Identification of glycan structure and glycosylation sites in cellobiohydrolase II and endoglucanases I and II from Trichoderma reesei

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 of Saccharomyces cerevisiae RER2 Gene Encoding Cis-Prenyltransferase in Trichoderma atroviride Increases the Activity of Secretory Hydrolases and Enhances Antimicrobial Features

Some Trichoderma spp. exhibit natural abilities to reduce fungal diseases of plants through their mycoparasitic and antagonistic properties. In this study, we created new Trichoderma atroviride strains with elevated antifungal activity. This effect was achieved by improving the activity of cis-prenyltransferase, the main enzyme in dolichol synthesis, by expressing the RER2 gene from Saccharomyces cerevisiae. Since dolichyl phosphate is the carrier of carbohydrate residues during protein glycosylation, activation of its synthesis enhanced the activities of dolichyl-dependent enzymes, DPM synthase and N-acetylglucosamine transferase, as well as stimulated glycosylation of secretory proteins. Cellulases secreted by the transformants revealed significantly higher levels or activities compared to the control strain. Consequently, the resulting Trichoderma strains were more effective against the plant pathogens Pythium ultimum.

Fungal cellulases: protein engineering and post-translational modifications

Enzymatic degradation of lignocelluloses into fermentable sugars to produce biofuels and other biomaterials is critical for environmentally sustainable development and energy resource supply. However, there are problems in enzymatic cellulose hydrolysis, such as the complex cellulase composition, low degradation efficiency, high production cost, and post-translational modifications (PTMs), all of which are closely related to specific characteristics of cellulases that remain unclear. These problems hinder the practical application of cellulases. Due to the rapid development of computer technology in recent years, computer-aided protein engineering is being widely used, which also brings new opportunities for the development of cellulases. Especially in recent years, a large number of studies have reported on the application of computer-aided protein engineering in the development of cellulases; however, these articles have not been systematically reviewed. This article focused on the aspect of protein engineering and PTMs of fungal cellulases. In this manuscript, the latest literatures and the distribution of potential sites of cellulases for engineering have been systematically summarized, which provide reference for further improvement of cellulase properties. KEY POINTS: •Rational design based on virtual mutagenesis can improve cellulase properties. •Modifying protein side chains and glycans helps obtain superior cellulases. •N-terminal glutamine-pyroglutamate conversion stabilizes fungal cellulases.

Biochemical properties of a native β-1,4-mannanase from Aspergillus aculeatus QH1 and partial characterization of its N-glycosylation

N-glycosylation plays critical roles in protein secretion, sorting, stability, activity modulation, and interactions to other molecules in the eukaryotic organisms. Fungal β-1,4-mannanases have been widely used in the agri-food industry and contribute to the pathogenesis on plants. However, the information on N-glycosylation of a specific fungal carbohydrate-active enzyme (CAZyme) is currently limited. Herein, a cDNA was cloned from Aspergillus aculeatus QH1, displaying a full length of 1302 bp with an open reading frame of 1134 bp encoding for a GH5 subfamily 7 β-1, 4-mannanase, namely AacMan5_7A. The enzyme was purified and exhibited an optimal activity at pH 4.6 and 60 °C, hydrolyzing glucomannan and galactomannan, but not yeast mannan. AacMan5_7A is an N-glycosylated protein decorated with a high-mannose type glycan. Further through UPLC-ESI-MS/MS analysis, one of the four predicted N-glycosylation sites at N255 position was experimentally verified. The present study expands the information of N-glycosylation in fungal CAZymes, providing scientific bases for enhancing the production of fungal enzymes and their applications in food, feed, and plant biomass conversions.

•

A cDNA was cloned from Aspergillus aculeatus QH1 for a GH5 subfamily 7 β-1, 4-mannanase, namely AacMan5_7A.

•

AacMan5_7A was characterized for its general enzyme properties.

•

AacMan5_7A is an N-glycosylated protein decorated with a high-mannose type glycan.

•

One of the four predicted N-glycosylation sites at N255 position was experimentally verified.

Heterologous expression of Phanerochaete chrysosporium cellobiose dehydrogenase in Trichoderma reesei

Background

Cellobiose dehydrogenase from Phanerochaete chrysosporium (PcCDH) is a key enzyme in lignocellulose depolymerization, biosensors and biofuel cells. For these applications, it should retain important molecular and catalytic properties when recombinantly expressed. While homologous expression is time-consuming and the prokaryote Escherichia coli is not suitable for expression of the two-domain flavocytochrome, the yeast Pichia pastoris is hyperglycosylating the enzyme. Fungal expression hosts like Aspergillus niger and Trichoderma reesei were successfully used to express CDH from the ascomycete Corynascus thermophilus. This study describes the expression of basidiomycetes PcCDH in T. reesei (PcCDH_Tr) and the detailed comparison of its molecular, catalytic and electrochemical properties in comparison with PcCDH expressed by P. chrysosporium and P. pastoris (PcCDH_Pp).

Results

PcCDH_Tr was recombinantly produced with a yield of 600 U L⁻¹ after 4 days, which is fast compared to the secretion of the enzyme by P. chrysosporium. PcCDH_Tr and PcCDH were purified to homogeneity by two chromatographic steps. Both enzymes were comparatively characterized in terms of molecular and catalytic properties. The pH optima for electron acceptors are identical for PcCDH_Tr and PcCDH. The determined FAD cofactor occupancy of 70% for PcCDH_Tr is higher than for other recombinantly produced CDHs and its catalytic constants are in good accordance with those of PcCDH. Mass spectrometry showed high mannose-type N-glycans on PcCDH, but only single N-acetyl-d-glucosamine additions at the six potential N-glycosylation sites of PcCDH_Tr, which indicates the presence of an endo-N-acetyl-β-d-glucosaminidase in the supernatant.

Conclusions

Heterologous production of PcCDH_Tr is faster and the yield higher than secretion by P. chrysosporium. It also does not need a cellulose-based medium that impedes efficient production and purification of CDH by binding to the polysaccharide. The obtained high uniformity of PcCDH_Tr glycoforms will be very useful to investigate electron transfer characteristics in biosensors and biofuel cells, which are depending on the spatial restrictions inflicted by high-mannose N-glycan trees. The determined catalytic and electrochemical properties of PcCDH_Tr are very similar to those of PcCDH and the FAD cofactor occupancy is good, which advocates T. reesei as expression host for engineered PcCDH for biosensors and biofuel cells.

Glycoproteomics Technologies in Glycobiotechnology

Glycosylation is a key factor determining the pharmacological properties of biotherapeutics, including their stability, solubility, bioavailability, pharmacokinetics, and immunogenicity. As such, comprehensive information about glycosylation of biotherapeutics is critical to demonstrate similarity. Regulatory agencies also require extensive documentation of the comprehensive analyses of glycosylation-related critical quality attributes (CQAs) during the development, manufacturing, and release of biosimilars. Mass spectrometry has catalysed tremendous advancements in the characterisation of glycosylation CQAs of biotherapeutics. Here we provide a perspective overview on the MS-based technologies relevant for biotherapeutic product characterisation with an emphasis on the recent developments that allow determination of glycosylation features such as site of glycosylation, sialic acid linkage, glycan structure, and content.

A linker of the proline-threonine repeating motif sequence is bimodal

The linker of the endoglucanase from Xanthomonas campestris pv. campestris ((PT)₁₂) has a specific sequence, a repeating proline-threonine motif. In order to understand its role, it has been compared to a regular sequence linker, in this work-the cellobiohydrolase 2 from Trichoderma reesei (CBH2). Elastic properties of the two linkers have been estimated by calculating free energy profile along the linker length from an enhanced sampling molecular dynamics simulation. The (PT)₁₂ exhibits more pronounced elastic behaviour than CBH2. The PT repeating motif results in a two-mode energy profile which could be very useful in the enzyme motions along the substrate during hydrolytic catalysis.

An integrated strategy for high-sensitive and multi-level glycoproteome analysis from low micrograms of protein samples

Glycosylation, as a biologically important protein post-translational modification, often alters on both glycosites and glycans, simultaneously. However, most of current approaches focused on biased profiling of either glycosites or glycans, and limited by time-consuming process and milligrams of starting protein material. We describe here a simple and integrated spintip-based glycoproteomics technology (termed Glyco-SISPROT) for achieving a comprehensive view of glycoproteome with shorter sample processing time and low microgram starting material. By carefully integrating and optimizing SCX, C18 and Concanavalin A (Con A) packing material and their combination in spintip format, both predigested peptides and protein lysates could be processed by Glyco-SISPROT with high efficiency. More importantly, deglycopeptide, intact glycopeptide and glycans released by multiple glycosidases could be readily collected from the same Glyco-SISPROT workflow for LC-MS analysis. In total, above 1850 glycosites in ˜1770 unique deglycopeptides were characterized from mouse liver by using either 100 μg of predigested peptides or directly using 100 μg of protein lysates, in which about 30% of glycosites were released by both PNGase F and Endos. To the best of our knowledge, this approach should be one of the most comprehensive glycoproteomic approaches by using limited protein starting material. One significant benefit of Glyco-SISPROT is that whole processing time is dramatically reduced from a few days to less than 6 h with good reproducibility when protein lysates were directly processed by Glyco-SISPROT. We expect that this method will be suitable for multi-level glycoproteome analysis of rare biological samples with high sensitivity.

Human disease glycomics: technology advances enabling protein glycosylation analysis - part 1

INTRODUCTION

Protein glycosylation is recognized as an important post-translational modification, with specific substructures having significant effects on protein folding, conformation, distribution, stability and activity. However, due to the structural complexity of glycans, elucidating glycan structure-function relationships is demanding. The fine detail of glycan structures attached to proteins (including sequence, branching, linkage and anomericity) is still best analysed after the glycans are released from the purified or mixture of glycoproteins (glycomics). The technologies currently available for glycomics are becoming streamlined and standardized and many features of protein glycosylation can now be determined using instruments available in most protein analytical laboratories. Areas covered: This review focuses on the current glycomics technologies being commonly used for the analysis of the microheterogeneity of monosaccharide composition, sequence, branching and linkage of released N- and O-linked glycans that enable the determination of precise glycan structural determinants presented on secreted proteins and on the surface of all cells. Expert commentary: Several emerging advances in these technologies enabling glycomics analysis are discussed. The technological and bioinformatics requirements to be able to accurately assign these precise glycan features at biological levels in a disease context are assessed.

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.

Enhancing structural characterisation of glucuronidated O-linked glycans using negative mode ion trap higher energy collision-induced dissociation mass spectrometry

RATIONALE

High protein production and secretion with eukaryotic glycosylation machinery make T. reesei RUT-C30 a suitable expression host for recombinant proteins. The N-glycosylation of secreted proteins of RUT-C30 is known to vary depending on culture nutrients but O-glycosylation has been less extensively studied.

METHODS

O-Glycans and glycopeptides from secreted proteins were separated by porous graphitised carbon and C-18 liquid chromatography, respectively. O-Glycans were analysed in negative ion mode by electrospray ionisation linear ion trap mass spectrometry and glycopeptides in positive ion mode by electrospray ionisation hybrid quadrupole-orbitrap mass spectrometry. Tandem mass spectrometry was used on O-glycans and glycopeptides including ion trap higher energy collision-induced dissociation (tHCD) to detect glycan fragments not detectable with standard ion trap fragmentation. tHCD allowed targeted MS³ experiments to be performed on structures containing hexuronic acid, which was not possible with ion trap CID, validating this novel O-glycan composition. Positive mode C18-LC/ESI-MS/MS was used to identify and characterise glycopeptides found to be modified with this class of O-glycans, identifying cellobiohydrolase I as a carrier of these novel O-glycans.

RESULTS

Negative mode ion trap higher energy collision-induced dissociation allowed detection and targeted MS³ experiments to be performed on the hexuronic acid substituent of O-glycan structures, which was not possible with ion trap CID, validating the novel O-glycan composition to include hexuronic acid. Using glycopeptide analysis, this novel O-glycan composition was found to be present on the catalytic domain of cellobiohydrolase I, the most abundant secreted protein by T. reesei.

CONCLUSIONS

These are the first reported O-glycans to contain acidic sugars in fungi and they could have significant implications for cellobiohydrolase I structure and activity as well as the activity of recombinant proteins expressed in this host system. Copyright © 2017 John Wiley & Sons, Ltd.

Coexpression of cellulases in Pichia pastoris as a self-processing protein fusion

The term cellulase refers to any component of the enzymatic complex produced by some fungi, bacteria and protozoans which act serially or synergistically to catalyze the cleavage of cellulosic materials. Cellulases have been widely used in many industrial applications ranging from food industry to the production of second generation ethanol. In an effort to develop new strategies to minimize the costs of enzyme production we describe the development of a Pichia pastoris strain able to coproduce two different cellulases. For that purpose the eglII (endoglucanase II) and cbhII (cellobiohydrolase II) genes from Trichoderma reesei were fused in-frame separated by the self-processing 2A peptide sequence from the foot-and-mouth disease virus. The protein fusion construct was placed under the control of the strong inducible AOX1 promoter. Analysis of culture supernatants from methanol-induced yeast transformants showed that the protein fusion was effectively processed. Enzymatic assay showed that the processed enzymes were fully functional with the same catalytic properties of the individual enzymes produced separately. Furthermore, when combined both enzymes acted synergistically on filter paper to produce cellobiose as the main end-product. Based on these results we propose that P. pastoris should be considered as an alternative platform for the production of cellulases at competitive costs.

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.

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.

Mass spectrometric study of gas-phase ions of acid β-glucosidase (Cerezyme) and iminosugar pharmacological chaperones

The effect on the conformations and stability of gas-phase ions of Cerezyme, a glycoprotein, when bound to three small-molecule chaperones has been studied using intact ESI MS, collision cross section and MS/MS measurements. To distinguish between the peaks from apo and small-molecule complex ions, Cerezyme is deglycosylated (dg-Cer). ESI MS of dg-Cer reveals that glycosylation accounts for 8.5% of the molecular weight. When excess chaperone, either covalent (2FGF) or noncovalent (A and B iminosugars), is added to solutions of dg-Cer, mass spectra show peaks from 1:1 chaperone-enzyme complexes as well as free enzyme. On average, ions of the apoenzyme have 1.6 times higher cross sections when activated in the source region of the mass spectrometer. For a given charge state, ions of complexes of 2FGF and B have about 30% and 8.4% lower cross sections, respectively, compared to the apoenzyme. Thus, binding the chaperones causes the gas-phase protein to adopt more compact conformations. The noncovalent complex ions dissociate by the loss of charged chaperones. In the gas phase, the relative stability of dg-Cer with B is higher than that with the A, whereas in solution A binds enzyme more strongly than B. Nevertheless, the disagreement is explained based on the greater number of contacts between the B and dg-Cer than the A and dg-Cer (13 vs. 8), indicating the importance of noncovalent interactions within the protein-chaperone complex in the absence of solvent. Findings in this work suggest a hypothesis towards predicting a consistent correlation between gas-phase properties to solution binding properties.

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]

Biochemical properties of glycosylation and characterization of a histidine acid phosphatase (phytase) expressed in Pichia pastoris

Phytases catalyze the cleavage of phosphate groups from phytic acid. Here, we have studied the effects of glycosylation on the properties of Aspergillus japonicus C03 phytase expressed in Pichia pastoris. The enzyme ORF of 1338 nucleotides was cloned from genomic DNA, and encoded a secreted mature protein of 446 amino acids, which included the sequence motif RHGXRX and dipeptide HD, classifying the phytase as a histidine acid phosphate. After transformation and 72h of induction, P.pastoris GS115 expressed a 75kDa protein showing 526U/mg phytase activity and 143mg/L of protein. The amino acid sequence showed 8 and 3 potential N- and O-glycosylation sites, respectively. Analysis by ESMS showed two glycoform masses of 75,467 and 72,793, which after deglycosylation decreased to 54,327 and 54,128, respectively, indicating a carbohydrate content of 27-30%. A single GlcNAc was assigned at Asn6, Asn38, Asn84, Asn99, Asn209, Asn218, Asn355 and Asn367. The recombinant phytase showed maximum activity at 50°C, a half-life of 40min, and farUVCD spectroscopy indicated a secondary structure rich in α-helix. Thermal denaturation analyses reveal the melting temperature varied from 50°C at pH 6 to a maximum of 66°C at pH 3 and pH 4.

Cellulase linkers are optimized based on domain type and function: insights from sequence analysis, biophysical measurements, and molecular simulation

Cellulase enzymes deconstruct cellulose to glucose, and are often comprised of glycosylated linkers connecting glycoside hydrolases (GHs) to carbohydrate-binding modules (CBMs). Although linker modifications can alter cellulase activity, the functional role of linkers beyond domain connectivity remains unknown. Here we investigate cellulase linkers connecting GH Family 6 or 7 catalytic domains to Family 1 or 2 CBMs, from both bacterial and eukaryotic cellulases to identify conserved characteristics potentially related to function. Sequence analysis suggests that the linker lengths between structured domains are optimized based on the GH domain and CBM type, such that linker length may be important for activity. Longer linkers are observed in eukaryotic GH Family 6 cellulases compared to GH Family 7 cellulases. Bacterial GH Family 6 cellulases are found with structured domains in either N to C terminal order, and similar linker lengths suggest there is no effect of domain order on length. O-glycosylation is uniformly distributed across linkers, suggesting that glycans are required along entire linker lengths for proteolysis protection and, as suggested by simulation, for extension. Sequence comparisons show that proline content for bacterial linkers is more than double that observed in eukaryotic linkers, but with fewer putative O-glycan sites, suggesting alternative methods for extension. Conversely, near linker termini where linkers connect to structured domains, O-glycosylation sites are observed less frequently, whereas glycines are more prevalent, suggesting the need for flexibility to achieve proper domain orientations. Putative N-glycosylation sites are quite rare in cellulase linkers, while an N-P motif, which strongly disfavors the attachment of N-glycans, is commonly observed. These results suggest that linkers exhibit features that are likely tailored for optimal function, despite possessing low sequence identity. This study suggests that cellulase linkers may exhibit function in enzyme action, and highlights the need for additional studies to elucidate cellulase linker functions.

High resolution crystal structure of the endo-N-Acetyl-β-D-glucosaminidase responsible for the deglycosylation of Hypocrea jecorina cellulases

Endo-N-acetyl-β-D-glucosaminidases (ENGases) hydrolyze the glycosidic linkage between the two N-acetylglucosamine units that make up the chitobiose core of N-glycans. The endo-N-acetyl-β-D-glucosaminidases classified into glycoside hydrolase family 18 are small, bacterial proteins with different substrate specificities. Recently two eukaryotic family 18 deglycosylating enzymes have been identified. Here, the expression, purification and the 1.3Å resolution structure of the ENGase (Endo T) from the mesophilic fungus Hypocrea jecorina (anamorph Trichoderma reesei) are reported. Although the mature protein is C-terminally processed with removal of a 46 amino acid peptide, the protein has a complete (β/α)8 TIM-barrel topology. In the active site, the proton donor (E131) and the residue stabilizing the transition state (D129) in the substrate assisted catalysis mechanism are found in almost identical positions as in the bacterial GH18 ENGases: Endo H, Endo F1, Endo F3, and Endo BT. However, the loops defining the substrate-binding cleft vary greatly from the previously known ENGase structures, and the structures also differ in some of the α-helices forming the barrel. This could reflect the variation in substrate specificity between the five enzymes. This is the first three-dimensional structure of a eukaryotic endo-N-acetyl-β-D-glucosaminidase from glycoside hydrolase family 18. A glycosylation analysis of the cellulases secreted by a Hypocrea jecorina Endo T knock-out strain shows the in vivo function of the protein. A homology search and phylogenetic analysis show that the two known enzymes and their homologues form a large but separate cluster in subgroup B of the fungal chitinases. Therefore the future use of a uniform nomenclature is proposed.

A new stoichiometric miniaturization strategy for screening of industrial microbial strains: application to cellulase hyper-producing Trichoderma reesei strains

Background

During bioprocess development, secondary screening is a key step at the boundary between laboratory and industrial conditions. To ensure an effective high-throughput screening, miniaturized laboratory conditions must mimic industrial conditions, especially for oxygen transfer, feeding capacity and pH stabilization.

Results

A feeding strategy has been applied to develop a simple screening procedure, in which a stoichiometric study is combined with a standard miniaturization procedure. Actually, the knowledge of all nutriments and base or acid requirements leads to a great simplification of pH stabilization issue of miniaturized fed-batch cultures. Applied to cellulase production by Trichoderma reesei, this strategy resulted in a stoichiometric mixed feed of carbon and nitrogen sources. While keeping the pH between shake flask and stirred bioreactor comparable, the developed shake flask protocol reproduced the strain behaviour under stirred bioreactor conditions. Compared to a an already existing miniaturized shake flasks protocol, the cellulase concentration was increased 5-fold, reaching about 10 g L^-1. Applied to the secondary screening of several clones, the newly developed protocol succeeded in selecting a clone with a high industrial potential.

Conclusions

The understanding of a bioprocess stoichiometry contributed to define a simpler and more effective miniaturization. The suggested strategy can potentially be applied to other fed-batch processes, for the screening of either strain collections or experimental conditions.

Effect of pH and temperature on the global compactness, structure, and activity of cellobiohydrolase Cel7A from Trichoderma harzianum

Due to its elevated cellulolytic activity, the filamentous fungus Trichoderma harzianum (T. harzianum) has considerable potential in biomass hydrolysis application. Cellulases from Trichoderma reesei have been widely used in studies of cellulose breakdown. However, cellulases from T. harzianum are less-studied enzymes that have not been characterized biophysically and biochemically as yet. Here, we examined the effects of pH and temperature on the secondary and tertiary structures, compactness, and enzymatic activity of cellobiohydrolase Cel7A from T. harzianum (Th Cel7A) using a number of biophysical and biochemical techniques. Our results show that pH and temperature perturbations affect Th Cel7A stability by two different mechanisms. Variations in pH modify protonation of the enzyme residues, directly affecting its activity, while leading to structural destabilization only at extreme pH limits. Temperature, on the other hand, has direct influence on mobility, fold, and compactness of the enzyme, causing unfolding of Th Cel7A just above the optimum temperature limit. Finally, we demonstrated that incubation with cellobiose, the product of the reaction and a competitive inhibitor, significantly increased the thermal stability of Th Cel7A. Our studies might provide insights into understanding, at a molecular level, the interplay between structure and activity of Th Cel7A at different pH and temperature conditions.

A structural study of Hypocrea jecorina Cel5A

Interest in generating lignocellulosic biofuels through enzymatic hydrolysis continues to rise as nonrenewable fossil fuels are depleted. The high cost of producing cellulases, hydrolytic enzymes that cleave cellulose into fermentable sugars, currently hinders economically viable biofuel production. Here, we report the crystal structure of a prevalent endoglucanase in the biofuels industry, Cel5A from the filamentous fungus Hypocrea jecorina. The structure reveals a general fold resembling that of the closest homolog with a high-resolution structure, Cel5A from Thermoascus aurantiacus. Consistent with previously described endoglucanase structures, the H. jecorina Cel5A active site contains a primarily hydrophobic substrate binding groove and a series of hydrogen bond networks surrounding two catalytic glutamates. The reported structure, however, demonstrates stark differences between side-chain identity, loop regions, and the number of disulfides. Such structural information may aid efforts to improve the stability of this protein for industrial use while maintaining enzymatic activity through revealing nonessential and immutable regions.

Expression and secretion of the human erythropoietin using an optimized cbh1 promoter and the native CBH I signal sequence in the industrial fungus Trichoderma reesei

The human erythropoietin (HuEPO) structural gene was fused with the secretion signal of the Trichoderma reesei cellobiohydrolase I and controlled by a newly optimized cbh1 promoter in an integrated expression vector pTrCBH-EPO. The recombinant HuEPO construct was transformed into two different T. reesei strains, a protease-deficient strain RutC-30 M3, and a glycosylation-modified strain T108. After lactose induction, the heterologous rHuEPO was found to be stably expressed in the selected transformants T47 (derived from RutC-30 M3) and T112 (derived from T108), which were shown to have high genetic stability. Secretion of erythropoietin in these transformants was further confirmed by SDS-PAGE and western blot analysis. Moreover, the secreted rHuEPO from T112 had an apparent molecular weight of 32 kDa, which was higher than from T47 (28 kDa) and similar to that of mammals (more than 30 kDa). These results demonstrate the potential of using industrial filamentous fungi for the production of human-derived erythropoietin.

Effect of codon optimization on the expression of Trichoderma reesei endoglucanase 1 in Pichia pastoris

Trichoderma reesei cellulases are important biocatalysts for a wide range of industrial applications that include the paper, feed, and textile industries. T. reesei endoglucanase 1 (egl1) was successfully expressed as an active and stable catalyst in Pichia pastoris for the first time. Codon optimization was applied to egl1 of T. reesei to enhance its expression levels in P. pastoris. When compared with the originally cloned egl1 gene of T. reesei, the synthetic codon optimized egl1 gene (egl1s) was expressed at a higher level in P. pastoris. Batch fermentations of both clones with the same copy number under controlled conditions indicated that codon optimized EGI enzyme activity increased to 1.24 fold after 72 h of methanol induction. Our research indicated that P. pastoris is a suitable host for cellulase production.

Capillary zone electrophoresis for the analysis of glycoforms of cellobiohydrolase

Cellobiohydrolase (CBH) is an important enzyme for the conversion of lignocellulosic biomass to ethanol. This work separated the glycoforms of CBH possessing different numbers of neutral mannoses using capillary zone electrophoresis (CZE) in a 50 mM, pH 7.5 phosphate buffer. The method analysed CBH in an intact form using a polyacrylamide coated fused silica capillary without requiring additives or labelling of the enzyme. The migration time of the major peak was found to be 21.6±0.1 min (n=3) and the approach is suitable for testing of batch-to-batch consistency of CBH. Ease-of-use, automation and speed are the other benefits due to which the use of CZE for analysing glycoforms of CBH was concluded to be ideal.

A specific, robust, and automated method for routine at-line monitoring of the concentration of cellulases in genetically modified sugarcane plants

Bagasse is one of the waste crop materials highlighted as commercially viable for cellulosic bio-ethanol production via enzymatic conversion to release fermentable sugars. Genetically modified sugarcane expressing cellobiohydrolases (CBH), endoglucanase (EG), and β-glucosidases (BG) provide a more cost-effective route to cellulose breakdown compared to culturing these enzymes in microbial tanks. Hence, process monitoring of the concentration profile of these key cellulases in incoming batches of sugarcane is required for fiscal measures and bio-ethanol process control. The existing methods due to their non-specificity, requirement of trained analysts, low sample throughput, and low amenability to automation are unsuitable for this purpose. Therefore, this paper explores a membrane-based sample preparation method coupled to capillary zone electrophoresis (CZE) to quantify these enzymes. The maximum enzyme extraction efficiency was obtained by using a polyethersulfone membrane with molecular cut-off of 10 kDa. The use of 15 mM, pH 7.75, phosphate buffer resulted in CZE separation and quantification of CBH, EG, and BG within 10 min. Migration time reproducibility was between 0.56% and 0.7% and hence, suitable for use with automatic peak detection software. Therefore, the developed CZE method is suitable for at-line analysis of BG, CBH, and EG in every batch of harvested sugarcane.

Purification, crystallization and preliminary crystallographic analysis of the catalytic domain of the extracellular cellulase CBHI from Trichoderma harzianum

The filamentous fungus Trichoderma harzianum has a considerable cellulolytic activity that is mediated by a complex of enzymes which are essential for the hydrolysis of microcrystalline cellulose. These enzymes were produced by the induction of T. harzianum with microcrystalline cellulose (Avicel) under submerged fermentation in a bioreactor. The catalytic core domain (CCD) of cellobiohydrolase I (CBHI) was purified from the extracellular extracts and submitted to robotic crystallization. Diffraction-quality CBHI CCD crystals were grown and an X-ray diffraction data set was collected under cryogenic conditions using a synchrotron-radiation source.

Identification of a gene coding for a deglycosylating enzyme in Hypocrea jecorina

An enzyme with mannosyl glycoprotein endo-N-acetyl-beta-D-glucosaminidase (ENGase)-type activity was partially purified from the extracellular medium of the mould Hypocrea jecorina (Trichoderma reesei). Internal peptides were generated and used to identify the gene in the T. reesei genome. The active enzyme is processed both at the N- and at the C-terminus. High-mannose-type glycoproteins are good substrates, whereas complex-type glycans are not hydrolysed. The enzyme represents the first fungal member of glycoside hydrolase family 18 with ENGase-type activity. Bacterial ENGases and the fungal chitinases belonging to the same family show very low homology with Endo T. Database searches identify several highly homologous genes in fungi and the activity is also found within other Trichoderma species. This ENGase activity, not coregulated with cellulase production, could be responsible for the extensive N-deglycosylation observed for several T. reesei cellulases.

A multistage process to enhance cellobiose production from cellulosic materials

Cellobiose, a disaccharide, is a valuable product that can be obtained from cellulose hydrolysis. In this study, a simple methodology is presented to enhance the production and improve the selectivity of cellobiose during enzymatic hydrolysis of cellulose. The approach consisted of a multistage removal of filtrate via vacuum filtration and resuspension of the retentate. By this process, the remaining solid was further hydrolyzed without additional enzyme loading. Compared to the continuous hydrolysis process, the production of cellobiose increased by 45%. Increased selectivity of cellobiose is due to the loss of beta-glucosidases in the filtrate, while enhanced productivity is likely due to mitigated product inhibition.

A family of thermostable fungal cellulases created by structure-guided recombination

SCHEMA structure-guided recombination of 3 fungal class II cellobiohydrolases (CBH II cellulases) has yielded a collection of highly thermostable CBH II chimeras. Twenty-three of 48 genes sampled from the 6,561 possible chimeric sequences were secreted by the Saccharomyces cerevisiae heterologous host in catalytically active form. Five of these chimeras have half-lives of thermal inactivation at 63 degrees C that are greater than the most stable parent, CBH II enzyme from the thermophilic fungus Humicola insolens, which suggests that this chimera collection contains hundreds of highly stable cellulases. Twenty-five new sequences were designed based on mathematical modeling of the thermostabilities for the first set of chimeras. Ten of these sequences were expressed in active form; all 10 retained more activity than H. insolens CBH II after incubation at 63 degrees C. The total of 15 validated thermostable CBH II enzymes have high sequence diversity, differing from their closest natural homologs at up to 63 amino acid positions. Selected purified thermostable chimeras hydrolyzed phosphoric acid swollen cellulose at temperatures 7 to 15 degrees C higher than the parent enzymes. These chimeras also hydrolyzed as much or more cellulose than the parent CBH II enzymes in long-time cellulose hydrolysis assays and had pH/activity profiles as broad, or broader than, the parent enzymes. Generating this group of diverse, thermostable fungal CBH II chimeras is the first step in building an inventory of stable cellulases from which optimized enzyme mixtures for biomass conversion can be formulated.

Cellulase production from spent lignocellulose hydrolysates by recombinant Aspergillus niger

A recombinant Aspergillus niger strain expressing the Hypocrea jecorina endoglucanase Cel7B was grown on spent hydrolysates (stillage) from sugarcane bagasse and spruce wood. The spent hydrolysates served as excellent growth media for the Cel7B-producing strain, A. niger D15[egI], which displayed higher endoglucanase activities in the spent hydrolysates than in standard medium with a comparable monosaccharide content (e.g., 2,100 nkat/ml in spent bagasse hydrolysate compared to 480 nkat/ml in standard glucose-based medium). In addition, A. niger D15[egI] was also able to consume or convert other lignocellulose-derived compounds, such as acetic acid, furan aldehydes, and phenolic compounds, which are recognized as inhibitors of yeast during ethanolic fermentation. The results indicate that enzymes can be produced from the stillage stream as a high-value coproduct in second-generation bioethanol plants in a way that also facilitates recirculation of process water.

Three microbial strategies for plant cell wall degradation

Cellulolytic bacteria and fungi have been shown to use two different approaches to degrade cellulose. Most aerobic microbes secrete sets of individual cellulases, many of which contain a carbohydrate binding molecule (CBM), which act synergistically on native cellulose. Most anaerobic microorganisms produce large multienzyme complexes called cellulosomes, which are usually attached to the outer surface of the microorganism. Most of the cellulosomal enzymes lack a CBM, but the cohesin subunit, to which they are bound, does contain a CBM. The cellulases present in each class show considerable overlap in their catalytic domains, and processive cellulases (exocellulases and processive endocellulases) are the most abundant components of both the sets of free enzymes and of the cellulosomal cellulases. Analysis of the genomic sequences of two cellulolytic bacteria, Cytophaga hutchinsonii, an aerobe, and Fibrobacter succinogenes, an anaerobe, suggest that these organisms must use a third mechanism. This is because neither of these organisms, encodes processive cellulases and most of their many endocellulase genes do not encode CBMs. Furthermore, neither organism appears to encode the dockerin and cohesin domains that are key components of cellulosomes.

Protein glycosylation pathways in filamentous fungi

Glycosylation of proteins is important for protein stability, secretion, and localization. In this study, we have investigated the glycan synthesis pathways of 12 filamentous fungi including those of medical/agricultural/industrial importance for which genomes have been recently sequenced. We have adopted a systems biology approach to combine the results from comparative genomics techniques with high confidence information on the enzymes and fungal glycan structures, reported in the literature. From this, we have developed a composite representation of the glycan synthesis pathways in filamentous fungi (both N- and O-linked). The N-glycosylation pathway in the cytoplasm and endoplasmic reticulum was found to be highly conserved evolutionarily across all the filamentous fungi considered in the study. In the final stages of N-glycan synthesis in the Golgi, filamentous fungi follow the high mannose pathway as in Saccharomyces cerevisiae, but the level of glycan mannosylation is reduced. Highly specialized N-glycan structures with galactofuranose residues, phosphodiesters, and other insufficiently trimmed structures have also been identified in the filamentous fungi. O-Linked glycosylation in filamentous fungi was seen to be highly conserved with many mannosyltransferases that are similar to those in S. cerevisiae. However, highly variable and diverse O-linked glycans also exist. We have developed a web resource for presenting the compiled data with user-friendly query options, which can be accessed at www.fungalglycans.org. This resource can assist attempts to remodel glycosylation of recombinant proteins expressed in filamentous fungal hosts.