The three-dimensional structure of the cellobiohydrolase Cel7A from Aspergillus fumigatus at 1.5 Å resolution

Powerful cell wall biomass degradation enzymatic system from saprotrophic Aspergillus fumigatus

Cell wall biomass, Earth's most abundant natural resource, holds significant potential for sustainable biofuel production. Composed of cellulose, hemicellulose, lignin, pectin, and other polymers, the plant cell wall provides essential structural support to diverse organisms in nature. In contrast, non-plant species like insects, crustaceans, and fungi rely on chitin as their primary structural polysaccharide. The saprophytic fungus Aspergillus fumigatus has been widely recognized for its adaptability to various environmental conditions. It achieves this by secreting different cell wall biomass degradation enzymes to obtain essential nutrients. This review compiles a comprehensive collection of cell wall degradation enzymes derived from A. fumigatus, including cellulases, hemicellulases, various chitin degradation enzymes, and other polymer degradation enzymes. Notably, these enzymes exhibit biochemical characteristics such as temperature tolerance or acid adaptability, indicating their potential applications across a spectrum of industries.

The S-S bridge mutation between the A2 and A4 loops (T416C-I432C) of Cel7A of Aspergillus fumigatus enhances catalytic activity and thermostability

Disulfide bonds are important for maintaining the structural conformation and stability of the protein. The introduction of the disulfide bond is a promising strategy to increase the thermostability of the protein. In this report, cysteine residues are introduced to form disulfide bonds in the Glycoside Hydrolase family GH 7 cellobiohydrolase (GH7 CBHs) or Cel7A of Aspergillus fumigatus. Disulfide by Design 2.0 (DbD2), an online tool is used for the detection of the mutation sites. Mutations are created (D276C-G279C; DSB1, D322C-G327C; DSB2, T416C-I432C; DSB3, G460C-S465C; DSB4) inside and outside of the peripheral loops but, not in the catalytic region. The introduction of cysteine in the A2 and A4 loop of DSB3 mutant showed higher thermostability (70% activity at 70°C), higher substrate affinity (Km = 0.081 mM) and higher catalytic activity (Kcat = 9.75 min-1; Kcat/Km = 120.37 mM min-1) compared to wild-type AfCel7A (50% activity at 70°C; Km = 0.128 mM; Kcat = 4.833 min-1; Kcat/Km = 37.75 mM min-1). The other three mutants with high B factor showed loss of thermostability and catalytic activity. Molecular dynamic simulations revealed that the mutation T416C-I432C makes the tunnel wider (DSB3: 13.6 Å; Wt: 5.3 Å) at the product exit site, giving flexibility in the entrance region or mobility of the substrate in the exit region. It may facilitate substrate entry into the catalytic tunnel and release the product faster than the wild type, whereas in other mutants, the tunnel is not prominent (DSB4), the exit is lost (DSB1), and the ligand binding site is absent (DSB2). This is the first report of the gain of function of both thermostability and enzyme activity of cellobiohydrolase Cel7A by disulfide bond engineering in the loop.IMPORTANCEBioethanol is one of the cleanest renewable energy and alternatives to fossil fuels. Cost efficient bioethanol production can be achieved through simultaneous saccharification and co-fermentation that needs active polysaccharide degrading enzymes. Cellulase enzyme complex is a crucial enzyme for second-generation bioethanol production from lignocellulosic biomass. Cellobiohydrolase (Cel7A) is an important member of this complex. In this work, we engineered (disulfide bond engineering) the Cel7A to increase its thermostability and catalytic activity which is required for its industrial application.

Heterologous expression and characterization of glycoside hydrolase with its potential applications in hyperthermic environment

With the progressive focus on renewable energy via biofuels production from lignocellulosic biomass, cellulases are the key enzymes that play a fundamental role in this regard. This study aims to unravel the characteristics of Thermotoga maritima MSB8 (Tma) (a hyperthermophile from hot springs) thermostable glycoside hydrolase enzyme. Here, a glycoside hydrolase gene of Thermotoga maritima (Tma) was heterologously expressed and characterized. The gene was placed in the pQE-30 expression vector under the T5 promotor, and the construct pQE-30-Gh was then successfully integrated into Escherichia coli BL21 (DH5α) genome by transformation. Sequence of the glycoside hydrolase contained an open reading frame of 2.124 kbp, encoded a polypeptide of 721 amino acid residues. The molecular weight of the recombinant protein estimated was 79 kDa. The glycoside hydrolase was purified by Ni⁺²-NTA affinity chromatography and its enzymatic activity was investigated. The recombinant enzyme is highly stable within an extreme pH range (2.0–7.0) and highly thermostable at 80 °C for 72 h indicating its viability in hyperthermic environment and acidic nature. Moreover, the Ca²⁺ and Mn²⁺ introduction stimulated the residual activity of recombinant enzyme. Conclusively, the thermostable glycoside hydrolase possesses potential to be exploited for industrial applications at hyperthermic environment.

Machine learning reveals sequence-function relationships in family 7 glycoside hydrolases

Family 7 glycoside hydrolases (GH7) are among the principal enzymes for cellulose degradation in nature and industrially. These enzymes are often bimodular, including a catalytic domain and carbohydrate-binding module (CBM) attached via a flexible linker, and exhibit an active site that binds cello-oligomers of up to ten glucosyl moieties. GH7 cellulases consist of two major subtypes: cellobiohydrolases (CBH) and endoglucanases (EG). Despite the critical importance of GH7 enzymes, there remain gaps in our understanding of how GH7 sequence and structure relate to function. Here, we employed machine learning to gain data-driven insights into relationships between sequence, structure, and function across the GH7 family. Machine-learning models, trained only on the number of residues in the active-site loops as features, were able to discriminate GH7 CBHs and EGs with up to 99% accuracy, demonstrating that the lengths of loops A4, B2, B3, and B4 strongly correlate with functional subtype across the GH7 family. Classification rules were derived such that specific residues at 42 different sequence positions each predicted the functional subtype with accuracies surpassing 87%. A random forest model trained on residues at 19 positions in the catalytic domain predicted the presence of a CBM with 89.5% accuracy. Our machine learning results recapitulate, as top-performing features, a substantial number of the sequence positions determined by previous experimental studies to play vital roles in GH7 activity. We surmise that the yet-to-be-explored sequence positions among the top-performing features also contribute to GH7 functional variation and may be exploited to understand and manipulate function.

A Novel Cellobiohydrolase I (CBHI) from Penicillium digitatum: Production, Purification, and Characterization

A new cellulase producer strain of Penicillium digitatum (RV 06) was previously obtained from rotten maize grains. This work aim was to optimize the production and characterize this microorganism produced cellulase. A CMCase maximum production (1.6 U/mL) was obtained in stationary liquid culture, with an initial pH of 5.0, at 25 °C, with 1% lactose as carbon source, and cultured for 5 days. The produced enzyme was purified by ammonium sulfate precipitation and exclusion chromatography. The purified enzyme optimal temperature and pH were 60 °C and 5.2, respectively. The experimental T_m of thermal inactivation was 63.68 °C, and full activity was recovered after incubation of 7 h at 50 °C. The purified 74 kDa CMCase presented K_M for CMC of 11.2 mg/mL, V_max of 0.13 μmol/min, k_cat of 52 s^-1, and k_cat/K_M of 4.7 (mg/mL)^-1 s^-1. The purified enzyme had a high specificity for CMC and p-nitrophenyl cellobioside and released glucose and cellobiose as final products of the CMC hydrolysis. The enzyme trypsin digestion produced peptides whose masses were obtained by MALDI-TOF/TOF mass spectrometry, which was also used to obtain two peptide sequences. These peptide sequences and the mass peak profile retrieved a CBHI within the annotated genome of P. digitatum PD1. Sequence alignments and phylogenetic analysis confirmed this enzyme as a CBHI of the glycoside hydrolase family 7. The P. digitatum PD1 protein in silico structural model revealed a coil and β-conformation predominance, which was confirmed by circular dichroism of the P. digitatum RV 06 purified enzyme.

A Thermostable Aspergillus fumigatus GH7 Endoglucanase Over-Expressed in Pichia pastoris Stimulates Lignocellulosic Biomass Hydrolysis

In the context of avoiding the use of non-renewable energy sources, employing lignocellulosic biomass for ethanol production remains a challenge. Cellulases play an important role in this scenario: they are some of the most important industrial enzymes that can hydrolyze lignocellulose. This study aims to improve on the characterization of a thermostable Aspergillus fumigatus endo-1,4-β-glucanase GH7 (Af-EGL7). To this end, Af-EGL7 was successfully expressed in Pichia pastoris X-33. The kinetic parameters K_m and V_max were estimated and suggested a robust enzyme. The recombinant protein was highly stable within an extreme pH range (3.0-8.0) and was highly thermostable at 55 °C for 72 h. Low Cu²⁺ concentrations (0.1-1.0 mM) stimulated Af-EGL7 activity up to 117%. Af-EGL7 was tolerant to inhibition by products, such as glucose and cellobiose. Glucose at 50 mM did not inhibit Af-EGL7 activity, whereas 50 mM cellobiose inhibited Af-EGL7 activity by just 35%. Additionally, the Celluclast^® 1.5L cocktail supplemented with Af-EGL7 provided improved hydrolysis of sugarcane bagasse "in natura", sugarcane exploded bagasse (SEB), corncob, rice straw, and bean straw. In conclusion, the novel characterization of Af-EGL7 conducted in this study highlights the extraordinary properties that make Af-EGL7 a promising candidate for industrial applications.

Cellulases from Thermophiles Found by Metagenomics

Cellulases are a heterogeneous group of enzymes that synergistically catalyze the hydrolysis of cellulose, the major component of plant biomass. Such reaction has biotechnological applications in a broad spectrum of industries, where they can provide a more sustainable model of production. As a prerequisite for their implementation, these enzymes need to be able to operate in the conditions the industrial process requires. Thus, cellulases retrieved from extremophiles, and more specifically those of thermophiles, are likely to be more appropriate for industrial needs in which high temperatures are involved. Metagenomics, the study of genes and gene products from the whole community genomic DNA present in an environmental sample, is a powerful tool for bioprospecting in search of novel enzymes. In this review, we describe the cellulolytic systems, we summarize their biotechnological applications, and we discuss the strategies adopted in the field of metagenomics for the discovery of new cellulases, focusing on those of thermophilic microorganisms.

Thermal Inactivation Kinetics and Secondary Structure Change of a Low Molecular Weight Halostable Exoglucanase from a Marine Aspergillus niger at High Salinities

Two kinds of exoglucanase were purified from a marine Aspergillus niger. Catalytic ability of halophilic exoglucanase with a lower molecular weight and secondary structure change was analyzed at different salinities. Activity of the low molecular weight exoglucanase in 10% NaCl solution (w/v) was 1.69-fold higher of that in NaCl-free solution. Half-life time in 10% NaCl solution (w/v) was over 1.27-fold longer of that in NaCl-free solution. Free energy change of the low molecular weight exoglucanase denaturation, △G, in 10% NaCl solution (w/v) was 0.54 kJ/mol more than that in NaCl-free solution. Melt point in 10% NaCl solution (w/v), 52.01 °C, was 4.21 °C higher than that in NaCl-free solution, 47.80 °C. K _m value, 0.179 mg/ml in 10% NaCl solution (w/v) was less 0.044 mg/ml than that, 0.224 mg/ml, in NaCl-free solution. High salinity made content of α-helix increased. Secondary structure change caused by high salinities improved exoglucanase thermostability and catalysis activity. The halophilic exoglucanase from a marine A. niger was valuable for hydrolyzing cellulose at high salinities.

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.

Human Chitotriosidase Is an Endo-Processive Enzyme

Human chitotriosidase (HCHT) is involved in immune response to chitin-containing pathogens in humans. The enzyme is able to degrade chitooligosaccharides as well as crystalline chitin. The catalytic domain of HCHT is connected to the carbohydrate binding module (CBM) through a flexible hinge region. In humans, two active isoforms of HCHT are found-the full length enzyme and its truncated version lacking CBM and the hinge region. The active site architecture of HCHT is reminiscent to that of the reducing-end exo-acting processive chitinase ChiA from bacterium Serratia marcescens (SmChiA). However, the presence of flexible hinge region and occurrence of two active isoforms are reminiscent to that of non-processive endo-chitinase from S. marcescens, SmChiC. Although the studies on soluble chitin derivatives suggest the endo-character of HCHT, the mode of action of the enzyme on crystalline chitin is not known. Here, we made a thorough characterization of HCHT in terms of the mode of action, processivity, binding, and rate constants for the catalysis and dissociation using α-chitin as substrate. HCHT efficiently released the end-label from reducing-end labelled chitin and had also high probability (95%) of endo-mode initiation of processive run. These results qualify HCHT as an endo-processive enzyme. Processivity and the rate constant of dissociation of HCHT were found to be in-between those, characteristic to processive exo-enzymes, like SmChiA and randomly acting non-processive endo-enzymes, like SmChiC. Apart from increasing the affinity for chitin, CBM had no major effect on kinetic properties of HCHT.

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.