Mode of action of endoglucanase III from Trichoderma reesei

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.

Glycosyl hydrolases family 5, subfamily 5: Relevance and structural insights for designing improved biomass degrading cocktails

Endoglucanases are carbohydrate-degrading enzymes widely used for bioethanol production as part of the enzymatic cocktail. However, family 5 subfamily 5 (GH5_5) endoglucanases are still poorly explored in depth. The Trichoderma reesei representative is the most studied enzyme, presenting catalytic activity in acidic media and mild temperature conditions. Though biochemically similar, its modular structure and synergy with other components vary greatly compared to other GH5_5 members and there is still a lack of specific studies regarding their interaction with other cellulases and application on novel and better mixtures. In this regard, the threedimensional structure elucidation is a highly valuable tool to both uncover basic catalytic mechanisms and implement engineering techniques, proved by the high success rate GH5_5 endoglucanases show. GH5_5 enzymes must be carefully evaluated to fully uncover their potential in biomass-degrading cocktails: the optimal industrial conditions, synergy with other cellulases, structural studies, and enzyme engineering approaches. We aimed to provide the current understanding of these main topics, collecting all available information about characterized GH5_5 endoglucanases function, structure, and bench experiments, in order to suggest future directions to a better application of these enzymes in the industry.

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.

Saccharification of rice straw by cellulase from a local Trichoderma harzianum SNRS3 for biobutanol production

BACKGROUND

Rice straw has shown to be a promising agricultural by-product in the bioconversion of biomass to value-added products. Hydrolysis of cellulose, a main constituent of lignocellulosic biomass, is a requirement for fermentable sugar production and its subsequent bioconversion to biofuels such as biobutanol. The high cost of commercial enzymes is a major impediment to the industrial application of cellulases. Therefore, the use of local microbial enzymes has been suggested. Trichoderma harzianum strains are potential CMCase and β-glucosidase producers. However, few researches have been reported on cellulase production by T. harzianum and the subsequent use of the crude cellulase for cellulose enzymatic hydrolysis. For cellulose hydrolysis to be efficiently performed, the presence of the whole set of cellulase components including exoglucanase, endoglucanase, and β-glucosidase at a considerable concentration is required. Biomass recalcitrance is also a bottleneck in the bioconversion of agricultural residues to value-added products. An effective pretreatment could be of central significance in the bioconversion of biomass to biofuels.

RESULTS

Rice straw pretreated using various concentrations of NaOH was subjected to enzymatic hydrolysis. The saccharification of rice straw pretreated with 2% (w/v) NaOH using crude cellulase from local T. harzianum SNRS3 resulted in the production of 29.87 g/L reducing sugar and a yield of 0.6 g/g substrate. The use of rice straw hydrolysate as carbon source for biobutanol fermentation by Clostridium acetobutylicum ATCC 824 resulted in an ABE yield, ABE productivity, and biobutanol yield of 0.27 g/g glucose, 0.04 g/L/h and 0.16 g/g glucose, respectively. As a potential β-glucosidase producer, T. harzianum SNRS3 used in this study was able to produce β-glucosidase at the activity of 173.71 U/g substrate. However, for cellulose hydrolysis to be efficient, Filter Paper Activity at a considerable concentration is also required to initiate the hydrolytic reaction. According to the results of our study, FPase is a major component of cellulose hydrolytic enzyme complex system and the reducing sugar rate-limiting enzyme.

CONCLUSION

Our study revealed that rice straw hydrolysate served as a potential substrate for biobutanol production and FPase is a rate-limiting enzyme in saccharification.

Asn124 of Cel5A from Hypocrea jecorina not only provides the N-glycosylation site but is also essential in maintaining enzymatic activity

To investigate the function of N-glycosylation of Cel5A (endoglucanase II) from Hypocrea jecorina, two N-glycosylation site deletion Cel5A mutants (rN124D and rN124H) were expressed in Saccharomyces cerevisiae. The weights of these recombinant mutants were 54 kDa, which were lower than that of rCel5A. This result was expected to be attributed to deglycosylation. The enzyme activity of rN124H was greatly reduced to 60.6% compared with rCel5A, whereas rN124D showed slightly lower activity (10%) than that of rCel5A. rN124D and rN124H showed different thermal stabilities compared with the glycosylated rCel5A, especially at lower pH value. Thermal stabilities were reduced and improved for rN124D and rN124H, respectively. Circular dichroism spectroscopy showed that the modification of secondary structure by mutation may be the reason for the change in enzymatic activity and thermal stability. [BMB Reports 2014; 47(5): 256-261]

Cloning and identification of novel hydrolase genes from a dairy cow rumen metagenomic library and characterization of a cellulase gene

BACKGROUND

Interest in cellulose degrading enzymes has increased in recent years due to the expansion of the ellulosic biofuel industry. The rumen is a highly adapted environment for the degradation of cellulose and a promising source of enzymes for industrial use. To identify cellulase enzymes that may be of such use we have undertaken a functional metagenomic screen to identify cellulase enzymes from the bacterial community in the rumen of a grass-hay fed dairy cow.

RESULTS

Twenty five clones specifying cellulose activity were identified. Subcloning and sequence analysis of a subset of these hydrolase-positive clones identified 10 endoglucanase genes. Preliminary characterization of the encoded cellulases was carried out using crude extracts of each of the subclones. Zymogram analysis using carboxymethylcellulose as a substrate showed a single positive band for each subclone, confirming that only one functional cellulase gene was present in each. One cellulase gene, designated Cel14b22, was expressed at a high level in Escherichia coli and purified for further characterization. The purified recombinant enzyme showed optimal activity at pH 6.0 and 50°C. It was stable over a broad pH range, from pH 4.0 to 10.0. The activity was significantly enhanced by Mn2+ and dramatically reduced by Fe3+ or Cu2+. The enzyme hydrolyzed a wide range of beta-1,3-, and beta-1,4-linked polysaccharides, with varying activities. Activities toward microcrystalline cellulose and filter paper were relatively high, while the highest activity was toward Oat Gum.

CONCLUSION

The present study shows that a functional metagenomic approach can be used to isolate previously uncharacterized cellulases from the rumen environment.

Two structurally discrete GH7-cellobiohydrolases compete for the same cellulosic substrate fiber

BACKGROUND

Cellulose consisting of arrays of linear beta-1,4 linked glucans, is the most abundant carbon-containing polymer present in biomass. Recalcitrance of crystalline cellulose towards enzymatic degradation is widely reported and is the result of intra- and inter-molecular hydrogen bonds within and among the linear glucans. Cellobiohydrolases are enzymes that attack crystalline cellulose. Here we report on two forms of glycosyl hydrolase family 7 cellobiohydrolases common to all Aspergillii that attack Avicel, cotton cellulose and other forms of crystalline cellulose.

RESULTS

Cellobiohydrolases Cbh1 and CelD have similar catalytic domains but only Cbh1 contains a carbohydrate-binding domain (CBD) that binds to cellulose. Structural superpositioning of Cbh1 and CelD on the Talaromyces emersonii Cel7A 3-dimensional structure, identifies the typical tunnel-like catalytic active site while Cbh1 shows an additional loop that partially obstructs the substrate-fitting channel. CelD does not have a CBD and shows a four amino acid residue deletion on the tunnel-obstructing loop providing a continuous opening in the absence of a CBD. Cbh1 and CelD are catalytically functional and while specific activity against Avicel is 7.7 and 0.5 U.mg prot-1, respectively specific activity on pNPC is virtually identical. Cbh1 is slightly more stable to thermal inactivation compared to CelD and is much less sensitive to glucose inhibition suggesting that an open tunnel configuration, or absence of a CBD, alters the way the catalytic domain interacts with the substrate. Cbh1 and CelD enzyme mixtures on crystalline cellulosic substrates show a strong combinatorial effort response for mixtures where Cbh1 is present in 2:1 or 4:1 molar excess. When CelD was overrepresented the combinatorial effort could only be partially overcome. CelD appears to bind and hydrolyze only loose cellulosic chains while Cbh1 is capable of opening new cellulosic substrate molecules away from the cellulosic fiber.

CONCLUSION

Cellobiohydrolases both with and without a CBD occur in most fungal genomes where both enzymes are secreted, and likely participate in cellulose degradation. The fact that only Cbh1 binds to the substrate and in combination with CelD exhibits strong synergy only when Cbh1 is present in excess, suggests that Cbh1 unties enough chains from cellulose fibers, thus enabling processive access of CelD.

Crystallization and preliminary X-ray diffraction analysis of endoglucanase III from Trichoderma harzianum

Endoglucanases are enzymes that hydrolyze cellulose and are important components of the cellulolytic complex. In contrast to other members of the complex, they cleave internal β-1,4-glycosidic bonds in the cellulose polymer, allowing cellulose to be used as an energy source. Since biomass is an important renewable source of energy, the structural and functional characterization of these enzymes is of interest. In this study, endoglucanase III from Trichoderma harzianum was produced in Pichia pastoris and purified. Crystals belonging to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 47.54, b = 55.57, c = 157.3 Å, were obtained by the sitting-drop vapour-diffusion method and an X-ray diffraction data set was collected to 2.07 Å resolution.

Optimization of a synthetic mixture composed of major Trichoderma reesei enzymes for the hydrolysis of steam-exploded wheat straw

BACKGROUND

An efficient hydrolysis of lignocellulosic substrates to soluble sugars for biofuel production necessitates the interplay and synergistic interaction of multiple enzymes. An optimized enzyme mixture is crucial for reduced cost of the enzymatic hydrolysis step in a bioethanol production process and its composition will depend on the substrate and type of pretreatment used. In the present study, an experimental design was used to determine the optimal composition of a Trichoderma reesei enzyme mixture, comprising the main cellulase and hemicellulase activities, for the hydrolysis of steam-exploded wheat straw.

METHODS

Six enzymes, CBH1 (Cel7a), CBH2 (Cel6a), EG1 (Cel7b), EG2 (Cel5a), as well as the xyloglucanase Cel74a and the xylanase XYN1 (Xyl11a) were purified from a T. reesei culture under lactose/xylose-induced conditions. Sugar release was followed in milliliter-scale hydrolysis assays for 48 hours and the influence of the mixture on initial conversion rates and final yields is assessed.

RESULTS

The developed model could show that both responses were strongly correlated. Model predictions suggest that optimal hydrolysis yields can be obtained over a wide range of CBH1 to CBH2 ratios, but necessitates a high proportion of EG1 (13% to 25%) which cannot be replaced by EG2. Whereas 5% to 10% of the latter enzyme and a xylanase content above 6% are required for highest yields, these enzymes are predicted to be less important in the initial stage of hydrolysis.

CONCLUSIONS

The developed model could reliably predict hydrolysis yields of enzyme mixtures in the studied domain and highlighted the importance of the respective enzyme components in both the initial and the final hydrolysis phase of steam-exploded wheat straw.

High-temperature enzymatic breakdown of cellulose

Cellulose is an abundant and renewable biopolymer that can be used for biofuel generation; however, structural entrapment with other cell wall components hinders enzyme-substrate interactions, a key bottleneck for ethanol production. Biomass is routinely subjected to treatments that facilitate cellulase-cellulose contacts. Cellulases and glucosidases act by hydrolyzing glycosidic bonds of linear glucose β-1,4-linked polymers, producing glucose. Here we describe eight high-temperature-operating cellulases (TCel enzymes) identified from a survey of thermobacterial and archaeal genomes. Three TCel enzymes preferentially hydrolyzed soluble cellulose, while two preferred insoluble cellulose such as cotton linters and filter paper. TCel enzymes had temperature optima ranging from 85°C to 102°C. TCel enzymes were stable, retaining 80% of initial activity after 120 h at 85°C. Two modes of cellulose breakdown, i.e., with endo- and exo-acting glucanases, were detected, and with two-enzyme combinations at 85°C, synergistic cellulase activity was observed for some enzyme combinations.

Adsorption of monocomponent enzymes in enzyme mixture analyzed quantitatively during hydrolysis of lignocellulose substrates

The adsorption of purified Trichoderma reesei cellulases (TrCel7A, TrCel6A and TrCel5A) and xylanase TrXyn11 and Aspergillus niger β-glucosidase AnCel3A was studied in enzyme mixture during hydrolysis of two pretreated lignocellulosic materials, steam pretreated and catalytically delignified spruce, along with microcrystalline cellulose (Avicel). The enzyme mixture was compiled to resemble the composition of commercial cellulase preparations. The hydrolysis was carried out at 35 °C to mimic the temperature of the simultaneous saccharification and fermentation (SSF). Enzyme adsorption was followed by analyzing the activity and the protein amount of the individual free enzymes in the hydrolysis supernatant. Most enzymes adsorbed quickly at early stages of the hydrolysis and remained bound throughout the hydrolysis, although the conversion reached was fairly high. Only with the catalytically oxidized spruce samples, the bound enzymes started to be released as the hydrolysis degree reached 80%. The results based on enzyme activities and protein assay were in good accordance.

Cloning and heterologous expression of a novel endoglucanase gene egVIII from Trichoderma viride in Saccharomyces cerevisiae

Endoglucanase is a major cellulolytic enzyme produced by the fungus Trichoderma viride. The 1,317 bp cDNA of endoglucanase gene egVIII was cloned from T. viride AS3.3711, encoding a 438 amino acid protein with a calculated molecular mass of 46.86 kDa and isoelectric point of 4.32. Sequence analysis suggested that EGVIII belonged to the glycosyl hydrolase family 5. The N-terminal region of EGVIII contains a signal peptide sequence of 19 amino acid residues, indicating that it is an extracellular enzyme. Transcription of the egVIII gene in T. viride AS3.3711 can be induced by carboxymethyl cellulose sodium (CMC-Na), sucrose, microcrystalline cellulose, and corn stalk, and inhibited by glucose and fructose. The alpha-mating factor signal can effectively enhance the secretion of the recombinant EGVIII in Saccharomyces cerevisiae, as demonstrated by the enzymatic activity of recombinant yeast IpYEMalpha-xegVIII in the supernatant, which was 0.86 times higher than that of the IpYES2-egVIII. Recombinant endoglucanase EGVIII showed optimal activity at a temperature of 60 degrees C and pH of 6.0. It was stable when incubated from 35 degrees C to 70 degrees C for 1 h. The enzymatic activity of recombinant EGVIII was stable at a pH 3.0 to 7.5 at 50 degrees C and reached the highest level at 0.174U when activated by 75 mM of Zn(2+). The Michaelis-Menten constant (Km) and Kcat values for CMC-Na and cellotriose hydrolysis were 3.82 mg/ml, 9.56 s(-1) and 1.75 mg/ml, 7.08 s(-1), respectively. Transgenic yeast strain IpYEMalpha-xegVIII might be useful for renewable fuels industries.

Surface character of pulp fibres studied using endoglucanases

The endoglucanase Cel5A from Trichoderma reesei and an endoglucanase from Aspergillus sp. (Novozym 476 from Novozyme A/S) were evaluated as probes for the surface properties of soft- and hardwood chemical pulp fibres. The hydrolysis time curves were in accordance with a two-phase degradation model described by a biexponential function. The kinetic parameters corresponding to the amount of fast and slow degraded parts of the substrate correlated to tensile index, relative bonded area and z-strength of the paper. All paper properties showing a correlation with enzyme kinetic parameters were related to fibre-fibre interactions. Fluorescence labelling of the reducing end groups in pulp fibres followed by enzyme treatment indicated that the fast substrate class corresponds to the population of "loose" cellulose chain ends not tightly associated with the bulk cellulose. The correlation between the parameters of enzyme kinetics and mechanical properties of the paper produced from the corresponding pulp found in this study should allow a rapid evaluation of the raw fibre material used in paper making process.

Directed evolution for engineering pH profile of endoglucanase III from Trichoderma reesei

The potential of cellulase has been revealed not only in biomass conversion but also in various industrial processes, including food, textiles, laundry, pulp, and paper. Due to the need for alkali-tolerant cellulase with high specific activity at alkaline pH, for example, for application in detergent industry an error-prone PCR approach was employed for enhancing the alkali-tolerant ability of endoglucanase III (EG III) from Trichoderma reesei by error-prone PCR. One mutant (N321T) which exhibited an optimal activity at pH 5.4, corresponded to a basic shift of 0.6 pH unit compared to the wild-type enzyme, was selected and characterized. In addition, two site-directed mutations, N321D and N321H, were designed to study the role of residue at position 321. As expected, the N321D mutation changed enzyme's optimal activity to pH 4.0, resulting in a large decrease in the specific activity. However, the N321H mutated enzyme was active over a broader pH range compared to the wild type, with no much change in the specific activity. These properties suggest that the residue at position 321 is important amino acid residue in determining the pH activity profile of the EG III from T. reesei.

Biochemical characterization and mode of action of a thermostable endoglucanase purified from Thermoascus aurantiacus

A major extracellular endoglucanase purified to homogeneity from Thermoascus aurantiacus had a M(r) of 34 kDa and a pI of 3.7 and was optimally active at 70-80 degrees C and pH 4.0-4.4. It was stable at pH 2.8-6.8 at 50 degrees C for 48 h and maintained its secondary structure and folded conformation up to 70 degrees C at pH 5.0 and 2.8, respectively. A 33-amino acid sequence at the N terminus showed considerable homology with 14 microbial endoglucanases having highly conserved 8 amino acids (positions 10-17) and Gly, Pro, Gly, and Pro at positions 8, 22, 23, and 32, respectively. The enzyme is rich in Asp (15%) and Glu (10%) with a carbohydrate content of 2.7%. Polyclonal antibodies of endoglucanase cross-reacted with their own antigen and with other purified cellulases from T. aurantiacus. The endoglucanase was specific for polymeric substrates with highest activity toward carboxymethyl cellulose followed by barley beta-glucan and lichenan. It preferentially cleaved the internal glycosidic bonds of Glc(n) and MeUmbGlc(n) and possessed an extended substrate-binding site with five subsites. The data indicate that the endoglucanase from T. aurantiacus is a member of glycoside hydrolase family 5.

Biochemical characterization of MI-ENG1, a family 5 endoglucanase secreted by the root-knot nematode Meloidogyne incognita

A beta-1,4-endoglucanase named MI-ENG1, homologous to the family 5 glycoside hydrolases, was previously isolated from the plant parasitic root-knot nematode Meloidogyne incognita. We describe here the detection of the enzyme in the nematode homogenate and secretion and its complete biochemical characterization. This study is the first comparison of the enzymatic properties of an animal glycoside hydrolase with plant and microbial enzymes. MI-ENG1 shares many enzymatic properties with known endoglucanases from plants, free-living or rumen-associated microorganisms and phytopathogens. In spite of the presence of a cellulose-binding domain at the C-terminus, the ability of MI-ENG1 to bind cellulose could not be demonstrated, whatever the experimental conditions used. The biochemical characterization of the enzyme is a first step towards the understanding of the molecular events taking place during the plant-nematode interaction.

Overproduction of recombinant Trichoderma reesei cellulases by Aspergillus oryzae and their enzymatic properties

We have established an expression system of Trichoderma reesei cellulase genes using Aspergillus oryzae as a host. In this system, the expression of T. reesei cellulase genes were regulated under the control of A. oryzae Taka-amylase promoter and the cellulase genes were highly expressed when maltose was used as a main carbon source for inducer. The production of recombinant cellulases by A. oryzae transformants reached a maximum after 3-4 days of cultivation. In some cases, proteolysis of recombinant cellulases was observed in the late stage of cultivation. The recombinant cellulases were purified and characterized. The apparent molecular weights of recombinant cellulases were more or less larger than those of native enzymes. The optimal temperatures and pHs of recombinant cellulases were 50-70 degrees C and 4-5, respectively. Among the recombinant cellulases, endoglucanase I showed broad substrate specificities and high activity when compared with the other cellulases investigated here.

Stability and identification of active-site residues of carboxymethylcellulases from Aspergillus niger and Cellulomonas biazotea

Determination of the apparent pKa's of purified carboxymethylcellulases from Aspergillus niger and Cellulomonas biazotea at different temperatures and in the presence of dioxane indicated two side chain carboxyl groups which controlled the limiting rate in both organisms. The thermostability of both enzymes slightly decreased with increasing pH from 5 to 75 but was unaffected in the presence of 0.5 mmol/L Mn2+. The CMCase from C. biazotea had an activation energy of 35 kJ/mol and a half-life of 89 min in the presence of 8 mol/L urea at 40 degrees C. The half-life of CMCase from A. niger in 8 mol/L urea and at 37 degrees C was 125 min as determined by a 0-9 mol/L transverse urea gradient PAGE. The CMCases from A. niger and C. biazotea had the same thermostabilities in the absence of CMC although the enzyme from the former was more thermostable in the presence of the substrate. The CMCase from A. niger was also more efficient in hydrolyzing CMC than the enzyme from C. biazotea.

Definition of the substrate specificity of the 'sensing' xylanase of Streptomyces cyaneus using xylooligosaccharide and cellooligosaccharide glycosides of 3,4-dinitrophenol

The title compounds, (Xylp beta (1-->4))nXylp beta-3,4-DNP (n = 0-4) have been made by selective anomeric deprotection of peracetylated xylose oligosaccharides with hydrazine, followed by formation of the trichloroacetimidate, uncatalysed reaction with 3,4-dinitrophenol, and Zemplén deacetylation. The values of k(cat)/K(m) for 3,4-dinitrophenol release from these substrates by xylanase III of Streptomyces cyaneus, expressed in Escherichia coli, increase with increasing n up to n = 2 and then slightly decrease. Since it is known from previous work that in its normal host, the enzyme is produced constitutively at low levels and excreted, these results suggest that the biological function of the enzyme may be to produce small molecule inducers, predominantly xylotriose, from the non-reducing end of the xylan. Activity on cellooligosaccharide glycosides (Glcp beta (1-->4))nGlcp beta-3,4-DNP (n = 0-3) was detected, at a rate about two-and-a-half orders of magnitude less than that observed on the corresponding xylooligosaccharides, indicating that the enzyme is a true xylanase.

Action patterns and mapping of the substrate-binding regions of endo-(1-->5)-alpha-L-arabinanases from Aspergillus niger and Aspergillus aculeatus

The substrate binding sites of endo-(1-->5)-alpha-L-arabinanases (EC 3.2.1.99) from Aspergillus niger and Aspergillus aculeatus were investigated using reduced and regular (1-->5)-alpha-L-arabino-oligosaccharides and high performance anion exchange chromatographic analysis. Calculation of bond cleavage frequencies and kcat/K(m) parameters for these substrates enabled the determination of the number of arabinofuranosyl binding subsites and the estimation of the binding affinities of each subsite. The A. aculeatus endo-arabinanase has six subsites arranged symmetrically around the catalytic site, while the A. niger endo-arabinanase has five subsites; two from the catalytic site towards the non-reducing end of the bound substrate and three toward the reducing end. The two subsites directly adjacent to the catalytic sites in both the A. niger and A. aculeatus endo-arabinanase have near-zero net free energy of binding. These results are unlike most glycopyranosyl endo-hydrolases studied which have net negative (unfavourable) energies of interaction at these two subsites, and may be related to the greater conformational flexibility of arabinofuranosyl residues than glycopyranosyl residues. The complete subsite maps are also rationalized with regard to the observed action patterns of these enzymes on linear (1-->5)-alpha-L-arabinan.

Role of four major cellulases in triggering of cellulase gene expression by cellulose in Trichoderma reesei

The relative contributions of four major cellulases of Trichoderma reesei (1,4-beta-D-glucan cellobiohydrolase I [CBH I], CBH II, endo-1,4-beta-D-glucanase I [EG I], and EG II) to the generation of the cellulase inducer from cellulose were studied with isogenic strains in which the corresponding genes (cbh1, cbh2, egl1, and egl2) had been deleted by insertion of the Aspergillus nidulans amdS marker gene. During growth on lactose (a soluble carbon source provoking cellulase gene expression), these strains showed no significant alterations in their ability to express the respective other cellulase genes, with the exception of the strain containing delta cbh1, which exhibited an increased steady-state level of cbh2 mRNA. On crystalline cellulose as the only carbon source, however, significant differences were apparent: strains in which cbh2 and egl2, respectively, had been deleted showed no expression of the other cellulase genes, whereas strains carrying the cbh1 or egl1 deletion showed these transcripts. The delta cbh1-containing strain also showed enhanced cbh2 mRNA levels under these conditions. A strain in which both cbh1 and cbh2 had been deleted, however, was unable to initiate growth on cellulose. Addition of 2 mM sophorose, a putative inducer of cellulase gene expression, to such cultures induced the transcription of egl1 and egl2 and restored the ability to grow on cellulose. We conclude that CBH II and EG II are of major importance for the efficient formation of the inducer from cellulose in T. reesei and that removal of both cellobiohydrolases renders T. reesei unable to attack crystalline cellulose.

Substrate specificity of endoglucanases: what determines xyloglucanase activity?

Endoglucanases from Trichoderma viride differ in their activity and mode of action towards xyloglucans. In order to explain the basis for their different behavior, the number of substrate-binding sites of three endoglucanases (endoI, endoIV, and endoV) were determined using bond cleavage frequencies of both normal and reduced cellodextrins and Ko/K(m). EndoIV differed from other endoglucanases described so far, in having at least nine putative binding sites. The specificities of the three endoglucanases towards various xyloglucans derived from apple fruit and potato were determined. Also, the release of oligosaccharides from these substrates in time was monitored. It was concluded that the endoglucanases prefer to bind unbranched glucosyl residues. Because most xyloglucans are composed of XXXG-type of building units, distant subsites are needed to bind xyloglucan. Having at least nine substrate-binding sites, endoIV seems to be well equipped to degrade xyloglucans which was confirmed by its high xyloglucanase activity.

Family A cellulases: two essential tryptophan residues in endoglucanase III from Trichoderma reesei

Three tryptophan residues are readily oxidised by N-bromosuccinimide in endoglucanase III from Trichoderma reesei. Evidence was obtained that the residue first modified is situated in the cellulose-binding domain and the second in the enzyme's catalytic site. The latter influences the binding and hydrolysis of soluble substrates. The modification of a third residue does not further affect the catalytic properties. The present results complement published data concerning other identified catalytic residues, and help to clarify the active site structure of family A cellulases.

Cloning, characterization and functional expression of an endoglucanase-encoding gene from the phytopathogenic fungus Macrophomina phaseolina

An endoglucanase-encoding clone (egl2) was isolated from the phytopathogenic soilborne deuteromycete fungus Macrophomina phaseolina (Mp). Clones were obtained from a cDNA library by functional expression in Escherichia coli. The egl2 clone hybridized to a 1.3-kb mRNA. Expression is induced by carboxymethylcellulose (CMC) and repressed by glucose. The deduced amino acid (aa) sequence revealed strong similarity to the egl3 from Trichoderma reesei (Tr) (72% for identical residues and 81% with conservative substitution over a span of 324 aa). The Mp egl2 lacks the cellulose-binding domain and linker region found in the Tr egl3. Different codon usage between the two fungi resulted in a much shorter span of nucleotide homology. The Egl2 protein cleaves cellodextrins with continguous beta, 1-4 linkages of four and larger, and shows activity against CMC and birchwood xylan.

A unique endoglucanase-encoding gene cloned from the phytopathogenic fungus Macrophomina phaseolina

The deduced amino acid sequence derived from a Macrophomina phaseolina beta-1,4-endoglucanase-encoding gene revealed 48% identity (over 119 amino acids) with egl1 from the phytopathogen Pseudomonas solanacearum. Its similarity to saprophyte endoglucanases was not significant. Its minimum substrate size, unlike that of any known saprophyte endoglucanase, was cellopentaose. The unique characteristics of M. phaseolina egl1-encoded endoglucanase suggest that it is phytopathogen specific.

Isolation of four major subunits from Clostridium thermocellum cellulosome and their synergism in the hydrolysis of crystalline cellulose

The cellulosome of Clostridium thermocellum, purified by affinity chromatography, was dissociated under mild conditions and separated by SDS-PAGE. Two major p-nitrophenylcellobiosidases (PNPCases I and II) corresponding to the S5 (103 kDa) and S8 (78 kDa) subunits and one major carboxymethylcellulase (CMCase) coinciding with the S11 (60.5 kDa) subunit were isolated and characterized using carboxymethylcellulose (CMC), H3PO4-swollen cellulose and cello-oligosaccharides. Both PNPCases showed little effect on the viscosity of CMC and released twice as much total sugar as reducing sugar from H3PO4-swollen cellulose. The CMCase released ten times more total sugar than reducing sugar from H3PO4-swollen cellulose and reduced the viscosity of CMC rapidly. None of these enzymes was active on cellotriose. Both PNPCases released cellobiose from cellotetraose, and cellobiose and cellotriose from cellopentaose. In contrast, CMCase was active only on cellopentaose and released mainly glucose. Use of MeUmb(Glc)n revealed that both PNPCases cleaved preferentially either the second or fourth linkage from the non-reducing end while the CMCase was specific for the internal glycosidic bonds. Thus, the PNPCases and CMCase behaved as typical exo- and endoglucanases, respectively. When tested individually, all three enzymes degraded Avicel only to a small extent. A 1.5-2.0-fold increase in sugar release was observed when CMCase was combined with either PNPCase I, II or both. Combining S1 with either PNPCase II or CMCase resulted in fourfold synergism in the hydrolysis of Avicel. Synergism was sevenfold when all three enzymes were combined with S1.

Cellulose hydrolysis by the cellulases from Trichoderma reesei: adsorptions of two cellobiohydrolases, two endocellulases and their core proteins on filter paper and their relation to hydrolysis

Separate binding of several purified cellulolytic components of Trichoderma reesei on to filter paper was studied and concomitant hydrolysis rates evaluated. Enhancement of mass transfer from the bulk liquid to the solid substrate by agitation has two different effects on adsorption depending on the type of enzyme: (i) the fraction of cellobiohydrolase II (CBH II) and endoglucanase III (EG III) bound at equilibrium is increased, whereas (ii) the rate but not the extent of cellobiohydrolase I (CBH I) and endoglucanase I (EG I) adsorption is affected. The adsorption of CBH I core, a component lacking the cellulose-binding domain (CBD), is, however, not significantly influenced by mass transfer. The CBH I interdomain peptide (present in CBH I core b) does not participate in adsorption but enhances stability. The adsorption of CBH I core proteins is a fully reversible process whereas that of the intact CBH I is not. Thus, the interaction of the CBD with filter paper apparently accounts for the mass-transfer-limited binding rate and also for the irreversible adsorption of intact CBH I. Adsorption isotherms at 50 degrees C indicate very similar relative association constants for the intact cellulases (0.24-0.30 l/g of cellulose), but drastically reduced values for CBH I core proteins (0.03 l/g of cellulose). The specific activities of adsorbed CBH I and of its core proteins are identical and a linear relationship between adsorption and rates of hydrolysis is found only for these enzymes. Thus, non-productive binding on to cellulose seems evident in the case of CBH II and EG III but not CBH I.

Cellulose hydrolysis by the cellulases from Trichoderma reesei: a new model for synergistic interaction

The hydrolysis of Whatman no. 1 filter paper by purified cellulolytic components from Trichoderma reesei and the synergistic action of binary combinations of these enzymes on the same substrate were investigated. At 20 milligrams filter paper, enzyme concentrations needed to obtain half-maximal hydrolysis rates (KE values) were in the 3-4 microM range for the cellobiohydrolases (CBHs) and 0.05-0.10 microM for the endoglucanases (EGs). Catalytic-core proteins of CBH I and EG III, lacking the cellulose-binding domain, exhibit KE values 2.3 and 5.1 times higher than those of the intact enzymes. In synergistic combinations of two cellulases, the KE value of at least one enzyme was 3-10-fold reduced. CBH I/CBH II and CBH I/EG III combinations showed the most powerful synergism, and optimal ratios were a function of the total protein concentration. Results obtained in activity and adsorption assays using filter paper pretreated with one component, followed by inactivation and subsequent hydrolysis with the same or another cellulase component, point to a sequential enzymic attack of the cellulose and seems consistent with the mathematical model presented.

The biological degradation of cellulose

Cellulolytic microorganisms play an important role in the biosphere by recycling cellulose, the most abundant carbohydrate produced by plants. Cellulose is a simple polymer, but it forms insoluble, crystalline microfibrils, which are highly resistant to enzymatic hydrolysis. All organisms known to degrade cellulose efficiently produce a battery of enzymes with different specificities, which act together in synergism. The study of cellulolytic enzymes at the molecular level has revealed some of the features that contribute to their activity. In spite of a considerable diversity, sequence comparisons show that the catalytic cores of cellulases belong to a restricted number of families. Within each family, available data suggest that the various enzymes share a common folding pattern, the same catalytic residues, and the same reaction mechanism, i.e. either single substitution with inversion of configuration or double substitution resulting in retention of the beta-configuration at the anomeric carbon. An increasing number of three-dimensional structures is becoming available for cellulases and xylanases belonging to different families, which will provide paradigms for molecular modeling of related enzymes. In addition to catalytic domains, many cellulolytic enzymes contain domains not involved in catalysis, but participating in substrate binding, multi-enzyme complex formation, or possibly attachment to the cell surface. Presumably, these domains assist in the degradation of crystalline cellulose by preventing the enzymes from being washed off from the surface of the substrate, by focusing hydrolysis on restricted areas in which the substrate is synergistically destabilized by multiple cutting events, and by facilitating recovery of the soluble degradation products by the cellulolytic organism. In most cellulolytic organisms, cellulase synthesis is repressed in the presence of easily metabolized, soluble carbon sources and induced in the presence of cellulose. Induction of cellulases appears to be effected by soluble products generated from cellulose by cellulolytic enzymes synthesized constitutively at a low level. These products are presumably converted into true inducers by transglycosylation reactions. Several applications of cellulases or hemicellulases are being developed for textile, food, and paper pulp processing. These applications are based on the modification of cellulose and hemicellulose by partial hydrolysis. Total hydrolysis of cellulose into glucose, which could be fermented into ethanol, isopropanol or butanol, is not yet economically feasible. However, the need to reduce emissions of greenhouse gases provides an added incentive for the development of processes generating fuels from cellulose, a major renewable carbon source.

Stereochemistry, specificity and kinetics of the hydrolysis of reduced cellodextrins by nine cellulases

The catalytic activity of nine enzymes (endoglucanases I-III, V, VI and cellobiohydrolases I and II from Humicola insolens; endoglucanases A and C from Bacillus lautus), representative of cellulase families A-C, H, J and K, has been investigated using a series of reduced cellooligosaccharides (cellotriitol to cellohexaitol) as substrates. For each enzyme, the specificity of cleavage was determined by analytical HPLC while the kinetic constants were obtained from a kinetic assay involving a cellobiose dehydrogenase purified from H. insolens as a coupled enzyme using 2,6-dichloroindophenol as the electron acceptor. These data were used to estimate the number of subsites in the enzymes. The stereochemical course of hydrolysis by seven enzymes, representing the six different families, was assessed using 1H-NMR. The enzymes belonging to families which had already been investigated (A-C), showed results in agreement with previous studies. The three other families (H, J and K), for which no mechanistic data was previously available, gave results which indicated that enzymes in group H had retaining-type activity and enzymes in groups J and K had inverting-type activity. The retaining endoglucanases I and III displayed a high glycosyl-transferase activity under the conditions used during the NMR experiments resulting in precipitates of higher oligomers.