Heterologous β-glucosidase in a fungal cellulase system: Comparison of different methods for development of multienzyme cocktails

Heterologous protein production in filamentous fungi

Filamentous fungi are able to produce a wide range of valuable proteins and enzymes for many industrial applications. Recent advances in fungal genomics and experimental technologies are rapidly changing the approaches for the development and use of filamentous fungi as hosts for the production of both homologous and heterologous proteins. In this review, we highlight the benefits and challenges of using filamentous fungi for the production of heterologous proteins. We review various techniques commonly employed to improve the heterologous protein production in filamentous fungi, such as strong and inducible promoters, codon optimization, more efficient signal peptides for secretion, carrier proteins, engineering of glycosylation sites, regulation of the unfolded protein response and endoplasmic reticulum associated protein degradation, optimization of the intracellular transport process, regulation of unconventional protein secretion, and construction of protease-deficient strains. KEY POINTS: • This review updates the knowledge on heterologous protein production in filamentous fungi. • Several fungal cell factories and potential candidates are discussed. • Insights into improving heterologous gene expression are given.

A novel sucrose-inducible expression system and its application for production of biomass-degrading enzymes in Aspergillus niger

BACKGROUND

Filamentous fungi are extensively exploited as important enzyme producers due to the superior secretory capability. However, the complexity of their secretomes greatly impairs the titer and purity of heterologous enzymes. Meanwhile, high-efficient evaluation and production of bulk enzymes, such as biomass-degrading enzymes, necessitate constructing powerful expression systems for bio-refinery applications.

RESULTS

A novel sucrose-inducible expression system based on the host strain Aspergillus niger ATCC 20611 and the β-fructofuranosidase promoter (PfopA) was constructed. A. niger ATCC 20611 preferentially utilized sucrose for rapid growth and β-fructofuranosidase production. Its secretory background was relatively clean because β-fructofuranosidase, the key enzyme responsible for sucrose utilization, was essentially not secreted into the medium and the extracellular protease activity was low. Furthermore, the PfopA promoter showed a sucrose concentration-dependent induction pattern and was not subject to glucose repression. Moreover, the strength of PfopA was 7.68-fold higher than that of the commonly used glyceraldehyde-3-phosphate dehydrogenase promoter (PgpdA) with enhanced green fluorescence protein (EGFP) as a reporter. Thus, A. niger ATCC 20611 coupled with the PfopA promoter was used as an expression system to express a β-glucosidase gene (bgla) from A. niger C112, allowing the production of β-glucosidase at a titer of 17.84 U/mL. The crude β-glucosidase preparation could remarkably improve glucose yield in the saccharification of pretreated corncob residues when added to the cellulase mixture of Trichoderma reesei QM9414. The efficacy of this expression system was further demonstrated by co-expressing the T. reesei-derived chitinase Chi46 and β-N-acetylglucosaminidase Nag1 to obtain an efficient chitin-degrading enzyme cocktail, which could achieve the production of N-acetyl-D-glucosamine from colloidal chitin with a conversion ratio of 91.83%. Besides, the purity of the above-secreted biomass-degrading enzymes in the crude culture supernatant was over 86%.

CONCLUSIONS

This PfopA-driven expression system expands the genetic toolbox of A. niger and broadens the application field of the traditional fructo-oligosaccharides-producing strain A. niger ATCC 20611, advancing it to become a high-performing enzyme-producing cell factory. In particular, the sucrose-inducible expression system possessed the capacity to produce biomass-degrading enzymes at a high level and evade endogenous protein interference, providing a potential purification-free enzyme production platform for bio-refinery applications.

Designing enzyme cocktails from Penicillium and Aspergillus species for the enhanced saccharification of agro-industrial wastes

The aim of this study was to develop optimized enzyme cocktails, containing native and recombinant purified enzymes from five fungal species, for the saccharification of alkali- and acid-pretreated sugarcane bagasse (SCB), soybean hulls (SBH) and oil palm empty fruit bunches (EFB). Basic cellulases were represented by cellobiohydrolase I (CBH) and endo-glucanase II (EG) from Penicillium verruculosum and β-glucosidase (BG) from Aspergillus niger. Auxiliary enzymes were represented by endo-xylanase A (Xyl), pectin lyase (PNL) and arabinoxylanhydrolase (AXH) from Penicillium canescens, β-xylosidase (BX) from Aspergillus japonicus, endo-arabinase (ABN) from A. niger and arabinofuranosidase (Abf) from Aspergillus foetidus. Enzyme loads were 5 mg protein/g dry substrate (basic cellulases) and 1 mg/g (each auxiliary enzyme). The best choice for SCB and EFB saccharification was alkaline pretreatment and addition of Xyl + BX, AXH + BX or ABN + BX + Abf to basic cellulases. For SBH, acid pretreatment and basic cellulases combined with ABN + BX + Abf or Xyl + BX performed better than other enzyme preparations.

CRISPR/Cas Genome Editing in Filamentous Fungi

The review describes the CRISPR/CAS system and its adaptation for the genome editing in filamentous fungi commonly used for production of enzyme complexes, enzymes, secondary metabolites, and other compounds used in industrial biotechnology and agriculture. In the second part of this review, examples of the CRISPR/CAS technology application for improving properties of the industrial strains of fungi from the Trichoderma, Aspergillus, Penicillium, and other genera are presented. Particular attention is given to the efficiency of genome editing, as well as system optimization for specific industrial producers.

Properties of recombinant endo-β-1,6-glucanase from Trichoderma harzianum and its application in the pustulan hydrolysis

The gene encoding Trichoderma harzianum fungus pustulanase (ThBGL1.6, GH5 family, endo-β-1,6-glucanase, EC 3.2.1.75) was cloned and heterologously expressed by the highly productive Penicillium verruculosum fungus. The recombinant ThBGL1.6 was purified and its properties were studied. The ThBGL1.6 had an observed molecular mass of 46 kDa (SDS-PAGE data) and displayed maximum of the enzyme activity at pH 5.0 and 50 °C. At 45 °C, the ThBGL1.6 was stable for at least 3 h. The K_m was 1.0 g/L with pustulan as the substrate. Reaction product analysis by HPLC clearly indicated that ThBGL1.6 has an endo-hydrolytic mode of action against pustulan as specific substrate. It was also identified that gentiobiose is the main reaction product at studying of long-term pustulan hydrolysis.

Effect of Novel Penicillium verruculosum Enzyme Preparations on the Saccharification of Acid- and Alkali-Pretreated Agro-Industrial Residues

This study aimed at evaluating different enzyme combinations in the saccharification of sugarcane bagasse (SCB), soybean husks (SBH) and oil palm empty fruit bunches (EFB) submitted to mild acid and alkaline pretreatments. Enzyme pools were represented by B1 host (crude cellulase/xylanase complexes of Penicillium verruculosum); B1-XylA (Penicillium canescens xylanase A expressed in P. verruculosum B1 host strain); and F10 (Aspergillus niger β-glucosidase expressed in B1 host strain). Enzyme loading was 10 mg protein/g dry substrate and 40 U/g of β-glucosidase (F10) activity. SCB was efficiently hydrolyzed by B1 host after alkaline pretreatment, yielding glucose and reducing sugars at 71 g/L or 65 g/100 g of dry pretreated substrate and 91 g/L or 83 g/100 g, respectively. B1 host performed better also for EFB, regardless of the pretreatment method, but yields were lower (glucose 27–30 g/L, 25–27 g/100 g; reducing sugars 37–42 g/L, 34–38 g/100 g). SBH was efficiently saccharified by the combination of B1 host and B1-XylA, yielding similar concentrations of reducing sugars for both pretreatments (92–96 g/L, 84–87 g/100 g); glucose recovery, however, was higher with alkaline pretreatment (81 g/L, 74 g/100 g). Glucose and reducing sugar yields from initial substrate mass were 42% and 54% for SCB, 36% and 42–47% for SBH and 16–18% and 21–26% for EFB, respectively.

Designing a cellulolytic enzyme cocktail for the efficient and economical conversion of lignocellulosic biomass to biofuels

Concerns about dwindling fossil fuels and their unfavorable environmental impacts shifted the global focus towards the development of biofuels from lignocellulosic feedstocks. The structure of this biomass is very complex due to which variety of enzymes (cellulolytic, hemicellulolytic, auxiliary/AA9) and proteins (e.g. swollenin) required for efficient deconstruction. Major impediments in large-scale commercial production of cellulosic ethanol are the cost of cellulases and inability of any single microorganism to produce all cellulolytic components in sufficient titers. In the recent past, various methods for reducing the enzyme cost during cellulosic ethanol production have been attempted. These include designing optimal synergistic enzyme blends/cocktail, having certain ratios of enzymes from different microbial sources, for efficient hydrolysis of pretreated biomass. However, the mechanisms underlying the development, strategies for production and evaluation of optimal cellulolytic cocktails still remain unclear. This article aims to explore the technical and economic benefits of using cellulolytic enzyme cocktail, basic enzymatic and non-enzymatic components required for its development and various strategies employed for efficient cellulolytic cocktail preparation. Consideration was also given to the ways of evaluation of commercially available and in-house developed cocktails. Discussion about commercially available cellulolytic cocktails, current challenges and possible avenues in the development of cellulolytic cocktails included.

Using an Inducible Promoter of a Gene Encoding Penicillium verruculosum Glucoamylase for Production of Enzyme Preparations with Enhanced Cellulase Performance

Background

Penicillium verruculosum is an efficient producer of highly active cellulase multienzyme system. One of the approaches for enhancing cellulase performance in hydrolysis of cellulosic substrates is to enrich the reaction system with β -glucosidase and/or accessory enzymes, such as lytic polysaccharide monooxygenases (LPMO) displaying a synergism with cellulases.

Results

Genes bglI, encoding β-glucosidase from Aspergillus niger (AnBGL), and eglIV, encoding LPMO (formerly endoglucanase IV) from Trichoderma reesei (TrLPMO), were cloned and expressed by P. verruculosum B1-537 strain under the control of the inducible gla1 gene promoter. Content of the heterologous AnBGL in the secreted multienzyme cocktails (hBGL1, hBGL2 and hBGL3) varied from 4 to 10% of the total protein, while the content of TrLPMO in the hLPMO sample was ~3%. The glucose yields in 48-h hydrolysis of Avicel and milled aspen wood by the hBGL1, hBGL2 and hBGL3 preparations increased by up to 99 and 80%, respectively, relative to control enzyme preparations without the heterologous AnBGL (at protein loading 5 mg/g substrate for all enzyme samples). The heterologous TrLPMO in the hLPMO preparation boosted the conversion of the lignocellulosic substrate by 10–43%; however, in hydrolysis of Avicel the hLPMO sample was less effective than the control preparations. The highest product yield in hydrolysis of aspen wood was obtained when the hBGL2 and hLPMO preparations were used at the ratio 1:1.

Conclusions

The enzyme preparations produced by recombinant P. verruculosum strains, expressing the heterologous AnBGL or TrLPMO under the control of the gla1 gene promoter in a starch-containing medium, proved to be more effective in hydrolysis of a lignocellulosic substrate than control enzyme preparations without the heterologous enzymes. The enzyme composition containing both AnBGL and TrLPMO demonstrated the highest performance in lignocellulose hydrolysis, providing a background for developing a fungal strain capable to express both heterologous enzymes simultaneously.

Construction of Aspergillus niger integrated with cellulase gene from Ampullaria gigas Spix for improved enzyme production and saccharification of alkaline-pretreated rice straw

Aspergillus niger is an important microorganism that has been used for decades to produce extracellular enzymes. In this study, a novel Aspergillus niger strain integrated with a eukaryotic expression vector harboring the gpd-Shi promoter of shiitake mushrooms and cellulase gene of Ampullaria gigas Spix was engineered to improve cellulase production for the achievement of highly efficient saccharification of agricultural residues. In one strain, designated ACShi27, which exhibited the highest total cellulase expression, total cellulase, endoglucanase, exoglucanase, and xylanase expression levels were 1.73, 16.23, 17.73, and 150.83 U ml⁻¹, respectively; these values were 14.5, 22.3, 24.6, and 17.3% higher than those of the wild-type Aspergillus niger M85 using wheat bran as an induction substrate. Production of cellulases and xylanase by solid-state fermentation followed by in situ saccharification of ACShi27 was investigated with alkaline-pretreated rice straw as a substrate. After 2 days of enzyme induction at 30 °C, followed by 48 h of saccharification at 50 °C, the conversion rate of carbon polymers into reducing sugar reached 293.2 mg g⁻¹, which was 1.23-fold higher than that of the wild-type strain. The expression of sestc in Aspergillus niger can improve the total cellulase and xylanase activity and synergism, thereby enhancing the lignocellulose in situ saccharification.