Characterization of the cross-linked enzyme aggregates of a novel β-galactosidase, a potential catalyst for the synthesis of galacto-oligosaccharides

Magnetic CLEAs of β-Galactosidase from Aspergillus oryzae as a Potential Biocatalyst to Produce Tagatose from Lactose

β-galactosidase is an enzyme capable of hydrolysing lactose, used in various branches of industry, mainly the food industry. As the efficient industrial use of enzymes depends on their reuse, it is necessary to find an effective method for immobilisation, maintaining high activity and stability. The present work proposes cross-linked magnetic cross-linked enzyme aggregates (mCLEAs) to prepare heterogeneous biocatalysts of β-galactosidase. Different concentrations of glutaraldehyde (0.6%, 1.0%, 1.5%), used as a cross-linking agent, were studied. The use of dextran-aldehyde as an alternative cross-linking agent was also evaluated. The mCLEAs presented increased recovered activity directly related to the concentration of glutaraldehyde. Modifications to the protocol to prepare mCLEAs with glutaraldehyde, adding a competitive inhibitor or polymer coating, have not been effective in increasing the recovered activity of the heterogeneous biocatalysts or its thermal stability. The biocatalyst prepared using dextran-aldehyde presented 73.6% recovered activity, aside from substrate affinity equivalent to the free enzyme. The thermal stability at 60 °C was higher for the biocatalyst prepared with glutaraldehyde (mCLEA-GLU-1.5) than the one produced with dextran-aldehyde (mCLEA-DEX), and the opposite happened at 50 °C. Results obtained for lactose hydrolysis, the use of its product to produce a rare sugar (D-tagatose) and operational and storage stability indicate that heterogeneous biocatalysts have adequate characteristics for industrial use.

Co-Immobilizing Two Glycosidases Based on Cross-Linked Enzyme Aggregates to Enhance Enzymatic Properties for Achieving High Titer Icaritin Biosynthesis

Icaritin is a rare and high-value isopentane flavonoid compound with remarkable activities. Increasing yields while reducing cost has been a great challenge in icaritin production. Herein, we first reported a high titer icaritin biosynthesis strategy from epimedin C through co-immobilizing α-l-rhamnosidase (Rha1) and β-glucosidase (Glu4) using cross-linked enzyme aggregates (CLEAs). The created CLEAs exhibited excellent performances in terms of catalytic activity, thermal stability, pH stability, and reusability. Notably, Rha1-CLEAs (K_i: 1 M) and Glu4-CLEAs (K_i: 0.1 M) were more tolerant to sugars (glucose or rhamnose) than free enzymes (0.1 M for Rha1 and 0.007 M for Glu4) by immobilization, achieving the highest icaritin productivity under the highest substrate concentration ever reported. Finally, about 34.24 g/L icaritin could be obtained from 100 g/L epimedin C within 8 h, indicating the great potential for industrialization. This study also provides a promising strategy for the low-cost production of other high-value aglycone compounds by solving poor stability and sugar inhibition of glycosidase.

Cross-linked enzyme aggregates immobilization: preparation, characterization, and applications

Enzymes are commonly used as biocatalysts for various biological and chemical processes. However, some major drawbacks of free enzymes (e.g. poor reusability and instability) significantly restrict their industrial practices. How to overcome these weaknesses remain considerable challenges. Enzyme immobilization is one of the most effective ways to improve the reusability and stability of enzymes. Cross-linked enzyme aggregates (CLEAs) has been known as a novel and versatile carrier-free immobilization method. CLEAs is attractive due to its simplicity and robustness, without purification. It generally shows: high catalytic specificity and selectivity, good operational and storage stabilities, and good reusability. Moreover, co-immobilization of different kinds of enzymes can be acquired. These CLEAs advantages provide opportunities for further industrial applications. Herein, the preparation parameters of CLEAs were first summarized. Next, characterization of structural and catalytic properties, stability and reusability are also proposed. Finally, some important applications of this technique in: environmental protection, industrial chemistry, food industry, and pharmaceutical synthesis and delivery are introduced. Potential challenges and future research directions, such as improving cross-linking efficiency and internal mass transfer efficiency, are also presented. This implies that CLEAs provide an efficient and feasible technique to improve the properties of enzymes for use in the industry.

An Insight in Developing Carrier-Free Immobilized Enzymes

Enzymes play vital roles in all organisms. The enzymatic process is progressively at its peak, mainly for producing biochemical products with a higher value. The immobilization of enzymes can sometimes tremendously improve the outcome of biocatalytic processes, making the product(s) relatively pure and economical. Carrier-free immobilized enzymes can increase the yield of the product and the stability of the enzyme in biocatalysis. Immobilized enzymes are easier to purify. Due to these varied advantages, researchers are tempted to explore carrier-free methods used for the immobilization of enzymes. In this review article, we have discussed various aspects of enzyme immobilization, approaches followed to design a process used for immobilization of an enzyme and the advantages and disadvantages of various common processes used for enzyme immobilization.

Complementary experimental/docking approach for determining chitosan and carboxymethylchitosan ability for the formation of active polymer-β-galactosidase adducts

The spontaneous aggregation of chitosan and carboxymethylchitosan polymers can be advantageous for the enzyme confinement on these colloidal systems during immobilization processes. The initial crucial step involves the polymer-enzyme adduct formation. The objective here is to determine the interactions that drive the adduct formation between these polymers and β-galactosidase from Bacillus circulans. The chemical characterization of chitosan and its carboxymethyl-derivate allowed to explain their colloidal behavior and design the four-unit fragments ligands used for the docking study. The deacetylation degree (0.6 times lower), isoelectric point (5.2 instead 6.4) and substitution degree (DS_O = 1.779 and DS_2N = 0.441) of carboxymenthylchitosan are due to the hydroxide concentration (>25%) and 30 °C modification conditions. Favorable Van der Waals and H-bond interactions between chitosan-β-galactosidase and contribution of electrostatic attraction mediated by calcium ions for carboxymethylchitosan-β-galactosidase explained the zeta potential and dynamic light scattering results at pH 7.0. These interactions occur onto the external surface of this galactosidase, without affecting the catalytic activity. A cross-linked enzyme aggregates-type model was proposed for the formation of the adducts, based on the complementary experimental-docking results. They contribute understanding the behavior of polyelectrolyte chitosan-derived matrices for enzyme immobilization.

Characterization of Cross-Linked Enzyme Aggregates of the Y509E Mutant of a Glycoside Hydrolase Family 52 β-xylosidase from G. stearothermophilus

Cross-linked enzyme aggregates (CLEAs) of the Y509E mutant of glycoside hydrolase family 52 β-xylosidase from Geobacillus stearothermophilus with dual activity of β-xylosidase and xylanase (XynB2^Y509E) were prepared. Ammonium sulfate was used as the precipitant agent, and glutaraldehyde as cross-linking agent. The optimum conditions were found to be 90% ammonium sulfate, 12.5 mM glutaraldehyde, 3 h of cross-linking reaction at 25 °C, and pH 8.5. Under these (most effective) conditions, XynB2^Y509E-CLEAs retained 92.3% of their original β-xylosidase activity. Biochemical characterization of both crude and immobilized enzymes demonstrated that the maximum pH and temperature after immobilization remained unchanged (pH 6.5 and 65 °C). Moreover, an improvement in pH stability and thermostability was also found after immobilization. Analysis of kinetic parameters shows that the K _m value of XynB2^Y509E-CLEAs obtained was slightly higher than that of free XynB2^Y509E (1.2 versus 0.9 mM). Interestingly, the xylanase activity developed by the mutation was also conserved after the immobilization process.

Preparation, characterization and stability studies of cross-linked α-amylase aggregates (CLAAs) for continuous liquefaction of starch

In current study, α-amylase of fungal origin was immobilized using cross-linking strategy. The influence of precipitant (ammonium sulphate) and cross-linker (glutaraldehyde) concentration revealed that 60% (w/v) precipitant and 1.5% (v/v) cross-linker saturation was required to attain optimum activity. Cross-linked amylase aggregates (CLAAs) were characterized and 10-degree shift in optimum temperature (soluble enzyme: 50 °C; cross-linked: 60 °C) and 1-unit shift in pH (soluble enzyme: pH -6; cross-linked: pH -7) was observed after immobilization. The V_max for soluble α-amylase and its cross-linked form was 1225 U ml^-1 and 3629 U ml^-1, respectively. The CLAAs was more thermostable than its soluble form and retained its 30% activity even after 60 min of incubation at 70 °C. Moreover, cross-linked amylase retained its activity after two months while its soluble counterpart lost its complete activity after 10 and 20 days at 30 °C and 4 °C storage, respectively. Reusability test showed that cross-linked amylase could retain 13% of its residual activity after 10 repeated cycles. Therefore, 10 times more glucose was produced after cross-linking than soluble amylase when it was utilized multiple times. This study indicates that amylase aggregates are highly effective for continuous liquefaction of starch, hence have strong potential to be used for different industrial processes.

Development of recyclable magnetic cross-linked enzyme aggregates for the synthesis of high value rare sugar d-tagatose in aqueous phase catalysis

In this study, a high value rare sugar d-tagatose was synthesized using recyclable magnetic catalysts.

Engineering of β-Glucosidase Bgl15 with Simultaneously Enhanced Glucose Tolerance and Thermostability To Improve Its Performance in High-Solid Cellulose Hydrolysis

In this study, a Petri-dish-based double-layer high-throughput screening method was established to improve glucose tolerance of β-glucosidase Bgl15. Two beneficial mutations were identified, and the joint mutant 2R1 improved the half-maximal inhibitory concentration of glucose from 0.04 to 2.1 M. The crystal structure of 2R1 was subsequently determined at 2.7 Å. Structure analysis revealed that enhancement of glucose tolerance may be due to improved transglycosylation activity made possible by a hydrophobic binding site for glucose as an acceptor and more stringent control of a putative water channel. To further ameliorate the application potential of the enzyme, it was engineered to increase the half-life at 50 °C from 0.8 h (Bgl15) to 180 h (mutant 5R1). Furthermore, supplementation of 5R1 to the cellulase cocktail significantly improved glucose production from pretreated sugar cane bagasse by 38%. Consequently, this study provided an efficient approach to enhance glucose tolerance and generated a promising catalyst for cellulose saccharification.

Improving Galactooligosaccharide Synthesis Efficiency of β-Galactosidase Bgal1-3 by Reshaping the Active Site with an Intelligent Hydrophobic Amino Acid Scanning

There are ongoing interests in improving the galactooligosaccharide (GOS) synthesis efficiency of β-galactosidase by protein engineering. In this study, an intelligent double-hydrophobic amino acid scanning strategy was proposed and employed to target nine residues forming the glycon-binding site (-1 subsite) of β-galactosidase Bgal1-3. Two mutants C510V and H512I with significantly improved GOS synthesis efficiency were obtained. When 40% (w/v) lactose was used as a substrate, Bgal1-3 reached a maximum GOS yield of 45.3% at 16 h, while the mutants reached higher yields in a much shorter time (59.1% at 10 h for C510V, 51.5% at 2 h for H512I). When skim milk was treated with these enzymes, more GOS was produced (19.9 g/L for C510V, 12.7 g/L for H512I) than that for Bgal1-3 (10.3 g/L) at a lactose conversion of 90%. These results validated hydrophobicity scanning as an efficient method to engineer β-galactosidases into promising catalysts for the preparation of GOS and GOS-enriched milk.

Poly-lysine supported cross-linked enzyme aggregates of penicillin G acylase and its application in synthesis of β-lactam antibiotics

Penicillin G acylase (PGA) from Providencia rettgeri PX04 (PrPGA) was utilized to synthesize β-lactam antibiotics. Poly-lysine supported cross-linked enzyme aggregates (PL-CLEAs) were prepared using PGA. Addition of poly-lysine significantly increased retention of PGA activity in CLEAs, with a decrease in the synthesis/hydrolysis (S/H) ratio. PL-CLEAs with 0.56 mg/mL poly-lysine retained 83% of free PGA activity, and displayed a higher S/H ratio than that of the free enzyme. Both PL-CLEAs and CLEAs exhibited high pH and thermal stabilities. PL-CLEAs possessed the best stability profile, and the lowest α value [(k_cat/K_m)_Ps/(k_cat/K_m)_AD], and was most effective at amoxicillin synthesis. A >94% yield of amoxicillin was achieved using a D-HPGME/6-APA ratio of 1.2:1 (240 mM, 200 mM), with fed-batch addition of D-HPGME. PL-CLEAs displayed excellent operational stability during amoxicillin synthesis. Over 97% of initial conversion was retained after twenty rounds of catalysis. PL-CLEAs exhibited greater potency than CLEAs in practical catalysis, permitting a higher concentration of reactants.

Preparation of Cross-Linked Enzyme Aggregates (CLEAs) of an Inulosucrase Mutant for the Enzymatic Synthesis of Inulin-Type Fructooligosaccharides

Fructooligosaccharides are well-known carbohydrate molecules that exhibit good probiotic activity and are widely used as sweeteners. Inulin-type fructooligosaccharides (IFOs) can be synthesized from sucrose using inulosucrase. In this study, cross-linked enzyme aggregates (CLEAs) of Lactobacillus reuteri 121 inulosucrase (R483A-LrInu) were prepared and used as a biocatalyst for IFOs production. Under optimum conditions, R483A-LrInu CLEAs retained 42% of original inulosucrase activity. Biochemical characterization demonstrated that the optimum pH of inulosucrase changed from 5 to 4 after immobilization, while the optimum temperature was unchanged. Furthermore, the pH stability and thermostability of the R483A-LrInu CLEAs was significantly improved. IFOs product characterization indicated that the product specificity of the enzyme was impacted by CLEA generation, producing a narrower range of IFOs than the soluble enzyme. In addition, the R483A-LrInu CLEAs showed operational stability in the batch synthesis of IFOs.

CLEAs, Combi-CLEAs and ‘Smart’ Magnetic CLEAs: Biocatalysis in a Bio-Based Economy

Biocatalysis has emerged in the last decade as a pre-eminent technology for enabling the envisaged transition to a more sustainable bio-based economy. For industrial viability it is essential that enzymes can be readily recovered and recycled by immobilization as solid, recyclable catalysts. One method to achieve this is via carrier-free immobilization as cross-linked enzyme aggregates (CLEAs). This methodology proved to be very effective with a broad selection of enzymes, in particular carbohydrate-converting enzymes. Methods for optimizing CLEA preparations by, for example, adding proteic feeders to promote cross-linking, and strategies for making the pores accessible for macromolecular substrates are critically reviewed and compared. Co-immobilization of two or more enzymes in combi-CLEAs enables the cost-effective use of multiple enzymes in biocatalytic cascade processes and the use of “smart” magnetic CLEAs to separate the immobilized enzyme from other solids has raised the CLEA technology to a new level of industrial and environmental relevance. Magnetic-CLEAs of polysaccharide-converting enzymes, for example, are eminently suitable for use in the conversion of first and second generation biomass.

Enhancing the Thermostability of Highly Active and Glucose-Tolerant β-Glucosidase Ks5A7 by Directed Evolution for Good Performance of Three Properties

A high-performance β-glucosidase for efficient cellulose hydrolysis needs to excel in thermostability, catalytic efficiency, and resistance to glucose inhibition. However, it is challenging to achieve superb properties in all three aspects in a single enzyme. In this study, a hyperactive and glucose-tolerant β-glucosidase Ks5A7 was employed as the starting point. Four rounds of random mutagenesis were then performed, giving rise to a thermostable mutant 4R1 with five amino acid substitutions. The half-life of 4R1 at 50 °C is 8640-fold that of Ks5A7 (144 h vs 1 min). Meanwhile, 4R1 had a higher specific activity (374.26 vs 243.18 units·mg^-1) than the wild type with a similar glucose tolerance. When supplemented to Celluclast 1.5L, the mutant significantly enhanced the hydrolysis of pretreated sugar cane bagasse, improving the released glucose concentration by 44%. With excellent performance in thermostability, activity, and glucose tolerance, 4R1 will serve as an exceptional catalyst for industrial applications.

Transforming food waste: how immobilized enzymes can valorize waste streams into revenue streams

Food processing generates byproduct and waste streams rich in lipids, carbohydrates, and proteins, which contribute to its negative environmental impact. However, these compounds hold significant economic potential if transformed into revenue streams such as biofuels and ingredients. Indeed, the high protein, sugar, and fat content of many food waste streams makes them ideal feedstocks for enzymatic valorization. Compared to synthetic catalysts, enzymes have higher specificity, lower energy requirement, and improved environmental sustainability in performing chemical transformations, yet their poor stability and recovery limits their performance in their native state. This review article surveys the current state-of-the-art in enzyme stabilization & immobilization technologies, summarizes opportunities in enzyme-catalyzed valorization of waste streams with emphasis on streams rich in mono- and disaccharides, polysaccharides, lipids, and proteins, and highlights challenges and opportunities in designing commercially translatable immobilized enzyme systems towards the ultimate goals of sustainable food production and reduced food waste.

Preparation and Characterization of Sugar-Assisted Cross-Linked Enzyme Aggregates (CLEAs) of Recombinant Cellobiose 2-epimerase from Caldicellulosiruptor saccharolyticus ( CsCE)

High-efficiency lactulose-producing enzyme of Caldicellulosiruptor saccharolyticus cellobiose 2-epimerase (WT- CsCE) was immobilized in the form of cross-linked enzyme aggregates (CLEAs). Conditions for enzyme aggregation and cross-linking were optimized, and a sugar-assisted strategy with less damage to enzyme secondary structures was developed to improve the activity yield of CLEAs up to approximately 65%. The resulting CLEAs with multiple-layer network structures exhibited an enlarged optimal temperature range (70-80 °C) and maintained higher activity at 50-90 °C. Besides, CLEAs retained more than 95% of their initial activity after 10 successive batches at 60 °C, demonstrating superior reusability. Moreover, CLEAs displayed an equivalent or higher catalytic ability to free WT- CsCE in lactulose biosynthesis, and the final sugar ratios were similar, lactulose 58.8-61.7%, epilactose 9.3-10.2%, and lactose 27.8-30%, with a constant isomerization selectivity of 0.84-0.87 regardless of enzymes used and temperature applied. The proposed strategy is the first trial for enzymatic synthesis of lactulose catalyzed by CLEAs of WT- CsCE.

Designing cross-linked xylanase aggregates for bioconversion of agroindustrial waste biomass towards potential production of nutraceuticals

Immobilized biocatalysts design has the potential to efficiently produce valuable bioproducts from lignocellulosic biomass. Among them, the carrier-free immobilization through the cross-linked enzyme aggregates technology is a simple and low-cost alternative. A two steps statistical approach was utilized to evaluate the synthesis of a cross-linked enzyme aggregate from a xylanolytic preparation, which was produced by Cohnella sp. AR92 grown in a peptone-based culture medium. The resulting immobilized biocatalyst, Xyl-CLEA, was significate more stable (25 to 45%) towards temperatures up to 50°C with respect to the free enzyme, and retained over 50% of its initial activity after 5 consecutive cycles of reuse. By means of infrared spectroscopy and electron microscopy, the Xyl-CLEA showed architectural features described as signature of type I and type II of protein aggregates. These, were the result of the simultaneous aggregation of a multiplicity of proteins from the crude enzymatic extract. The enzymatic activity was assessed using alkali pretreated sugar cane bagasse as substrate. Whereas the free enzymatic preparation released xylose as the main product, the immobilized xylanase produced xylooligosaccharides, thus showing that the immobilization procedure modified the potential application of the extracellular xylanase from Conhella sp. AR92.

Improving the Secretion Yield of the β-Galactosidase Bgal1-3 in Pichia pastoris for Use as a Potential Catalyst in the Production of Prebiotic-Enriched Milk

In this study, three kinds of milk were treated with the β-galactosidase Bgal1-3 (4 U/mL), resulting in 7.2-9.5 g/L galactooligosaccharides (GOS) at a lactose conversion of 90-95%. Then, Bgal1-3 was secreted from Pichia pastoris X33 under the direction of an α-factor signal peptide. After cultivation for 144 h in a flask culture with shaking, the extracellular activity of Bgal1-3 was 4.4 U/mL. Five more signal peptides (HFBI, apre, INU1A, MF4I, and W1) were employed to direct the secretion, giving rise to a more efficient signal peptide, W1 (11.2 U/mL). To further improve the secretion yield, recombinant strains harboring two copies of the bgal1-3 gene were constructed, improving the extracellular activity to 22.6 U/mL (about 440 mg/L). This study successfully constructed an engineered strain for the production of the β-galactosidase Bgal1-3, which is a promising catalyst in the preparation of prebiotic-enriched milk.

Improved synthesis of isomaltooligosaccharides using immobilized α-glucosidase in organic-aqueous media

α-Glucosidase was immobilized onto an epoxy-activated resin (Eupergit C) to catalyze maltose into isomaltooligosaccharides (IMO), and then the effects of organic-aqueous media on the enzymatic properties of immobilized α-glucosidase were examined. An immobilization efficiency of 79.61% was obtained under the condition of pH 6.0, ionic strength of 2.0 M, and 30 mg of protein/g of resin. The butyl acetate-aqueous biphasic system was found to significantly improve the catalytic activity of the immobilized enzyme and the yield of IMO. The highest yield of IMO (50.83%, w/w) was obtained at pH 4.5 and 55 °C in a butyl acetate/buffer system (25:75, v/v). In addition, the immobilized enzyme particles were distributed into the organic phase after the completion of transglycosylation, which facilitates the separation and recycling use of the immobilized enzyme. Immobilized α-glucosidase retains a robust reusability in this continuous operation model. The present findings are of potential in improving the IMO manufacturing process.

Preparation and characterization of gellan gum microspheres containing a cold-adapted β-galactosidase from Rahnella sp. R3

R-β-Gal is a cold-adapted β-galactosidase that is able to hydrolyze lactose and has the potential to produce low-lactose or lactose-free dairy products at low temperatures (4°C). Cold-adapted enzymes unfold at moderate temperatures due to the lower intramolecular stabilizing interactions necessary for flexibility at low temperatures. To increase stability and usage-performance, R-β-Gal was encapsulated in gellan gum by injecting an aqueous solution into two different hardening solutions (10mM CaCl₂ or 10mM MgCl₂). Enzyme characteristics of both free and encapsulated R-β-Gal were carried out, and the different effects of two cations were investigated. R-β-Gal showed better thermal and pH stability after encapsulation. Ca²⁺ gels had higher encapsulation efficiency (71.4%) than Mg²⁺ (66.7%) gels, and Ca²⁺ formed larger inner and surface pores. R-β-Gal was released from the Ca²⁺ hydrogel beads more rapidly than the Mg²⁺ hydrogels during storage in aqueous solution due to the larger inner/surface pores of the matrix.

Cross-Linked Enzyme Aggregates for Applications in Aqueous and Nonaqueous Media

Extensive cross-linking of a precipitate of a protein by a cross-linking reagent (glutaraldehyde has been most commonly used) creates an insoluble enzyme preparation called cross-linked enzyme aggregates (CLEAs). CLEAs show high stability and performance in conventional aqueous as well as nonaqueous media. These are also stable at fairly high temperatures. CLEAs with more than one kind of enzyme activity can be prepared, and such CLEAs are called combi-CLEAs or multipurpose CLEAs. Extent of cross-linking often influences their morphology, stability, activity, and enantioselectivity.

A new cross-linked enzyme aggregate biocatalyst for NAD+-booster production

Cross-linked enzyme aggregates of NMN deamidase were produced with enhanced stability and reusability to obtain nicotinic acid mononucleotide.

Optimizing the preparation conditions and characterization of cross-linked enzyme aggregates of a monoamine oxidase

Monoamine oxidases are useful in determination of biogenic monoamines, particularly histamine and tyramine. In this study, cross-linked enzyme aggregates (CLEAs) technique was applied to improve the stability of a monoamine oxidase from Arthrobacter aurescens (AMAO). Under the optimized condition (50% of saturated ammonium sulfate, 5 mM glutaraldehyde, 2.0 mg/mL AMAO, 4 h-cross-linking at 25°C, pH 8.0), CLEAs-AMAO was recovered with a yield of 82% based on the subjected total enzyme activity. Both pH activity and stability at alkaline pHs of CLEAs-AMAO were significantly improved compared to those of the free enzyme, resulting in the shift of optimum pH to pH 8.0 and a broader pH profile. The half-life of the CLEAs at 65°C was elongated by 1.7-fold compared to that of the free enzyme, suggesting the thermal stability of AMAO was also improved by the CLEAs formation.

Efficient Secretion of the β-Galactosidase Bgal1-3 via both Tat-Dependent and Tat-Independent Pathways in Bacillus subtilis

In this study, the twin-arginine (Tat) signal peptide PhoD was used to direct the secretion of the β-galactosidase Bgal1-3 into the growth medium of an engineered strain of Bacillus subtilis 168. After 24 h of cultivation, the extracellular activity reached 1.15 U/mL, representing 78% of the total activity. Bgal1-3 was exported via both Tat-dependent and Tat-independent pathways. To improve the secretion amounts, two more copies of the target gene were inserted into the designated loci on the chromosome, further improving the extracellular enzymatic activity to 2.15 U/mL. The engineered strain with three copies of bgal1-3 was genetically stable after 150 generations. To the best of our knowledge, this is the first report on the functional secretion of a heterologous protein via both Tat-dependent and Tat-independent pathways mediated by a Tat signal peptide in B. subtilis. Furthermore, this study provides us with a markerless engineered strain for the production of β-galactosidase.

A multifunctional thermophilic glycoside hydrolase from Caldicellulosiruptor owensensis with potential applications in production of biofuels and biochemicals

BACKGROUND

Thermophilic enzymes have attracted much attention for their advantages of high reaction velocity, exceptional thermostability, and decreased risk of contamination. Exploring efficient thermophilic glycoside hydrolases will accelerate the industrialization of biofuels and biochemicals.

RESULTS

A multifunctional glycoside hydrolase (GH) CoGH1A, belonging to GH1 family with high activities of β-d-glucosidase, exoglucanase, β-d-xylosidase, β-d-galactosidase, and transgalactosylation, was cloned and expressed from the extremely thermophilic bacterium Caldicellulosiruptor owensensis. The enzyme exerts excellent thermostability by retaining 100 % activity after 12-h incubation at 75 °C. The catalytic coefficients (k cat/K m) of the enzyme against pNP-β-D-galactopyranoside, pNP-β-D-glucopyranoside, pNP-β-D-cellobioside, pNP-β-D-xylopyranoside, and cellobiose were, respectively, 7450.0, 2467.5, 1085.4, 90.9, and 137.3 mM(-1) s(-1). When CoGH1A was supplemented at the dosage of 20 Ucellobiose g(-1) biomass for hydrolysis of the pretreated corn stover, comparing with the control, the glucose and xylose yields were, respectively, increased 37.9 and 42.1 %, indicating that the enzyme contributed not only for glucose but also for xylose release. The efficiencies of lactose decomposition and synthesis of galactooligosaccharides (GalOS) by CoGH1A were investigated at low (40 g L(-1)) and high (500 g L(-1)) initial lactose concentrations. At low lactose concentration, the time for decomposition of 83 % lactose was 10 min, which is much shorter than the reported 2-10 h for reaching such a decomposition rate. At high lactose concentration, after 50-min catalysis, the GalOS concentration reached 221 g L(-1) with a productivity of 265.2 g L(-1) h(-1). This productivity is at least 12-fold higher than those reported in literature.

CONCLUSIONS

The multifunctional glycoside hydrolase CoGH1A has high capabilities in saccharification of lignocellulosic biomass, decomposition of lactose, and synthesis of galactooligosaccharides. It is a promising enzyme to be used for bioconversion of carbohydrates in industrial scale. In addition, the results of this study indicate that the extremely thermophilic bacteria are potential resources for screening highly efficient glycoside hydrolases for the production of biofuels and biochemicals.

Engineering a novel glucose-tolerant β-glucosidase as supplementation to enhance the hydrolysis of sugarcane bagasse at high glucose concentration

BACKGROUND

Most β-glucosidases reported are sensitive to the end product (glucose), making it the rate limiting component of cellulase for efficient degradation of cellulose through enzymatic route. Thus, there are ongoing interests in searching for glucose-tolerant β-glucosidases, which are still active at high glucose concentration. Although many β-glucosidases with different glucose-tolerance levels have been isolated and characterized in the past decades, the effects of glucose-tolerance on the hydrolysis of cellulose are not thoroughly studied.

RESULTS

In the present study, a novel β-glucosidase (Bgl6) with the half maximal inhibitory concentration (IC 50) of 3.5 M glucose was isolated from a metagenomic library and characterized. However, its poor thermostability at 50 °C hindered the employment in cellulose hydrolysis. To improve its thermostability, random mutagenesis was performed. A thermostable mutant, M3, with three amino acid substitutions was obtained. The half-life of M3 at 50 °C is 48 h, while that of Bgl6 is 1 h. The K cat/K m value of M3 is 3-fold higher than that of Bgl6. The mutations maintained its high glucose-tolerance with IC 50 of 3.0 M for M3. In a 10-h hydrolysis of cellobiose, M3 completely converted cellobiose to glucose, while Bgl6 reached a conversion of 80 %. Then their synergistic effects with the commercial cellulase (Celluclast 1.5 L) on hydrolyzing pretreated sugarcane bagasse (SCB) were investigated. The supplementation of Bgl6 or mutant M3 to Celluclast 1.5 L significantly improved the SCB conversion from 64 % (Celluclast 1.5 L alone) to 79 % (Bgl6) and 94 % (M3), respectively. To further evaluate the application potential of M3 in high-solids cellulose hydrolysis, such reactions were performed at initial glucose concentration of 20-500 mM. Results showed that the supplementation of mutant M3 enhanced the glucose production from SCB under all the conditions tested, improving the SCB conversion by 14-35 %.

CONCLUSIONS

These results not only clearly revealed the significant role of glucose-tolerance in cellulose hydrolysis, but also showed that mutant M3 may be a potent candidate for high-solids cellulose refining.