Extracellular beta-D-glucosidase from Chaetomium thermophilum var. coprophilum: production, purification and some biochemical properties

Optimization of extracellular ethanol-tolerant β-glucosidase production from a newly isolated Aspergillus sp. DHE7 via solid state fermentation using jojoba meal as substrate: purification and biochemical characterization for biofuel preparation

BACKGROUND

The increasing demand and the continuous depletion in fossil fuels have persuaded researchers to investigate new sources of renewable energy. Bioethanol produced from cellulose could be a cost-effective and a viable alternative to petroleum. It is worth note that β-glucosidase plays a key role in the hydrolysis of cellulose and therefore in the production of bioethanol. This study aims to investigate a simple and standardized method for maximization of extracellular β-glucosidase production from a novel fungal isolate under solid-state fermentation using agro-industrial residues as the sole source of carbon and nitrogen. Furthermore, purification and characterization of β-glucosidase were performed to determine the conditions under which the enzyme displayed the highest performance.

RESULTS

A fungus identified genetically as a new Aspergillus sp. DHE7 was found to exhibit the highest extracellular β-glucosidase production among the sixty fungal isolates tested. Optimization of culture conditions improved the enzyme biosynthesis by 2.1-fold (174.6 ± 5.8 U/g of dry substrate) when the fungus grown for 72 h at 35 °C on jojoba meal with 60% of initial substrate moisture, pH 6.0, and an inoculum size of 2.54 × 107 spores/mL. The enzyme was purified to homogeneity through a multi-step purification process. The purified β-glucosidase is monomeric with a molecular mass of 135 kDa as revealed by the SDS-PAGE analysis. Optimum activity was observed at 60 °C and pH of 6.0, with a remarkable pH and thermal stability. The enzyme retained about 79% and 53% of its activity, after 1 h at 70 °C and 80 °C, respectively. The purified β-glucosidase hydrolysed a wide range of substrates but displaying its greater activity on p-nitrophenyl-β-D-glucopyranoside and cellobiose. The values of Km and Vmax on p-nitrophenyl β-D-glucopyranoside were 0.4 mM and 232.6 U/mL, respectively. Purified β-glucosidase displayed high catalytic activity (improved by 25%) in solutions contained ethanol up to 15%.

CONCLUSION

β-glucosidase characteristics associated with its ability to hydrolyse cellobiose, underscore its utilization in improving the quality of food and beverages. In addition, taking into consideration that the final concentration of ethanol produced by the conventional methods is about 10%, suggests its use in ethanol-containing industrial processes and in the saccharification processes for bioethanol production.

Intracellular cellobiose metabolism and its applications in lignocellulose-based biorefineries

Complete hydrolysis of cellulose has been a key characteristic of biomass technology because of the limitation of industrial production hosts to use cellodextrin, the partial hydrolysis product of cellulose. Cellobiose, a β-1,4-linked glucose dimer, is a major cellodextrin of the enzymatic hydrolysis (via endoglucanase and exoglucanase) of cellulose. Conversion of cellobiose to glucose is executed by β-glucosidase. The complete extracellular hydrolysis of celluloses has several critical barriers in biomass technology. An alternative bioengineering strategy to make the bioprocessing less challenging is to engineer microbes with the abilities to hydrolyze and assimilate the cellulosic-hydrolysate cellodextrin. Microorganisms engineered to metabolize cellobiose rather than the monomeric glucose can provide several advantages for lignocellulose-based biorefineries. This review describes the recent advances and challenges in engineering efficient intracellular cellobiose metabolism in industrial hosts. This review also describes the limitations of and future prospectives in engineering intracellular cellobiose metabolism.

Production and Characterization of Highly Thermostable β-Glucosidase during the Biodegradation of Methyl Cellulose by Fusarium oxysporum

Production of β-glucosidase from Fusarium oxysporum was investigated during degradation of some cellulosic substrates (Avicel, α-cellulose, carboxymethyl cellulose (CMC), and methylcellulose). Optimized production of β-glucosidase using the cellulosic substrate that supported highest yield of enzyme was examined over 192 h fermentation period and varied pH of 3.0–11.0. The β-glucosidase produced was characterized for its suitability for industrial application. Methyl cellulose supported the highest yield of β-glucosidase (177.5 U/mg) at pH 6.0 and 30°C at 96 h of fermentation with liberation of 2.121 μmol/mL glucose. The crude enzyme had optimum activity at pH 5.0 and 70°C. The enzyme was stable over broad pH range of 4.0–7.0 with relative residual activity above 60% after 180 min of incubation. β-glucosidase demonstrated high thermostability with 83% of its original activity retained at 70°C after 180 min of incubation. The activity of β-glucosidase was enhanced by Mn²⁺ and Fe²⁺ with relative activities of 167.67% and 205.56%, respectively, at 5 mM and 360% and 315%, respectively, at 10 mM. The properties shown by β-glucosidase suggest suitability of the enzyme for industrial applications in the improvement of hydrolysis of cellulosic compounds into fermentable sugars that can be used in energy generation and biofuel production.

Molecular characterization of a highly-active thermophilic β-glucosidase from Neosartorya fischeri P1 and its application in the hydrolysis of soybean isoflavone glycosides

Isoflavone occurs abundantly in leguminous seeds in the form of glycoside and aglycone. However, isoflavone glycoside has anti-nutritional effect and only the free type is beneficial to human health. In the present study we identified a β-glucosidase from thermophilic Neosartorya fischeri P1, termed NfBGL1, capable of efficiently converting isoflavone glycosides into free isoflavones. The gene, belonging to glycoside hydrolase family 3, was successfully overexpressed in Pichia pastoris at high cell density in a 3.7-l fermentor. Purified recombinant NfBGL1 had higher specific activity (2189 ± 1.7 U/mg) and temperature optimum (80 °C) than other fungal counterparts when using p-nitrophenyl β-D-glucopyranoside as the substrate. It retained stable at temperatures up to 70 °C and over a broad pH range of 3.0-10.0. NfBGL1 had broad substrate specificity including glucosidase, cellobiase, xylanase and glucanase activities, and displayed preference for hydrolysis of β-1,2 glycosidic bond rather than β-1,3, β-1,4, β-1,6 bonds. The enzyme showed high bioconversion ability for major soybean isoflavone glycosides (daidin, gensitin and glycitin) into free forms. These properties make NfBGL1 potential for the wide use in the food, feed, pharmacy and biofuel industries.

Cellulolytic potential of thermophilic species from four fungal orders

Elucidation of fungal biomass degradation is important for understanding the turnover of biological materials in nature and has important implications for industrial biomass conversion. In recent years there has been an increasing interest in elucidating the biological role of thermophilic fungi and in characterization of their industrially useful enzymes. In the present study we investigated the cellulolytic potential of 16 thermophilic fungi from the three ascomycete orders Sordariales, Eurotiales and Onygenales and from the zygomycete order Mucorales thus covering all fungal orders that include thermophiles. Thermophilic fungi are the only described eukaryotes that can grow at temperatures above 45°C. All 16 fungi were able to grow on crystalline cellulose but their secreted enzymes showed widely different cellulolytic activities, pH optima and thermostabilities. Interestingly, in contrast to previous reports, we found that some fungi such as Melanocarpus albomyces readily grew on crystalline cellulose and produced cellulases. These results indicate that there are large differences in the cellulolytic potential of different isolates of the same species. Furthermore, all the selected species were able to degrade cellulose but the differences in cellulolytic potential and thermostability of the secretome did not correlate to the taxonomic position. PCR amplification and sequencing of 22 cellulase genes from the fungi showed that the level of thermostability of the cellulose-degrading activity could not be inferred from the phylogenetic relationship of the cellulases.

Cloning, expression and characterization of an ethanol tolerant GH3 β-glucosidase from Myceliophthora thermophila

The β-glucosidase gene bgl3a from Myceliophthora thermophila, member of the fungal glycosyl hydrolase (GH) family 3, was cloned and expressed in Pichia pastoris. The mature β-glucosidase gene, which results after the excision of one intron and the secreting signal peptide, was placed under the control of the strong alcohol oxidase promoter (AOX1) in the plasmid pPICZαC. The recombinant enzyme (90 kDa) was purified and characterized in order to evaluate its biotechnological potential. Recombinant P. pastoris efficiently secreted β-glucosidase into the medium and produced high level of enzymatic activity (41 U/ml) after 192 h of growth, under methanol induction. MtBgl3a was able to hydrolyze low molecular weight substrates and polysaccharides containing β-glucosidic residues. The K_m was found to be 0.39 mM on p-β-NPG and 2.64 mM on cellobiose. Optimal pH and temperature for the p-β-NPG hydrolysis were 5.0 and 70 °C. The β-glucosidase exhibits a half life of 143 min at 60 °C. Kinetic parameters of inhibition were determined for D-glucose, D-xylose and D-gluconic acid, indicating tolerance of the enzyme for these sugars and oxidized products. The recombinant enzyme was stimulated by short chain alcohols and has been shown to efficiently synthesize methyl-D-glucoside in the presence of methanol due to its transglycosylation activity. The stability of MtBgl3a in ethanol was prominent, and it retained most of its original activity after we exposed it to 50% ethanol for 6 h. The high catalytic performance, good thermal stability and tolerance to elevated concentrations of ethanol, D-xylose and D-glucose qualify this enzyme for use in the hydrolysis of lignocellulosic biomass for biofuel production, as part of an efficient complete multi-enzyme cocktail.

Optimization of β-glucosidase, β-xylosidase and xylanase production by Colletotrichum graminicola under solid-state fermentation and application in raw sugarcane trash saccharification

Efficient, low-cost enzymatic hydrolysis of lignocellulosic residues is essential for cost-effective production of bioethanol. The production of β-glucosidase, β-xylosidase and xylanase by Colletotrichum graminicola was optimized using Response Surface Methodology (RSM). Maximal production occurred in wheat bran. Sugarcane trash, peanut hulls and corncob enhanced β-glucosidase, β-xylosidase and xylanase production, respectively. Maximal levels after optimization reached 159.3 ± 12.7 U g⁻¹, 128.1 ± 6.4 U g⁻¹ and 378.1 ± 23.3 U g⁻¹, respectively, but the enzymes were produced simultaneously at good levels under culture conditions optimized for each one of them. Optima of pH and temperature were 5.0 and 65 °C for the three enzymes, which maintained full activity for 72 h at 50 °C and for 120 min at 60 °C (β-glucosidase) or 65 °C (β-xylosidase and xylanase). Mixed with Trichoderma reesei cellulases, C. graminicola crude extract hydrolyzed raw sugarcane trash with glucose yield of 33.1% after 48 h, demonstrating good potential to compose efficient cocktails for lignocellulosic materials hydrolysis.

Heterologous expression in Pichia pastoris and characterization of an endogenous thermostable and high-glucose-tolerant β-glucosidase from the termite Nasutitermes takasagoensis

Termites are well-known cellulose decomposers and can give researchers insights into how to utilize lignocellulosic biomass in the actual scenario of energy consumption. In this work, an endogenous β-glucosidase from the midgut of the higher termite Nasutitermes takasagoensis was purified to homogeneity by Ni(2+) affinity chromatography and its properties were characterized. This β-glucosidase (G1mgNtBG1), which belongs to glycoside hydrolase family 1, is a homotrimer in its native form, with a molecular mass of 169.5 kDa, as demonstrated by gel filtration chromatography. The enzyme displayed maximum activity at pH 5.5 and had broad substrate specificities toward several saccharides, including cellobiose. G1mgNtBG1 showed a relatively high temperature optimum of 65°C and one of the highest levels of glucose tolerance among several β-glucosidases already characterized, with a K(i) of 600 mM glucose. To examine the applicability of G1mgNtBG1 in biomass conversion, we compared the thermostability and glucose tolerance of G1mgNtBG1 with those of Novozym 188. We found that G1mgNtBG1 was more thermostable after 5 h of incubation at 60°C and more resistant to glucose inhibition than Novozym 188. Furthermore, our result suggests that G1mgNtBG1 acts synergistically with Celluclast 1.5 L in releasing reducing sugars from Avicel. Thus, G1mgNtBG1 seems to be a potential candidate for use as a supplement in the hydrolysis of biomass.

Cloning of a gene encoding β-glucosidase from Chaetomium thermophilum CT2 and its expression in Pichia pastoris

A new thermostable β-glucosidase gene (bgl) from Chaetomium thermophilum CT2 was cloned, sequenced and expressed. The full-length DNA of bgl was 3,101 bp and included three introns. The full-length cDNA contained an open reading frame of 2,604-bp nucleotides, encoding 867 amino acids with a potential secretion signal. The C. thermophilum CT2 β-glucosidase gene was functionally expressed in Pichia pastoris. The purified recombinant β-glucosidase was a 119-kDa glycoprotein with an optimum catalytic activity at pH 5.0 and 60°C. The enzyme was stable at 50°C, and retained 67.7% activity after being kept at 60°C for 1 h; the half-time of the enzyme at 65°C was approximately 55 min, and even retained 29.7% activity after incubation at 70°C for 10 min.

Purification and biochemical properties of a glucose-stimulated beta-D-glucosidase produced by Humicola grisea var. thermoidea grown on sugarcane bagasse

The effect of several carbon sources on the production of mycelial-bound beta-glucosidase by Humicola grisea var. thermoidea in submerged fermentation was investigated. Maximum production occurred when cellulose was present in the culture medium, but higher specific activities were achieved with cellobiose or sugarcane bagasse. Xylose or glucose (1%) in the reaction medium stimulated beta-glucosidase activity by about 2-fold in crude extracts from mycelia grown in sugarcane bagasse. The enzyme was purified by ammonium sulfate precipitation, followed by Sephadex G-200 and DEAE-cellulose chromatography, showing a single band in PAGE and SDS-PAGE. The beta-glucosidase had a carbohydrate content of 43% and showed apparent molecular masses of 57 and 60 kDa, as estimated by SDS-PAGE and gel filtration, respectively. The optimal pH and temperature were 6.0 and 50 degrees C, respectively. The purified enzyme was thermostable up to 60 min in water at 55 degrees C and showed half-lives of 7 and 14 min when incubated in the absence or presence of 50 mM glucose, respectively, at 60 degrees C. The enzyme hydrolyzed p-nitrophenyl-beta-D-glucopyranoside, p-nitrophenyl-beta-D-galactopyranoside, p-nitrophenyl-beta-D-fucopyranoside, p-nitrophenyl-beta-D-xylopyranoside, o-nitrophenyl-beta-D-galactopyranoside, lactose, and cellobiose. The best synthetic and natural substrates were p-nitrophenyl-beta-D-fucopyranoside and cellobiose, respectively. Purified enzyme activity was stimulated up to 2-fold by glucose or xylose at concentrations from 25 to 200 mM. The addition of purified or crude beta-glucosidase to a reaction medium containing Trichoderma reesei cellulases increased the saccharification of sugarcane bagasse by about 50%. These findings suggest that H. grisea var. thermoidea beta-glucosidase has a potential for biotechnological applications in the bioconversion of lignocellulosic materials.

Characterization of a thermostable extracellular beta-glucosidase with activities of exoglucanase and transglycosylation from Paecilomyces thermophila

The purification and characterization of a novel extracellular beta-glucosidase from Paecilomyces thermophila J18 was studied. The beta-glucosidase was purified to 105-fold apparent homogeneity with a recovery yield of 21.7% by DEAE 52 and Sephacryl S-200 chromatographies. Its molecular masses were 116 and 197 kDa when detected by SDS-PAGE and gel filtration, respectively. It was a homodimeric glycoprotein with a carbohydrate content of 82.3%. The purified enzyme exhibited an optimal activity at 75 degrees C and pH 6.2. It was stable up to 65 degrees C and in the pH range of 5.0-8.5. The enzyme exhibited a broad substrate specificity and significantly hydrolyzed p-nitrophenyl-beta- d-glucopyranoside ( pNPG), cellobiose, gentiobiose, sophorose, amygdalin, salicin, daidzin, and genistin. Moreover, it displayed substantial activity on beta-glucans such as laminarin and lichenan, indicating that the enzyme has some exoglucanase activity. The rate of glucose released by the purified enzyme from cellooligosaccharides with a degree of polymerization (DP) ranging between 2 and 5 decreased with increasing chain length. Glucose and glucono-delta-lactone inhibited the beta-glucosidase competitively with Ki values of 73 and 0.49 mM, respectively. The beta-glucosidase hydrolyzed pNPG, cellobiose, gentiobiose, sophorose, salicin, and amygdalin, exhibiting apparent Km values of 0.26, 0.65, 0.77, 1.06, 1.39, and 1.45 mM, respectively. Besides, the enzyme showed transglycosylation activity, producing oligosaccharides with higher DP than the substrates when cellooligosaccharides were hydrolyzed. These properties make this beta-glucosidase useful for various biotechnological applications.

Thermostable enzymes in lignocellulose hydrolysis

Thermostable enzymes offer potential benefits in the hydrolysis of lignocellulosic substrates; higher specific activity decreasing the amount of enzymes, enhanced stability allowing improved hydrolysis performance and increased flexibility with respect to process configurations, all leading to improvement of the overall economy of the process. New thermostable cellulase mixtures were composed of cloned fungal enzymes for hydrolysis experiments. Three thermostable cellulases, identified as the most promising enzymes in their categories (cellobiohydrolase, endoglucanase and beta-glucosidase), were cloned and produced in Trichoderma reesei and mixed to compose a novel mixture of thermostable cellulases. Thermostable xylanase was added to enzyme preparations used on substrates containing residual hemicellulose. The new optimised thermostable enzyme mixtures were evaluated in high temperature hydrolysis experiments on technical steam pretreated raw materials: spruce and corn stover. The hydrolysis temperature could be increased by about 10-15 degrees C, as compared with present commercial Trichoderma enzymes. The same degree of hydrolysis, about 90% of theoretical, measured as individual sugars, could be obtained with the thermostable enzymes at 60 degrees C as with the commercial enzymes at 45 degrees C. Clearly more efficient hydrolysis per assayed FPU unit or per amount of cellobiohydrolase I protein used was obtained. The maximum FPU activity of the novel enzyme mixture was about 25% higher at the optimum temperature at 65 degrees C, as compared with the highest activity of the commercial reference enzyme at 60 degrees C. The results provide a promising basis to produce and formulate improved enzyme products. These products can have high temperature stability in process conditions in the range of 55-60 degrees C (with present industrial products at 45-50 degrees C) and clearly improved specific activity, essentially decreasing the protein dosage required for an efficient hydrolysis of lignocellulosic substrates. New types of process configurations based on thermostable enzymes are discussed.

Structural and kinetic alterations of constitutive conidial alkaline phosphatase from the osmotically-sensitive mutant ofNeurospora crassa

The osmotically-sensitive os-1 mutant of Neurospora crassa overproduced conidial alkaline phosphatase. The enzyme was purified by Phenyl-Sepharose CL-4B chromatography and Sephadex G-200 gel filtration. PAGE analysis of the purified enzyme suggested the occurrence of aggregation and/or disaggregation phenomena. The enzyme is a glycoprotein containing 16% saccharide, with apparent molar mass of 137 kDa. Two protein bands (36 and 62 kDa) were observed in SDS-PAGE, suggesting that the native enzyme was a trimer. The pI was estimated to be 2.7, and optima of pH and temperature were 9.5 and 65 degrees C, respectively. The enzyme showed broad substrate specificity, hydrolyzing preferentially 4-nitrophenyl phosphate, O-phosphoamino-acids and 2-phosphoglycerate. The hydrolysis of 4-nitrophenyl phosphate was stimulated by Co(II) (26%), Ni(II) (23%) and Mg(II) ions (80%). The enzyme was stable for up to 6 months at 4 degrees C in 5 mmol/L Tris-HCl buffer and also upon storage at 25 degrees C for 10 d. The kinetic and structural properties of the conidial enzyme purified from the os-1 mutant were quite different from those of the wild type strain. The enzyme overproduction observed in the mutant may be related to cell wall alterations that affect the process of enzyme secretion.