A lichenase-like family 12 endo-(1→4)-β-glucanase from Aspergillus japonicus: study of the substrate specificity and mode of action on β-glucans in comparison with other glycoside hydrolases

Two β-glucanases from bacterium Cellulomonas flavigena: expression in Pichia pastoris, properties, biotechnological potential

In the genome of Cellulomonas flavigena, two genes that potentially encode endoglucanases - Cfla_2912 and Cfla_2913 were identified. We cloned the genes and created Pichia pastoris-based recombinant producers of two proteins that were expressed from the AOX1 promoter. Each of the endoglucanase molecules contains a GH6 catalytic domain, CBM2 carbohydrate-binding module, and TAT signal peptide. The fermentation of the producers was carried out in a 10 L fermenter; Cfla_2912 and Cfla_2913 were purified using affinity chromatography. The yield comprised 10.3 mg/ml (430 U/ml) for Cfla_2913 and 9 mg/ml (370 U/ml) for Cfla_2912. Cfla_2912 and Cfla_2913 were found to have a high activity against barley β-glucan and lichenan, a weak activity against carboxymethyl cellulose (CMC), phosphoric-acid treated cellulose, and no activity against laminarin, xylan, soluble starch, microcrystalline cellulose, cellobiose, and cellotriose. Thus, the proteins exhibited β-glucanase activity. Both proteins had a neutral pH optimum of about 7.0 and were more stable at neutral and slightly alkaline pH ranging from 7.0 to 9.0. Cfla_2912 and Cfla_2913 showed a moderate thermal stability. The products of barley β-glucan hydrolysis by Cfla_2912 and Cfla_2913 were trisaccharide, tetrasaccharide, and cellobiose. Cfla_2912 and Cfla_2913 efficiently hydrolyzed cereal polysaccharides, which indicate that they may have biotechnological potential.

Unusual substrate specificity in GH family 12: structure-function analysis of glucanases Bgh12A and Xgh12B from Aspergillus cervinus, and Egh12 from Thielavia terrestris

In this study, we compared the properties and structures of three fungal GH12 enzymes: the strict endoglucanase Bgh12A and the xyloglucanase Xgh12B from Aspergillus cervinus, and the endoglucanase Egh12 from Thielavia terrestris combining activity on linear β-glucan and branched xyloglucan. Egh12 from T. terrestris was produced in Pichia pastoris, purified, and characterized as a thermostable enzyme with maximal activity at 70 ºC and a half-life time of 138 min at 65 °C. We for the first time demonstrated that the GH12 endoglucanases Egh12 and Bgh12A, but not the strict xyloglucanase Xgh12B, hydrolyzed (1,3)-β-linkages in (1,3;1,4)-β-D-glucooligosaccharides and had transglycosylase activity on (1,3)-β-D-glucooligosaccharides. Phylogenetic analysis indicated that Egh12 from T. terrestris and Bgh12A from A. cervinus are more related than Bgh12A and Xgh12B isolated from one strain. The X-ray structure of Bgh12A was determined with 2.17 Å resolution and compared with 3D-homology models of Egh12 and Xgh12B. The enzymes have a β-jelly roll structure with a catalytic cleft running across the protein. Comparative analysis and a docking study demonstrated the importance of endoglucanase-specific loop 1 partly covering the catalytic cleft for correct placement of the linear substrates. Variability in substrate specificity between the GH12 endoglucanases is determined by non-conservative residues in structural loops framing the catalytic cleft. A residue responsible for the thermostability of Egh12 was predicted. The key structural elements and residues described in this study may serve as potential targets for modification aimed at the improvement of enzymatic properties. KEY POINTS: • Thermostable endoglucanase Egh12 from T. terrestris was produced in P. pastoris, purified, and characterized • The X-ray structure of GH12 endoglucanase Bgh12A from A. cervinus was resolved • GH12 endoglucanases, but not GH12 xyloglucanases, hydrolyze (1,3)-β-linkages in (1,3;1,4)-β-D-glucooligosaccharides.

A trimodular family 16 glycoside hydrolase from the cellulosome of Ruminococcus flavefaciens displays highly specific licheninase (EC 3.2.1.73) activity

Cellulosomes are highly complex cell-bound multi-enzymatic nanomachines used by anaerobes to break down plant carbohydrates. The genome sequence of Ruminococcus flavefaciens revealed a remarkably diverse cellulosome composed of more than 200 cellulosomal enzymes. Here we provide a detailed biochemical characterization of a highly elaborate R. flavefaciens cellulosomal enzyme containing an N-terminal dockerin module, which anchors the enzyme into the multi-enzyme complex through binding of cohesins located in non-catalytic cell-bound scaffoldins, and three tandemly repeated family 16 glycoside hydrolase (GH16) catalytic domains. The DNA sequence encoding the three homologous catalytic domains was cloned and hyper-expressed in Escherichia coli BL21 (DE3) cells. SDS-PAGE analysis of purified His₆ tag containing RfGH16_21 showed a single soluble protein of molecular size ~89 kDa, which was in agreement with the theoretical size, 89.3 kDa. The enzyme RfGH16_21 exhibited activity over a wide pH range (pH 5.0-8.0) and a broad temperature range (50-70 °C), displaying maximum activity at an optimum pH of 7.0 and optimum temperature of 55 °C. Substrate specificity analysis of RfGH16_21 revealed maximum activity against barley β-d-glucan (257 U mg^-1) followed by lichenan (247 U mg^-1), but did not show significant activity towards other tested polysaccharides, suggesting that it is specifically a β-1,3-1,4-endoglucanase. TLC analysis revealed that RfGH16_21 hydrolyses barley β-d-glucan to cellotriose, cellotetraose and a higher degree of polymerization of gluco-oligosaccharides indicating an endo-acting catalytic mechanism. This study revealed a fairly high, active and thermostable bacterial endo-glucanase which may find considerable biotechnological potentials.

A novel bifunctional glucanase exhibiting high production of glucose and cellobiose from rumen bacterium

Herbivores gastrointestinal microbiota is of tremendous interest for mining novel lignocellulosic enzymes for bioprocessing. We previously reported a set of potential carbohydrate-active enzymes from the metatranscriptome of the Hu sheep rumen microbiome. In this study, we isolated and heterologously expressed two novel glucanase genes, Cel5A-h38 and Cel5A-h49, finding that both recombinant enzymes showed the optimum temperatures of 50 °C. Substrate-specificity determination revealed that Cel5A-h38 was exclusively active in the presence of mixed-linked glucans, such as barley β-glucan and Icelandic moss lichenan, whereas Cel5A-h49 (EC 3.2.1.4) exhibited a wider substrate spectrum. Surprisingly, Cel5A-h38 initially released only cellotriose from lichenan and further converted it into an equivalent amount of glucose and cellobiose, suggesting a dual-function as both endo-β-1,3-1,4-glucanase (EC 3.2.1.73) and exo-cellobiohydrolase (EC 3.2.1.91). Additionally, we performed enzymatic hydrolysis of sheepgrass (Leymus chinensis) and rice (Orysa sativa) straw using Cel5A-h38, revealing liberation of 1.91 ± 0.30 mmol/mL and 2.03 ± 0.09 mmol/mL reducing sugars, respectively, including high concentrations of glucose and cellobiose. These results provided new insights into glucanase activity and lay a foundation for bioconversion of lignocellulosic biomass.

Transcriptional analysis of the lichenase-like gene cel12A of the filamentous fungus Stachybotrys atra BP-A and its relevance for lignocellulose depolymerization

To rationally optimize the production of industrial enzymes by molecular means requires previous knowledge of the regulatory circuits controlling the expression of the corresponding genes. The genus Stachybotrys is an outstanding producer of cellulose-degrading enzymes. Previous studies isolated and characterized the lichenase-like/non-typical cellulase Cel12A of S. atra (AKA S. chartarum) belonging to glycosyl hydrolase family 12 (GH12). In this study, we used RT-qPCR to determine the pattern of expression of cel12A under different carbon sources and initial ambient pH. Among the carbon sources examined, rice straw triggered a greater increase in the expression of cel12A than 1% lactose or 0.1% glucose, indicating specific induction by rice straw. In contrast, cel12A was repressed in the presence of glucose even when combined with this inducer. The proximity of 2 adjacent 5'-CTGGGGTCTGGGG-3' CreA consensus target sites to the translational start site of cel12A strongly suggests that the carbon catabolite repression observed is directly mediated by CreA. Ambient pH did not have a significant effect on cel12A expression. These findings present new knowledge on transcriptional regulatory networks in Stachybotrys associated with cellulose/hemicellulose depolymerization. Rational engineering of CreA to remove CCR could constitute a novel strategy for improving the production of Cel12A.

Three glycoside hydrolase family 12 enzymes display diversity in substrate specificities and synergistic action between each other

PoCel12A, PoCel12B, and PoCel12C are genes that encode glycoside hydrolase family 12 (GH12) enzymes in Penicillium oxalicum. PoCel12A and PoCel12B are typical GH12 enzymes that belong to fungal subfamilies 12-1 and 12-2, respectively. PoCel12C contains a low-complexity region (LCR) domain, which is not found in PoCel12A or PoCel12B and independent of fungal subfamily 12-1 or 12-2. Recombinant enzymes (named rCel12A, rCel12B and rCel12C) demonstrate existing diversity in the substrate specificities. Although most members in GH family 12 are typical endoglucanases and preferentially hydrolyze β-1,4-glucan (e.g., carboxymethylcellulose), recombinant PoCel12A is a non-typical endo-(1-4)-β-glucanase; it preferentially hydrolyzes mix-linked β-glucan (barley β-glucan, β-1,3-1,4-glucan) and slightly hydrolyzes β-1,4-glucan (carboxymethylcellulose). Recombinant PoCel12B possesses a significantly high activity against xyloglucan. A specific activity of rCel12B toward xyloglucan (239 µmol/min/mg) is the second-highest value known. Recombinant PoCel12C shows low activity toward β-glucan, carboxymethylcellulose, or xyloglucan. All three enzymes can degrade phosphoric acid-swollen cellulose (PASC). However, the hydrolysis products toward PASC by enzymes are different: the main hydrolysis products are cellotriose, cellotetraose, and cellobiose for rCel12A, rCel12B, and rCel12C, correspondingly. A synergistic action toward PASC among rCel12A and rCel12B is observed, thereby suggesting a potential application for preparing enzyme cocktails used in lignocellulose hydrolysis.

Novel endo-(1,4)-β-glucanase Bgh12A and xyloglucanase Xgh12B from Aspergillus cervinus belong to GH12 subgroup I and II, respectively

In spite of intensive exploitation of aspergilli for the industrial production of carbohydrases, little is known about hydrolytic enzymes of fungi from the section Cervini. Novel glycoside hydrolases Bgh12A and Xgh12B from Aspergillus cervinus represent examples of divergent activities within one enzyme family and belong to the GH12 phylogenetic subgroup I (endo-(1,4)-β-glucanases) and II (endo-xyloglucanases), respectively. The bgh12A and xgh12B genes were identified in the unsequenced genome of A. cervinus using primers designed for conservative regions of the corresponding subgroups and a genome walking approach. The recombinant enzymes were heterologously produced in Pichia pastoris, purified, and characterized. Bgh12A was an endo-(1,4)-β-glucanase (EC 3.2.1.4) hydrolyzing the unbranched soluble β-(1,4)-glucans and mixed linkage β-(1,3;1,4)-D-glucans. Bgh12A exhibited maximum activity on barley β-glucan (BBG), which amounted to 614 ± 30 U/mg of protein. The final products of BBG and lichenan hydrolysis were glucose, cellobiose, cellotriose, 4-O-β-laminaribiosyl-glucose, and a range of higher mixed-linkage gluco-oligosaccharides. In contrast, the activity of endo-xyloglucanase Xgh12B (EC 3.2.1.151) was restricted to xyloglucan, with 542 ± 39 U/mg protein. The enzyme cleaved the (1,4)-β-glycosidic bonds of the xyloglucan backbone at the unsubstituted glucose residues finally generating cellotetraose-based hepta-, octa, and nona-oligosaccharides. Bgh12A and Xgh12B had maximal activity at 55 °C, pH 5.0. At these conditions, the half-time of Xgh12B inactivation was 158 min, whereas the half-life of Bgh12A was 5 min. Recombinant P. pastoris strains produced up to 10⁶ U/L of the target enzymes with at least 75% of recombinant protein in the total extracellular proteins. The Bgh12A and Xgh12B sequences show 43% identity. Strict differences in substrate specificity of Bgh12A and Xgh12B were in congruence with the presence of subgroup-specific structural loops and substrate-binding aromatic residues in the catalytic cleft of the enzymes. Individual composition of aromatic residues in the catalytic cleft defined variability in substrate selectivity within GH12 subgroups I and II.

Fungal β-1,3-1,4-glucanases: production, proprieties and biotechnological applications

β-1,3-1,4-glucanases (or lichenases; EC 3.2.1.73) comprise one of the main enzymes used in industry during recent decades. These enzymes hydrolyze β-glucans containing β-1,3 and β-1,4 linkages, such as cereal β-glucans and lichenan. The β-1,3-1,4-glucanases are produced by a variety of bacteria, fungi, plants and animals. A large number of microbial β-1,3-1,4-glucanases have potential application in industrial processes, such as feed, food and detergent industries. The present review summarizes the available studies with respect to β-1,3-1,4-glucanases production conditions, enzyme biochemical properties and potential industrial application. © 2018 Society of Chemical Industry.

Functional Analysis of a Highly Active β-Glucanase from Bispora sp. MEY-1 Using Its C-terminally Truncated Mutant

A β-1,3-1,4-glucanase-encoding gene, Bisglu16B, was identified in Bispora sp. MEY-1. The deduced BisGlu16B consists of an N-terminal signal peptide, a catalytic module of glycoside hydrolase family 16 (GH16), and a C-terminal serine/proline-rich module. After expression in Pichia pastoris GS115, the purified recombinant BisGlu16B showed maximal activity at pH 4.0 and 55 °C and had broad substrate specificity (β-1,3-/β-1,4-mixed, β-1,3-, β-1,4-, and β-1,6-linked glucan, and β-1,4-mannan). The enzyme possessed high specific activities toward barley β-glucan (34 700 U·mg^-1), lichenan (23 900 U·mg^-1), and laminarin (9 000 U·mg^-1). After removing the C-terminal module, the truncated mutant, BisGlu16B-ΔC, retained similar enzymatic properties to the wild type but displayed significantly enhanced activities (up to 2.5-fold). Functional and structural analyses indicated that the C-terminal module plays a key role in the substrate binding of BisGlu16B. This study provided an excellent candidate glucanase for industrial purposes and revealed the functions of a C-terminal serine/proline-rich region.

Production and partial characterization of β-1,3-glucanase obtained from Rhodotorula oryzicola

The current study aims to assess the kinetics of population growth of Rhodotorula oryzicola and the production of β-1,3-glucanase (EC 3.2.1.39) enzyme by this yeast. It also aims to obtain the optimum conditions of β-1,3-glucanase enzymatic activity by varying the pH as well as to study the enzyme thermostability. R. oryzicola population doubled within 12 hr. During this period, 9.26 generations were obtained, with 1 hr and 29 min of interval from one generation to the other, with specific growth rate (µ) of 0.15 (hr^-1). The entire microorganism growth process was monitored during β-1,3-glucanases production, and the maximum value was obtained in the stationary phase in the 48-hr fermentation period. pH and temperature optimum values were 4.7 and 96°C, respectively. The enzyme maintained 88% of its activity when submitted to the temperature of 90°C for an incubation period of 1 hr. The results show that the enzyme can be used in industrial processes that require high temperatures and acidic pH.

Bonds broken and formed during the mixed-linkage glucan : xyloglucan endotransglucosylase reaction catalysed by Equisetum hetero-trans-β-glucanase

Mixed-linkage glucan∶xyloglucan endotransglucosylase (MXE) is one of the three activities of the recently characterised hetero-trans-β-glucanase (HTG), which among land plants is known only from Equisetum species. The biochemical details of the MXE reaction were incompletely understood - details that would promote understanding of MXE's role in vivo and enable its full technological exploitation. We investigated HTG's site of attack on one of its donor substrates, mixed-linkage (1→3),(1→4)-β-d-glucan (MLG), with radioactive oligosaccharides of xyloglucan as the acceptor substrate. Comparing three different MLG preparations, we showed that the enzyme favours those with a high content of cellotetraose blocks. The reaction products were analysed by enzymic digestion, thin-layer chromatography (TLC), high-pressure liquid chromatography (HPLC) and gel-permeation chromatography (GPC). Equisetum HTG consistently cleaved the MLG at the third consecutive β-(1→4)-bond following (towards the reducing terminus) a β-(1→3)-bond. It then formed a β-(1→4)-bond between the MLG and the non-reducing terminal glucose residue of the xyloglucan oligosaccharide, consistent with its xyloglucan endotransglucosylase/hydrolase subfamily membership. Using size-homogeneous barley MLG as the donor substrate, we showed that HTG does not favour any particular region of the MLG chain relative to the polysaccharide's reducing and non-reducing termini; rather, it selects its target cellotetraosyl unit stochastically along the MLG molecule. This work improves our understanding of how enzymes can exhibit promiscuous substrate specificities and provides the foundations to explore strategies for engineering novel substrate specificities into transglycanases.

Loop 3 of Fungal Endoglucanases of Glycoside Hydrolase Family 12 Modulates Catalytic Efficiency

Glycoside hydrolase (GH) family 12 comprises enzymes with a wide range of activities critical for the degradation of lignocellulose. However, the important roles of the loop regions of GH12 enzymes in substrate specificity and catalytic efficiency remain poorly understood. This study examined how the loop 3 region affects the enzymatic properties of GH12 glucanases using NfEG12A from Neosartorya fischeri P1 and EG (PDB 1KS4) from Aspergillus niger Acidophilic and thermophilic NfEG12A had the highest catalytic efficiency (k_cat/K_m , 3,001 and 263 ml/mg/s toward lichenin and carboxymethyl cellulose sodium [CMC-Na], respectively) known so far. Based on the multiple-sequence alignment and homology modeling, two specific sequences (FN and STTQA) were identified in the loop 3 region of GH12 endoglucanases from fungi. To determine their functions, these sequences were introduced into NfEG12A, or the counterpart sequence STTQA was removed from EG. These modifications had no effects on the optimal pH and temperature or substrate specificity but changed the catalytic efficiency (k_cat/K_m ) of these enzymes (in descending order, NfEG12A [100%], NfEG12A-FN [140%], and NfEG12A-STTQA [190%]; EG [100%] and EGΔSTTQA [41%]). Molecular docking and dynamic simulation analyses revealed that the longer loop 3 in GH12 may strengthen the hydrogen-bond interactions between the substrate and protein, thereby increasing the turnover rate (k_cat). This study provides a new insight to understand the vital roles of loop 3 for GH12 endoglucanases in catalysis.IMPORTANCE Loop structures play critical roles in the substrate specificity and catalytic hydrolysis of GH12 enzymes. Three typical loops exist in these enzymes. Loops 1 and 2 are recognized as the catalytic loops and are closely related to the substrate specificity and catalytic efficiency. Loop 3 locates in the -1 or +1 subsite and varies a lot in amino acid composition, which may play a role in catalysis. In this study, two GH12 glucanases, NfEG12A and EG, which were mutated by introducing or deleting partial loop 3 sequences FN and/or STTQA, were selected to identify the function of loop 3. It revealed that the longer loop 3 of GH12 glucanases may strengthen the hydrogen network interactions between the substrate and protein, consequently increasing the turnover rate (k_cat). This study proposes a strategy to increase the catalytic efficiency of GH12 glucanases by improving the hydrogen network between substrates and catalytic loops.

Improvement of the thermostability and catalytic efficiency of a highly active β-glucanase from Talaromyces leycettanus JCM12802 by optimizing residual charge-charge interactions

BACKGROUND

β-Glucanase is one of the most extensively used biocatalysts in biofuel, food and animal feed industries. However, the poor thermostability and low catalytic efficiency of most reported β-glucanases limit their applications. Currently, two strategies are used to overcome these bottlenecks, i.e., mining for novel enzymes from extremophiles and engineering existing enzymes.

RESULTS

A novel endo-β-1,3-1,4-glucanase of GH16 (Tlglu16A) from the thermophilic fungus Talaromyces leycettanus JCM12802 was produced in Pichia pastoris and characterized. For potential industrial applications, recombinant TlGlu16A exhibits favorable enzymatic properties over most reported glucanases, i.e., remarkable stability over a wide pH range from 1.0 to 10.0 and superior activity on glucan substrates (up to 15,197 U/mg). The only weakness of TlGlu16A is the thermolability at 65 °C and higher. To improve the thermostability, the enzyme thermal stability system was then used to engineer TlGlu16A through optimization of residual charge-charge interactions. Eleven mutants were constructed and compared to the wild-type TlGlu16A. Four mutants, H58D, E134R, D235G and D296K, showed longer half-life time at 80 °C (31, 7, 25, 22 vs. 0.5 min), and two mutants, D235G and D296K, had greater specific activities (158.2 and 122.2 %, respectively) and catalytic efficiencies (k cat/K m, 170 and 114 %, respectively).

CONCLUSIONS

The engineered TlGlu16A has great application potentials from the perspectives of enzyme yield and properties. Its thermostability and activity were apparently improved in the engineered enzymes through charge optimization. This study spans the genetic, functional and structural fields, and provides a combination of gene mining and protein engineering approaches for the systematic improvement of enzyme performance.

Cloning, Expression, and Characterization of a Thermophilic Endoglucanase, AcCel12B from Acidothermus cellulolyticus 11B

The gene ABK52392 from the thermophilic bacterium Acidothermus cellulolyticus 11B was predicted to be endoglucanase and classified into glycoside hydrolase family 12. ABK52392 encodes a protein containing a catalytic domain and a carbohydrate binding module. ABK52392 was cloned and functionally expressed in Escherichia coli. After purification by Ni-NTA agarose affinity chromatography and Q-Sepharose^® Fast Flow chromatography, the properties of the recombinant protein (AcCel12B) were characterized. AcCel12B exhibited optimal activity at pH 4.5 and 75 °C. The half-lives of AcCel12B at 60 and 70 °C were about 90 and 2 h, respectively, under acidic conditions. The specific hydrolytic activities of AcCel12B at 70 °C and pH 4.5 for sodium carboxymethylcellulose (CMC) and regenerated amorphous cellulose (RAC) were 118.3 and 104.0 U·mg⁻¹, respectively. The K_m and V_max of AcCel12B for CMC were 25.47 mg·mL⁻¹ and 131.75 U·mg⁻¹, respectively. The time course of hydrolysis for RAC was investigated by measuring reducing ends in the soluble and insoluble phases. The total hydrolysis rate rapidly decreased after the early stage of incubation and the generation of insoluble reducing ends decreased earlier than that of soluble reducing ends. High thermostability of the cellulase indicates its potential commercial significance and it could be exploited for industrial application in the future.

Purification and characterization of a novel alkaline β-1,3-1,4-glucanase (lichenase) from thermophilic fungus Malbranchea cinnamomea

A novel alkaline β-1,3-1,4-glucanase (McLic1) from a thermophilic fungus, Malbranchea cinnamomea, was purified and biochemically characterized. McLic1 was purified to homogeneity with a purification fold of 3.1 and a recovery yield of 3.7 %. The purified enzyme was most active at pH 10.0 and 55 °C, and exhibited a wide range of pH stability (pH 4.0–10.0). McLic1 displayed strict substrate specificity for barley β-glucan, oat β-glucan and lichenan, but did not show activity towards other tested polysaccharides and synthetic p-nitrophenyl derivates, suggesting that it is a specific β-1,3-1,4-glucanase. The K  m values for barley β-glucan, oat β-glucan and lichenan were determined to be 0.69, 1.11 and 0.63 mg mL−1, respectively. Moreover, the enzyme was stable in various non ionic surfactants, oxidizing agents and several commercial detergents. Thus, the alkaline β-1,3-1,4-glucanase may have potential in industrial applications, such as detergent, paper and pulp industries.

The characterization of the endoglucanase Cel12A from Gloeophyllum trabeum reveals an enzyme highly active on β-glucan

The basidiomycete fungus Gloeophyllum trabeum causes a typical brown rot and is known to use reactive oxygen species in the degradation of cellulose. The extracellular Cel12A is one of the few endo-1,4-β-glucanase produced by G. trabeum. Here we cloned cel12A and heterologously expressed it in Aspergillus niger. The identity of the resulting recombinant protein was confirmed by mass spectrometry. We used the purified GtCel12A to determine its substrate specificity and basic biochemical properties. The G. trabeum Cel12A showed highest activity on β-glucan, followed by lichenan, carboxymethylcellulose, phosphoric acid swollen cellulose, microcrystalline cellulose, and filter paper. The optimal pH and temperature for enzymatic activity were, respectively, 4.5 and 50°C on β-glucan. Under these conditions specific activity was 239.2±9.1 U mg⁻¹ and the half-life of the enzyme was 84.6±3.5 hours. Thermofluor studies revealed that the enzyme was most thermal stable at pH 3. Using β-glucan as a substrate, the K_m was 3.2±0.5 mg mL⁻¹ and the V_max was 0.41±0.02 µmol min⁻¹. Analysis of the effects of GtCel12A on oat spelt and filter paper by scanning electron microscopy revealed the morphological changes taking place during the process.

Homologous cloning, purification and characterization of highly active cellobiohydrolase I (Cel7A) from Penicillium canescens

Penicillium canescens is a filamentous fungus that normally does not secrete notable levels of cellulase activity. Cellobiohydrolase I of P. canescens (PcCel7A) was homologously cloned into a host strain RN3-11-7 (niaD-) and then expressed under the control of a strong xylA promoter. Using three steps of chromatography, PcCel7A was purified. The enzyme displayed maximum activity at pH 4.0-4.5. PcCel7A was stable at 50°C and pH 4.5 at least for 3h, while at 60°C it lost 45% of activity after 30min of incubation. When equalized by protein concentration, PcCel7A demonstrated a higher performance in prolonged hydrolysis of Avicel and milled aspen wood than CBH I (Cel7A) from Trichoderma reesei, the most industrially utilized cellulase at this moment. The high catalytic efficiency of the PcCel7A makes it a potential candidate for industrial applications.

Purification and characterization of a novel lichenase from Bacillus licheniformis GZ-2

A novel lichenase from Bacillus licheniformis GZ-2 was purified to homogeneity by two steps ion-exchange chromatography with a specific activity of 8231.3 U/mg. The purified enzyme showed as a single protein band with a molecular mass of 25 kDa. The optimum pH and temperature for the enzyme activity were 6.5 and 60 °C, respectively. The enzyme exhibited strict specificity for β-1,3-1,4-d-glucans. The kinetic parameters Km and Vmax were 5.11 mg/mL and 2097 µmol/Min/mg for lichenan and 7.42 mg/mL and 1440 µmol/Min/mg for barley β-glucan. Compared to most of the reported β-1,3-1,4-glucanases (lichenase), the activity of the purified enzyme for lichenan was much higher than that for barley β-glucan. The main products of β-glucan hydrolyzed by the lichenase were cellubiosyltriose (DP3) and cellutriosyltraose (DP4). The lichenase gene from B. licheniformis GZ-2 was cloned and sequenced. The open reading frame of gene gz-2 contained 642 bp coding for a 214 amino acid mature protein. The gene was cloned into an expression vector pET 28a and expressed in Escherichia coli BL21. The activity in cell lysate supernatant was 137.9 U/mg.

Overexpression of an acidic endo-β-1,3-1,4-glucanase in transgenic maize seed for direct utilization in animal feed

BACKGROUND

Incorporation of exogenous glucanase into animal feed is common practice to remove glucan, one of the anti-nutritional factors, for efficient nutrition absorption. The acidic endo-β-1,3-1,4-glucanase (Bgl7A) from Bispora sp. MEY-1 has excellent properties and represents a potential enzyme supplement to animal feed.

METHODOLOGY/PRINCIPAL FINDINGS

Here we successfully developed a transgenic maize producing a high level of Bgl7AM (codon modified Bgl7A) by constructing a recombinant vector driven by the embryo-specific promoter ZM-leg1A. Southern and Western blot analysis indicated the stable integration and specific expression of the transgene in maize seeds over four generations. The β-glucanase activity of the transgenic maize seeds reached up to 779,800 U/kg, about 236-fold higher than that of non-transgenic maize. The β-glucanase derived from the transgenic maize seeds had an optimal pH of 4.0 and was stable at pH 1.0-8.0, which is in agreement with the normal environment of digestive tract.

CONCLUSION/SIGNIFICANCE

Our study offers a transgenic maize line that could be directly used in animal feed without any glucanase production, purification and supplementation, consequently simplifying the feed enzyme processing procedure.

Considerations in the search for mixed-linkage (1→3),(1→4)-β-D-glucan-active endotransglycosylases

Xyloglucan endotransglucosylase, catalyzed by XTH subfamily members, is thought to play crucial roles in plant cell wall physiology. Recent discovery of endotransglycosylases active on other hemicelluloses extend our understanding of the physiological scope of endotransglycosylation in general. Discovery in Poaceaen XTHs of endotransglycosylases which act on Poaceaen-prevalent hemicelluloses, such as MLG, could reconcile the apparent incongruence between the large size of Poaceaen putative XTH families and the low xyloglucan content of their cell walls. Here, I speculate on hypothetical MLG-active endotransglycosylases and highlight potential hindrances to their discovery. It is suggested that because the location of β-(1→3) bonds within MLG oligosaccharides (MLGOs) could define their ability to act as endotranglycosylase acceptor substrates: a) thorough probing of substrate specificities necessitates the use of MLGOs created using different endo-glycanases; and b) endogenous plant exo-glycosidases, which can hinder endotranglycosylase assays by degrading acceptor substrates, might prove particularly troublesome where MLGOs are concerned.

High-level expression of a novel Penicillium endo-1,3(4)-β-d-glucanase with high specific activity in Pichia pastoris

A novel endo-1,3(4)-β-d-glucanase gene (bgl16C1) from Penicillium pinophilum C1 was cloned and sequenced. The 945-bp full-length gene encoded a 315-residue polypeptide consisting of a putative signal peptide of 18 residues and a catalytic domain belonging to glycosyl hydrolase family 16. The deduced amino acid sequence showed the highest identity (82%) with the putative endo-1,3(4)-β-glucanase from Talaromyces stipitatus ATCC 10500 and 60% identity with the characterized β-1,3(4)-glucanase from Paecilomyces sp. FLH30. The gene was successfully overexpressed in Pichia pastoris. Recombinant Bgl16C1 constituted 95% of total secreted proteins (2.61 g l−1) with activity of 28,721 U ml−1 in a 15-l fermentor. The purified recombinant Bgl16C1 had higher specific activity toward barley β-glucan (12,622 U mg−1) than all known glucanases and also showed activity against lichenan and laminarin. The enzyme was optimally active at pH 5.0 and 55°C and exhibited good stability over a broad acid and alkaline pH range (>85% activity at pH 3.0–7.0 and even 30% at pH 11.0). All these favorable enzymatic properties make it attractive for potential applications in various industries.

Purification and characterization of a novel β-1,3-1,4-glucanase (lichenase) from thermophilic Rhizomucor miehei with high specific activity and its gene sequence

Production, purification, and characterization of a novel β-1,3-1,4-glucanase (lichenase) from thermophilic Rhizomucor miehei CAU432 were investigated. High-level extracellular β-1,3-1,4-glucanase production of 6230 U/mL was obtained when oat flour (3%, w/v) was used as a carbon source at 50 °C. The crude enzyme was purified to homogeneity with a specific activity of 28818 U/mg. The molecular weight of purified enzyme was estimated to be 35.4 kDa and 33.7 kDa by SDS-PAGE and gel filtration, respectively. The optimal pH and temperature of the enzyme were pH 5.5 and 60 °C, respectively. The K(m) values of purified β-1,3-1,4-glucanase for barley β-glucan and lichenan were 2.0 mM and 1.4 mM, respectively. Furthermore, the gene (RmLic16A) encoding the β-1,3-1,4-glucanase was cloned and its deduced amino acid sequence showed the highest identity (50%) to characterized β-1,3-1,4-glucanase from Paecilomyces thermophila. The high-level production and biochemical properties of the enzyme enable its potential industrial applications.

Expression of novel β-glucanase Cel12A from Stachybotrys atra in bacterial and fungal hosts

β-glucanase Cel12A from Stachybotrys atra has been cloned and expressed in Aspergillus niger. The purified enzyme showed high activity of β-1,3-1,4-mixed glucans, was also active on carboxymethylcellulose (CMC), while it did not hydrolyze crystalline cellulose or β-1,3 glucans as laminarin. Cel12A showed a marked substrate preference for β-1,3-1,4 glucans, showing maximum activity on barley β-glucans (27.69 U mg(-1)) while the activity on CMC was much lower (0.51 U mg(-1)). Analysis by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), isoelectric focussing (IEF), and zymography showed the recombinant enzyme has apparent molecular weight of 24 kDa and a pI of 8.2. Optimal temperature and pH for enzyme activity were 50°C and pH 6.5. Thin layer chromatography analysis showed that major hydrolysis products from barley β-glucan and lichean were 3-O-β-cellotriosyl-D-glucose and 3-O-β-cellobiosyl-D-glucose, while glucose and cellobiose were released in smaller amounts. The amino acid sequence deduced from cel12A revealed that it is a single domain enzyme belonging to the GH12 family, a family that contains several endoglucanases with substrate preference for β-1,3-1,4 glucans. We believe that S. atra Cel12A should be considered as a lichenase-like or nontypical endoglucanase.

Purification, molecular cloning, and enzymatic properties of a family 12 endoglucanase (EG-II) from fomitopsis palustris: role of EG-II in larch holocellulose hydrolysis

A family 12 endoglucanase with a molecular mass of 23,926 Da (EG-II) from the brown-rot basidiomycete Fomitopsis palustris was purified and characterized. One of the roles of EG-II in wood degradation is thought to be to loosen the polysaccharide network in cell walls by disentangling hemicelluloses that are associated with cellulose.

Biochemical characterization of a novel thermostable beta-1,3-1,4-glucanase (lichenase) from Paecilomyces thermophila

The purification and characterization of a novel extracellular beta-1,3-1,4-glucanase from the thermophilic fungus Paecilomyces thermophila J18 were studied. The strain produced the maximum level of extracellular beta-glucanase (135.6 U mL(-1)) when grown in a medium containing corncob (5%, w/v) at 50 degrees C for 4 days. The crude enzyme solution was purified by 122.5-fold with an apparent homogeneity and a recovery yield of 8.9%. The purified enzyme showed as a single protein band on SDS-PAGE with a molecular mass of 38.6 kDa. The molecular masses were 34.6 kDa and 31692.9 Da when detected by gel filtration and mass spectrometry, respectively, suggesting that it is a monomeric protein. The enzyme was a glycoprotein with a carbohydrate content of 19.0% (w/w). Its N-terminal sequence of 10 amino acid residues was determined as H2N-A(?)GYVSNIVVN. The purified enzyme was optimally active at pH 7.0 and 70 degrees C. It was stable within pH range 4.0-10.0 and up to 65 degrees C, respectively. Substrate specificity studies revealed that the enzyme is a true beta-1,3-1,4-D-glucanase. The K m values determined for barley beta-D-glucan and lichenan were 2.46 and 1.82 mg mL(-1), respectively. The enzyme hydrolyzed barley beta-D-glucan and lichenan to yield bisaccharide, trisaccharide, and tetrasaccharide as the main products. Circular dichroism studies indicated that the protein contains 28% alpha-helix, 24% beta-sheet, and 48% random coil. Circular dichroism spectroscopy is also used to investigate the thermostability of the purified enzyme. This is the first report on the purification and characterization of a beta-1,3-1,4-glucanase from Paecilomyces sp. These properties make the enzyme highly suitable for industrial applications.

Mixed-linkage (1-->3,1-->4)-beta-D-glucan is a major hemicellulose of Equisetum (horsetail) cell walls

Mixed-linkage (1-->3,1-->4)-beta-d-glucan (MLG) is a hemicellulose reputedly confined to certain Poales. Here, the taxonomic distribution of MLG, and xyloglucan, especially in early-diverging pteridophytes, has been re-investigated. Polysaccharides were digested with lichenase and xyloglucan endoglucanase (XEG), which specifically hydrolyse MLG and xyloglucan, respectively. The oligosaccharides produced were analysed by thin-layer chromatography (TLC), high-pressure liquid chromatography (HPLC) and alkaline peeling. Lichenase yielded oligo-beta-glucans from all Equisetum species tested (Equisetum arvense, Equisetum fluviatile, Equisetum scirpoides, Equisetum sylvaticum and Equisetum xtrachyodon). The major product was the tetrasaccharide beta-glucosyl-(1-->4)-beta-glucosyl-(1-->4)-beta-glucosyl-(1-->3)-glucose (G4G4G3G), which was converted to cellotriose by alkali, confirming its structure. Minor products included G3G, G4G3G and a nonasaccharide. By contrast, poalean MLGs yielded G4G3G > G4G4G3G > nonasaccharide > dodecasaccharide. No other pteridophytes tested contained MLG, including Psilotum and eusporangiate ferns. No MLG was found in lycopodiophytes, bryophytes, Chara or Nitella. XEG digestion showed that Equisetum xyloglucan has unusual repeat units. Equisetum, an exceedingly isolated genus whose closest living relatives diverged > 380 million years ago, has evolved MLG independently of the Poales. Equisetum and poalean MLGs share basic structural motifs but also exhibit clear-cut differences. Equisetum MLG is firmly wall-bound, and may tether neighbouring microfibrils. It is also suggested that MLG acts as a template for silica deposition, characteristic of grasses and horsetails.

Production, purification and application-relevant characterisation of an endo-1,3(4)-β-glucanase from Rhizomucor miehei

Growth on a wheat bran media induced production of an extracellular beta-glucanase by Rhizomucor miehei (DSM 1330). The enzyme was purified to homogeneity. Substrate specificity studies coupled with protein database similarity searching using mass spectrometry-derived sequence data indicate it to be an endo-1,3(4)-beta-glucanase (EC 3.2.1.6). The enzyme was characterised in terms of potential suitability for use in animal (poultry) feed. Significant activity was observed over the entire pH range typical of the avian upper digestive tract (pH 2.6-6.5). The enzyme was also found to be more thermostable than current commercialized beta-glucanases, particularly when heated at a high enzyme concentration, and retained twice as much residual activity as the latter upon exposure to simulated avian digestive tract conditions. There are no previous reports of the production, purification or characterization of a beta-glucanase from a Rhizomucor, and the enzyme's application-relevant physicochemical characteristics render it potentially suited for use in animal feed.