Extracellular secretion of β-glucosidase in ethanologenic E. coli enhances ethanol fermentation of cellobiose

Heterologous Expression of Lignocellulose-Modifying Enzymes in Microorganisms: Current Status

Heterologous expression of the carbohydrate-active enzymes in microorganisms is a promising approach to produce bio-based compounds, such as fuels, nutraceuticals and other value-added products from sustainable lignocellulosic sources. Several microorganisms, including Saccharomyces cerevisiae, Escherichia coli, and the filamentous fungi Aspergillus nidulans, have unique characteristics desirable for a biorefinery production approach like well-known genetic tools, thermotolerance, high fermentative capacity and product tolerance, and high amount of recombinant enzyme secretion. These microbial factories are already stablished in the heterologous production of the carbohydrate-active enzymes to produce, among others, ethanol, xylooligosaccharides and the valuable coniferol. A complete biocatalyst able to heterologous express the CAZymes of glycoside hydrolases, carbohydrate esterases and auxiliary activities families could release these compounds faster, with higher yield and specificity. Recent advances in the synthetic biology tools could expand the number and diversity of enzymes integrated in these microorganisms, and also modify those already integrated. This review outlines the heterologous expression of carbohydrate-active enzymes in microorganisms, as well as recent updates in synthetic biology.

Engineering of a new Escherichia coli strain efficiently metabolizing cellobiose with promising perspectives for plant biomass-based application design

The necessity to decrease our fossil energy dependence requests bioprocesses based on biomass degradation. Cellobiose is the main product released by cellulases when acting on the major plant cell wall polysaccharide constituent, the cellulose. Escherichia coli, one of the most common model organisms for the academy and the industry, is unable to metabolize this disaccharide. In this context, the remodeling of E. coli to catabolize cellobiose should thus constitute an important progress for the design of such applications. Here, we developed a robust E. coli strain able to metabolize cellobiose by integration of a small set of modifications in its genome. Contrary to previous studies that use adaptative evolution to achieve some growth on this sugar by reactivating E. coli cryptic operons coding for cellobiose metabolism, we identified easily insertable modifications impacting the cellobiose import (expression of a gene coding a truncated variant of the maltoporin LamB, modification of the expression of lacY encoding the lactose permease) and its intracellular degradation (genomic insertion of a gene encoding either a cytosolic β-glucosidase or a cellobiose phosphorylase). Taken together, our results provide an easily transferable set of mutations that confers to E. coli an efficient growth phenotype on cellobiose (doubling time of 2.2 ​h in aerobiosis) without any prior adaptation.

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Production of a cytosolic β-glucosidase or cellobiose phosphorylase allows E. coli to metabolize cellobiose.

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Optimization of cellobiose uptake through ​E. coli ​enveloppe to sustain an efficient growth on this carbon source.

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Genetic engineering of E. coli can provide the capacity of an efficient cellobiose catabolism.

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.

Enhancing the selective extracellular location of a recombinant E. coli domain antibody by management of fermentation conditions

The preparation of a recombinant protein using Escherichia coli often involves a challenging primary recovery sequence. This is due to the inability to secrete the protein to the extracellular space without a significant degree of cell lysis. This results in the release of nucleic acids, leading to a high viscosity, difficulty to clarify, broth and also to contamination with cell materials such as lipopolysaccharides and host cell proteins. In this paper, we present different fermentation strategies to facilitate the recovery of a V H domain antibody (13.1 kDa) by directing it selectively to the extracellular space and changing the balance between domain antibody to nucleic acid release. The manipulation of the cell growth rate in order to increase the outer cell membrane permeability gave a small ~1.5-fold improvement in released domain antibody to nucleic acid ratio without overall loss of yield. The introduction during fermentation of release agents such as EDTA gave no improvement in the ratio of released domain antibody to nucleic acid and a loss of overall productivity. The use of polyethyleneimine (PEI) during fermentation was with the aim to (a) permeabilise the outer bacterial membrane to release selectively domain antibody and (b) remove selectively by precipitation nucleic acids released during cell lysis. This strategy resulted in up to ~4-fold increase in the ratio of domain antibody to soluble nucleic acid with no reduction in domain antibody overall titre. In addition, a reduction in host cell protein contamination was achieved and there was no increase in endotoxin levels. Similar results were demonstrated with a range of other antibody products prepared in E. coli.