to include those capable of synthesising structurally homogenenous xyloglucans for applications.