Role of carbohydrate binding module (CBM3c) of GH9 β-1,4 endoglucanase (Cel9W) from Hungateiclostridium thermocellum ATCC 27405 in catalysis

Carbohydrate binding modules: Compact yet potent accessories in the specific substrate binding and performance evolution of carbohydrate-active enzymes

Carbohydrate binding modules (CBMs) are independent non-catalytic domains widely found in carbohydrate-active enzymes (CAZymes), and they play an essential role in the substrate binding process of CAZymes by guiding the appended catalytic modules to the target substrates. Owing to their precise recognition and selective affinity for different substrates, CBMs have received increasing research attention over the past few decades. To date, CBMs from different origins have formed a large number of families that show a variety of substrate types, structural features, and ligand recognition mechanisms. Moreover, through the modification of specific sites of CBMs and the fusion of heterologous CBMs with catalytic domains, improved enzymatic properties and catalytic patterns of numerous CAZymes have been achieved. Based on cutting-edge technologies in computational biology, gene editing, and protein engineering, CBMs as auxiliary components have become portable and efficient tools for the evolution and application of CAZymes. With the aim to provide a theoretical reference for the functional research, rational design, and targeted utilization of novel CBMs in the future, we systematically reviewed the function-related characteristics and potentials of CAZyme-derived CBMs in this review, including substrate recognition and binding mechanisms, non-catalytic contributions to enzyme performances, module modifications, and innovative applications in various fields.

Structure elucidation of a multi-modular recombinant endoglucanase, AtGH9C-CBM3A-CBM3B from Acetivibrio thermocellus ATCC 27405 and its substrate binding analysis

Cellulases from GH9 family show endo-, exo- or processive endocellulase activity, but the reason behind the variation is unclear. A GH9 recombinant endoglucanase, AtGH9C-CBM3A-CBM3B from Acetivibrio thermocellus was structurally characterized for conformation, binding and dynamics assessment. Modeled AtGH9C-CBM3A-CBM3B depicted (α/α)6-barrel structure with Asp98, Asp101 and Glu489 acting as catalytic triad. CD results revealed 25.2 % α-helix, 18.4 % β-sheet and rest 56.4 % of random coils, corroborating with predictions from PSIPRED and SOPMA. MD simulation of AtGH9C-CBM3A-CBM3B bound cellotetraose showed structural stability and global compactness with lowered RMSD values (1.5 nm) as compared with only AtGH9C-CBM3A-CBM3B (1.8 nm) for 200 ns. Higher fluctuation in RMSF values in far-positioned CBM3B pointed to its redundancy in substrate binding. Docking studies showed maximum binding with cellotetraose (ΔG = -5.05 kcal/mol), with reduced affinity towards ligands with degree of polymerization (DP) lower (DP < 4) or higher than 4 (DP > 4). Processivity index displayed the enzyme to be processive with loop 3 (342-379 aa) possibly blocking the non-reducing end of cellulose chain, resulting in cellotetraose release. SAXS analysis of AtGH9C-CBM3A-CBM3B at 5 mg/mL displayed monodispersed state with fist-and-elbow shape in solution. Negative zeta potential of -24 mV at 5 mg/mL indicated stability and free from aggregation.

Molecular mechanism of cellulose depolymerization by the two-domain BlCel9A enzyme from the glycoside hydrolase family 9

Carbohydrate-active enzymes from the glycoside hydrolase family 9 (GH9) play a key role in processing lignocellulosic biomass. Although the structural features of some GH9 enzymes are known, the molecular mechanisms that drive their interactions with cellulosic substrates remain unclear. To investigate the molecular mechanisms that the two-domain Bacillus licheniformis BlCel9A enzyme utilizes to depolymerize cellulosic substrates, we used a combination of biochemical assays, X-ray crystallography, small-angle X-ray scattering, and molecular dynamics simulations. The results reveal that BlCel9A breaks down cellulosic substrates, releasing cellobiose and glucose as the major products, but is highly inefficient in cleaving oligosaccharides shorter than cellotetraose. In addition, fungal lytic polysaccharide oxygenase (LPMO) TtLPMO9H enhances depolymerization of crystalline cellulose by BlCel9A, while exhibiting minimal impact on amorphous cellulose. The crystal structures of BlCel9A in both apo form and bound to cellotriose and cellohexaose were elucidated, unveiling the interactions of BlCel9A with the ligands and their contribution to substrate binding and products release. MD simulation analysis reveals that BlCel9A exhibits higher interdomain flexibility under acidic conditions, and SAXS experiments indicate that the enzyme flexibility is induced by pH and/or temperature. Our findings provide new insights into BlCel9A substrate specificity and binding, and synergy with the LPMOs.

Role of carbohydrate binding modules, CBM3A and CBM3B in stability and catalysis by a β-1,4 endoglucanase, AtGH9C-CBM3A-CBM3B from Acetivibrio thermocellus ATCC 27405

A recombinant β-1,4 endoglucanase, AtGH9C-CBM3A-CBM3B from Acetivibrio thermocellus ATCC27405 was explored for biochemical properties and the role of its associated CBMs in catalysis. The gene expressing full-length multi-modular β-1,4-endoglucanase (AtGH9C-CBM3A-CBM3B) and its truncated derivatives (AtGH9C-CBM3A, AtGH9C, CBM3A and CBM3B) were independently cloned and expressed in Escherichia coli BL21(DE3) cells and purified. AtGH9C-CBM3A-CBM3B showed maximal activity at 55 °C and pH 7.5. AtGH9C-CBM3A-CBM3B exhibited highest activity against carboxy methyl cellulose (58.8 U/mg) followed by lichenan (44.5 U/mg), β-glucan (36.2 U/mg) and hydroxy ethyl cellulose (17.9 U/mg). Catalytic module, AtGH9C showed insignificant activity against the substrates, signifying the essential requirement of CBMs in catalysis. AtGH9C-CBM3A-CBM3B displayed stability in pH range, 6.0-9.0 and thermostability up to 60 °C for 90 min with unfolding transition midpoint (T_m) of 65 °C. The generation of cellotetraose and other higher oligosaccharides by AtGH9C-CBM3A-CBM3B confirmed it as an endo-β-1,4-glucanase. AtGH9C activity was partially recovered by the addition of equimolar concentration of CBM3A, CBM3B or CBM3A + CBM3B by 47 %, 13 % or 50 %, respectively. Moreover, the associated CBMs imparted thermostability to the catalytic module, AtGH9C. These results showed that the physical association of AtGH9C with its associated CBMs and the cross-talk between CBMs are necessary for AtGH9C-CBM3A-CBM3B in effective cellulose catalysis.

Processive binding mechanism of Cel9G from Clostridium cellulovorans: molecular dynamics and free energy landscape investigations

The processive binding mechanism of cellulose by Cel9G from C. cellulovorans was investigated by MD and metadynamics simulations.

A thermo-alkali stable and detergent compatible processive β-1,4-glucanase from Himalayan Bacillus sp. PCH94

Present study reports a novel and robust GH9 processive endoglucanase β-1,4-glucanase from Bacillus sp. PCH94 (EGaseBL) with thermo-alkali stable properties. The EGaseBL gene was cloned in pET-28b(+) and expressed in Escherichia coli BL21(DE3) cells. The recombinant protein was purified 94-fold with a yield of 67.8%. The biochemical characterization revealed an active enzyme at a wide pH (4.0–10.0) and temperature (4–100°C). It showed a Km and Vmax of 1.10 mg/ml and 208.24 IU/mg, respectively, using β-glucan as a substrate. The EGaseBL showed dual activities for endoglucanase (134.17 IU/mg) and exoglucanase (28.76 IU/mg), assayed using substrates β-glucan and Avicel, respectively. The enzyme is highly stable in neutral and alkaline pH and showed a half-life of 11.29 h, and 8.31 h in pH 7.0 and 9.0, respectively. The enzyme is also compatible with commercial detergents (Tide, Surf, Ghadi, Raj, and Healing tree) of the Indian market and retained &gt; 85% enzyme activity. Concisely, robustness, extreme functionality, and detergent compatibility endorse EGaseBL as a potential bioresource for the detergent industry, in addition to its implications for the bioethanol industry.

A processive GH9 family endoglucanase of Bacillus licheniformis and the role of its carbohydrate-binding domain

One of the critical steps in lignocellulosic deconstruction is the hydrolysis of crystalline cellulose by cellulases. Endoglucanases initially facilitate the breakdown of cellulose in lignocellulosic biomass and are further aided by other cellulases to produce fermentable sugars. Furthermore, if the endoglucanase is processive, it can adsorb to the smooth surface of crystalline cellulose and release soluble sugars during repeated cycles of catalysis before dissociating. Most glycoside hydrolase family 9 (GH9) endoglucanases have catalytic domains linked to a CBM (carbohydrate-binding module) (mostly CBM3) and present the second-largest cellulase family after GH5. GH9 endoglucanases are relatively less characterized. Bacillus licheniformis is a mesophilic soil bacterium containing many glycoside hydrolase (GH) enzymes. We identified an endoglucanase gene, gh9A, encoding the GH9 family enzyme H1AD14 in B. licheniformis and cloned and overexpressed H1AD14 in Escherichia coli. The purified H1AD14 exhibited very high enzymatic activity on endoglucanase substrates, such as β-glucan, lichenan, Avicel, CMC-Na (sodium carboxymethyl cellulose) and PASC (phosphoric acid swollen cellulose), across a wide pH range. The enzyme is tolerant to 2 M sodium chloride and retains 74% specific activity on CMC after 10 days, the highest amongst the reported GH9 endoglucanases. The full-length H1AD14 is a processive endoglucanase and efficiently saccharified sugarcane bagasse. The deletion of the CBM reduces the catalytic activity and processivity. The results add to the sparse knowledge of GH9 endoglucanases and offer the possibility of characterizing and engineering additional enzymes from B. licheniformis toward developing a cellulase cocktail for improved biomass deconstruction. KEY POINTS: • H1AD14 is a highly active and processive GH9 endoglucanase from B. licheniformis. • H1AD14 is thermostable and has a very long half-life. • H1AD14 showed higher saccharification efficiency than commercial endoglucanase.

QM/MM investigation of the catalytic mechanism of processive endoglucanase Cel9G from Clostridium cellulovorans

Carbohydrate degradation catalyzed by glucoside hydrolases (GHs) is a major mechanism in biomass conversion. GH family 9 endoglucanase (Cel9G) from Clostridium cellulovorans, a typical multimodular enzyme, contains a catalytic domain closely linked to a family 3c carbohydrate-binding module (CBM3c). Unlike the conventional behavior proposed for other carbohydrate-binding modules, CBM3c has a direct impact on catalytic activity. In this work, extensive molecular dynamics (MD) simulations were employed to clarify the functional role of CBM3c. Furthermore, the detailed catalytic mechanism of Cel9G was investigated at the atomistic level using the combined quantum mechanical and molecular mechanical (QM/MM) method. Based on these simulations, owing to the rigidity of the peptide linker, CBM3c may affect the enzymatic activity via direct interactions with alpha helix 4 of GH9, especially with the K123 and H125 residues. In addition, using cellohexaose as a substrate, the QM/MM MD simulations confirmed that this enzyme can cleave the β-1,4-glycosidic linkage via an inverting mechanism. An oxocarbenium ion-like transition state was located with a barrier height of 19.6 kcal mol^-1. Furthermore, the G(-1) pyranose unit preferentially adopted a distorted ¹S₅/⁴H₅ conformer in the enzyme-substrate complex. For the cleavage of the glycosidic bond, we were able to identify a plausible route (¹S₅/⁴H₅ → [⁴H₅/⁴E]^# → ⁴C₁) from the reactant to the product at the G(-1) site.

Highly efficient, processive and multifunctional recombinant endoglucanase RfGH5_4 from Ruminococcus flavefaciens FD-1 v3 for recycling lignocellulosic plant biomasses

Gene encoding endoglucanase, RfGH5_4 from R. flavefaciens FD-1 v3 was cloned, expressed in Escherichia coli BL21(DE3) cells and purified. RfGH5_4 showed molecular size 41 kDa and maximum activity at pH 5.5 and 55 °C. It was stable between pH 5.0-8.0, retaining 85% activity and between 5 °C-45 °C, retaining 75% activity, after 60 min. RfGH5_4 displayed maximum activity (U/mg) against barley β-D-glucan (665) followed by carboxymethyl cellulose (450), xyloglucan (343), konjac glucomannan (285), phosphoric acid swollen cellulose (86), beechwood xylan (21.7) and carob galactomannan (16), thereby displaying the multi-functionality. Catalytic efficiency (mL.mg^-1 s^-1) of RfGH5_4 against carboxymethyl cellulose (146) and konjac glucomannan (529) was significantly high. TLC and MALDI-TOF-MS analyses of RfGH5_4 treated hydrolysates of cellulosic and hemicellulosic polysaccharides displayed oligosaccharides of degree of polymerization (DP) between DP2-DP11. TLC, HPLC and Processivity-Index analyses revealed RfGH5_4 to be a processive endoglucanase as initially, for 30 min it hydrolysed cellulose to cellotetraose followed by persistent release of cellotriose and cellobiose. RfGH5_4 yielded sufficiently high Total Reducing Sugar (TRS, mg/g) from saccharification of alkali pre-treated sorghum (72), finger millet (62), sugarcane bagasse (38) and cotton (27) in a 48 h saccharification reaction. Thus, RfGH5_4 can be considered as a potential endoglucanase for renewable energy applications.

Engineering of a chitosanase fused to a carbohydrate-binding module for continuous production of desirable chitooligosaccharides

Chitooligosaccharides (CHOS) with multiple biological activities are usually produced through enzymatic hydrolysis of chitosan or chitin. However, purification and recycling of the enzyme have largely limited the advancement of CHOS bioproduction. Here, we engineered a novel enzyme by fusing the native chitosanase Csn75 with a carbohydrate-binding module (CBM) that can specifically bind to curdlan. The recombinase Csn75-CBM was successfully expressed by Pichia pastoris and allowed one-step purification and immobilization in the chitosanase immobilized curdlan packed-bed reactor (CICPR), where a maximum adsorption capacity of 39.59 mg enzyme/g curdlan was achieved. CHOS with degrees of polymerization of 2-5 (a hydrolysis yield of 97.75%), 3-6 (75.45%), and 3-7 (73.2%) were continuously produced by adjusting the ratio of enzyme and chitosan or the flow rate of chitosan. Moreover, the CICPR exhibited good stability and reusability after several cycles. The recombinase Csn75-CBM has greatly improved the efficiency of the bioproduction of CHOS.

Effect of carbohydrate binding modules alterations on catalytic activity of glycoside hydrolase family 6 exoglucanase from Chaetomium thermophilum to cellulose

Exoglucanase (CBH) is the rate limiting enzyme in the process of cellulose degradation. The carbohydrate binding module (CBM) can improve the accessibility of cellulase to substrate, thereby promoting the enzymatic hydrolysis of cellulase. In this study, the influence of CBM on the properties of GH6 exoglucanase from Chaetomium thermophilum (CtCBH) is systematically explored from three perspectives: the fusion of exogenous CBM, the exogenous CBM replacement of its own CBM, and the deletion of its own CBM. The parental and reconstructed CtCBH presented the same optimum pH (6.0) and temperature (60 °C) for maximum activity. Fusion of exogenous CBM increased the binding capacity of CtCBH to Avicel by 8% and 9%, respectively, but it had no significant effect on its catalytic activity. The exogenous CBM replacement of its own CBM resulted in a 12% reduction in the binding ability of CtCBH to Avicel, and a 26% reduction in the catalytic activity of Avicel. The deletion of its own CBM significantly reduced the binding ability of CtCBH to Avicel by approximately 53%, but its catalytic activity was not obviously reduced. These observations suggest that binding ability of CBM is not necessary for the catalysis of CtCBH.

Characterization of three bacterial glycoside hydrolase family 9 endoglucanases with different modular architectures isolated from a compost metagenome

BACKGROUND

Environmental bacteria express a wide diversity of glycoside hydrolases (GH). Screening and characterization of GH from metagenomic sources provides an insight into biomass degradation strategies of non-cultivated prokaryotes.

METHODS

In the present report, we screened a compost metagenome for lignocellulolytic activities and identified six genes encoding enzymes belonging to family GH9 (GH9a-f). Three of these enzymes (GH9b, GH9d and GH9e) were successfully expressed and characterized.

RESULTS

A phylogenetic analysis of the catalytic domain of pro- and eukaryotic GH9 enzymes suggested the existence of two major subgroups. Bacterial GH9s displayed a wide variety of modular architectures and those harboring an N-terminal Ig-like domain, such as GH9b and GH9d, segregated from the remainder. We purified and characterized GH9 endoglucanases from both subgroups and examined their stabilities, substrate specificities and product profiles. GH9e exhibited an original hydrolysis pattern, liberating an elevated proportion of oligosaccharides longer than cellobiose. All of the enzymes exhibited processive behavior and a synergistic action on crystalline cellulose. Synergy was also evidenced between GH9d and a GH48 enzyme identified from the same metagenome.

CONCLUSIONS

The characterized GH9 enzymes displayed different modular architectures and distinct substrate and product profiles. The presence of a cellulose binding domain was shown to be necessary for binding and digestion of insoluble cellulosic substrates, but not for processivity.

GENERAL SIGNIFICANCE

The identification of six GH9 enzymes from a compost metagenome and the functional variety of three characterized members highlight the importance of this enzyme family in bacterial biomass deconstruction.

Enzyme systems of thermophilic anaerobic bacteria for lignocellulosic biomass conversion

Economic production of lignocellulose degrading enzymes for biofuel industries is of considerable interest to the biotechnology community. While these enzymes are widely distributed in fungi, their industrial production from other sources, particularly by thermophilic anaerobic bacteria (growth T_opt ≥ 60 °C), is an emerging field. Thermophilic anaerobic bacteria produce a large number of lignocellulolytic enzymes having unique structural features and employ different schemes for biomass degradation, which can be classified into four systems namely; 'free enzyme system', 'cell anchored enzymes', 'complex cellulosome system', and 'multifunctional multimodular enzyme system'. Such enzymes exhibit high specific activity and have a natural ability to withstand harsh bioprocessing conditions. However, achieving a higher production of these thermostable enzymes at current bioprocessing targets is challenging. In this review, the research opportunities for these distinct enzyme systems in the biofuel industry and the associated technological challenges are discussed. The current status of research findings is highlighted along with a detailed description of the categorization of the different enzyme production schemes. It is anticipated that high temperature-based bioprocessing will become an integral part of sustainable bioenergy production in the near future.

Computational modeling and small-angle X-ray scattering based structure analysis and identifying ligand cleavage mechanism by processive endocellulase of family 9 glycoside hydrolase (HtGH9) from Hungateiclostridium thermocellum ATCC 27405

The cellulases of family 9 glycoside hydrolase with subtle difference in amino acid sequence have shown different types of catalytic activities such as endo-, exo- or processive endocellulase. However, the reason behind the different types of catalytic activities still unclear. In this study, the processive endocellulase, HtGH9 of family 9 GH from Hungateiclostridium thermocellum was modeled by homology modeling. The catalytic module (HtGH9t) of HtGH9 modeled structure displayed the (α/α)₆ barrel topology and associated family 3 carbohydrate binding module (HtCBM3c) displayed β-sandwich fold. Ramachandran plot of HtGH9 modeled structure displayed all the amino acid residues in allowed region except Asn225 and Asp317. Secondary structure analysis of modeled HtGH9 showed the presence of 41.3% α-helices and 11.0% β-strands which was validated through circular dichroism analysis that showed the presence of 42.6% α-helices and 14.5% β-strands. Molecular Dynamic (MD) simulation of HtGH9 structure for 50 ns showed Root Mean Square Deviation (RMSD), 0.84 nm and radius of gyration (Rg) 3.1 nm. The Small-angle X-ray scattering of HtGH9 confirmed the monodisperse state. The radius of gyration for globular shape (Rg) was 5.50 ± 0.15 nm and for rod shape (Rc) by Guinier plot was 2.0 nm. The loop formed by amino acid residues, 264-276 towards one end of the catalytic site of HtGH9 forms a barrier, that blocks the non-reducing end of the cellulose chain causing the processive cleavage resulting in the release of cellotetraose. The position of the corresponding loop in cellulases of family 9 GH is responsible for different types of cleavage patterns.