Implications of cellobiohydrolase glycosylation for use in biomass conversion

N-linked glycosylation affects catalytic parameters and fluctuation of the active center of Aspergillus awamori exo-inulinase

Heterogeneous expression of enzymes allows large-scale production with reduced costs. Changes in glycosylation often occur due to changes in the expression host. In the study, the catalytic and biochemical properties of Aspergillus awamori exo-inulinase 1 are compared for A. awamori and Penicillium verruculosum expression hosts. The tertiary structure contains seven sites of N-glycosylation, with two of them located near the active center. If expressed in P. verruculosum, the enzyme was four times less glycosylated and two times more active toward sucrose, raffinose, and stachyose due to an increase in kcat. These substrates with a short chain of 2-4 monosaccharide units were used to characterize the interaction of the substrate with the amino acid residues in the active center while preventing the interaction of the substrate with N-linked glycans. Molecular dynamics simulations showed an increase in the fluctuation of the active center with an increase in the length of N-linked glycans. The fluctuation of the residues N40 and Q57, which interact with the hydroxyl group O5 of the fructose unit in the -1 subsite of the active center, was increased by 1.6 times. The fluctuation of the residue W335, which interacts with the hydroxyl group O1 of the fructose unit together with the catalytic residue D41 and affects the torsion angle geometry of the substrate molecules, was increased by 1.5 times. The residue R188, which analogously to W335 affects the torsion angle geometry of the substrate molecules, was also among the affected residues with a 1.2-fold increase in the fluctuation.

The role of glycosylation in non-productive adsorption of cellulase to lignin isolated from pretreated corn stover

Glycosylation, a general post-translational modification for fungal cellulase, has been shown to affect cellulase binding to its substrate. However, the exact impact of glycosylation on cellulase-lignin interaction remain unclear. Here, we demonstrated that the lignin isolated from tetrahydrofuran-pretreated corn stover exhibits strong adsorption capability to cellulase due to its negatively charged and porous structure. For the cellulases with varying glycosylation levels, the less-glycosylated protein showed high adsorption capability to lignin, and that trend was observed for the main cellulase components secreted by Penicillium oxilicum, including endoglucanase PoCel5B, cellobiohydrolase PoCel7A-2, and β-glucosidase PoBgl1. Additionally, N-glycan sites and motifs were examined using mass spectrometry, and protein structures with N-glycans were constructed, where PoBgl1 and PoCel7A-2 contained 13 and 1 glycosylated sites respectively. The results of molecular dynamics simulations indicated that the N-glycans impacted on the solvent-accessible surface area and secondary structure of protein, and the binding conformation of lignin fragment on cellulase, resulting in a decrease in binding energy (14 kcal/mol for PoBgl1 and 13 kcal/mol for PoCel7A-2), particularly for van der Waals and electrostatic interaction. Those findings suggested that glycosylation negatively impacted the lignin-cellulase interaction, providing a theoretical basis for the rational engineering of enzymes to reduce lignin-enzyme interaction.

Glycosylation of cellulase: a novel strategy for improving cellulase

Protein glycosylation is the most complex posttranslational modification process. Most cellulases from filamentous fungi contain N-glycosylation and O-glycosylation. Here, we discuss the potential roles of glycosylation on the characteristics and function of cellulases. The use of certain cultivation, inducer, and alteration of engineering glycosylation pathway can enable the rational control of cellulase glycosylation. Glycosylation does not occur arbitrarily and may tend to modify the 3D structure of cellulases by using specially distributed glycans. Therefore, glycoengineering should be considered comprehensively along with the spatial structure of cellulases. Cellulase glycosylation may be an evolution phenomenon, which has been considered as an economical way for providing different functions from identical proteins. In addition to gene and transcription regulations, glycosylation may be another regulation on the protein expression level. Enhanced understanding of the potential regulatory role of cellulase glycosylation will enable synthetic biology approaches for the development of commercial cellulase.

Significance of glycans in cellulolytic enzymes for lignocellulosic biorefinery - A review

Lignocellulosic (LC) biomass is the most abundant renewable resource for mankind gravitating society towards sustainable solution for energy that can reduce the carbon footprint. The economic feasibility of 'biomass biorefinery' depends upon the efficiency cellulolytic enzymes which is the main crux. Its high production cost and low efficiencies are the major limitations, that need to be resolved. As the complexity of the genome increases, so does the complexity of the proteome, further facilitated by protein post-translational modifications (PTMs). Glycosylation is regarded the major PTMs and hardly any recent work is focused on importance of glycosylation in cellulase. By modifying protein side chains and glycans, superior cellulases with improved stability and efficiency can be obtained. Functional proteomics relies heavily on PTMs because they regulate activity, localization, and interactions with protein, lipid, nucleic acid, and cofactor molecules. O- and N- glycosylation in cellulases influences its characteristics adding positive attributes to the enzymes.

Expression, characterization, and activity optimization of a novel cellulase from the thermophilic bacteria Cohnella sp. A01

Cellulases are hydrolytic enzymes with wide scientific and industrial applications. We described a novel cellulase, CelC307, from the thermophilic indigenous Cohnella sp. A01. The 3-D structure of the CelC307 was predicted by comparative modeling. Docking of CelC307 with specific inhibitors and molecular dynamic (MD) simulation revealed that these ligands bound in a non-competitive manner. The CelC307 protein was purified and characterized after recombinant expression in Escherichia coli (E. coli) BL21. Using CMC 1% as the substrate, the thermodynamic values were determined as K_m 0.46 mM, k_cat 104.30 × 10^-3 (S^-1), and k_cat/K_m 226.73 (M^-1 S^-1). The CelC307 was optimally active at 40 °C and pH 7.0. The culture condition was optimized for improved CelC307 expression using Plackett-Burman and Box-Behnken design as follows: temperature 20 °C, pH 7.5, and inoculation concentration with an OD₆₀₀ = 1. The endoglucanase activity was positively modulated in the presence of Na⁺, Li⁺, Ca²⁺, 2-mercaptoethanol (2-ME), and glycerol. The thermodynamic parameters calculated for CelC307 confirmed its inherent thermostability. The characterized CelC307 may be a suitable candidate for various biotechnological applications.

Heterologous production of cellulose- and starch-degrading hydrolases to expand Saccharomyces cerevisiae substrate utilization: Lessons learnt

Selected strains of Saccharomyces cerevisiae are used for commercial bioethanol production from cellulose and starch, but the high cost of exogenous enzymes for substrate hydrolysis remains a challenge. This can be addressed through consolidated bioprocessing (CBP) where S. cerevisiae strains are engineered to express recombinant glycoside hydrolases during fermentation. Looking back at numerous strategies undertaken over the past four decades to improve recombinant protein production in S. cerevisiae, it is evident that various steps in the protein production "pipeline" can be manipulated depending on the protein of interest and its anticipated application. In this review, we briefly introduce some of the strategies and highlight lessons learned with regards to improved transcription, translation, post-translational modification and protein secretion of heterologous hydrolases. We examine how host strain selection and modification, as well as enzyme compatibility, are crucial determinants for overall success. Finally, we discuss how lessons from heterologous hydrolase expression can inform modern synthetic biology and genome editing tools to provide process-ready yeast strains in future. However, it is clear that the successful expression of any particular enzyme is still unpredictable and requires a trial-and-error approach.

Non-productive binding of cellobiohydrolase i investigated by surface plasmon resonance spectroscopy

Future biorefineries are facing the challenge to separate and depolymerize biopolymers into their building blocks for the production of biofuels and basic molecules as chemical stock. Fungi have evolved lignocellulolytic enzymes to perform this task specifically and efficiently, but a detailed understanding of their heterogeneous reactions is a prerequisite for the optimization of large-scale enzymatic biomass degradation. Here, we investigate the binding of cellulolytic enzymes onto biopolymers by surface plasmon resonance (SPR) spectroscopy for the fast and precise characterization of enzyme adsorption processes. Using different sensor architectures, SPR probes modified with regenerated cellulose as well as with lignin films were prepared by spin-coating techniques. The modified SPR probes were analyzed by atomic force microscopy and static contact angle measurements to determine physical and surface molecular properties. SPR spectroscopy was used to study the activity and affinity of Trichoderma reesei cellobiohydrolase I (CBHI) glycoforms on the modified SPR probes. N-glycan removal led to no significant change in activity or cellulose binding, while a slightly higher tendency for non-productive binding to SPR probes modified with different lignin fractions was observed. The results suggest that the main role of the N-glycosylation in CBHI is not to prevent non-productive binding to lignin, but probably to increase its stability against proteolytic degradation. The work also demonstrates the suitability of SPR-based techniques for the characterization of the binding of lignocellulolytic enzymes to biomass-derived polymers.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10570-021-04002-6.

Efficient Constitutive Expression of Cellulolytic Enzymes in Penicillium oxalicum for Improved Efficiency of Lignocellulose Degradation

Efficient cellulolytic enzyme production is important for the development of lignocellulose-degrading enzyme mixtures. However, purification of cellulases from their native hosts is time- and labor-consuming. In this study, a constitutive expression system was developed in Penicillium oxalicum for the secreted production of proteins. Using a constitutive polyubiquitin gene promoter and cultivating with glucose as the sole carbon source, nine cellulolytic enzymes of different origins with relatively high purity were produced within 48 h. When supplemented to a commercial cellulase preparation, cellobiohydrolase I from P. funiculosum and cellobiohydrolase II from Talaromyces verruculosus showed remarkable enhancing effects on the hydrolysis of steam-exploded corn stover. Additionally, a synergistic effect was observed for these two cellobiohydrolases during the hydrolysis. Taken together, the constitutive expression system provides a convenient tool for the production of cellulolytic enzymes, which is expected to be useful in the development of highly efficient lignocellulose-degrading enzyme mixtures.

In-depth characterization of Trichoderma reesei cellobiohydrolase TrCel7A produced in Nicotiana benthamiana reveals limitations of cellulase production in plants by host-specific post-translational modifications

Sustainable production of biofuels from lignocellulose feedstocks depends on cheap enzymes for degradation of such biomass. Plants offer a safe and cost-effective production platform for biopharmaceuticals, vaccines and industrial enzymes boosting biomass conversion to biofuels. Production of intact and functional protein is a prerequisite for large-scale protein production, and extensive host-specific post-translational modifications (PTMs) often affect the catalytic properties and stability of recombinant enzymes. Here we investigated the impact of plant PTMs on enzyme performance and stability of the major cellobiohydrolase TrCel7A from Trichoderma reesei, an industrially relevant enzyme. TrCel7A was produced in Nicotiana benthamiana using a vacuum-based transient expression technology, and this recombinant enzyme (TrCel7Arec ) was compared with the native fungal enzyme (TrCel7Anat ) in terms of PTMs and catalytic activity on commercial and industrial substrates. We show that the N-terminal glutamate of TrCel7Arec was correctly processed by N. benthamiana to a pyroglutamate, critical for protein structure, while the linker region of TrCel7Arec was vulnerable to proteolytic digestion during protein production due to the absence of O-mannosylation in the plant host as compared with the native protein. In general, the purified full-length TrCel7Arec had 25% lower catalytic activity than TrCel7Anat and impaired substrate-binding properties, which can be attributed to larger N-glycans and lack of O-glycans in TrCel7Arec . All in all, our study reveals that the glycosylation machinery of N. benthamiana needs tailoring to optimize the production of efficient cellulases.

Removal of N-linked glycans in cellobiohydrolase Cel7A from Trichoderma reesei reveals higher activity and binding affinity on crystalline cellulose

BACKGROUND

Cellobiohydrolase from glycoside hydrolase family 7 is a major component of commercial enzymatic mixtures for lignocellulosic biomass degradation. For many years, Trichoderma reesei Cel7A (TrCel7A) has served as a model to understand structure-function relationships of processive cellobiohydrolases. The architecture of TrCel7A includes an N-glycosylated catalytic domain, which is connected to a carbohydrate-binding module through a flexible, O-glycosylated linker. Depending on the fungal expression host, glycosylation can vary not only in glycoforms, but also in site occupancy, leading to a complex pattern of glycans, which can affect the enzyme's stability and kinetics.

RESULTS

Two expression hosts, Aspergillus oryzae and Trichoderma reesei, were utilized to successfully express wild-types TrCel7A (WT _Ao and WT _Tr ) and the triple N-glycosylation site deficient mutants TrCel7A N45Q, N270Q, N384Q (ΔN-glyc _Ao and ΔN-glyc _Tr ). Also, we expressed single N-glycosylation site deficient mutants TrCel7A (N45Q _Ao , N270Q _Ao , N384Q _Ao ). The TrCel7A enzymes were studied by steady-state kinetics under both substrate- and enzyme-saturating conditions using different cellulosic substrates. The Michaelis constant (K _M ) was consistently found to be lowered for the variants with reduced N-glycosylation content, and for the triple deficient mutants, it was less than half of the WTs' value on some substrates. The ability of the enzyme to combine productively with sites on the cellulose surface followed a similar pattern on all tested substrates. Thus, site density (number of sites per gram cellulose) was 30-60% higher for the single deficient variants compared to the WT, and about twofold larger for the triple deficient enzyme. Molecular dynamic simulation of the N-glycan mutants TrCel7A revealed higher number of contacts between CD and cellulose crystal upon removal of glycans at position N45 and N384.

CONCLUSIONS

The kinetic changes of TrCel7A imposed by removal of N-linked glycans reflected modifications of substrate accessibility. The presence of N-glycans with extended structures increased K _M and decreased attack site density of TrCel7A likely due to steric hindrance effect and distance between the enzyme and the cellulose surface, preventing the enzyme from achieving optimal conformation. This knowledge could be applied to modify enzyme glycosylation to engineer enzyme with higher activity on the insoluble substrates.

A novel starch-binding laccase from the wheat pathogen Zymoseptoria tritici highlights the functional diversity of ascomycete laccases

BACKGROUND

Laccases are multicopper oxidases, which are assigned into auxiliary activity family 1 (AA1) in the CAZy database. These enzymes, catalyzing the oxidation of phenolic and nonphenolic substrates coupled to reduction of O2 to H2O, are increasingly attractive as eco-friendly oxidation biocatalysts. Basidiomycota laccases are well characterized due to their potential in de-lignification of lignocellulose. By contrast, insight into the biochemical diversity of Ascomycota counterparts from saprophytes and plant pathogens is scarce.

RESULTS

Here, we report the properties of the laccase from the major wheat pathogen Zymoseptoria tritici (ZtrLac1A), distinguished from common plant fungal pathogens by an apoplastic infection strategy. We demonstrate that ZtrLac1A is appended to a functional starch-binding module and displays an activity signature disfavoring relatively apolar phenolic redox mediators as compared to the related biochemically characterized laccases. By contrast, the redox potential of ZtrLac1A (370 mV vs. SHE) is similar to ascomycetes counterparts. The atypical specificity is consistent with distinctive sequence substitutions and insertions in loops flanking the T1 site and the enzyme C-terminus compared to characterized laccases.

CONCLUSIONS

ZtrLac1A is the first reported modular laccase appended to a functional starch-specific carbohydrate binding module of family 20 (CBM20). The distinct specificity profile of ZtrLac1A correlates to structural differences in the active site region compared to previously described ascomycetes homologues. These differences are also highlighted by the clustering of the sequence of ZtrLac1A in a distinct clade populated predominantly by plant pathogens in the phylogenetic tree of AA1 laccases. The possible role of these laccases in vivo merits further investigations. These findings expand our toolbox of laccases for green oxidation and highlight the binding functionality of CBM-appended laccases as versatile immobilization tags.

The diversity of glycosylation of cellobiohydrolase I from Trichoderma reesei determined with mass spectrometry

Cellulases are glycosylated enzymes that have wide applications in fields like biofuels. It has been widely accepted that glycosylation of cellulases impact their performance. Trichoderma reesei is the most important cellulase-producer and cellobiohydrolase I (CBHI) is the most important cellulase from T. reesei. Therefore, the glycosylation of T. reesei CBHI has been a focus of research. However, investigations have been focused on N-glycosylation of three of the four potential glycosylation sites, as well as O-glycosylation on the linker region, while a full picture of glycosylation of T. reesei CBHI is still needed. In this work, with extensive mass spectrometric investigations on CBHI from two T. reesei strains grown under three conditions, several new discoveries were made: 1) N45 and N64 are N-glycosylated with high mannose type glycans; 2) the catalytic domain of CBHI is extensively O-glycosylated with hexoses and N-acetylhexosamines; 3) experimental evidence on the mannosylation of carbohydrate binding domain (other than the linker adjacent region) was found. With structural analysis, we found several glycosylation sites (such as T383, S8, and S46) are located at the openings of the substrate-binding tunnel, and potentially involve in the binding of cellulose. These investigations provide a full and comprehensive picture on the glycosylation of CBHI from T. reesei, which benefits the engineering of CBHI by raising potential sites for modification.

The Promoter Toolbox for Recombinant Gene Expression in Trichoderma reesei

The ascomycete Trichoderma reesei is one of the main fungal producers of cellulases and xylanases based on its high production capacity. Its enzymes are applied in food, feed, and textile industry or in lignocellulose hydrolysis in biofuel and biorefinery industry. Over the last years, the demand to expand the molecular toolbox for T. reesei to facilitate genetic engineering and improve the production of heterologous proteins grew. An important instrument to modify the expression of key genes are promoters to initiate and control their transcription. To date, the most commonly used promoter for T. reesei is the strong inducible promoter of the main cellobiohydrolase cel7a. Beside this one, there is a number of alternative inducible promoters derived from other cellulase- and xylanase encoding genes and a few constitutive promoters. With the advances in genomics and transcriptomics the identification of new constitutive and tunable promoters with different expression strength was simplified. In this review, we will discuss new developments in the field of promoters and compare their advantages and disadvantages. Synthetic expression systems constitute a new option to control gene expression and build up complex gene circuits. Therefore, we will address common structural features of promoters and describe options for promoter engineering and synthetic design of promoters. The availability of well-characterized gene expression control tools is essential for the analysis of gene function, detection of bottlenecks in gene networks and yield increase for biotechnology applications.

Engineering enhanced cellobiohydrolase activity

Glycoside Hydrolase Family 7 cellobiohydrolases (GH7 CBHs) catalyze cellulose depolymerization in cellulolytic eukaryotes, making them key discovery and engineering targets. However, there remains a lack of robust structure–activity relationships for these industrially important cellulases. Here, we compare CBHs from Trichoderma reesei (TrCel7A) and Penicillium funiculosum (PfCel7A), which exhibit a multi-modular architecture consisting of catalytic domain (CD), carbohydrate-binding module, and linker. We show that PfCel7A exhibits 60% greater performance on biomass than TrCel7A. To understand the contribution of each domain to this improvement, we measure enzymatic activity for a library of CBH chimeras with swapped subdomains, demonstrating that the enhancement is mainly caused by PfCel7A CD. We solve the crystal structure of PfCel7A CD and use this information to create a second library of TrCel7A CD mutants, identifying a TrCel7A double mutant with near-equivalent activity to wild-type PfCel7A. Overall, these results reveal CBH regions that enable targeted activity improvements.

Cellobiohydrolases (CBHs) are critical for natural and industrial biomass degradation but their structure–activity relationships are not fully understood. Here, the authors present the biochemical and structural characterization of two CBHs, identifying protein regions that confer enhanced CBH activity.

Distinct roles of N- and O-glycans in cellulase activity and stability

In nature, many microbes secrete mixtures of glycoside hydrolases, oxidoreductases, and accessory enzymes to deconstruct polysaccharides and lignin in plants. These enzymes are often decorated with N- and O-glycosylation, the roles of which have been broadly attributed to protection from proteolysis, as the extracellular milieu is an aggressive environment. Glycosylation has been shown to sometimes affect activity, but these effects are not fully understood. Here, we examine N- and O-glycosylation on a model, multimodular glycoside hydrolase family 7 cellobiohydrolase (Cel7A), which exhibits an O-glycosylated carbohydrate-binding module (CBM) and an O-glycosylated linker connected to an N- and O-glycosylated catalytic domain (CD)-a domain architecture common to many biomass-degrading enzymes. We report consensus maps for Cel7A glycosylation that include glycan sites and motifs. Additionally, we examine the roles of glycans on activity, substrate binding, and thermal and proteolytic stability. N-glycan knockouts on the CD demonstrate that N-glycosylation has little impact on cellulose conversion or binding, but does have major stability impacts. O-glycans on the CBM have little impact on binding, proteolysis, or activity in the whole-enzyme context. However, linker O-glycans greatly impact cellulose conversion via their contribution to proteolysis resistance. Molecular simulations predict an additional role for linker O-glycans, namely that they are responsible for maintaining separation between ordered domains when Cel7A is engaged on cellulose, as models predict α-helix formation and decreased cellulose interaction for the nonglycosylated linker. Overall, this study reveals key roles for N- and O-glycosylation that are likely broadly applicable to other plant cell-wall-degrading enzymes.

Cellobiohydrolase B of Aspergillus niger over-expressed in Pichia pastoris stimulates hydrolysis of oil palm empty fruit bunches

BACKGROUND

Aspergillus niger, along with many other lignocellulolytic fungi, has been widely used as a commercial workhorse for cellulase production. A fungal cellulase system generally includes three major classes of enzymes i.e., β-glucosidases, endoglucanases and cellobiohydrolases. Cellobiohydrolases (CBH) are vital to the degradation of crystalline cellulose present in lignocellulosic biomass. However, A. niger naturally secretes low levels of CBH. Hence, recombinant production of A. niger CBH is desirable to increase CBH production yield and also to allow biochemical characterisation of the recombinant CBH from A. niger.

METHODS

In this study, the gene encoding a cellobiohydrolase B (cbhB) from A. niger ATCC 10574 was cloned and expressed in the methylotrophic yeast Pichia pastoris X-33. The recombinant CBHB was purified and characterised to study its biochemical and kinetic characteristics. To evaluate the potential of CBHB in assisting biomass conversion, CBHB was supplemented into a commercial cellulase preparation (Cellic^® CTec2) and was used to hydrolyse oil palm empty fruit bunch (OPEFB), one of the most abundant lignocellulosic waste from the palm oil industry. To attain maximum saccharification, enzyme loadings were optimised by response surface methodology and the optimum point was validated experimentally. Hydrolysed OPEFB samples were analysed using attenuated total reflectance FTIR spectroscopy (ATR-FTIR) to screen for any compositional changes upon enzymatic treatment.

RESULTS

Recombinant CBHB was over-expressed as a hyperglycosylated protein attached to N-glycans. CBHB was enzymatically active towards soluble substrates such as 4-methylumbelliferyl-β-D-cellobioside (MUC), p-nitrophenyl-cellobioside (pNPC) and p-nitrophenyl-cellobiotrioside (pNPG3) but was not active towards crystalline substrates like Avicel^® and Sigmacell cellulose. Characterisation of purified CBHB using MUC as the model substrate revealed that optimum catalysis occurred at 50 °C and pH 4 but the enzyme was stable between pH 3 to 10 and 30 to 80 °C. Although CBHB on its own was unable to digest crystalline substrates, supplementation of CBHB (0.37%) with Cellic^® CTec2 (30%) increased saccharification of OPEFB by 27%. Compositional analyses of the treated OPEFB samples revealed that CBHB supplementation reduced peak intensities of both crystalline cellulose Iα and Iβ in the treated OPEFB samples.

DISCUSSION

Since CBHB alone was inactive against crystalline cellulose, these data suggested that it might work synergistically with other components of Cellic^® CTec2. CBHB supplements were desirable as they further increased hydrolysis of OPEFB when the performance of Cellic^® CTec2 was theoretically capped at an enzyme loading of 34% in this study. Hence, A. niger CBHB was identified as a potential supplementary enzyme for the enzymatic hydrolysis of OPEFB.

A comparative study on the activity of fungal lytic polysaccharide monooxygenases for the depolymerization of cellulose in soybean spent flakes

Lytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes capable of the oxidative breakdown of polysaccharides. They are of industrial interest due to their ability to enhance the enzymatic depolymerization of recalcitrant substrates by glycoside hydrolases. In this paper, twenty-four lytic polysaccharide monooxygenases (LPMOs) expressed in Trichoderma reesei were evaluated for their ability to oxidize the complex polysaccharides in soybean spent flakes, an abundant and industrially relevant substrate. TrCel61A, a soy-polysaccharide-active AA9 LPMO from T. reesei, was used as a benchmark in this evaluation. In total, seven LPMOs demonstrated activity on pretreated soy spent flakes, with the products from enzymatic treatments evaluated using mass spectrometry and high performance anion exchange chromatography. The hydrolytic boosting effect of the top-performing enzymes was evaluated in combination with endoglucanase and beta-glucosidase. Two enzymes (TrCel61A and Aspte6) showed the ability to release more than 36% of the pretreated soy spent flake glucose - a greater than 75% increase over the same treatment without LPMO addition.

Molecular and biochemical characterization of recombinant cel12B, cel8C, and peh28 overexpressed in Escherichia coli and their potential in biofuel production

BACKGROUND

The high crystallinity of cellulosic biomass myofibrils as well as the complexity of their intermolecular structure is a significant impediment for biofuel production. Cloning of celB-, celC-encoded cellulases (cel12B and cel8C) and peh-encoded polygalacturonase (peh28) from Pectobacterium carotovorum subsp. carotovorum (Pcc) was carried out in our previous study using Escherichia coli as a host vector. The current study partially characterizes the enzymes' molecular structures as well as their catalytic performance on different substrates which can be used to improve their potential for lignocellulosic biomass conversion.

RESULTS

β-Jelly roll topology, (α/α)₆ antiparallel helices and right-handed β-helices were the folds identified for cel12B, cel8C, and peh28, respectively, in their corresponding protein model structures. Purifications of 17.4-, 6.2-, and 6.0-fold, compared to crude extract, were achieved for cel12B and cel8C, and peh28, respectively, using specific membrane ultrafiltrations and size-exclusion chromatography. Avicel and carboxymethyl cellulose (CMC) were substrates for cel12B, whereas for cel8C catalytic activity was only shown on CMC. The enzymes displayed significant synergy on CMC but not on Avicel when tested for 3 h at 45 °C. No observed β-glucosidase activities were identified for cel8C and cel12B when tested on p-nitrophenyl-β-d-glucopyranoside. Activity stimulation of 130% was observed when a recombinant β-glucosidase from Pcc was added to cel8C and cel12B as tested for 3 h at 45 °C. Optimum temperature and pH of 45 °C and 5.4, respectively, were identified for all three enzymes using various substrates. Catalytic efficiencies (k_cat/K_m) were calculated for cel12B and cel8C on CMC as 0.141 and 2.45 ml/mg/s respectively, at 45 °C and pH 5.0 and for peh28 on polygalacturonic acid as 4.87 ml/mg/s, at 40 °C and pH 5.0. Glucose and cellobiose were the end-products identified for cel8C, cel12B, and β-glucosidase acting together on Avicel or CMC, while galacturonic acid and other minor co-products were identified for peh28 action on pectin.

CONCLUSIONS

This study provides some insight into which parameters should be optimized when application of cel8C, cel12B, and peh28 to biomass conversion is the goal.

Xyloglucan breakdown by endo-xyloglucanase family 74 from Aspergillus fumigatus

Xyloglucan is the most abundant hemicellulose in primary walls of spermatophytes except for grasses. Xyloglucan-degrading enzymes are important in lignocellulosic biomass hydrolysis because they remove xyloglucan, which is abundant in monocot-derived biomass. Fungal genomes encode numerous xyloglucanase genes, belonging to at least six glycoside hydrolase (GH) families. GH74 endo-xyloglucanases cleave xyloglucan backbones with unsubstituted glucose at the -1 subsite or prefer xylosyl-substituted residues in the -1 subsite. In this work, 137 GH74-related genes were detected by examining 293 Eurotiomycete genomes and Ascomycete fungi contained one or no GH74 xyloglucanase gene per genome. Another interesting feature is that the triad of tryptophan residues along the catalytic cleft was found to be widely conserved among Ascomycetes. The GH74 from Aspergillus fumigatus (AfXEG74) was chosen as an example to conduct comprehensive biochemical studies to determine the catalytic mechanism. AfXEG74 has no CBM and cleaves the xyloglucan backbone between the unsubstituted glucose and xylose-substituted glucose at specific positions, along the XX motif when linked to regions deprived of galactosyl branches. It resembles an endo-processive activity, which after initial random hydrolysis releases xyloglucan-oligosaccharides as major reaction products. This work provides insights on phylogenetic diversity and catalytic mechanism of GH74 xyloglucanases from Ascomycete fungi.

Single-molecule Imaging Analysis of Binding, Processive Movement, and Dissociation of Cellobiohydrolase Trichoderma reesei Cel6A and Its Domains on Crystalline Cellulose

Trichoderma reesei Cel6A (TrCel6A) is a cellobiohydrolase that hydrolyzes crystalline cellulose into cellobiose. Here we directly observed the reaction cycle (binding, surface movement, and dissociation) of single-molecule intact TrCel6A, isolated catalytic domain (CD), cellulose-binding module (CBM), and CBM and linker (CBM-linker) on crystalline cellulose I_α The CBM-linker showed a binding rate constant almost half that of intact TrCel6A, whereas those of the CD and CBM were only one-tenth of intact TrCel6A. These results indicate that the glycosylated linker region largely contributes to initial binding on crystalline cellulose. After binding, all samples showed slow and fast dissociations, likely caused by the two different bound states due to the heterogeneity of cellulose surface. The CBM showed much higher specificity to the high affinity site than to the low affinity site, whereas the CD did not, suggesting that the CBM leads the CD to the hydrophobic surface of crystalline cellulose. On the cellulose surface, intact molecules showed slow processive movements (8.8 ± 5.5 nm/s) and fast diffusional movements (30-40 nm/s), whereas the CBM-Linker, CD, and a catalytically inactive full-length mutant showed only fast diffusional movements. These results suggest that both direct binding and surface diffusion contribute to searching of the hydrolysable point of cellulose chains. The duration time constant for the processive movement was 7.7 s, and processivity was estimated as 68 ± 42. Our results reveal the role of each domain in the elementary steps of the reaction cycle and provide the first direct evidence of the processive movement of TrCel6A on crystalline cellulose.

Functional characterization of the native swollenin from Trichoderma reesei: study of its possible role as C1 factor of enzymatic lignocellulose conversion

BACKGROUND

Through binding to cellulose, expansin-like proteins are thought to loosen the structural order of crystalline surface material, thus making it more accessible for degradation by hydrolytic enzymes. Swollenin SWO1 is the major expansin-like protein from the fungus Trichoderma reesei. Here, we have performed a detailed characterization of a recombinant native form of SWO1 with respect to its possible auxiliary role in the enzymatic saccharification of lignocellulosic substrates.

RESULTS

The swo1 gene was overexpressed in T. reesei QM9414 Δxyr1 mutant, featuring downregulated cellulase production, and the protein was purified from culture supernatant. SWO1 was N-glycosylated and its circular dichroism spectrum suggested a folded protein. Adsorption isotherms (25 °C, pH 5.0, 1.0 mg substrate/mL) revealed SWO1 to be 120- and 20-fold more specific for binding to birchwood xylan and kraft lignin, respectively, than for binding to Avicel PH-101. The SWO1 binding capacity on lignin (25 µmol/g) exceeded 12-fold that on Avicel PH-101 (2.1 µmol/g). On xylan, not only the binding capacity (22 µmol/g) but also the affinity of SWO1 (K d = 0.08 µM) was enhanced compared to Avicel PH-101 (K d = 0.89 µM). SWO1 caused rapid release of a tiny amount of reducing sugars (<1 % of total) from different substrates (Avicel PH-101, nanocrystalline cellulose, steam-pretreated wheat straw, barley β-glucan, cellotetraose) but did not promote continued saccharification. Atomic force microscopy revealed that amorphous cellulose films were not affected by SWO1. Also with AFM, binding of SWO1 to cellulose nanocrystallites was demonstrated at the single-molecule level, but adsorption did not affect this cellulose. SWO1 exhibited no synergy with T. reesei cellulases in the hydrolysis of the different celluloses. However, SWO1 boosted slightly (1.5-fold) the reducing sugar release from a native grass substrate.

CONCLUSIONS

SWO1 is a strongly glycosylated protein, which has implications for producing it in heterologous hosts. Although SWO1 binds to crystalline cellulose, its adsorption to xylan is much stronger. SWO1 is not an auxiliary factor of the enzymatic degradation of a variety of cellulosic substrates. Effect of SWO1 on sugar release from intact plant cell walls might be exploitable with certain (e.g., mildly pretreated) lignocellulosic feedstocks.

Mapping N-linked glycosylation of carbohydrate-active enzymes in the secretome of Aspergillus nidulans grown on lignocellulose

BACKGROUND

The genus Aspergillus includes microorganisms that naturally degrade lignocellulosic biomass, secreting large amounts of carbohydrate-active enzymes (CAZymes) that characterize their saprophyte lifestyle. Aspergillus has the capacity to perform post-translational modifications (PTM), which provides an additional advantage for the use of these organisms as a host for the production of heterologous proteins. In this study, the N-linked glycosylation of CAZymes identified in the secretome of Aspergillus nidulans grown on lignocellulose was mapped.

RESULTS

Aspergillus nidulans was grown in glucose, xylan and pretreated sugarcane bagasse (SCB) for 96 h, after which glycoproteomics and glycomics were carried out on the extracellular proteins (secretome). A total of 265 proteins were identified, with 153, 210 and 182 proteins in the glucose, xylan and SCB substrates, respectively. CAZymes corresponded to more than 50 % of the total secretome in xylan and SCB. A total of 182 N-glycosylation sites were identified, of which 121 were detected in 67 CAZymes. A prevalence of the N-glyc sequon N-X-T (72.2 %) was observed in N-glyc sites compared with N-X-S (27.8 %). The amino acids flanking the validated N-glyc sites were mainly composed of hydrophobic and polar uncharged amino acids. Selected proteins were evaluated for conservation of the N-glyc sites in Aspergilli homologous proteins, but a pattern of conservation was not observed. A global analysis of N-glycans released from the proteins secreted by A. nidulans was also performed. While the proportion of N-glycans with Hex5 to Hex9 was similar in the xylan condition, a prevalence of Hex5 was observed in the SCB and glucose conditions.

CONCLUSIONS

The most common and frequent N-glycosylated motifs, an overview of the N-glycosylation of the CAZymes and the number of mannoses found in N-glycans were analyzed. There are many bottlenecks in protein production by filamentous fungi, such as folding, transport by vesicles and secretion, but N-glycosylation in the correct context is a fundamental event for defining the high levels of secretion of target proteins. A comprehensive analysis of the protein glycosylation processes in A. nidulans will assist with a better understanding of glycoprotein structures, profiles, activities and functions. This knowledge can help in the optimization of heterologous expression and protein secretion in the fungal host.

Substrate affinity and catalytic efficiency are improved by decreasing glycosylation sites in Trichoderma reesei cellobiohydrolase I expressed in Pichia pastoris

OBJECTIVES

To modify two main N-glycosylation residues of cellobiohydrolase I from Trichoderma reesei by site-directed mutagenesis for decreasing the extent of glycosylation and exploring possible effects on its properties.

RESULTS

Asparagine 45 and 64 residues were mutated to alanine to make single/double mutants and expressed in P. pastoris. Decreasing N-glycosylation of the recombinant CBH I resulted in an increased affinity of the enzyme for carboxymethylcellulose and also improved the Kcat/Km while the specific activity was decreased. Also, the enzymes were stable up to 80 °C. There was no significant change of the optimum pH and temperature by decrease of glycosylation in the mutated enzymes in comparison to the wild-type at constant incubation time of assay.

CONCLUSION

Post-translational glycan-modification of CBH I in P. pastoris has different impacts on the properties of the secreted enzymes. Substrate affinity and catalytic efficiency were improved significantly while the activity and high temperature stability were negatively affected.

Recombinant cellulase accumulation in the leaves of mature, vegetatively propagated transgenic sugarcane

The cost of enzymes that hydrolyse lignocellulosic substrates to fermentable sugars needs to be reduced to make cellulosic ethanol a cost-competitive liquid transport fuel. Sugarcane is a perennial crop and the successful integration of cellulase transgenes into the sugarcane production system requires that transgene expression is stable in the ratoon. Herein, we compared the accumulation of recombinant fungal cellobiohydrolase I (CBH I), fungal cellobiohydrolase II (CBH II), and bacterial endoglucanase (EG) in the leaves of mature, initial transgenic sugarcane plants and their mature ratoon. Mature ratoon events containing equivalent or elevated levels of active CBH I, CBH II, and EG in the leaves were identified. Further, we have demonstrated that recombinant fungal CBH I and CBH II can resist proteolysis during sugarcane leaf senescence, while bacterial EG cannot. These results demonstrate the stability of cellulase enzyme transgene expression in transgenic sugarcane and the utility of sugarcane as a biofactory crop for production of cellulases.

Analysis of the role of O-glycosylation in GH51 α-L-arabinofuranosidase from Pleurotus ostreatus

In this study, the recombinant α-L-arabinofuranosidase from the fungus Pleurotus ostreatus (rPoAbf) was subjected to site-directed mutagenesis with the aim of elucidating the role of glycosylation on the properties of the enzyme at the level of S160 residue. As a matter of fact, previous mass spectral analyses had led to the localization of a single O-glycosylation at this site. Recombinant expression and characterization of the rPoAbf mutant S160G was therefore performed. It was shown that the catalytic properties are slightly changed by the mutation, with a more evident modification of the Kcat and KM toward the synthetic substrate pN-glucopyranoside. More importantly, the mutation negatively affected the stability of the enzyme at various pHs and temperatures. Circular dichroism (CD) analyses showed a minimum at 210 nm for wild-type (wt) rPoAbf, typical of the beta-sheets structure, whereas this minimum is shifted for rPoAbf S160G, suggesting the presence of an unfolded structure. A similar behavior was revealed when wt rPoAbf was enzymatically deglycosylated. CD structural analyses of both the site-directed mutant and the enzymatically deglycosylated wild-type enzyme indicate a role of the glycosylation at the S160 residue in rPoAbf secondary structure stability.

Free Energy Diagram for the Heterogeneous Enzymatic Hydrolysis of Glycosidic Bonds in Cellulose

Kinetic and thermodynamic data have been analyzed according to transition state theory and a simplified reaction scheme for the enzymatic hydrolysis of insoluble cellulose. For the cellobiohydrolase Cel7A from Hypocrea jecorina (Trichoderma reesei), we were able to measure or collect relevant values for all stable and activated complexes defined by the reaction scheme and hence propose a free energy diagram for the full heterogeneous process. For other Cel7A enzymes, including variants with and without carbohydrate binding module (CBM), we obtained activation parameters for the association and dissociation of the enzyme-substrate complex. The results showed that the kinetics of enzyme-substrate association (i.e. formation of the Michaelis complex) was almost entirely entropy-controlled and that the activation entropy corresponded approximately to the loss of translational and rotational degrees of freedom of the dissolved enzyme. This implied that the transition state occurred early in the path where the enzyme has lost these degrees of freedom but not yet established extensive contact interactions in the binding tunnel. For dissociation, a similar analysis suggested that the transition state was late in the path where most enzyme-substrate contacts were broken. Activation enthalpies revealed that the rate of dissociation was far more temperature-sensitive than the rates of both association and the inner catalytic cycle. Comparisons of one- and two-domain variants showed that the CBM had no influence on the transition state for association but increased the free energy barrier for dissociation. Hence, the CBM appeared to promote the stability of the complex by delaying dissociation rather than accelerating association.

Characterization of Cryptopygus antarcticus endo-β-1,4-glucanase from Bombyx mori expression systems

Endo-β-1,4-glucanase (CaCel) from Antarctic springtail, Cryptopygus antarcticus, a cellulase with high activity at low temperature, shows potential industrial use. To obtain sufficient active cellulase for characterization, CaCel gene was expressed in Bombyx mori-baculovirus expression systems. Recombinant CaCel (rCaCel) has been expressed in Escherichia coli (Ec-CaCel) at temperatures below 10°C, but the expression yield was low. Here, rCaCel with a silkworm secretion signal (Bm-CaCel) was successfully expressed and secreted into pupal hemolymph and purified to near 90% purity by Ni-affinity chromatography. The yield and specific activity of rCaCel purified from B. mori were estimated at 31 mg/l and 43.2 U/mg, respectively, which is significantly higher than the CaCel yield obtained from E. coli (0.46 mg/l and 35.8 U/mg). The optimal pH and temperature for the rCaCels purified from E. coli and B. mori were 3.5 and 50°C. Both rCaCels were active at a broad range of pH values and temperatures, and retained more than 30% of their maximal activity at 0°C. Oligosaccharide structural analysis revealed that Bm-CaCel contains elaborated N- and O-linked glycans, whereas Ec-CaCel contains putative O-linked glycans. Thermostability of Bm-CaCel from B. mori at 60°C was higher than that from E. coli, probably due to glycosylation.

Efficient production and evaluation of lignocellulolytic enzymes using a constitutive protein expression system in Penicillium oxalicum

Native lignocellulolytic enzyme systems secreted by filamentous fungi can be further optimized by protein engineering or supplementation of exogenous enzyme components. We developed a protein production and evaluation system in cellulase-producing fungus Penicillium oxalicum. First, by deleting the major amylase gene amy15A, a strain Δ15A producing few extracellular proteins on starch was constructed. Then, three lignocellulolytic enzymes (BGL4, Xyn10B, and Cel12A) with originally low expression levels were successfully expressed with selected constitutive promoters in strain Δ15A. BGL4 and Cel12A overexpression resulted in increased specific filter paper activity (FPA), while the overexpression of Xyn10B improved volumetric FPA but not specific FPA. By switching the culture medium, this platform is convenient to produce originally low-expressed lignocellulolytic enzymes in relatively high purities on starch and to evaluate the effect of their supplementation on the performance of a complex cellulase system on cellulose.

Homologous expression of the Caldicellulosiruptor bescii CelA reveals that the extracellular protein is glycosylated

Members of the bacterial genus Caldicellulosiruptor are the most thermophilic cellulolytic microbes described with ability to digest lignocellulosic biomass without conventional pretreatment. The cellulolytic ability of different species varies dramatically and correlates with the presence of the multimodular cellulase CelA, which contains both a glycoside hydrolase family 9 endoglucanase and a glycoside hydrolase family 48 exoglucanase known to be synergistic in their activity, connected by three cellulose-binding domains via linker peptides. This architecture exploits the cellulose surface ablation driven by its general cellulase processivity as well as excavates cavities into the surface of the substrate, revealing a novel paradigm for cellulase activity. We recently reported that a deletion of celA in C. bescii had a significant effect on its ability to utilize complex biomass. To analyze the structure and function of CelA and its role in biomass deconstruction, we constructed a new expression vector for C. bescii and were able, for the first time, to express significant quantities of full-length protein in vivo in the native host. The protein, which contains a Histidine tag, was active and excreted from the cell. Expression of CelA protein with and without its signal sequence allowed comparison of protein retained intracellularly to protein transported extracellularly. Analysis of protein in culture supernatants revealed that the extracellular CelA protein is glycosylated whereas the intracellular CelA is not, suggesting that either protein transport is required for this post-translational modification or that glycosylation is required for protein export. The mechanism and role of protein glycosylation in bacteria is poorly understood and the ability to express CelA in vivo in C. bescii will allow the study of the mechanism of protein glycosylation in this thermophile. It will also allow the study of glycosylation of CelA itself and its role in the structure and function of this important enzyme in biomass deconstruction.

A constitutive expression system for glycosyl hydrolase family 7 cellobiohydrolases in Hypocrea jecorina

BACKGROUND

One of the primary industrial-scale cellulase producers is the ascomycete fungus, Hypocrea jecorina, which produces and secretes large quantities of diverse cellulolytic enzymes. Perhaps the single most important biomass degrading enzyme is cellobiohydrolase I (cbh1or Cel7A) due to its enzymatic proficiency in cellulose depolymerization. However, production of Cel7A with native-like properties from heterologous expression systems has proven difficult. In this study, we develop a protein expression system in H. jecorina (Trichoderma reesei) useful for production and secretion of heterologous cellobiohydrolases from glycosyl hydrolase family 7. Building upon previous work in heterologous protein expression in filamentous fungi, we have integrated a native constitutive enolase promoter with the native cbh1 signal sequence.

RESULTS

The constitutive eno promoter driving the expression of Cel7A allows growth on glucose and results in repression of the native cellulase system, severely reducing background endo- and other cellulase activity and greatly simplifying purification of the recombinant protein. Coupling this system to a Δcbh1 strain of H. jecorina ensures that only the recombinant Cel7A protein is produced. Two distinct transformant colony morphologies were observed and correlated with high and null protein production. Production levels in 'fast' transformants are roughly equivalent to those in the native QM6a strain of H. jecorina, typically in the range of 10 to 30 mg/L when grown in continuous stirred-tank fermenters. 'Slow' transformants showed no evidence of Cel7A production. Specific activity of the purified recombinant Cel7A protein is equivalent to that of native protein when assayed on pretreated corn stover, as is the thermal stability and glycosylation level. Purified Cel7A produced from growth on glucose demonstrated remarkably consistent specific activity. Purified Cel7A from the same strain grown on lactose demonstrated significantly higher variability in activity.

CONCLUSIONS

The elimination of background cellulase induction provides much more consistent measured specific activity compared to a traditional cbh1 promoter system induced with lactose. This expression system provides a powerful tool for the expression and comparison of mutant and/or phylogenetically diverse cellobiohydrolases in the industrially relevant cellulase production host H. jecorina.

Mutagenesis of Trichoderma reesei endoglucanase I: impact of expression host on activity and stability at elevated temperatures

BACKGROUND

Trichoderma reesei is a key cellulase source for economically saccharifying cellulosic biomass for the production of biofuels. Lignocellulose hydrolysis at temperatures above the optimum temperature of T. reesei cellulases (~50°C) could provide many significant advantages, including reduced viscosity at high-solids loadings, lower risk of microbial contamination during saccharification, greater compatibility with high-temperature biomass pretreatment, and faster rates of hydrolysis. These potential advantages motivate efforts to engineer T. reesei cellulases that can hydrolyze lignocellulose at temperatures ranging from 60-70°C.

RESULTS

A B-factor guided approach for improving thermostability was used to engineer variants of endoglucanase I (Cel7B) from T. reesei (TrEGI) that are able to hydrolyze cellulosic substrates more rapidly than the recombinant wild-type TrEGI at temperatures ranging from 50-70°C. When expressed in T. reesei, TrEGI variant G230A/D113S/D115T (G230A/D113S/D115T Tr_TrEGI) had a higher apparent melting temperature (3°C increase in Tm) and improved half-life at 60°C (t1/2 = 161 hr) than the recombinant (T. reesei host) wild-type TrEGI (t1/2 = 74 hr at 60°C, Tr_TrEGI). Furthermore, G230A/D113S/D115T Tr_TrEGI showed 2-fold improved activity compared to Tr_TrEGI at 65°C on solid cellulosic substrates, and was as efficient in hydrolyzing cellulose at 60°C as Tr_TrEGI was at 50°C. The activities and stabilities of the recombinant TrEGI enzymes followed similar trends but differed significantly in magnitude depending on the expression host (Escherichia coli cell-free, Saccharomyces cerevisiae, Neurospora crassa, or T. reesei). Compared to N.crassa-expressed TrEGI, S. cerevisiae-expressed TrEGI showed inferior activity and stability, which was attributed to the lack of cyclization of the N-terminal glutamine in Sc_TrEGI and not to differences in glycosylation. N-terminal pyroglutamate formation in TrEGI expressed in S. cerevisiae was found to be essential in elevating its activity and stability to levels similar to the T. reesei or N. crassa-expressed enzyme, highlighting the importance of this ubiquitous modification in GH7 enzymes.

CONCLUSION

Structure-guided evolution of T. reesei EGI was used to engineer enzymes with increased thermal stability and activity on solid cellulosic substrates. Production of TrEGI enzymes in four hosts highlighted the impact of the expression host and the role of N-terminal pyroglutamate formation on the activity and stability of TrEGI enzymes.

Glycosylation of Cellulases: Engineering Better Enzymes for Biofuels

Cellulose in plant cell walls is the largest reservoir of renewable carbon on Earth. The saccharification of cellulose from plant biomass into soluble sugars can be achieved using fungal and bacterial cellulolytic enzymes, cellulases, and further converted into fuels and chemicals. Most fungal cellulases are both N- and O-glycosylated in their native form, yet the consequences of glycosylation on activity and structure are not fully understood. Studying protein glycosylation is challenging as glycans are extremely heterogeneous, stereochemically complex, and glycosylation is not under direct genetic control. Despite these limitations, many studies have begun to unveil the role of cellulase glycosylation, especially in the industrially relevant cellobiohydrolase from Trichoderma reesei, Cel7A. Glycosylation confers many beneficial properties to cellulases including enhanced activity, thermal and proteolytic stability, and structural stabilization. However, glycosylation must be controlled carefully as such positive effects can be dampened or reversed. Encouragingly, methods for the manipulation of glycan structures have been recently reported that employ genetic tuning of glycan-active enzymes expressed from homogeneous and heterologous fungal hosts. Taken together, these studies have enabled new strategies for the exploitation of protein glycosylation for the production of enhanced cellulases for biofuel production.

Structural analysis of N- and O-glycans using ZIC-HILIC/dialysis coupled to NMR detection

Protein glycosylation, an important and complex post-translational modification (PTM), is involved in various biological processes, including the receptor-ligand and cell-cell interaction, and plays a crucial role in many biological functions. However, little is known about the glycan structures of important biological complex samples, and the conventional glycan enrichment strategy (i.e., size-exclusion column [SEC] separation) prior to nuclear magnetic resonance (NMR) detection is time-consuming and tedious. In this study, we developed a glycan enrichment strategy that couples Zwitterionic hydrophilic interaction liquid chromatography (ZIC-HILIC) with dialysis to enrich the glycans from the pronase E digests of RNase B, followed by NMR analysis of the glycoconjugate. Our results suggest that the ZIC-HILIC enrichment coupled with dialysis is a simple, fast, and efficient sample preparation approach. The approach was thus applied to analysis of a biological complex sample, the pronase E digest of the secreted proteins from the fungus Aspergillus niger. The NMR spectra revealed that the secreted proteins from A. niger contain both N-linked glycans with a high-mannose core similar to the structure of the glycan from RNase B, and O-linked glycans bearing mannose and glucose with 1→3 and 1→6 linkages. In all, our study provides compelling evidence that ZIC-HILIC separation coupled with dialysis is very effective and accessible in preparing glycans for the downstream NMR analysis, which could greatly facilitate the future NMR-based glycoproteomics research.

Purification and characterization of recombinant Cel7A from maize seed

The corn grain biofactory was used to produce Cel7A, an exo-cellulase (cellobiohydrolase I) from Hypocrea jecorina. The enzymatic activity on small molecule substrates was equivalent to its fungal counterpart. The corn grain-derived enzyme is glycosylated and 6 kDa smaller than the native fungal protein, likely due to more sugars added in the glycosylation of the fungal enzyme. Our data suggest that corn seed-derived cellobiohydrolase (CBH) I performs as well as or better than its fungal counterpart in releasing sugars from complex substrates such as pre-treated corn stover or wood. This recombinant protein product can enter and expand current reagent enzyme markets as well as create new markets in textile or pulp processing. The purified protein is now available commercially.

Glycosylation variants of a β-glucosidase secreted by a Taiwanese fungus, Chaetomella raphigera, exhibit variant-specific catalytic and biochemical properties

Cellulosic biomass is an abundant and promising energy source. To make cellulosic biofuels competitive against conventional fuels, conversion of rigid plant materials into sugars must become efficient and cost-effective. During cellulose degradation, cellulolytic enzymes generate cellobiose (β-(1→4)-glucose dimer) molecules, which in turn inhibit such enzymes by negative feedback. β-Glucosidases (BGLs) cleave cellobiose into glucose monomers, assisting overall cellulolytic activities. Therefore, BGLs are essential for efficient conversion of cellulosic biomass into biofuels, and it is important to characterize newly isolated BGLs for useful traits. Here, we report our discovery that the indigenous Taiwanese fungus Chaetomella raphigera strain D2 produces two molecular weight variants of a single BGL, D2-BGL (shortened to “D2”), which differ in O-glycosylation. The more extensively O-glycosylated form of native D2 (nD2L) has increased activity toward the natural substrate, cellobiose, compared to the less O-glycosylated form (nD2S). nD2L is more stable at 60°C, in acidic pH, and in the presence of the ionic detergent sodium dodecyl sulfate than nD2S. Furthermore, unlike nD2S, nD2L does not display substrate inhibition by an artificial substrate p-nitrophenyl glucopyranoside (pNPG), and the glucose feedback inhibition kinetics of nD2L is competitive (while it is non-competitive for nD2S), suggesting that these two glycovariants of D2 bind substrates differently. Interestingly, D2 produced in a heterologous system, Pichia pastoris, closely mimics properties of nD2S. Our studies suggest that O-glycosylation of D2 is important in determining its catalytic and biochemical properties.

Biochemical characterization and crystal structures of a fungal family 3 β-glucosidase, Cel3A from Hypocrea jecorina

Cellulase mixtures from Hypocrea jecorina are commonly used for the saccharification of cellulose in biotechnical applications. The most abundant β-glucosidase in the mesophilic fungus Hypocrea jecorina is HjCel3A, which hydrolyzes the β-linkage between two adjacent molecules in dimers and short oligomers of glucose. It has been shown that enhanced levels of HjCel3A in H. jecorina cellulase mixtures benefit the conversion of cellulose to glucose. Biochemical characterization of HjCel3A shows that the enzyme efficiently hydrolyzes (1,4)- as well as (1,2)-, (1,3)-, and (1,6)-β-D-linked disaccharides. For crystallization studies, HjCel3A was produced in both H. jecorina (HjCel3A) and Pichia pastoris (Pp-HjCel3A). Whereas the thermostabilities of HjCel3A and Pp-HjCel3A are the same, Pp-HjCel3A has a higher degree of N-linked glycosylation. Here, we present x-ray structures of HjCel3A with and without glucose bound in the active site. The structures have a three-domain architecture as observed previously for other glycoside hydrolase family 3 β-glucosidases. Both production hosts resulted in HjCel3A structures that have N-linked glycosylations at Asn(208) and Asn(310). In H. jecorina-produced HjCel3A, a single N-acetylglucosamine is present at both sites, whereas in Pp-HjCel3A, the P. pastoris-produced HjCel3A enzyme, the glycan chains consist of 8 or 4 saccharides. The glycosylations are involved in intermolecular contacts in the structures derived from either host. Due to the different sizes of the glycosylations, the interactions result in different crystal forms for the two protein forms.

Asn124 of Cel5A from Hypocrea jecorina not only provides the N-glycosylation site but is also essential in maintaining enzymatic activity

To investigate the function of N-glycosylation of Cel5A (endoglucanase II) from Hypocrea jecorina, two N-glycosylation site deletion Cel5A mutants (rN124D and rN124H) were expressed in Saccharomyces cerevisiae. The weights of these recombinant mutants were 54 kDa, which were lower than that of rCel5A. This result was expected to be attributed to deglycosylation. The enzyme activity of rN124H was greatly reduced to 60.6% compared with rCel5A, whereas rN124D showed slightly lower activity (10%) than that of rCel5A. rN124D and rN124H showed different thermal stabilities compared with the glycosylated rCel5A, especially at lower pH value. Thermal stabilities were reduced and improved for rN124D and rN124H, respectively. Circular dichroism spectroscopy showed that the modification of secondary structure by mutation may be the reason for the change in enzymatic activity and thermal stability. [BMB Reports 2014; 47(5): 256-261]