Generating bifunctional fusion enzymes composed of heat-active endoglucanase (Cel5A) and endoxylanase (XylT)

Design and genome engineering of microbial cell factories for efficient conversion of lignocellulose to fuel

The gradually increasing need for fossil fuels demands renewable biofuel substitutes. This has fascinated an increasing investigation to design innovative energy fuels that have comparable Physico-chemical and combustion characteristics with fossil-derived fuels. The efficient microbes for bioenergy synthesis desire the proficiency to consume a large quantity of carbon substrate, transfer various carbohydrates through efficient metabolic pathways, capability to withstand inhibitory components and other degradation compounds, and improve metabolic fluxes to synthesize target compounds. Metabolically engineered microbes could be an efficient methodology for synthesizing biofuel from cellulosic biomass by cautiously manipulating enzymes and metabolic pathways. This review offers a comprehensive perspective on the trends and advances in metabolic and genetic engineering technologies for advanced biofuel synthesis by applying various heterologous hosts. Probable technologies include enzyme engineering, heterologous expression of multiple genes, CRISPR-Cas technologies for genome editing, and cell surface display.

Characterization of a novel bifunctional enzyme from buffalo rumen metagenome and its effect on in vitro ruminal fermentation and microbial community composition

To efficiently use lignocellulosic materials in ruminants, it is crucial to explore effective enzymes, especially bifunctional enzymes. In this study, a novel stable bifunctional cellulase-xylanase protein from buffalo rumen metagenome was expressed and characterized, CelXyn2. The enzyme displayed optimal activity at pH 6.0 and 45 °C. The residual endoglucanase and xylanase activities were 90.6% and 86.4% after a 60-min pre-incubation at 55 °C. Hydrolysis of rice straw, wheat straw, sheepgrass and sugar beet pulp by CelXyn2 showed its ability to degrade both cellulose and hemicellulose polymers. Treatment with CelXyn2 improved the hydrolysis of agricultural residues with an evident increase in production of total gas, lactate and volatile fatty acids. The results of 16S rRNA and real-time PCR showed that the effect on in vitro ruminal microbial community depended on fermentation substrates. This study demonstrated that CelXyn2 could strengthen lignocellulose hydrolysis and in vitro ruminal fermentation. These characteristics of CelXyn2 distinguish it as a promising candidate for agricultural application.

Artificial fusions between P450 BM3 and an alcohol dehydrogenase for efficient (+)-nootkatone production

Multi‐enzyme cascades enable the production of valuable chemical compounds, and fusion of the enzymes that catalyze these reactions can improve the reaction outcome. In this work, P450 BM3 from Bacillus megaterium and an alcohol dehydrogenase from Sphingomonas yanoikuyae were fused to bifunctional constructs to enable cofactor regeneration and improve the in vitro two‐step oxidation of (+)‐valencene to (+)‐nootkatone. An up to 1.5‐fold increased activity of P450 BM3 was achieved with the fusion constructs compared to the individual enzyme. Conversion of (+)‐valencene coupled to cofactor regeneration and performed in the presence of the solubilizing agent cyclodextrin resulted in up to 1080 mg L⁻¹ (+)‐nootkatone produced by the fusion constructs as opposed to 620 mg L⁻¹ produced by a mixture of the separate enzymes. Thus, a two‐step (+)‐valencene oxidation was considerably improved through the simple method of enzyme fusion.

Engineering multifunctional enzymes for agro-biomass utilization

Lignocellulosic biomass is a plentiful renewable resource that can be converted into a wide range of high-value-added industrial products. However, the complexity of its structural integrity is one of the major constraints and requires combinations of different fibrolytic enzymes for the cost-effective, industrially and environmentally feasible transformation. An interesting approach is constructing multifunctional enzymes, either in a single polypeptide or by joining multiple domains with linkers and performing diverse reactions simultaneously, in a single host. The production of such chimera proteins multiplies the advantages of different enzymatic reactions in a single setup, in lesser time, at lower production cost and with desirable and improved catalytic activities. This review embodies the various domain-tailoring and extracellular secretion strategies, possible solutions to their challenges, and efforts to experimentally connect different catalytic activities in a single host, as well as their applications.

Design of novel enzyme biocatalysts for industrial bioprocess: Harnessing the power of protein engineering, high throughput screening and synthetic biology

Biocatalysts have wider applications in various industries. Biocatalysts are generating bigger attention among researchers due to their unique catalytic properties like activity, specificity and stability. However the industrial use of many enzymes is hindered by low catalytic efficiency and stability during industrial processes. Properties of enzymes can be altered by protein engineering. Protein engineers are increasingly study the structure-function characteristics, engineering attributes, design of computational tools for enzyme engineering, and functional screening processes to improve the design and applications of enzymes. The potent and innovative techniques of enzyme engineering deliver outstanding opportunities for tailoring industrially important enzymes for the versatile production of biochemicals. An overview of the current trends in enzyme engineering is explored with important representative examples.

Genetically Engineered Proteins to Improve Biomass Conversion: New Advances and Challenges for Tailoring Biocatalysts

Protein engineering emerged as a powerful approach to generate more robust and efficient biocatalysts for bio-based economy applications, an alternative to ecologically toxic chemistries that rely on petroleum. On the quest for environmentally friendly technologies, sustainable and low-cost resources such as lignocellulosic plant-derived biomass are being used for the production of biofuels and fine chemicals. Since most of the enzymes used in the biorefinery industry act in suboptimal conditions, modification of their catalytic properties through protein rational design and in vitro evolution techniques allows the improvement of enzymatic parameters such as specificity, activity, efficiency, secretability, and stability, leading to better yields in the production lines. This review focuses on the current application of protein engineering techniques for improving the catalytic performance of enzymes used to break down lignocellulosic polymers. We discuss the use of both classical and modern methods reported in the literature in the last five years that allowed the boosting of biocatalysts for biomass degradation.

Expression of an endoglucanase-cellobiohydrolase fusion protein in Saccharomyces cerevisiae, Yarrowia lipolytica, and Lipomyces starkeyi

The low secretion levels of cellobiohydrolase I (CBHI) in yeasts are one of the key barriers preventing yeast from directly degrading and utilizing lignocellulose. To overcome this obstacle, we have explored the approach of genetically linking an easily secreted protein to CBHI, with CBHI being the last to be folded. The Trichoderma reesei eg2 (TrEGII) gene was selected as the leading gene due to its previously demonstrated outstanding secretion in yeast. To comprehensively characterize the effects of this fusion protein, we tested this hypothesis in three industrially relevant yeasts: Saccharomyces cerevisiae, Yarrowia lipolytica, and Lipomyces starkeyi. Our initial assays with the L. starkeyi secretome expressing differing TrEGII domains fused to a chimeric Talaromyces emersonii-T. reesei CBHI (TeTrCBHI) showed that the complete TrEGII enzyme, including the glycoside hydrolase (GH) 5 domain is required for increased expression level of the fusion protein when linked to CBHI. We found that this new construct (TrEGII-TeTrCBHI, Fusion 3) had an increased secretion level of at least threefold in L. starkeyi compared to the expression level of the chimeric TeTrCBHI. However, the same improvements were not observed when Fusion 3 construct was expressed in S. cerevisiae and Y. lipolytica. Digestion of pretreated corn stover with the secretomes of Y. lipolytica and L. starkeyi showed that conversion was much better using Y. lipolytica secretomes (50% versus 29%, respectively). In Y. lipolytica, TeTrCBHI performed better than the fusion construct. Furthermore, S. cerevisiae expression of Fusion 3 construct was poor and only minimal activity was observed when acting on the substrate, pNP-cellobiose. No activity was observed for the pNP-lactose substrate. Clearly, this approach is not universally applicable to all yeasts, but works in specific cases. With purified protein and soluble substrates, the exoglucanase activity of the GH7 domain embedded in the Fusion 3 construct in L. starkeyi was significantly higher than that of the GH7 domain in TeTrCBHI expressed alone. It is probable that a higher fraction of fusion construct CBHI is in an active form in Fusion 3 compared to just TeTrCBHI. We conclude that the strategy of leading TeTrCBHI expression with a linked TrEGII module significantly improved the expression of active CBHI in L. starkeyi.

Fused dimerization increases expression, solubility, and activity of bacterial dehydratase enzymes

FabA and FabZ are the two dehydratase enzymes in Escherichia coli that catalyze the dehydration of acyl intermediates in the biosynthesis of fatty acids. Both enzymes form obligate dimers in which the active site contains key amino acids from both subunits. While FabA is a soluble protein that has been relatively straightforward to express and to purify from cultured E. coli, FabZ has shown to be mostly insoluble and only partially active. In an effort to increase the solubility and activity of both dehydratases, we made constructs consisting of two identical subunits of FabA or FabZ fused with a naturally occurring peptide linker, so as to force their dimerization. The fused dimer of FabZ (FabZ-FabZ) was expressed as a soluble enzyme with an ninefold higher activity in vitro than the unfused FabZ. This construct exemplifies a strategy for the improvement of enzymes from the fatty acid biosynthesis pathways, many of which function as dimers, catalyzing critical steps for the production of fatty acids.

Development of a bifunctional xylanase-cellulase chimera with enhanced activity on rice and barley straws using a modular xylanase and an endoglucanase procured from camel rumen metagenome

The camel rumen metagenome is an untapped source of glycoside hydrolases. In this study, novel genes encoding for a modular xylanase (XylC) and a cellulase (CelC) were isolated from a camel rumen metagenome and expressed in Escherichia coli BL21 (DE3). XylC with xylanase (Xyn), CBM, and carbohydrate esterase (CE) domains was characterized as a β-1,4-endoxylanase with remarkable catalytic activity on oat-spelt xylan (K cat = 2919 ± 57 s-1). The implication of XylC's modular structure in its high catalytic activity was analyzed by truncation and fusion construction with CelC. The resulting fusions including Cel-CBM, Cel-CBM-CE, and Xyn-CBM-Cel showed remarkable enhancement in CMCase activity with K cat values of 742 ± 12, 1289 ± 34.5, and 2799 ± 51 s-1 compared to CelC with a K cat of 422 ± 3.5 s-1. It was also shown that the bifunctional Xyn-CBM-Cel with synergistic xylanase/cellulase activities was more efficient than XylC and CelC in hydrolysis of rice and barley straws.

A review on chimeric xylanases: methods and conditions

Multi-functional enzymes are one of the nature's solutions to facilitate metabolic pathways, thus several reactions are regulated and performed simultaneously on one polypeptide chain. Inspired by nature, artificial chimeric proteins have been designed to reduce the production costs and improve the performance. One of the interesting applications of this method is in the plant-based industries such as feed additive, waste treatment, biofuel production, and pulp and paper bleaching. In fact, the heterogeneous texture of plants needs using a combination of different enzymes to achieve an optimal quality in the manufacturing process. Given that xylans are the most abundant non-cellulosic polysaccharides in nature, xylanases are widely utilized in the mentioned industries. In this regard, several studies have been conducted to develop the relevant chimeric enzymes. Despite the successes that have been attained in this field, misfolding, functional or structural interference, and linker breakage have been reported in some cases. The present paper reviews the research to introduce the prerequisites to design an appropriate chimeric xylanase.

Construction of a novel lipolytic fusion biocatalyst GDEst-lip for industrial application

The gene encoding esterase (GDEst-95) from Geobacillus sp. 95 was cloned and sequenced. The resulting open reading frame of 1497 nucleotides encoded a protein with calculated molecular weight of 54.7 kDa, which was classified as a carboxylesterase with an identity of 93-97% to carboxylesterases from Geobacillus bacteria. This esterase can be grouped into family VII of bacterial lipolytic enzymes, was active at broad pH (7-12) and temperature (5-85 °C) range and displayed maximum activity toward short acyl chain p-nitrophenyl (p-NP) esters. Together with GD-95 lipase from Geobacillus sp. strain 95, GDEst-95 esterase was used for construction of fused chimeric biocatalyst GDEst-lip. GDEst-lip esterase/lipase possessed high lipolytic activity (600 U/mg), a broad pH range of 6-12, thermoactivity (5-85 °C), thermostability and resistance to various organic solvents or detergents. For these features GDEst-lip biocatalyst has high potential for applications in various industrial areas. In this work the effect of additional homodomains on monomeric GDEst-95 esterase and GD-95 lipase activity, thermostability, substrate specificity and catalytic properties was also investigated. Altogether, this article shows that domain fusing strategies can modulate the activity and physicochemical characteristics of target enzymes for industrial applications.

Influence of Linker Length Variations on the Biomass-Degrading Performance of Heat-Active Enzyme Chimeras

Plant cell walls are composed of complex polysaccharides such as cellulose and hemicellulose. In order to efficiently hydrolyze cellulose, the synergistic action of several cellulases is required. Some anaerobic cellulolytic bacteria form multienzyme complexes, namely cellulosomes, while other microorganisms produce a portfolio of diverse enzymes that work in synergistic fashion. Molecular biological methods can mimic such effects through the generation of artificial bi- or multifunctional fusion enzymes. Endoglucanase and β-glucosidase from extremely thermophilic anaerobic bacteria Fervidobacterium gondwanense and Fervidobacterium islandicum, respectively, were fused end-to-end in an approach to optimize polysaccharide degradation. Both enzymes are optimally active at 90 °C and pH 6.0-7.0 representing excellent candidates for fusion experiments. The direct linkage of both enzymes led to an increased activity toward the substrate specific for β-glucosidase, but to a decreased activity of endoglucanase. However, these enzyme chimeras were superior over 1:1 mixtures of individual enzymes, because combined activities resulted in a higher final product yield. Therefore, such fusion enzymes exhibit promising features for application in industrial bioethanol production processes.

Is it possible to optimize the protein production yield by the generation of homomultimeric fusion enzymes?

Background

The supply of industrially relevant biocatalysts demands an easy and efficient protein production in high yield. In a conventional approach, a recombinant protein is produced in a heterologous host enabling the manipulation of multiple parameters including expression plasmids, growth conditions and regulation of protein biosynthesis. In this study, the generation of homomultimeric fusion genes is tested as an additional parameter to increase the production yield of a heat-stable cellulase.

Findings

The LE (LguI/Eco81I)-cloning strategy was used to generate a set of plasmids containing a single copy or two to four repetitions of the endoglucanase-encoding gene cel5A from the thermophilic anaerobe Fervidobacterium gondwanense. Serial up-scaling of shaking flask volumes from 50 to 500 mL were used to determine the production yield of active cellulolytic enzyme Cel5A in recombinant form in Escherichia coli. Monitoring the cellular wet weight and total protein proved that the bacterial growth rate is not depending on the production of fusion enzymes, however activity assays in combination with Western blotting analyses indicated instability effects of large homomultimeric fusion enzymes.

Conclusion

The production yield of fusion cellulases is constant with increasing molecular weights, but improved activities were not observed for recombinant Cel5A homomultimers. This strategy may serve as a starting point for further studies to generate more stable fusion proteins with improved catalytic activities and higher protein yield in the future.

Electronic supplementary material

The online version of this article (doi:10.1186/s40064-016-1968-0) contains supplementary material, which is available to authorized users.

Fast and reliable production, purification and characterization of heat-stable, bifunctional enzyme chimeras

Degradation of complex plant biomass demands a fine-regulated portfolio of glycoside hydrolases. The LE (LguI/Eco81I)-cloning approach was used to produce two enzyme chimeras CB and BC composed of an endoglucanase Cel5A (C) from the extreme thermophilic bacterium Fervidobacterium gondwanense and an archaeal β-glucosidase Bgl1 (B) derived from a hydrothermal spring metagenome. Recombinant chimeras and parental enzymes were produced in Escherichia coli and purified using a two-step affinity chromatography approach. Enzymatic properties revealed that both chimeras closely resemble the parental enzymes and physical mixtures, but Cel5A displayed lower temperature tolerance at 100°C when fused to Bgl1 independent of the conformational order. Moreover, the determination of enzymatic performances resulted in the detection of additive effects in case of BC fusion chimera. Kinetic measurements in combination with HPLC-mediated product analyses and site-directed mutation constructs indicated that Cel5A was strongly impaired when fused at the N-terminus, while activity was reduced to a slighter extend as C-terminal fusion partner. In contrast to these results, catalytic activity of Bgl1 at the N-terminus was improved 1.2-fold, effectively counteracting the slightly reduced activity of Cel5A by converting cellobiose into glucose. In addition, cellobiose exhibited inhibitory effects on Cel5A, resulting in a higher yield of cellobiose and glucose by application of an enzyme mixture (53.1%) compared to cellobiose produced from endoglucanase alone (10.9%). However, the overall release of cellobiose and glucose was even increased by catalytic action of BC (59.2%). These results indicate possible advantages of easily produced bifunctional fusion enzymes for the improved conversion of complex polysaccharide plant materials.

Genetically modified proteins: functional improvement and chimeragenesis

This review focuses on the emerging role of site-specific mutagenesis and chimeragenesis for the functional improvement of proteins in areas where traditional protein engineering methods have been extensively used and practically exhausted. The novel path for the creation of the novel proteins has been created on the farther development of the new structure and sequence optimization algorithms for generating and designing the accurate structure models in result of x-ray crystallography studies of a lot of proteins and their mutant forms. Artificial genetic modifications aim to expand nature's repertoire of biomolecules. One of the most exciting potential results of mutagenesis or chimeragenesis finding could be design of effective diagnostics, bio-therapeutics and biocatalysts. A sampling of recent examples is listed below for the in vivo and in vitro genetically improvement of various binding protein and enzyme functions, with references for more in-depth study provided for the reader's benefit.

Destructuring plant biomass: focus on fungal and extremophilic cell wall hydrolases

The use of plant biomass as feedstock for biomaterial and biofuel production is relevant in the current bio-based economy scenario of valorizing renewable resources. Fungi, which degrade complex and recalcitrant plant polymers, secrete different enzymes that hydrolyze plant cell wall polysaccharides. The present review discusses the current research trends on fungal, as well as extremophilic cell wall hydrolases that can withstand extreme physico-chemical conditions required in efficient industrial processes. Secretomes of fungi from the phyla Ascomycota, Basidiomycota, Zygomycota and Neocallimastigomycota are presented along with metabolic cues (nutrient sensing, coordination of carbon and nitrogen metabolism) affecting their composition. We conclude the review by suggesting further research avenues focused on the one hand on a comprehensive analysis of the physiology and epigenetics underlying cell wall degrading enzyme production in fungi and on the other hand on the analysis of proteins with unknown function and metagenomics of extremophilic consortia. The current advances in consolidated bioprocessing, altered secretory pathways and creation of designer plants are also examined. Furthermore, recent developments in enhancing the activity, stability and reusability of enzymes based on synergistic, proximity and entropic effects, fusion enzymes, structure-guided recombination between homologous enzymes and magnetic enzymes are considered with a view to improving saccharification.