Cocktail production of an endo-β-xylanase and a β-glucosidase from Trichoderma reesei QM 9414 in Escherichia coli

Efficient secretion of an enzyme cocktail in Escherichia coli for hemicellulose degradation

The use of host to secrete several hemicellulase is a cost-effective way for hemicellulose degradation. In this study, the xylose utilization gene xylAB of Escherichia coli BL21 was knocked out, and the xylanase (N20Xyl), β-xylosidase (Xys), and feruloyl esterase (FaeLam) were co-expressed in this strain. By measuring the content of reducing sugars generated by enzymatic hydrolysis of wheat bran in the fermentation supernatant, the order of the three enzymes was screened to obtain the optimal recombinant strain of E. coli BL21/∆xylAB/pDIII-2. Subsequently, fermentation conditions including culture medium, inducer concentration, induction timing, metal ions, and glycine concentration were optimized. Then, different concentrations of wheat bran and xylan were added to the fermentation medium for degradation. The results showed that the extracellular reducing sugars content reached the highest value of 33.70 ± 0.46 g/L when 50 g/L xylan was added. Besides, the scavenging rates of hydroxyl radical by the fermentation supernatant was 81.0 ± 1.41 %, and the total antioxidant capacity reached 2.289 ± 0.55. Furthermore, it showed the growth promotion effect on different lactic acid bacteria. These results provided a basis for constructing E. coli strain to efficiently degrade hemicellulose, and the strain obtained has great potential application to transform hemicellulose into fermentable carbon source.

Engineering cascade biocatalysis in whole cells for bottom-up synthesis of cello-oligosaccharides: flux control over three enzymatic steps enables soluble production

BACKGROUND

Soluble cello-oligosaccharides (COS, β-1,4-D-gluco-oligosaccharides with degree of polymerization DP 2-6) have been receiving increased attention in different industrial sectors, from food and feed to cosmetics. Development of large-scale COS applications requires cost-effective technologies for their production. Cascade biocatalysis by the three enzymes sucrose-, cellobiose- and cellodextrin phosphorylase is promising because it enables bottom-up synthesis of COS from expedient substrates such as sucrose and glucose. A whole-cell-derived catalyst that incorporates the required enzyme activities from suitable co-expression would represent an important step towards making the cascade reaction fit for production. Multi-enzyme co-expression to reach distinct activity ratios is challenging in general, but it requires special emphasis for the synthesis of COS. Only a finely tuned balance between formation and elongation of the oligosaccharide precursor cellobiose results in the desired COS.

RESULTS

Here, we show the integration of cellodextrin phosphorylase into a cellobiose-producing whole-cell catalyst. We arranged the co-expression cassettes such that their expression levels were upregulated. The most effective strategy involved a custom vector design that placed the coding sequences for cellobiose phosphorylase (CbP), cellodextrin phosphorylase (CdP) and sucrose phosphorylase (ScP) in a tricistron in the given order. The expression of the tricistron was controlled by the strong T7_lacO promoter and strong ribosome binding sites (RBS) for each open reading frame. The resulting whole-cell catalyst achieved a recombinant protein yield of 46% of total intracellular protein in an optimal ScP:CbP:CdP activity ratio of 10:2.9:0.6, yielding an overall activity of 315 U/g dry cell mass. We demonstrated that bioconversion catalyzed by a semi-permeabilized whole-cell catalyst achieved an industrial relevant COS product titer of 125 g/L and a space-time yield of 20 g/L/h. With CbP as the cellobiose providing enzyme, flux into higher oligosaccharides (DP ≥ 6) was prevented and no insoluble products were formed after 6 h of conversion.

CONCLUSIONS

A whole-cell catalyst for COS biosynthesis was developed. The coordinated co-expression of the three biosynthesis enzymes balanced the activities of the individual enzymes such that COS production was maximized. With the flux control set to minimize the share of insolubles in the product, the whole-cell synthesis shows a performance with respect to yield, productivity, product concentration and quality that is promising for industrial production.

Plasmid Design for Tunable Two-Enzyme Co-Expression Promotes Whole-Cell Production of Cellobiose

Catalyst development for biochemical cascade reactions often follows a "whole-cell-approach" in which a single microbial cell is made to express all required enzyme activities. Although attractive in principle, the approach can encounter limitations when efficient overall flux necessitates precise balancing between activities. This study shows an effective integration of major design strategies from synthetic biology to a coherent development of plasmid vectors, enabling tunable two-enzyme co-expression in E. coli, for whole-cell-production of cellobiose. An efficient transformation of sucrose and glucose into cellobiose by a parallel (countercurrent) cascade of disaccharide phosphorylases requires the enzyme co-expression to cope with large differences in specific activity of cellobiose phosphorylase (14 U mg^-1 ) and sucrose phosphorylase (122 U mg^-1 ). Mono- and bicistronic co-expression strategies controlling transcription, transcription-translation coupling or plasmid replication are analyzed for effect on activity and stable producibility of the whole-cell-catalyst. A key role of bom (basis of mobility) for plasmid stability dependent on the ori is reported and the importance of RBS (ribosome binding site) strength is demonstrated. Whole cell catalysts show high specific rates (460 µmol cellobiose min^-1  g^-1 dry cells) and performance metrics (30 g L^-1 ; ∼82% yield; 3.8 g L^-1 h^-1 overall productivity) promising for cellobiose production.

Functional Expression of a Thermostable Endoglucanase from Thermoascus aurantiacus RCKK in Pichia pastoris X-33 and Its Characterization

Thermostable cellulases offer several advantages like higher rates of substrate hydrolysis, lowered risk of contamination, and increased flexibility with respect to process design. In the present study, a thermostable native endoglucanase nEG (EC 3.2.1.4) was purified and characterized from T. aurantiacus RCKK. Further, it was cloned in P. pastoris X-33 and processed for over expression. Expression of recombinant endoglucanase (rEG) of molecular size ~ 33 kDa was confirmed by SDS-PAGE and western blotting followed by in gel activity determination by zymogram analysis. Similar to nEG, the purified rEG was characterized to harbor high thermostability while retaining 50% of its initial activity even after 6- and 10-h incubation at 80 and 70 °C, respectively, and exhibited considerable stability in pH range 3.0-7.0. CD spectroscopy revealed more than 20% β-sheets in protein structure consistently when incubated upto 85 °C as a speculated reason for protein high thermostability. Interestingly, both nEG and rEG were found tolerant up to 10% of the presence of 1-ethyl-3-methylimidazolium acetate [C2mim][OAc]. Values of the catalytic constants K_m and V_max for rEG were recorded as 2.5 mg/ml and 303.4 µmol/mg/min, respectively. Thermostability, pH stability, and resistance to the presence of ionic liquid signify the potential applicability of present enzyme in cellulose hydrolysis and enzymatic deinking of recycled paper pulp.

Construction of a trifunctional cellulase and expression in Saccharomyces cerevisiae using a fusion protein

Background

Cellulose is the most important component of lignocellulose, and its degradation requires three different types of enzymes to act synergistically. There have been reports of single gene duality, but no gene has been described to have more than two functions. Cloning and expression of fusion cellulases containing more than two kinds of catalytic domains has not been reported thus far.

Results

We synthesized three different cellulase genes and linked the three catalytic domains with a (G₄S)₃ flexible linker. The trifunctional cellulase gene (BCE) containing three types of cellulase functions was constructed and expressed in S. cerevisiae successfully. The β-glucosidase, the exoglucanase and the endoglucanase activity of the trifunctional cellulase BCE reached 16.80 IU/mg, 2.26 IU/mg and 20.67 IU/mg, which was 46.27, 6.73 and 46.20% higher than the activities of the β-glucosidase BG, the endoglucanase CBH and the endoglucanase EG. The filter paper enzyme activity of BCE was higher than those of BG, CBH and EG, reached 2.04 IU/mg.

Conclusions

The trifunctional cellulase BCE was designed based on β-glucosidase BG, endoglucanase EG and exoglucanase CBH, and it possessed β-glucosidase activity, endoglucanase activity and exoglucanase activity simultaneously. The BCE has better filter paper activity, it means the potential practical application.

Electronic supplementary material

The online version of this article (10.1186/s12896-018-0454-x) contains supplementary material, which is available to authorized users.

Directed Coevolution of Two Cellulosic Enzymes in Escherichia coli Based on Their Synergistic Reactions

Directed evolution is a widely used technique for improving enzymatic properties. The development of an efficient high-throughput screening method is a key procedure, which is however often unavailable for many enzyme reactions, including the cellulase-catalyzed cellulose hydrolysis. Here, we describe a high-throughput screening assay for directed coevolution of two cellulases (an endoglucanase and a β-glucosidase) in form of a bicistronic operon based on their synergistic reactions. Insoluble filter paper is used as the real cellulose substrate to screen for positive enzyme variants, facilitated by the colorimetric assay coupled to glucose liberated from cellulose under catalysis of endoglucanase and β-glucosidase. Directed coevolution saves the labor and time required for two independent directed evolution cycles, which might provide reference for the engineering of other cellulosic enzymes or multienzyme systems.

Two New Native β-Glucosidases from Clavispora NRRL Y-50464 Confer Its Dual Function as Cellobiose Fermenting Ethanologenic Yeast

Yeast strain Clavispora NRRL Y-50464 is able to produce cellulosic ethanol from lignocellulosic materials without addition of external β-glucosidase by simultaneous saccharification and fermentation. A β-glucosidase BGL1 protein from this strain was recently reported supporting its cellobiose utilization capability. Here, we report two additional new β-glucosidase genes encoding enzymes designated as BGL2 and BGL3 from strain NRRL Y-50464. Quantitative gene expression was analyzed and the gene function of BGL2 and BGL3 was confirmed by heterologous expression using cellobiose as a sole carbon source. Each gene was cloned and partially purified protein obtained separately for direct enzyme assay using varied substrates. Both proteins showed the highest specific activity at pH 5 and relatively strong affinity with a K_m of 0.08 and 0.18 mM for BGL2 and BGL3, respectively. The optimum temperature was found to be 50°C for BGL2 and 55°C for BGL3. Both proteins were able to hydrolyze 1,4 oligosaccharides evaluated in this study. They also showed a strong resistance to glucose product inhibition with a K_i of 61.97 and 38.33 mM for BGL2 and BGL3, respectively. While BGL3 was sensitive showing a significantly reduced activity to 4% ethanol, BGL2 demonstrated tolerance to ethanol. Its activity was enhanced in the presence of ethanol but reduced at concentrations greater than 16%. The presence of the fermentation inhibitors furfural and HMF did not affect the enzyme activity. Our results suggest that a β-glucosidase gene family exists in Clavispora NRRL Y-50464 with at least three members in this group that validate its cellobiose hydrolysis functions for lower-cost cellulosic ethanol production. Results of this study confirmed the cellobiose hydrolysis function of strain NRRL Y-50464, and further supported this dual functional yeast as a candidate for lower-cost cellulosic ethanol production and next-generation biocatalyst development in potential industrial applications.

Multiple Genes in a Single Host: Cost-Effective Production of Bacterial Laccase (cotA), Pectate Lyase (pel), and Endoxylanase (xyl) by Simultaneous Expression and Cloning in Single Vector in E. coli

This study attempted to reduce the enzyme production cost for exploiting lignocellulosic materials by expression of multiple genes in a single host. Genes for bacterial laccase (CotA), pectate lyase (Pel) and endoxylanase (Xyl), which hold significance in lignocellulose degradation, were cloned in pETDuet-1 vector containing two independent cloning sites (MCS). CotA and xyl genes were cloned in MCS1 and MCS 2, respectively. Pel gene was cloned by inserting complete cassette (T7 promoter, ribosome binding site, pel gene, His tag and complete gene ORF) preceded by cotA open reading frame in the MCS1. IPTG induction of CPXpDuet-1 construct in E. coli BL21(DE3) resulted in expression of all three heterologous proteins of ~65 kDa (CotA), ~45 kDa (Pel) and ~25 kDa (Xyl), confirmed by SDS-PAGE and western blotting. Significant portions of the enzymes were also found in culture supernatant (~16, ~720 and ~370 IU/ml activities of CotA, Pel and Xyl, respectively). Culture media optimization resulted in 2, 3 and 7 fold increased secretion of recombinant CotA, Pel and Xyl, respectively. Bioreactor level optimization of the recombinant cocktail expression resulted in production of 19 g/L dry cell biomass at OD600nm 74 from 1 L induced culture after 15 h of cultivation, from which 9, 627 and 1090 IU/ml secretory enzyme activities of CotA, Xyl and Pel were obtained, respectively. The cocktail was also found to increase the saccharification of orange peel in comparison to the xylanase alone. Thus, simultaneous expression as well as extra cellular secretion of these enzymes as cocktail can reduce the enzyme production cost which increases their applicability specially for exploiting lignocellulosic materials for their conversion to value added products like alcohol and animal feed.

Directed evolution of an exoglucanase facilitated by a co-expressed β-glucosidase and construction of a whole engineered cellulase system in Escherichia coli

A novel high-throughput screening method is proposed for the directed evolution of exoglucanase facilitated by the co-expression of β-glucosidase, using the glucose released from filter paper as the screening indicator. Three transformants (B1, D6 and G10) with improved activity were selected from 4,000 colonies. The specific activities of B1, D6 and G10 for releasing glucose were, respectively, 1.4-, 1.3- and 1.6-fold higher than that of the wild type. The engineered exoglucanase gene was inserted into an expression vector carrying the previously engineered endoglucanase and β-glucosidase genes, and transformed into Escherichia coli to form a completely engineered cellulase system that showed 8.2-fold increase in glucose production (relative activity) compared to the cells equipped with wild-type enzymes. To our knowledge, this is the first report for directed evolution of an exoglucanase using insoluble cellulose as the screening substrate.