Enzymatic synthesis of a library of β-(1→4) hetero- d-glucose and d-xylose-based oligosaccharides employing cellodextrin phosphorylase

Effect of Free Cysteine Residues to Serine Mutation on Cellodextrin Phosphorylase

Cellodextrin phosphorylase (CDP) plays a key role in energy-efficient cellulose metabolism of anaerobic bacteria by catalyzing phosphorolysis of cellodextrin to produce cellobiose and glucose 1-phosphate, which can be utilized for glycolysis without consumption of additional ATP. As the enzymatic phosphorolysis reaction is reversible, CDP is also employed to produce cellulosic materials in vitro. However, the enzyme is rapidly inactivated by oxidation, which hinders in vitro utilization in aerobic environments. It has been suggested that the cysteine residues of CDP, which do not form disulfide bonds, are responsible for the loss of activity, and the aim of the present work was to test this idea. For this purpose, we replaced all 11 free cysteine residues of CDP from Acetivibrio thermocellus (formerly known as Clostridium thermocellum) with serine, which structurally resembles cysteine in our previous work. Herein, we show that the resulting CDP variant, named CDP-CS, has comparable activity to the wild-type enzyme, but shows increased stability to oxidation during long-term storage. X-Ray crystallography indicated that the mutations did not markedly alter the overall structure of the enzyme. Ensemble refinement of the crystal structures of CDP and CDP-CS indicated that the C372S and C625S mutations reduce structural fluctuations in the protein main chain, which may contribute to the increased stability of CDP-CS to oxidation.

Precision synthesis of reducing-end thiol-modified cellulose enabled by enzyme selection

Enzyme-catalyzed iterative β-1,4-glycosylation of β-glycosides is promising for bottom-up polymerization of reducing-end-modified cello-oligosaccharide chains. Self-assembly of the chains from solution yields crystalline nanocellulose materials with properties that are tunable by the glycoside group used. Cellulose chains with a reducing-end thiol group are of interest to install a controllable pattern of site-selective modifications into the nanocellulose material. Selection of the polymerizing enzyme (cellodextrin phosphorylase; CdP) was pursued here to enhance the synthetic precision of β-1-thio-glucose conversion to generate pure “1-thio-cellulose” (≥95%) unencumbered by plain (unlabeled) cellulose resulting from enzymatic side reactions. The CdP from Clostridium stercorarium (CsCdP) was 21 times more active on β-1-thio-glucose (0.17 U/mg; 45 °C) than the CdP from Clostridium cellulosi (CcCdP), and it lacked hydrolase activity, which is substantial in CcCdP, against the α-d-glucose 1-phosphate donor substrate. The combination of these enzyme properties indicated that CsCdP is a practical catalyst for 1-thio-cellulose synthesis directly from β-1-thio-glucose (8 h; 25 mol% yield) that does not require a second enzyme (cellobiose phosphorylase), which was essential when using the less selective CcCdP. The 1-thio-cellulose chains had an average degree of polymerization of ∼10 and were assembled into highly crystalline cellulose II crystallinity material.

Molecular Recognition of Natural and Non-Natural Substrates by Cellodextrin Phosphorylase from Ruminiclostridium Thermocellum Investigated by NMR Spectroscopy

β‐1→4‐Glucan polysaccharides like cellulose, derivatives and analogues, are attracting attention due to their unique physicochemical properties, as ideal candidates for many different applications in biotechnology. Access to these polysaccharides with a high level of purity at scale is still challenging, and eco‐friendly alternatives by using enzymes in vitro are highly desirable. One prominent candidate enzyme is cellodextrin phosphorylase (CDP) from Ruminiclostridium thermocellum, which is able to yield cellulose oligomers from short cellodextrins and α‐d‐glucose 1‐phosphate (Glc‐1‐P) as substrates. Remarkably, its broad specificity towards donors and acceptors allows the generation of highly diverse cellulose‐based structures to produce novel materials. However, to fully exploit this CDP broad specificity, a detailed understanding of the molecular recognition of substrates by this enzyme in solution is needed. Herein, we provide a detailed investigation of the molecular recognition of ligands by CDP in solution by saturation transfer difference (STD) NMR spectroscopy, tr‐NOESY and protein‐ligand docking. Our results, discussed in the context of previous reaction kinetics data in the literature, allow a better understanding of the structural basis of the broad binding specificity of this biotechnologically relevant enzyme.

Kinetic modeling of phosphorylase-catalyzed iterative β-1,4-glycosylation for degree of polymerization-controlled synthesis of soluble cello-oligosaccharides

BACKGROUND

Cellodextrin phosphorylase (CdP; EC 2.4.1.49) catalyzes the iterative β-1,4-glycosylation of cellobiose using α-D-glucose 1-phosphate as the donor substrate. Cello-oligosaccharides (COS) with a degree of polymerization (DP) of up to 6 are soluble while those of larger DP self-assemble into solid cellulose material. The soluble COS have attracted considerable attention for their use as dietary fibers that offer a selective prebiotic function. An efficient synthesis of soluble COS requires good control over the DP of the products formed. A mathematical model of the iterative enzymatic glycosylation would be important to facilitate target-oriented process development.

RESULTS

A detailed time-course analysis of the formation of COS products from cellobiose (25 mM, 50 mM) and α-D-glucose 1-phosphate (10-100 mM) was performed using the CdP from Clostridium cellulosi. A mechanism-based, Michaelis-Menten type mathematical model was developed to describe the kinetics of the iterative enzymatic glycosylation of cellobiose. The mechanistic model was combined with an empirical description of the DP-dependent self-assembly of the COS into insoluble cellulose. The hybrid model thus obtained was used for kinetic parameter determination from time-course fits performed with constraints derived from initial rate data. The fitted hybrid model provided excellent description of the experimental dynamics of the COS in the DP range 3-6 and also accounted for the insoluble product formation. The hybrid model was suitable to disentangle the complex relationship between the process conditions used (i.e., substrate concentration, donor/acceptor ratio, reaction time) and the reaction output obtained (i.e., yield and composition of soluble COS). Model application to a window-of-operation analysis for the synthesis of soluble COS was demonstrated on the example of a COS mixture enriched in DP 4.

CONCLUSIONS

The hybrid model of CdP-catalyzed iterative glycosylation is an important engineering tool to study and optimize the biocatalytic synthesis of soluble COS. The kinetic modeling approach used here can be of a general interest to be applied to other iteratively catalyzed enzymatic reactions of synthetic importance.

β-Glucan phosphorylases in carbohydrate synthesis

Abstract

β-Glucan phosphorylases are carbohydrate-active enzymes that catalyze the reversible degradation of β-linked glucose polymers, with outstanding potential for the biocatalytic bottom-up synthesis of β-glucans as major bioactive compounds. Their preference for sugar phosphates (rather than nucleotide sugars) as donor substrates further underlines their significance for the carbohydrate industry. Presently, they are classified in the glycoside hydrolase families 94, 149, and 161 (www.cazy.org). Since the discovery of β-1,3-oligoglucan phosphorylase in 1963, several other specificities have been reported that differ in linkage type and/or degree of polymerization. Here, we present an overview of the progress that has been made in our understanding of β-glucan and associated β-glucobiose phosphorylases, with a special focus on their application in the synthesis of carbohydrates and related molecules.

Key points

• Discovery, characteristics, and applications of β-glucan phosphorylases.

• β-Glucan phosphorylases in the production of functional carbohydrates.

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.

Effect of C-6 Methylol Groups on Substrate Recognition of Glucose/Xylose Mixed Oligosaccharides by Cellobiose Dehydrogenase from the Basidiomycete Phanerochaete chrysosporium

Cellobiose dehydrogenase (CDH) is a flavocytochrome catalyzing oxidation of the reducing end of cellobiose and cellooligosaccharides, and has a key role in the degradation of cellulosic biomass by filamentous fungi. Here, we use a lineup of glucose/xylose-mixed β-1,4-linked disaccharides and trisaccharides, enzymatically synthesized by means of the reverse reaction of cellobiose phosphorylase and cellodextrin phosphorylase, to investigate the substrate recognition of CDH. We found that CDH utilizes β-D-xylopyranosyl-(1→4)-D-glucopyranose (Xyl-Glc) as an electron donor with similar K_m and k_cat values to cellobiose. β-D-Glucopyranosyl-(1→4)-D-xylopyranose (Glc-Xyl) shows a higher K_m value, while xylobiose does not serve as a substrate. Trisaccharides show similar behavior; i.e., trisaccharides with cellobiose and Xyl-Glc units at the reducing end show similar kinetics, while the enzyme was less active towards those with Glc-Xyl, and inactive towards those with xylobiose. We also use docking simulation to evaluate substrate recognition of the disaccharides, and we discuss possible molecular mechanisms of substrate recognition by CDH.

Preparative and Kinetic Analysis of β-1,4- and β-1,3-Glucan Phosphorylases Informs Access to Human Milk Oligosaccharide Fragments and Analogues Thereof

The enzymatic synthesis of oligosaccharides depends on the availability of suitable enzymes, which remains a limitation. Without recourse to enzyme engineering or evolution approaches, herein we demonstrate the ability of wild-type cellodextrin phosphorylase (CDP: β-1,4-glucan linkage-dependent) and laminaridextrin phosphorylase (Pro_7066: β-1,3-glucan linkage-dependent) to tolerate a number of sugar-1- phosphate substrates, albeit with reduced kinetic efficiency. In spite of catalytic efficiencies of <1 % of the natural reactions, we demonstrate the utility of given phosphorylase-sugar phosphate pairs to access new-to-nature fragments of human milk oligosaccharides, or analogues thereof, in multi-milligram quantities.

Cellodextrin phosphorylase from Ruminiclostridium thermocellum: X-ray crystal structure and substrate specificity analysis

The GH94 glycoside hydrolase cellodextrin phosphorylase (CDP, EC 2.4.1.49) produces cellodextrin oligomers from short β-1→4-glucans and α-D-glucose 1-phosphate. Compared to cellobiose phosphorylase (CBP), which produces cellobiose from glucose and α-D-glucose 1-phosphate, CDP is biochemically less well characterised. Herein, we investigate the donor and acceptor substrate specificity of recombinant CDP from Ruminiclostridium thermocellum and we isolate and characterise a glucosamine addition product to the cellobiose acceptor with the non-natural donor α-D-glucosamine 1-phosphate. In addition, we report the first X-ray crystal structure of CDP, along with comparison to the available structures from CBPs and other closely related enzymes, which contributes to understanding of the key structural features necessary to discriminate between monosaccharide (CBP) and oligosaccharide (CDP) acceptor substrates.

Biochemical properties of GH94 cellodextrin phosphorylase THA_1941 from a thermophilic eubacterium Thermosipho africanus TCF52B with cellobiose phosphorylase activity

A hypothetic gene (THA_1941) encoding a putative cellobiose phosphorylase (CBP) from Thermosipho africanus TCF52B has very low amino acid identities (less than 12%) to all known GH94 enzymes. This gene was cloned and over-expressed in Escherichia coli BL21(DE3). The recombinant protein was hypothesized to be a CBP enzyme and it showed an optimum temperature of 75 °C and an optimum pH of 7.5. Beyond its CBP activity, this enzyme can use cellobiose and long-chain cellodextrins with a degree of polymerization of greater than two as a glucose acceptor, releasing phosphate from glucose 1-phosphate. The catalytic efficiencies (k_cat/K_m) indicated that cellotetraose and cellopentaose were the best substrates for the phosphorolytic and reverse synthetic reactions, respectively. These results suggested that this enzyme was the first enzyme having both cellodextrin and cellobiose phosphorylases activities. Because it preferred cellobiose and cellodextrins to glucose in the synthetic direction, it was categorized as a cellodextrin phosphorylase (CDP). Due to its unique ability of the reverse synthetic reaction, this enzyme could be a potential catalyst for the synthesis of various oligosaccharides. The speculative function of this CDP in the carbohydrate metabolism of T. africanus TCF52B was also discussed.

Novel substrates for the automated and manual assay of endo-1,4-β-xylanase

endo-1,4-β-Xylanase (EC 3.2.1.8) is employed across a broad range of industries including animal feed, brewing, baking, biofuels, detergents and pulp (paper). Despite its importance, a rapid, reliable, reproducible, automatable assay for this enzyme that is based on the use of a chemically defined substrate has not been described to date. Reported herein is a new enzyme coupled assay procedure, termed the XylX6 assay, that employs a novel substrate, namely 4,6-O-(3-ketobutylidene)-4-nitrophenyl-β-4⁵-O-glucosyl-xylopentaoside. The development of the substrate and associated assay is discussed here and the relationship between the activity values obtained with the XylX6 assay versus traditional reducing sugar assays and its specificity and reproducibility were thoroughly investigated.

Enzymatic Synthesis of Oligo(ethylene glycol)-Bearing Cellulose Oligomers for in Situ Formation of Hydrogels with Crystalline Nanoribbon Network Structures

Enzymatic synthesis of cellulose and its derivatives has gained considerable attention for use in the production of artificial crystalline nanocelluloses with unique structural and functional properties. However, the poor colloidal stability of the nanocelluloses during enzymatic synthesis in aqueous solutions limits their crystallization-based self-assembly to greater architectures. In this study, oligo(ethylene glycol) (OEG)-bearing cellulose oligomers with different OEG chain lengths were systematically synthesized via cellodextrin phosphorylase-catalyzed oligomerization of α-d-glucose l-phosphate monomers against OEG-bearing β-d-glucose primers. The products were self-assembled into extremely well-grown crystalline nanoribbon network structures with the cellulose II allomorph, potentially due to OEG-derived colloidal stability of the nanoribbon's precursors, followed by the in situ formation of physically cross-linked hydrogels. The monomer conversions, average degree of polymerization, and morphologies of the nanoribbons changed significantly, depending on the OEG chain length. Taken together, our findings open a new avenue for the enzymatic reaction-based facile production of novel cellulosic soft materials with regular nanostructures.

Influence of Substrates on the Surface Characteristics and Membrane Proteome of Fibrobacter succinogenes S85

Although Fibrobacter succinogenes S85 is one of the most proficient cellulose degrading bacteria among all mesophilic organisms in the rumen of herbivores, the molecular mechanism behind cellulose degradation by this bacterium is not fully elucidated. Previous studies have indicated that cell surface proteins might play a role in adhesion to and subsequent degradation of cellulose in this bacterium. It has also been suggested that cellulose degradation machinery on the surface may be selectively expressed in response to the presence of cellulose. Based on the genome sequence, several models of cellulose degradation have been suggested. The aim of this study is to evaluate the role of the cell envelope proteins in adhesion to cellulose and to gain a better understanding of the subsequent cellulose degradation mechanism in this bacterium. Comparative analysis of the surface (exposed outer membrane) chemistry of the cells grown in glucose, acid-swollen cellulose and microcrystalline cellulose using physico-chemical characterisation techniques such as electrophoretic mobility analysis, microbial adhesion to hydrocarbons assay and Fourier transform infra-red spectroscopy, suggest that adhesion to cellulose is a consequence of an increase in protein display and a concomitant reduction in the cell surface polysaccharides in the presence of cellulose. In order to gain further understanding of the molecular mechanism of cellulose degradation in this bacterium, the cell envelope-associated proteins were enriched using affinity purification and identified by tandem mass spectrometry. In total, 185 cell envelope-associated proteins were confidently identified. Of these, 25 proteins are predicted to be involved in cellulose adhesion and degradation, and 43 proteins are involved in solute transport and energy generation. Our results supports the model that cellulose degradation in F. succinogenes occurs at the outer membrane with active transport of cellodextrins across for further metabolism of cellodextrins to glucose in the periplasmic space and inner cytoplasmic membrane.

Enzymatic synthesis using glycoside phosphorylases

Carbohydrate phosphorylases are readily accessible but under-explored catalysts for glycoside synthesis. Their use of accessible and relatively stable sugar phosphates as donor substrates underlies their potential. A wide range of these enzymes has been reported of late, displaying a range of preferences for sugar donors, acceptors and glycosidic linkages. This has allowed this class of enzymes to be used in the synthesis of diverse carbohydrate structures, including at the industrial scale. As more phosphorylase enzymes are discovered, access to further difficult to synthesise glycosides will be enabled. Herein we review reported phosphorylase enzymes and the glycoside products that they have been used to synthesise.

Enzymatic synthesis and post-functionalization of two-dimensional crystalline cellulose oligomers with surface-reactive groups

Two-dimensional crystalline cellulose oligomers with surface-reactive azide groups were synthesized by enzymatic reactions and covalently post-functionalized with alkyne-containing dye molecules through click reactions.

Generation and structural validation of a library of diverse xyloglucan-derived oligosaccharides, including an update on xyloglucan nomenclature

Xyloglucans are structurally complex plant cell wall polysaccharides that are involved in cell growth and expansion, energy metabolism, and signaling. Determining the structure-function relationships of xyloglucans would benefit from the availability of a comprehensive and structurally diverse collection of rigorously characterized xyloglucan oligosaccharides. Here, we present a workflow for the semi-preparative scale generation and purification of neutral and acidic xyloglucan oligosaccharides using a combination of enzymatic and chemical treatments and size-exclusion chromatography. Twenty-six of these oligosaccharides were purified to near homogeneity and their structures validated using a combination of matrix-assisted laser desorption/ionization mass spectrometry, high-performance anion exchange chromatography, and 1H nuclear magnetic resonance spectroscopy. Mass spectrometry and analytical chromatography were compared as methods for xyloglucan oligosaccharide quantification. 1H chemical shifts were assigned using two-dimensional correlation spectroscopy. A comprehensive update of the nomenclature describing xyloglucan side-chain structures is provided for reference.

Practical preparation of D-galactosyl-beta1-->4-L-rhamnose employing the combined action of phosphorylases

D-Galactosyl-beta1-->4-L-rhamnose (GalRha) was produced enzymatically from 1.1 M sucrose and 1.0 M L-rhamnose by the concomitant actions of four enzymes (sucrose phosphorylase, UDP-glucose-hexose 1-phosphate uridylyltransferase, UDP-glucose 4-epimerase, and D-galactosyl-beta1-->4-L-rhamnose phosphorylase) in the presence of 1.0 mM UDP-glucose and 30 mM inorganic phosphate. The accumulation of GalRha in 1 liter of the reaction mixture reached 230 g (the reaction yield was 71% from L-rhamnose). Sucrose and fructose in the reaction mixture were removed by yeast treatment, but isolation of GalRha by crystallization after yeast treatment was unsuccessful. Finally, 49 g of GalRha was isolated from part of the reaction mixture with yeast treatment by gel-filtration chromatography.

Efficient chemoenzymatic oligosaccharide synthesis by reverse phosphorolysis using cellobiose phosphorylase and cellodextrin phosphorylase from Clostridium thermocellum

Inverting cellobiose phosphorylase (CtCBP) and cellodextrin phosphorylase (CtCDP) from Clostridium thermocellum ATCC27405 of glycoside hydrolase family 94 catalysed reverse phosphorolysis to produce cellobiose and cellodextrins in 57% and 48% yield from α-d-glucose 1-phosphate as donor with glucose and cellobiose as acceptor, respectively. Use of α-d-glucosyl 1-fluoride as donor increased product yields to 98% for CtCBP and 68% for CtCDP. CtCBP showed broad acceptor specificity forming β-glucosyl disaccharides with β-(1→4)- regioselectivity from five monosaccharides as well as branched β-glucosyl trisaccharides with β-(1→4)-regioselectivity from three (1→6)-linked disaccharides. CtCDP showed strict β-(1→4)-regioselectivity and catalysed linear chain extension of the three β-linked glucosyl disaccharides, cellobiose, sophorose, and laminaribiose, whereas 12 tested monosaccharides were not acceptors. Structure analysis by NMR and ESI-MS confirmed two β-glucosyl oligosaccharide product series to represent novel compounds, i.e. β-D-glucopyranosyl-[(1→4)-β-D-glucopyranosyl](n)-(1→2)-D-glucopyranose, and β-D-glucopyranosyl-[(1→4)-β-D-glucopyranosyl](n)-(1→3)-D-glucopyranose (n = 1-7). Multiple sequence alignment together with a modelled CtCBP structure, obtained using the crystal structure of Cellvibrio gilvus CBP in complex with glucose as a template, indicated differences in the subsite +1 region that elicit the distinct acceptor specificities of CtCBP and CtCDP. Thus Glu636 of CtCBP recognized the C1 hydroxyl of β-glucose at subsite +1, while in CtCDP the presence of Ala800 conferred more space, which allowed accommodation of C1 substituted disaccharide acceptors at the corresponding subsites +1 and +2. Furthermore, CtCBP has a short Glu496-Thr500 loop that permitted the C6 hydroxyl of glucose at subsite +1 to be exposed to solvent, whereas the corresponding longer loop Thr637-Lys648 in CtCDP blocks binding of C6-linked disaccharides as acceptors at subsite +1. High yields in chemoenzymatic synthesis, a novel regioselectivity, and novel oligosaccharides including products of CtCDP catalysed oligosaccharide oligomerisation using α-d-glucosyl 1-fluoride, all together contribute to the formation of an excellent basis for rational engineering of CBP and CDP to produce desired oligosaccharides.

Enzymatic hydrolysis of 1,3-1,4-beta-glucosyl oligosaccharides by 1,3-1,4-beta-glucanase from Synechocystis PCC6803: a comparison with assays using polymer and chromophoric oligosaccharide substrates

The specificity of 1,3-1,4-beta-glucanase from Synechocystis PCC6803 (SsGlc) was investigated using novel substrates 1,3-1,4-beta-glucosyl oligosaccharides, in which 1,3- and 1,4-linkages are located in various arrangements. After the enzymatic reaction, the reaction products were separated and determined by high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD). As a result, SsGlc was found to hydrolyze the pentasaccharides, which possess three contiguous 1,4-beta-glycosidic linkages (cellotetraose sequence) adjacent to 1,3-beta-linkage, but none of the other oligosaccharides were hydrolyzed. To further analyze the specificity, kinetic measurements were performed using polymeric substrates and 4-methylumbelliferyl derivatives of laminaribiose and cellobiose (1,3-beta-(Glc)(2)-MU and 1,4-beta-(Glc)(2)-MU). The k(cat)/K(m) value obtained for barley beta-glucan was considerably larger than that for lichenan, indicating that SsGlc prefers 1,3-1,4-beta-glucan possessing a larger amount of cellotetraose sequence. This is consistent with the data obtained for 1,3-1,4-beta-glucosyl oligosaccharides. However, the k(cat)/K(m) value obtained for 1,4-beta-(Glc)(2)-MU was considerably lower than that for 1,3-beta-(Glc)(2)-MU, suggesting inconsistency with the data obtained from the other natural substrates. It is likely that the kinetic data obtained from such chromophoric substrates do not always reflect the true enzymatic properties.

A Method of Orthogonal Oligosaccharide Synthesis Leading to a Combinatorial Library Based on Stationary Solid-Phase Reaction

A new, efficient synthesis of oligosaccharides, which involves solid-phase reactions without mixing in combination with an orthogonal-glycosylation strategy, is described. Despite a great deal of biological interest, the combinatorial chemistry of oligosaccharides is an extremely difficult subject. The problems include 1) lengthy synthetic protocols required for the synthesis and 2) the variety of glycosylation conditions necessary for individual reactions. These issues were addressed and solved by using the orthogonal-coupling protocol and the application of a temperature gradient to provide appropriate conditions for individual reactions. Furthermore, we succeeded in carrying out solid-phase reactions with neither mechanical mixing nor flow. In this report, the synthesis of a series of trisaccharides, namely, alpha/beta-L-Fuc-(1-->6)-alpha/beta-D-Gal-(1-->2/3/4/6)-alpha/beta-D-Glc-octyl, is reported to demonstrate the eligibility of the synthetic method in combinatorial chemistry.

Outlook for cellulase improvement: screening and selection strategies

Cellulose is the most abundant renewable natural biological resource, and the production of biobased products and bioenergy from less costly renewable lignocellulosic materials is important for the sustainable development of human beings. A reduction in cellulase production cost, an improvement in cellulase performance, and an increase in sugar yields are all vital to reduce the processing costs of biorefineries. Improvements in specific cellulase activities for non-complexed cellulase mixtures can be implemented through cellulase engineering based on rational design or directed evolution for each cellulase component enzyme, as well as on the reconstitution of cellulase components. Here, we review quantitative cellulase activity assays using soluble and insoluble substrates, and focus on their advantages and limitations. Because there are no clear relationships between cellulase activities on soluble substrates and those on insoluble substrates, soluble substrates should not be used to screen or select improved cellulases for processing relevant solid substrates, such as plant cell walls. Cellulase improvement strategies based on directed evolution using screening on soluble substrates have been only moderately successful, and have primarily targeted improvement in thermal tolerance. Heterogeneity of insoluble cellulose, unclear dynamic interactions between insoluble substrate and cellulase components, and the complex competitive and/or synergic relationship among cellulase components limit rational design and/or strategies, depending on activity screening approaches. Herein, we hypothesize that continuous culture using insoluble cellulosic substrates could be a powerful selection tool for enriching beneficial cellulase mutants from the large library displayed on the cell surface.

Kinetics of substrate transglycosylation by glycoside hydrolase family 3 glucan (1→3)-β-glucosidase from the white-rot fungus Phanerochaete chrysosporium

To elucidate the interaction between substrate inhibition and substrate transglycosylation of retaining glycoside hydrolases (GHs), a steady-state kinetic study was performed for the GH family 3 glucan (1-->3)-beta-glucosidase from the white-rot fungus Phanerochaete chrysosporium, using laminarioligosaccharides as substrates. When laminaribiose was incubated with the enzyme, a transglycosylation product was detected by thin-layer chromatography. The product was purified by size-exclusion chromatography, and was identified as a 6-O-glucosyl-laminaribiose (beta-D-Glcp-(1-->6)-beta-D-Glcp-(1-->3)-D-Glc) by 1H NMR spectroscopy and electrospray ionization mass spectrometry analysis. In steady-state kinetic studies, an apparent decrease of laminaribiose hydrolysis was observed at high concentrations of the substrate, and the plots of glucose production versus substrate concentration were thus fitted to a modified Michaelis-Menten equation including hydrolytic and transglycosylation parameters (K(m), K(m2), k(cat), k(cat2)). The rate of 6-O-glucosyl-laminaribiose production estimated by high-performance anion-exchange chromatography coincided with the theoretical rate calculated using these parameters, clearly indicating that substrate inhibition of this enzyme is fully explained by substrate transglycosylation. Moreover, when K(m), k(cat), and affinity for glucosyl-enzyme intermediates (K(m2)) were estimated for laminarioligosaccharides (DP=3-5), the K(m) value of laminaribiose was approximately 5-9 times higher than those of the other oligosaccharides (DP=3-5), whereas the K(m2) values were independent of the DP of the substrates. The kinetics of transglycosylation by the enzyme could be well interpreted in terms of the subsite affinities estimated from the hydrolytic parameters (K(m) and k(cat)), and a possible mechanism of transglycosylation is proposed.

A family 8 glycoside hydrolase from Bacillus halodurans C-125 (BH2105) is a reducing end xylose-releasing exo-oligoxylanase

The gene encoding family 8 glycoside hydrolases from Bacillus halodurans C-125 (BH2105), an alkalophilic bacterium with a known genomic sequence, was expressed in Escherichia coli. The protein was expressed with the intact N-terminal sequence, suggesting that it did not possess a signal peptide and that it was an intracellular enzyme. The recombinant enzyme showed no hydrolytic activity on xylan, whereas it had been annotated as xylanase Y. It hydrolyzed xylooligosaccharide whose degree of polymerization is greater than or equal to 3 in an exo-splitting manner with anomeric inversion, releasing the xylose unit at the reducing end. Judging from its substrate specificity and reaction mechanism, we named the enzyme reducing end xylose-releasing exo-oligoxylanase (Rex). Rex was found to utilize only the beta-anomer of the substrate to form beta-xylose and alpha-xylooligosaccharide. The optimum pH of the enzymatic reaction (6.2-7.3) was found in the neutral range, a range beneficial for intracellular enzymes. The genomic sequence suggests that B. halodurans secretes two endoxylanases and possesses two alpha-arabinofuranosidases, one alpha-glucuronidase, and three beta-xylosidases intracellularly in addition to Rex. The extracellular enzymes supposedly hydrolyze xylan into arabino/glucurono-xylooligosaccharides that are then transported into the cells. Rex may play a role as a key enzyme in intracellular xylan metabolism in B. halodurans by cleaving xylooligosaccharides that were produced by the action of other intracellular enzymes from the arabino/glucurono-xylooligosaccharides.