Glycosylated linkers in multimodular lignocellulose-degrading enzymes dynamically bind to cellulose

The carbohydrate-binding module and linker of a modular lytic polysaccharide monooxygenase promote localized cellulose oxidation

Lytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes that catalyze the oxidative cleavage of polysaccharides such as cellulose and chitin, a feature that makes them key tools in industrial biomass conversion processes. The catalytic domains of a considerable fraction of LPMOs and other carbohydrate-active enzymes (CAZymes) are tethered to carbohydrate-binding modules (CBMs) by flexible linkers. These linkers preclude X-ray crystallographic studies, and the functional implications of these modular assemblies remain partly unknown. Here, we used NMR spectroscopy to characterize structural and dynamic features of full-length modular ScLPMO10C from Streptomyces coelicolor We observed that the linker is disordered and extended, creating distance between the CBM and the catalytic domain and allowing these domains to move independently of each other. Functional studies with cellulose nanofibrils revealed that most of the substrate-binding affinity of full-length ScLPMO10C resides in the CBM. Comparison of the catalytic performance of full-length ScLPMO10C and its isolated catalytic domain revealed that the CBM is beneficial for LPMO activity at lower substrate concentrations and promotes localized and repeated oxidation of the substrate. Taken together, these results provide a mechanistic basis for understanding the interplay between catalytic domains linked to CBMs in LPMOs and CAZymes in general.

Predominant Nonproductive Substrate Binding by Fungal Cellobiohydrolase I and Implications for Activity Improvement

Enzymatic conversion of the most abundant renewable source of organic compounds, cellulose to fermentable sugars is attractive for production of green fuels and chemicals. The major component of industrial enzyme systems, cellobiohydrolase I from Hypocrea jecorina (Trichoderma reesei) (HjCel7A) processively splits disaccharide units from the reducing ends of tightly packed cellulose chains. HjCel7A consists of a catalytic domain (CD) and a carbohydrate-binding module (CBM) separated by a linker peptide. A tunnel-shaped substrate-binding site in the CD includes nine subsites for β-d-glucose units, seven of which (-7 to -1) precede the catalytic center. Low catalytic activity of Cel7A is the bottleneck and the primary target for improvement. Here it is shown for the first time that, in spite of much lower apparent k_cat of HjCel7A at the hydrolysis of β-1,4-glucosidic linkages in the fluorogenic cellotetra- and -pentaose compared to the structurally related endoglucanase I (HjCel7B), the specificity constants (catalytic efficiency) k_cat /K_m for both enzymes are almost equal in these reactions. The observed activity difference appears from strong nonproductive substrate binding by HjCel7A, particularly significant for MU-β-cellotetraose (MUG₄ ). Interaction of substrates with the subsites -6 and -5 proximal to the nonconserved Gln101 residue in HjCel7A decreases K_m,ap by >1500 times. HjCel7A can be nonproductively bound onto cellulose surface with K_d ≈2-9 nM via CBM and CD that captures six terminal glucose units of cellulose chain. Decomposition of this nonproductive complex can determine the rate of cellulose conversion. MUG₄ is a promising substrate to select active cellobiohydrolase I variants with reduced nonproductive substrate binding.

Novel Insights from Comparative In Silico Analysis of Green Microalgal Cellulases

The assumption that cellulose degradation and assimilation can only be carried out by heterotrophic organisms was shattered in 2012 when it was discovered that the unicellular green alga, Chlamydomonas reinhardtii (Cr), can utilize cellulose for growth under CO₂-limiting conditions. Publications of genomes/transcriptomes of the colonial microalgae, Gonium pectorale (Gp) and Volvox carteri (Vc), between 2010⁻2016 prompted us to look for cellulase genes in these algae and to compare them to cellulases from bacteria, fungi, lower/higher plants, and invertebrate metazoans. Interestingly, algal catalytic domains (CDs), belonging to the family GH9, clustered separately and showed the highest (33⁻42%) and lowest (17⁻36%) sequence identity with respect to cellulases from invertebrate metazoans and bacteria, respectively, whereas the identity with cellulases from plants was only 27⁻33%. Based on comparative multiple alignments and homology models, the domain arrangement and active-site architecture of algal cellulases are described in detail. It was found that all algal cellulases are modular, consisting of putative novel cysteine-rich carbohydrate-binding modules (CBMs) and proline/serine-(PS) rich linkers. Two genes were found to encode a protein with a putative Ig-like domain and a cellulase with an unknown domain, respectively. A feature observed in one cellulase homolog from Gp and shared by a spinach cellulase is the existence of two CDs separated by linkers and with a C-terminal CBM. Dockerin and Fn-3-like domains, typically found in bacterial cellulases, are absent in algal enzymes. The targeted gene expression analysis shows that two Gp cellulases consisting, respectively, of a single and two CDs were upregulated upon filter paper addition to the medium.

The impact of O-glycan chemistry on the stability of intrinsically disordered proteins

Protein glycosylation is a diverse post-translational modification that serves myriad biological functions. O-linked glycans in particular vary widely in extent and chemistry in eukaryotes, with secreted proteins from fungi and yeast commonly exhibiting O-mannosylation in intrinsically disordered regions of proteins, likely for proteolysis protection, among other functions. However, it is not well understood why mannose is often the preferred glycan, and more generally, if the neighboring protein sequence and glycan have coevolved to protect against proteolysis in glycosylated intrinsically disordered proteins (IDPs). Here, we synthesized variants of a model IDP, specifically a natively O-mannosylated linker from a fungal enzyme, with α-O-linked mannose, glucose, and galactose moieties, along with a non-glycosylated linker. Upon exposure to thermolysin, O-mannosylation, by far, provides the highest extent of proteolysis protection. To explain this observation, extensive molecular dynamics simulations were conducted, revealing that the axial configuration of the C2-hydroxyl group (2-OH) of α-mannose adjacent to the glycan-peptide bond strongly influences the conformational features of the linker. Specifically, α-mannose restricts the torsions of the IDP main chain more than other glycans whose equatorial 2-OH groups exhibit interactions that favor perpendicular glycan-protein backbone orientation. We suggest that IDP stiffening due to O-mannosylation impairs protease action, with contributions from protein-glycan interactions, protein flexibility, and protein stability. Our results further imply that resistance to proteolysis is an important driving force for evolutionary selection of α-mannose in eukaryotic IDPs, and more broadly, that glycan motifs for proteolysis protection likely coevolve with the protein sequence to which they attach.

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.

An ascomycota coculture in batch bioreactor is better than polycultures for cellulase production

Efficient hydrolysis of holocellulose depends on a proper balance between cellulase (endoglucanase, exoglucanase, β-glucosidase) and xylanase activities. The present study aimed to induce the production of cellulases and xylanases using liquid cultures (one, two, three, and four fungal strains on the same bioreactor) of wild strains of Trichoderma harzianum, Aspergillus niger, and Fusarium oxysporum. The strains were identified by amplification and analysis of the ITS rDNA region and the obtained sequences were deposited in Genbank. Enzymes (endoglucanase, exoglucansae, β-glucosidase, and xylanase activities) and the profile of extracellular protein isoforms (SDS-PAGE) produced by different fungal combinations (N = 14) were analyzed by Pearson's correlation matrix and principal component analysis (PCA). According to our results, induction of endoglucanase (19.02%) and β-glucosidase (6.35%) were obtained after 4 days when A. niger and F. oxysporum were cocultured. The combination of A. niger-T. harzianum produced higher endoglucanase in a shorter time than monocultures. On the contrary, when more than two strains were cultured in the same reactor, the relationships of competition were established, trending to diminish the amount of enzymes and the extracellular protein isoforms produced. The xylanase production was sensible to stress produced by mixed cultures, decreasing their activity. This is important when the aim is to produce cellulase-free xylanase. In addition, exoglucanase activity did not change in the combinations tested.

Using Chemical Synthesis To Study and Apply Protein Glycosylation

Protein glycosylation is one of the most common post-translational modifications and can influence many properties of proteins. Abnormal protein glycosylation can lead to protein malfunction and serious disease. While appreciation of glycosylation's importance is growing in the scientific community, especially in recent years, a lack of homogeneous glycoproteins with well-defined glycan structures has made it difficult to understand the correlation between the structure of glycoproteins and their properties at a quantitative level. This has been a significant limitation on rational applications of glycosylation and on optimizing glycoprotein properties. Through the extraordinary efforts of chemists, it is now feasible to use chemical synthesis to produce collections of homogeneous glycoforms with systematic variations in amino acid sequence, glycosidic linkage, anomeric configuration, and glycan structure. Such a technical advance has greatly facilitated the study and application of protein glycosylation. This Perspective highlights some representative work in this research area, with the goal of inspiring and encouraging more scientists to pursue the glycosciences.

Deletion of a single glycosyltransferase in Caldicellulosiruptor bescii eliminates protein glycosylation and growth on crystalline cellulose

Protein glycosylation pathways have been identified in a variety of bacteria and are best understood in pathogens and commensals in which the glycosylation targets are cell surface proteins, such as S layers, pili, and flagella. In contrast, very little is known about the glycosylation of bacterial enzymes, especially those secreted by cellulolytic bacteria. Caldicellulosiruptor bescii secretes several unique synergistic multifunctional biomass-degrading enzymes, notably cellulase A which is largely responsible for this organism's ability to grow on lignocellulosic biomass without the conventional pretreatment. It was recently discovered that extracellular CelA is heavily glycosylated. In this work, we identified an O-glycosyltransferase in the C. bescii chromosome and targeted it for deletion. The resulting mutant was unable to grow on crystalline cellulose and showed no detectable protein glycosylation. Multifunctional biomass-degrading enzymes in this strain were rapidly degraded. With the genetic tools available in C. bescii, this system represents a unique opportunity to study the role of bacterial enzyme glycosylation as well an investigation of the pathway for protein glycosylation in a non-pathogen.

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.

Identification of heteromolecular binding sites in transcription factors Sp1 and TAF4 using high-resolution nuclear magnetic resonance spectroscopy

The expression of eukaryotic genes is precisely controlled by interactions between general transcriptional factors and promoter-specific transcriptional activators. The fourth element of TATA-box binding protein-associated factor (TAF4), an essential subunit of the general transcription factor TFIID, serves as a coactivator for various promoter-specific transcriptional regulators. Interactions between TAF4 and site-specific transcriptional activators, such as Sp1, are important for regulating the expression levels of genes of interest. However, only limited information is available on the molecular mechanisms underlying the interactions between these transcriptional regulatory proteins. We herein analyzed the interaction between the transcriptional factors Sp1 and TAF4 using high-resolution solution nuclear magnetic resonance spectroscopy. We found that four glutamine-rich (Q-rich) regions in TAF4 were largely disordered under nearly physiological conditions. Among them, the first Q-rich region in TAF4 was essential for the interaction with another Q-rich region in the Sp1 molecule, most of which was largely disordered. The residues responsible for this interaction were specific and highly localized in a defined region within a range of 20-30 residues. Nevertheless, a detailed analysis of 13 C-chemical shift values suggested that no significant conformational change occurred upon binding. These results indicate a prominent and exceptional binding mode for intrinsically disordered proteins other than the well-accepted concept of "coupled folding and binding."

The influence of different linker modifications on the catalytic activity and cellulose affinity of cellobiohydrolase Cel7A from Hypocrea jecorina

Various cellulases consist of a catalytic domain connected to a carbohydrate-binding module (CBM) by a flexible linker peptide. The linker if often strongly O-glycosylated and typically has a length of 20-50 amino acid residues. Functional roles, other than connecting the two folded domains, of the linker and its glycans, have been widely discussed, but experimental evidence remains sparse. One of the most studied cellulose degrading enzymes is the multi-domain cellobiohydrolase Cel7A from Hypocrea jecorina. Here, we designed variants of Cel7A with mutations in the linker region to elucidate the role of the linker. We found that moderate modification of the linker could result in significant changes in substrate affinity and catalytic efficacy. These changes were quite different for different linker variants. Thus, deletion of six residues near the catalytic domain had essentially no effects on enzyme function. Conversely, a substitution of four glycosylation sites near the middle of the linker reduced substrate affinity and increased maximal turnover. The observation of weaker binding provides some support of recent suggestions that linker glycans may be directly involved in substrate interactions. However, a variant with several inserted glycosylation sites near the CBM also showed lower affinity for the substrate compared to the wild-type, and we suggest that substrate interactions of the glycans depend on their exact location as well as other factors such as changes in structure and dynamics of the linker peptide.

High-resolution structure of a lytic polysaccharide monooxygenase from Hypocrea jecorina reveals a predicted linker as an integral part of the catalytic domain

For decades, the enzymes of the fungus Hypocrea jecorina have served as a model system for the breakdown of cellulose. Three-dimensional structures for almost all H. jecorina cellulose-degrading enzymes are available, except for HjLPMO9A, belonging to the AA9 family of lytic polysaccharide monooxygenases (LPMOs). These enzymes enhance the hydrolytic activity of cellulases and are essential for cost-efficient conversion of lignocellulosic biomass. Here, using structural and spectroscopic analyses, we found that native HjLPMO9A contains a catalytic domain and a family-1 carbohydrate-binding module (CBM1) connected via a linker sequence. A C terminally truncated variant of HjLPMO9A containing 21 residues of the predicted linker was expressed at levels sufficient for analysis. Here, using structural, spectroscopic, and biochemical analyses, we found that this truncated variant exhibited reduced binding to and activity on cellulose compared with the full-length enzyme. Importantly, a 0.95-Å resolution X-ray structure of truncated HjLPMO9A revealed that the linker forms an integral part of the catalytic domain structure, covering a hydrophobic patch on the catalytic AA9 module. We noted that the oxidized catalytic center contains a Cu(II) coordinated by two His ligands, one of which has a His-brace in which the His-1 terminal amine group also coordinates to a copper. The final equatorial position of the Cu(II) is occupied by a water-derived ligand. The spectroscopic characteristics of the truncated variant were not measurably different from those of full-length HjLPMO9A, indicating that the presence of the CBM1 module increases the affinity of HjLPMO9A for cellulose binding, but does not affect the active site.

Comparative insights into the saccharification potentials of a relatively unexplored but robust Penicillium funiculosum glycoside hydrolase 7 cellobiohydrolase

BACKGROUND

GH7 cellobiohydrolases (CBH1) are vital for the breakdown of cellulose. We had previously observed the enzyme as the most dominant protein in the active cellulose-hydrolyzing secretome of the hypercellulolytic ascomycete-Penicillium funiculosum (NCIM1228). To understand its contributions to cellulosic biomass saccharification in comparison with GH7 cellobiohydrolase from the industrial workhorse-Trichoderma reesei, we natively purified and functionally characterized the only GH7 cellobiohydrolase identified and present in the genome of the fungus.

RESULTS

There were marginal differences observed in the stability of both enzymes, with P. funiculosum (PfCBH1) showing an optimal thermal midpoint (Tm) of 68 °C at pH 4.4 as against an optimal Tm of 65 °C at pH 4.7 for T. reesei (TrCBH1). Nevertheless, PfCBH1 had an approximate threefold lower binding affinity (Km), an 18-fold higher turnover rate (kcat), a sixfold higher catalytic efficiency as well as a 26-fold higher enzyme-inhibitor complex equilibrium dissociation constant (Ki) than TrCBH1 on p-nitrophenyl-β-d-lactopyranoside (pNPL). Although both enzymes hydrolyzed cellooligomers (G2-G6) and microcrystalline cellulose, releasing cellobiose and glucose as the major products, the propensity was more with PfCBH1. We equally observed this trend during the hydrolysis of pretreated wheat straws in tandem with other core cellulases under the same conditions. Molecular dynamic simulations conducted on a homology model built using the TrCBH1 structure (PDB ID: 8CEL) as a template enabled us to directly examine the effects of substrate and products on the protein dynamics. While the catalytic triads-EXDXXE motifs-were conserved between the two enzymes, subtle variations in regions enclosing the catalytic path were observed, and relations to functionality highlighted.

CONCLUSION

To the best of our knowledge, this is the first report about a comprehensive and comparative description of CBH1 from hypercellulolytic ascomycete-P. funiculosum NCIM1228, against the backdrop of the same enzyme from the industrial workhorse-T. reesei. Our study reveals PfCBH1 as a viable alternative for CBH1 from T. reesei in industrial cellulase cocktails.

Structure of a Thermobifida fusca lytic polysaccharide monooxygenase and mutagenesis of key residues

BACKGROUND

Auxiliary activity (AA) enzymes are produced by numerous bacterial and fungal species to assist in the degradation of biomass. These enzymes are abundant but have yet to be fully characterized. Here, we report the X-ray structure of Thermobifida fusca AA10A (TfAA10A), investigate mutational characterization of key surface residues near its active site, and explore the importance of the various domains of Thermobifida fusca AA10B (TfAA10B). The structure of TfAA10A is similar to other bacterial LPMOs (lytic polysaccharide monooxygenases), including signs of photo-reduction and a distorted active site, with mixed features showing both type I and II copper coordination. The point mutation experiments of TfAA10A show that Trp82 and Asn83 are needed for binding, but only Trp82 affects activity. The TfAA10B domain truncation mutants reveal that CBM2 is crucial for the binding of substrate, but that the X1 module does not affect binding or activity.

RESULTS

In TfAA10A, Trp82 and Asn83 are needed for binding, but only Trp82 affects activity. The TfAA10B domain truncation mutants reveal that CBM2 is crucial for substrate binding, but that the X1 module does not affect binding or activity. The structure of TfAA10A is similar to other bacterial lytic polysaccharide monooxygenases with mixed features showing both type I and II copper coordination.

CONCLUSIONS

The role of LPMOs and the variability of abundance in genomes are not fully explored. LPMOs likely perform initial attacks into crystalline cellulose to allow larger processive cellulases to bind and attack, but the precise nature of their synergistic behavior remains to be definitively characterized.

Inter-domain Synergism Is Required for Efficient Feeding of Cellulose Chain into Active Site of Cellobiohydrolase Cel7A

Structural polysaccharides like cellulose and chitin are abundant and their enzymatic degradation to soluble sugars is an important route in green chemistry. Processive glycoside hydrolases (GHs), like cellobiohydrolase Cel7A of Trichoderma reesei (TrCel7A) are key components of efficient enzyme systems. TrCel7A consists of a catalytic domain (CD) and a smaller carbohydrate-binding module (CBM) connected through the glycosylated linker peptide. A tunnel-shaped active site rests in the CD and contains 10 glucose unit binding sites. The active site of TrCel7A is lined with four Trp residues with two of them, Trp-40 and Trp-38, in the substrate binding sites near the tunnel entrance. Although addressed in numerous studies the elucidation of the role of CBM and active site aromatics has been obscured by a complex multistep mechanism of processive GHs. Here we studied the role of the CBM-linker and Trp-38 of TrCel7A with respect to binding affinity, on- and off-rates, processivity, and synergism with endoglucanase. The CBM-linker increased the on-rate and substrate affinity of the enzyme. The Trp-38 to Ala substitution resulted in increased off-rates and decreased processivity. The effect of the Trp-38 to Ala substitution on on-rates was strongly dependent on the presence of the CBM-linker. This compensation between CBM-linker and Trp-38 indicates synergism between CBM-linker and CD in feeding the cellulose chain into the active site. The inter-domain synergism was pre-requisite for the efficient degradation of cellulose in the presence of endoglucanase.

Cellotriose-hydrolyzing activity conferred by truncating the carbohydrate-binding modules of Cel5 from Hahella chejuensis

Processivity is a typical characteristic of cellobiohydrolases (CBHs); it enables the enzyme to successively hydrolyze the ends of cellulose chains and to produce cellobiose as the major product. Some microbes, which do not have CBHs, utilize endoglucanases (EGs) that exhibit processivity, commonly referred to as processive EGs. A processive EG identified from Hahella chejuensis, HcCel5, has a catalytic domain (CD) belonging to the glycoside hydrolase family 5 (GH5) and two carbohydrate-binding modules (CBM6s). In this study, we compared HcCel5-CD with the CD of Saccharophagus degradans Cel5H (SdCel5H-CD), which is a processive EG reported previously. Our results showed that in comparison to SdCel5H-CD, HcCel5-CD has more suitable characteristics for cellulose hydrolysis, such as higher hydrolytic activity, thermostability (40-80 °C), and processivity. Noticeably, HcCel5-CD is capable of hydrolyzing cellotriose, unlike HcCel5. These features of HcCel5-CD for cellulose hydrolysis could be employed for efficient saccharification of lignocellulose to produce cellobiose and glucose, which may be used to produce renewable fuels and chemicals.

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.

Nanoscale Engineering of Designer Cellulosomes

Biocatalysts showcase the upper limit obtainable for high-speed molecular processing and transformation. Efforts to engineer functionality in synthetic nanostructured materials are guided by the increasing knowledge of evolving architectures, which enable controlled molecular motion and precise molecular recognition. The cellulosome is a biological nanomachine, which, as a fundamental component of the plant-digestion machinery from bacterial cells, has a key potential role in the successful development of environmentally-friendly processes to produce biofuels and fine chemicals from the breakdown of biomass waste. Here, the progress toward so-called "designer cellulosomes", which provide an elegant alternative to enzyme cocktails for lignocellulose breakdown, is reviewed. Particular attention is paid to rational design via computational modeling coupled with nanoscale characterization and engineering tools. Remaining challenges and potential routes to industrial application are put forward.

Structural and functional studies of the glycoside hydrolase family 3 β-glucosidase Cel3A from the moderately thermophilic fungus Rasamsonia emersonii

Cel3A from the thermophilic fungus R. emersonii has proven to be more efficient in the hydrolysis of β-glycosidic linkages than Cel3A from H. jecorina.

The filamentous fungus Hypocrea jecorina produces a number of cellulases and hemicellulases that act in a concerted fashion on biomass and degrade it into monomeric or oligomeric sugars. β-Glucosidases are involved in the last step of the degradation of cellulosic biomass and hydrolyse the β-glycosidic linkage between two adjacent molecules in dimers and oligomers of glucose. In this study, it is shown that substituting the β-glucosidase from H. jecorina (HjCel3A) with the β-glucosidase Cel3A from the thermophilic fungus Rasamsonia emersonii (ReCel3A) in enzyme mixtures results in increased efficiency in the saccharification of lignocellulosic materials. Biochemical characterization of ReCel3A, heterologously produced in H. jecorina, reveals a preference for disaccharide substrates over longer gluco-oligosaccharides. Crystallographic studies of ReCel3A revealed a highly N-glycosylated three-domain dimeric protein, as has been observed previously for glycoside hydrolase family 3 β-glucosidases. The increased thermal stability and saccharification yield and the superior biochemical characteristics of ReCel3A compared with HjCel3A and mixtures containing HjCel3A make ReCel3A an excellent candidate for addition to enzyme mixtures designed to operate at higher temperatures.

Effects of the linker region on the structure and function of modular GH5 cellulases

The association of glycosyl hydrolases with catalytically inactive modules is a successful evolutionary strategy that is commonly used by biomass-degrading microorganisms to digest plant cell walls. The presence of accessory domains in these enzymes is associated with properties such as higher catalytic efficiency, extension of the catalytic interface and targeting of the enzyme to the proper substrate. However, the importance of the linker region in the synergistic action of the catalytic and accessory domains remains poorly understood. Thus, this study examined how the inter-domain region affects the structure and function of modular GH5 endoglucanases, by using cellulase 5A from Bacillus subtilis (BsCel5A) as a model. BsCel5A variants featuring linkers with different stiffnesses or sizes were designed and extensively characterized, revealing that changes in flexibility or rigidity in this region differentially affect kinetic behavior. Regarding the linker length, we found that precise inter-domain spacing is required to enable efficient hydrolysis because excessively long or short linkers were equally detrimental to catalysis. Together, these findings identify molecular and structural features that may contribute to the rational design of chimeric and multimodular glycosyl hydrolases.

Extending the linker region increases the activity of the Bacillus subtilis cellulase CelI15

OBJECTIVES

To investigate the effect of the linker region (LR) on the enzymatic activity, stability, and flexibility of Bacillus subtilis cellulase CelI15, six mutants were constructed that contained increasing numbers of the LR.

RESULTS

The CelI15 mutant with three copies of the LR (approx. 57 amino acids) showed the highest activity, which was almost 20 % greater than that of wild type CelI15. The stability of the mutant enzymes increased as the copy number of the LR decreased. However, the substrate affinity of the mutant enzymes increased as the LR copy number increased, and the mutant with four copies of the LR exhibited the highest substrate affinity. Additionally, the flexibility of the CelI15 mutants increased as the LR copy number increased from zero to four copies, although it decreased sharply for the mutant with five copies of the LR.

CONCLUSION

The activity of CelI15 was increased by increasing the LR copy number, which could be a potential way to improve its enzymatic properties.

Biochemical and Structural Characterizations of Two Dictyostelium Cellobiohydrolases from the Amoebozoa Kingdom Reveal a High Level of Conservation between Distant Phylogenetic Trees of Life

Glycoside hydrolase family 7 (GH7) cellobiohydrolases (CBHs) are enzymes commonly employed in plant cell wall degradation across eukaryotic kingdoms of life, as they provide significant hydrolytic potential in cellulose turnover. To date, many fungal GH7 CBHs have been examined, yet many questions regarding structure-activity relationships in these important natural and commercial enzymes remain. Here, we present the crystal structures and a biochemical analysis of two GH7 CBHs from social amoeba: Dictyostelium discoideum Cel7A (DdiCel7A) and Dictyostelium purpureum Cel7A (DpuCel7A). DdiCel7A and DpuCel7A natively consist of a catalytic domain and do not exhibit a carbohydrate-binding module (CBM). The structures of DdiCel7A and DpuCel7A, resolved to 2.1 Å and 2.7 Å, respectively, are homologous to those of other GH7 CBHs with an enclosed active-site tunnel. Two primary differences between the Dictyostelium CBHs and the archetypal model GH7 CBH, Trichoderma reesei Cel7A (TreCel7A), occur near the hydrolytic active site and the product-binding sites. To compare the activities of these enzymes with the activity of TreCel7A, the family 1 TreCel7A CBM and linker were added to the C terminus of each of the Dictyostelium enzymes, creating DdiCel7ACBM and DpuCel7ACBM, which were recombinantly expressed in T. reesei DdiCel7ACBM and DpuCel7ACBM hydrolyzed Avicel, pretreated corn stover, and phosphoric acid-swollen cellulose as efficiently as TreCel7A when hydrolysis was compared at their temperature optima. The Ki of cellobiose was significantly higher for DdiCel7ACBM and DpuCel7ACBM than for TreCel7A: 205, 130, and 29 μM, respectively. Taken together, the present study highlights the remarkable degree of conservation of the activity of these key natural and industrial enzymes across quite distant phylogenetic trees of life.

IMPORTANCE

GH7 CBHs are among the most important cellulolytic enzymes both in nature and for emerging industrial applications for cellulose breakdown. Understanding the diversity of these key industrial enzymes is critical to engineering them for higher levels of activity and greater stability. The present work demonstrates that two GH7 CBHs from social amoeba are surprisingly quite similar in structure and activity to the canonical GH7 CBH from the model biomass-degrading fungus T. reesei when tested under equivalent conditions (with added CBM-linker domains) on an industrially relevant substrate.

O-Glycan analysis of cellobiohydrolase I from Neurospora crassa

We describe here the composition of the O-linked glycans on the Neurospora crassa cellobiohydrolase I (CBHI), which accounts for approximately 40% of the protein secreted by cells growing in the presence of cellulose. CBHI is O-glycosylated with six types of linear, and three types of branched, O-glycans containing approximately equal amounts of mannose and galactose. In addition to the classic fungal O-glycans with reducing end mannoses, we also identified reducing end galactoses which suggest the existence of a protein-O-galactosyltransferase in N. crassa Because of the excellent genetic resources available for N. crassa, the knowledge of the CBHI O-glycans may enable the future evaluation of the role of O-glycosylation on cellulase function and the development of directed O-glycan/cellulase engineering.

Influence of surface charge, binding site residues and glycosylation on Thielavia terrestris cutinase biochemical characteristics

Cutinases are esterases of industrial importance for applications in recycling and surface modification of polyesters. The cutinase from Thielavia terrestris (TtC) is distinct in terms of its ability to retain its stability and activity in acidic pH. Stability and activity in acidic pHs are desirable for esterases as the pH of the reaction tends to go down with the generation of acid. The pH stability and activity are governed by the charged state of the residues involved in catalysis or in substrate binding. In this study, we performed the detailed structural and biochemical characterization of TtC coupled with surface charge analysis to understand its acidic tolerance. The stability of TtC in acidic pH was rationalized by evaluating the contribution of charge interactions to the Gibbs free energy of unfolding at varying pHs. The activity of TtC was found to be limited by substrate binding affinity, which is a function of the surface charge. Additionally, the presence of glycosylation affects the biochemical characteristics of TtC owing to steric interactions with residues involved in substrate binding.

Investigation of the binding properties of a multi-modular GH45 cellulase using bioinspired model assemblies

BACKGROUND

Enzymes degrading plant biomass polymers are widely used in biotechnological applications. Their efficiency can be limited by non-specific interactions occurring with some chemical motifs. In particular, the lignin component is known to bind enzymes irreversibly. In order to determine interactions of enzymes with their substrates, experiments are usually performed on isolated simple polymers which are not representative of plant cell wall complexity. But when using natural plant substrates, the role of individual chemical and structural features affecting enzyme-binding properties is also difficult to decipher.

RESULTS

We have designed and used lignified model assemblies of plant cell walls as templates to characterize binding properties of multi-modular cellulases. These three-dimensional assemblies are modulated in their composition using the three principal polymers found in secondary plant cell walls (cellulose, hemicellulose, and lignin). Binding properties of enzymes are obtained from the measurement of their mobility that depends on their interactions with the polymers and chemical motifs of the assemblies. The affinity of the multi-modular GH45 cellulase was characterized using a statistical analysis to determine the role played by each assembly polymer. Presence of hemicellulose had much less impact on affinity than cellulose and model lignin. Depending on the number of CBMs appended to the cellulase catalytic core, binding properties toward cellulose and lignin were highly contrasted.

CONCLUSIONS

Model assemblies bring new insights into the molecular determinants that are responsible for interactions between enzymes and substrate without the need of complex analysis. Consequently, we believe that model bioinspired assemblies will provide relevant information for the design and optimization of enzyme cocktails in the context of biorefineries.

Development of Lignocellulosic Biorefinery Technologies: Recent Advances and Current Challenges

Recent developments of the biorefinery concept are described within this review, which focuses on the efforts required to make the lignocellulosic biorefinery a sustainable and economically viable reality. Despite the major research and development endeavours directed towards this goal over the past several decades, the integrated production of biofuel and other bio-based products still needs to be optimized from both technical and economical perspectives. This review will highlight recent progress towards the optimization of the major biorefinery processes, including biomass pretreatment and fractionation, saccharification of sugars, and conversion of sugars and lignin into fuels and chemical precursors. In addition, advances in genetic modification of biomass structure and composition for the purpose of enhancing the efficacy of conversion processes, which is emerging as a powerful tool for tailoring biomass fated for the biorefinery, will be overviewed. The continual improvement of these processes and their integration in the format of a modern biorefinery is paving the way for a sustainable bio-economy which will displace large portions of petroleum-derived fuels and chemicals with renewable substitutes.

Characterization of a feruloyl esterase B from Talaromyces cellulolyticus

A feruloyl esterase catalyzes the hydrolysis of the 4-hydroxy-3-methoxycinnamoyl (feruloyl) group from esterified sugars in plant cell walls. Talaromyces cellulolyticus is a high cellulolytic-enzyme producing fungus. However, there is no report for feruloyl esterase activity of T. cellulolyticus. Analysis of the genome database of T. cellulolyticus identified a gene encoding a putative feruloyl esterase B. The recombinant enzyme was prepared using a T. cellulolyticus homologous expression system and characterized. The purified enzyme exhibited hydrolytic activity toward p-nitrophenyl acetate, p-nitrophenyl trans-ferulate, methyl ferulate, rice husk, and bagasse. HPLC assays showed that the enzyme released ferulic acid and p-coumaric acid from hydrothermal-treated rice husk and bagasse. Trichoderma sp. is well-known high cellulolytic-enzyme producing fungus useful for the lignocellulosic biomass saccharification. Interestingly, no feruloyl esterase has been reported from Trichoderma sp. The results show that this enzyme is expected to be industrially useful for biomass saccharification.

Molecular-scale features that govern the effects of O-glycosylation on a carbohydrate-binding module

Protein glycosylation is a ubiquitous post-translational modification in all kingdoms of life. Despite its importance in molecular and cellular biology, the molecular-level ramifications of O-glycosylation on biomolecular structure and function remain elusive. Here, we took a small model glycoprotein and changed the glycan structure and size, amino acid residues near the glycosylation site, and glycosidic linkage while monitoring any corresponding changes to physical stability and cellulose binding affinity. The results of this study reveal the collective importance of all the studied features in controlling the most pronounced effects of O-glycosylation in this system. Going forward, this study suggests the possibility of designing proteins with multiple improved properties by simultaneously varying the structures of O-glycans and amino acids local to the glycosylation site.

O-glycosylation effects on family 1 carbohydrate-binding module solution structures

Family 1 carbohydrate-binding modules (CBMs) are ubiquitous components of multimodular fungal enzymes that degrade plant cell wall polysaccharides and bind specifically to cellulose. Native glycosylation of family 1 CBMs has been shown to substantially impact multiple physical properties, including thermal and proteolytic stability and cellulose binding affinity. To gain molecular insights into the changes in CBM properties upon glycosylation, solution structures of two glycoforms of a Trichoderma reesei family 1 CBM were studied by NMR spectroscopy: a glycosylated family 1 CBM with a mannose group attached to both Thr1 and Ser3 and a second family 1 CBM with single mannose groups attached to Thr1, Ser3 and Ser14. The structures clearly reveal that monosaccharides at both Ser3 and Ser14 on family 1 CBMs present additional cellulose binding platforms, similar to well-characterized aromatic residues at the binding interface, which align to the cellulose surface. These results are in agreement with previous experimental work demonstrating that glycans at Ser3 and Ser14 impart significant improvements in binding affinity. Additionally, detailed analysis of the NMR structures and molecular simulations indicates that the protein backbone of the CBM is not significantly altered by attachment of monosaccharides, and that the mannose attached to Ser14 may be more flexible than the mannose at Ser3. Overall, the present study reveals how family 1 CBM structures are affected by covalent attachment of monosaccharides, which are likely important post-translational modifications of these common subdomains of fungal plant cell wall degrading enzymes.

DATABASE

Structural data have been deposited in the RCSB Protein Data Bank (PDB codes: 2MWJ and 2MWK) and the BioMagRes Bank (BMRB codes: 25331 and 25332) for CBM_M2 and CBM_M3, respectively.

Reassembly and co-crystallization of a family 9 processive endoglucanase from its component parts: structural and functional significance of the intermodular linker

Non-cellulosomal processive endoglucanase 9I (Cel9I) from Clostridium thermocellum is a modular protein, consisting of a family-9 glycoside hydrolase (GH9) catalytic module and two family-3 carbohydrate-binding modules (CBM3c and CBM3b), separated by linker regions. GH9 does not show cellulase activity when expressed without CBM3c and CBM3b and the presence of the CBM3c was previously shown to be essential for endoglucanase activity. Physical reassociation of independently expressed GH9 and CBM3c modules (containing linker sequences) restored 60-70% of the intact Cel9I endocellulase activity. However, the mechanism responsible for recovery of activity remained unclear. In this work we independently expressed recombinant GH9 and CBM3c with and without their interconnecting linker in Escherichia coli. We crystallized and determined the molecular structure of the GH9/linker-CBM3c heterodimer at a resolution of 1.68 Å to understand the functional and structural importance of the mutual spatial orientation of the modules and the role of the interconnecting linker during their re-association. Enzyme activity assays and isothermal titration calorimetry were performed to study and compare the effect of the linker on the re-association. The results indicated that reassembly of the modules could also occur without the linker, albeit with only very low recovery of endoglucanase activity. We propose that the linker regions in the GH9/CBM3c endoglucanases are important for spatial organization and fixation of the modules into functional enzymes.

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.

Highly Active Biocatalytic Coatings from Protein-Polymer Diblock Copolymers

A method for fabricating nanostructured biocatalysts using bioconjugate block copolymer self-assembly is demonstrated, yielding very high protein loadings and activity per unit area, compared to more-established enzyme encapsulation methods. Self-assembled heterogeneous biocatalysts are fabricated by flow coating myoglobin-b-poly(N-isopropylacrylamide) (myoglobin-PNIPAM) block copolymers onto solid supports, and films are stabilized by lightly cross-linking with glutaraldehyde. The conjugates form weakly ordered, nonbirefringent micellar and lamellar assemblies in concentrated solution and disordered but micro-phase-separated structures in thin solid films. The low diffusion resistance in the bioconjugate film imparted by the water-swollen PNIPAM nanostructures, the high enzyme density within the film, and high retention of protein activity results in extremely high catalytic activity: 5-10 times greater than catalysts fabricated using other well-established methods.

Linker length and flexibility induces new cellobiohydrolase activity of PoCel6A from Penicillium oxalicum

In a previous study, a novel cellobiohydrolase, PoCel6A, with new enzymatic activity against p-nitrophenyl-β-D-cellobioside (pNPC), was purified from Penicillium oxalicum. The cellulose-binding module and catalytic domain of PoCel6A showed a high degree of sequence similarity with other fungal Cel6As. However, PoCel6A had 11 more amino acids in the linker region than other Cel6As. To evaluate the relationship between the longer linker of PoCel6A and its enzymatic activity, 11 amino acids were deleted from the linker region of PoCel6A. The shortened PoCel6A linker nullified the enzymatic activity against pNPC but dramatically increased the enzyme's capacity for crystalline cellulose degradation. The shortened linker segment appeared to have no effect on the secondary structural conformation of PoCel6A. Another variant (PoCel6A-6pro) with six consecutive proline residues in the interdomain linker had a higher rigid linker, and no enzymatic activity was observed against soluble and insoluble substrate. The flexibility of the linker had an important function in the formation of active cellulase. The length and flexibility of the linker is clearly able to modify the function of PoCel6A and induce new characteristics of Cel6A.

Effects of lytic polysaccharide monooxygenase oxidation on cellulose structure and binding of oxidized cellulose oligomers to cellulases

In nature, polysaccharide glycosidic bonds are cleaved by hydrolytic enzymes for a vast array of biological functions. Recently, a new class of enzymes that utilize an oxidative mechanism to cleave glycosidic linkages was discovered; these enzymes are called lytic polysaccharide monooxygenases (LPMO). These oxidative enzymes are synergistic with cocktails of hydrolytic enzymes and are thought to act primarily on crystalline regions, in turn providing new sites of productive attachment and detachment for processive hydrolytic enzymes. In the case of cellulose, the homopolymer of β-1,4-d-glucose, enzymatic oxidation occurs at either the reducing end or the nonreducing end of glucose, depending on enzymatic specificity, and results in the generation of oxidized chemical substituents at polymer chain ends. LPMO oxidation of cellulose is thought to produce either a lactone at the reducing end of glucose that can spontaneously or enzymatically convert to aldonic acid or 4-keto-aldose at the nonreducing end that may further oxidize to a geminal diol. Here, we use molecular simulation to examine the effect of oxidation on the structure of crystalline cellulose. The simulations highlight variations in behaviors depending on the chemical identity of the oxidized species and its location within the cellulose fibril, as different oxidized species introduce steric effects that disrupt local crystallinity and in some cases reduce the work needed for polymer decrystallization. Reducing-end oxidations are easiest to decrystallize when located at the end of the fibril, whereas nonreducing end oxidations readily decrystallize from internal cleavage sites despite their lower solvent accessibility. The differential in decrystallization free energy suggests a molecular mechanism consistent with experimentally observed LPMO/cellobiohydrolase synergy. Additionally, the soluble oxidized cellobiose products released by hydrolytic cellulases may bind to the active sites of cellulases with different affinities relative to cellobiose itself, which potentially affects hydrolytic turnover through product inhibition. To examine the effect of oxidation on cello-oligomer binding, we use thermodynamic integration to compute the relative change in binding free energy between the hydrolyzed and oxidized products in the active site of Family 7 and Family 6 processive glycoside hydrolases, Trichoderma reesei Cel7A and Cel6A, which are key industrial cellulases and commonly used model systems for fungal cellulases. Our results suggest that the equilibrium between the two reducing end oxidized products, favoring the linear aldonic acid, may increase product inhibition, which would in turn reduce processive substrate turnover. In the case of LMPO action at the nonreducing end, oxidation appears to lower affinity with the nonreducing end specific cellulase, reducing product inhibition and potentially promoting processive cellulose turnover. Overall, this suggests that oxidation of recalcitrant polysaccharides by LPMOs accelerates degradation not only by increasing the concentration of chain termini but also by reducing decrystallization work, and that product inhibition may be somewhat reduced as a result.

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.