Production of minicellulosomes for the enhanced hydrolysis of cellulosic substrates by recombinant Corynebacterium glutamicum

Production of a 135-residue long N-truncated human keratinocyte growth factor 1 in Escherichia coli

BACKGROUND

Palifermin (trade name Kepivance®) is an amino-terminally truncated recombinant human keratinocyte growth factor 1 (KGF-1) with 140 residues that has been produced using Escherichia coli to prevent and treat oral mucositis following radiation or chemotherapy. In this study, an amino-terminally shortened KGF-1 variant with 135 residues was produced and purified in E. coli, and its cell proliferation activity was evaluated.

RESULTS

We expressed soluble KGF-1 fused to thioredoxin (TRX) in the cytoplasmic fraction of E. coli to improve its production yield. However, three N-truncated forms (KGF-1 with 140, 138, and 135 residues) were observed after the removal of the TRX protein from the fusion form by cleavage of the human enterokinase light chain C112S (hEK_L C112S). The shortest KGF-1 variant, with 135 residues, was expressed by fusion with TRX via the hEK_L cleavage site in E. coli and purified at high purity (> 99%). Circular dichroism spectroscopy shows that purified KGF-1₁₃₅ had a structure similar to that of the KGF-1₁₄₀ as a random coiled form, and MCF-7 cell proliferation assays demonstrate its biological activity.

CONCLUSIONS

We identified variations in N-terminus-truncated KGF-1 and selected the most stable form. Furthermore, by a simple two-step purification, highly purified KGF-1₁₃₅ was obtained that showed biological activity. These results demonstrate that KGF-1₁₃₅ may be considered an alternative protein to KGF-1.

Synthetic biology tools for engineering Corynebacterium glutamicum

Corynebacterium glutamicum is a promising organism for the industrial production of amino acids, fuels, and various value-added chemicals. From the whole genome sequence release, C. glutamicum has been valuable in the field of industrial microbiology and biotechnology. Continuous discovery of genetic manipulations and regulation mechanisms has developed C. glutamicum as a synthetic biology platform chassis. This review summarized diverse genomic manipulation technologies and gene expression tools for static, dynamic, and multiplex control at transcription and translation levels. Moreover, we discussed the current challenges and applicable tools to C. glutamicum for future advancements.

Surface display of enzyme complex on Corynebacterium glutamicum as a whole cell biocatalyst and its consolidated bioprocessing using fungal-pretreated lignocellulosic biomass

A novel whole cell biocatalyst using fungal-pretreated lignocellulosic biomass was developed by displaying the enzyme complex consisting of N-acetylglucosaminidase (cNAG) and endoglucanse E (cCelE) on Corynebacterium glutamicum, hereafter called mNC. mNC showed a maximum 4.43-fold cNAG and 2.40-fold cCelE activity compared to single enzyme-secreting C. glutamicum. mNC also showed the highest efficiency of sugar production in various types of cellulose and fungal-pretreated biomass. The growth of mNC was 5.06-fold higher than that of the control. Then, the ability of mNC to produce a valuable chemical was confirmed. mNC overexpressing isopropanol biosynthesis genes showed a maximum titer of 218.9 ± 11.73 mg/L isopropanol and maintained high efficiency for isopropanol production in the recycling test, which was 90.07 ± 4.12 % during 4 cycles. This strategy can be applied to the direct saccharification of fungal-pretreated lignocellulosic biomass efficiently leading to the production of valuable products in various industrial fields.

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.

Consolidated Bioprocessing: Synthetic Biology Routes to Fuels and Fine Chemicals

The long road from emerging biotechnologies to commercial “green” biosynthetic routes for chemical production relies in part on efficient microbial use of sustainable and renewable waste biomass feedstocks. One solution is to apply the consolidated bioprocessing approach, whereby microorganisms convert lignocellulose waste into advanced fuels and other chemicals. As lignocellulose is a highly complex network of polymers, enzymatic degradation or “saccharification” requires a range of cellulolytic enzymes acting synergistically to release the abundant sugars contained within. Complications arise from the need for extracellular localisation of cellulolytic enzymes, whether they be free or cell-associated. This review highlights the current progress in the consolidated bioprocessing approach, whereby microbial chassis are engineered to grow on lignocellulose as sole carbon sources whilst generating commercially useful chemicals. Future perspectives in the emerging biofoundry approach with bacterial hosts are discussed, where solutions to existing bottlenecks could potentially be overcome though the application of high throughput and iterative Design-Build-Test-Learn methodologies. These rapid automated pathway building infrastructures could be adapted for addressing the challenges of increasing cellulolytic capabilities of microorganisms to commercially viable levels.

Research progress and the biotechnological applications of multienzyme complex

The multienzyme complex system has become a research focus in synthetic biology due to its highly efficient overall catalytic ability and has been applied to various fields. Multienzyme complexes are formed by cascading complexes, which are multiple functionally related enzymes that continuously and efficiently catalyze the production of substrates. Compared with current mainstream microbial cell catalytic systems, in vitro multienzyme molecular machines have many advantages, such as fewer side reactions, a high product yield, a fast reaction speed, easy product separation, a tolerable toxic environment, and robust system operability, showing increasing competitiveness in the field of biomanufacturing. In this review, the research progress of multienzyme complexes in nature and multienzyme cascades in vivo or in vitro will be introduced, and the discovered enzyme cascades concerning scaffolding proteins will also be discussed. This review is expected to provide a more theoretical basis for the modification of multienzyme complexes and broaden their application in the field of synthetic biology. KEY POINTS: • The cascade reactions of some natural multienzyme complexes are reviewed. • The main approaches of constructing artificial multienzyme complexes are summarized. • The structure and application of cellulosomes are discussed and prospected.

Cellulosomes: Highly Efficient Cellulolytic Complexes

Cellulosomes are elaborate multienzyme complexes capable of efficiently deconstructing lignocellulosic substrates, produced by cellulolytic anaerobic microorganisms, colonizing a large variety of ecological niches. These macromolecular structures have a modular architecture and are composed of two main elements: the cohesin-bearing scaffoldins, which are non-catalytic structural proteins, and the various dockerin-bearing enzymes that tenaciously bind to the scaffoldins. Cellulosome assembly is mediated by strong and highly specific interactions between the cohesin modules, present in the scaffoldins, and the dockerin modules, present in the catalytic units. Cellulosomal architecture and composition varies between species and can even change within the same organism. These differences seem to be largely influenced by external factors, including the nature of the available carbon-source. Even though cellulosome producing organisms are relatively few, the development of new genomic and proteomic technologies has allowed the identification of cellulosomal components in many archea, bacteria and even some primitive eukaryotes. This reflects the importance of this cellulolytic strategy and suggests that cohesin-dockerin interactions could be involved in other non-cellulolytic processes. Due to their building-block nature and highly cellulolytic capabilities, cellulosomes hold many potential biotechnological applications, such as the conversion of lignocellulosic biomass in the production of biofuels or the development of affinity based technologies.

Conjugated Protein Domains as Engineered Scaffold Proteins

Assembly of proteins into higher-order complexes generates specificity and selectivity in cellular signaling. Signaling complex formation is facilitated by scaffold proteins that use modular scaffolding domains, which recruit specific pathway enzymes. Multimerization and recombination of these conjugated native domains allows the generation of libraries of engineered multidomain scaffold proteins. Analysis of these engineered proteins has provided molecular insight into the regulatory mechanism of the native scaffold proteins and the applicability of these synthetic variants. This topical review highlights the use of engineered, conjugated multidomain scaffold proteins on different length scales in the context of synthetic signaling pathways, metabolic engineering, liquid-liquid phase separation, and hydrogel formation.

Metabolic engineering advances and prospects for amino acid production

Amino acid fermentation is one of the major pillars of industrial biotechnology. The multi-billion USD amino acid market is rising steadily and is diversifying. Metabolic engineering is no longer focused solely on strain development for the bulk amino acids L-glutamate and L-lysine that are produced at the million-ton scale, but targets specialty amino acids. These demands are met by the development and application of new metabolic engineering tools including CRISPR and biosensor technologies as well as production processes by enabling a flexible feedstock concept, co-production and co-cultivation schemes. Metabolic engineering advances are exemplified for specialty proteinogenic amino acids, cyclic amino acids, omega-amino acids, and amino acids functionalized by hydroxylation, halogenation and N-methylation.

Creating a New Pathway in Corynebacterium glutamicum for the Production of Taurine as a Food Additive

Taurine is a biologically and physiologically valuable food additive. However, commercial taurine production mainly relies on environmentally harmful chemical synthesis. Herein, for the first time in bacteria, we attempted to produce taurine in metabolically engineered Corynebacterium glutamicum. The taurine-producing strain was developed by introducing cs, cdo1, and csad genes. Interestingly, while the control strain could not produce taurine, the engineered strains successfully produced taurine via the newly introduced metabolic pathway. Furthermore, we investigated the effect of a deletion strain of the transcriptional repressor McbR gene on taurine production. As a result, sulfur accumulation and l-cysteine biosynthesis were reinforced by the McbR deletion strain, which further increased the taurine production by 2.3-fold. Taurine production of the final engineered strain Tau11 was higher than in other previously reported strains. This study demonstrated a potential approach for eco-friendly biosynthesis as an alternative to the chemical synthesis of a food additive.

Integration of Bacterial Expansin on Agarolytic Complexes to Enhance the Degrading Activity of Red Algae by Control of Gelling Properties

Expansin act by loosening hydrogen bonds in densely packed polysaccharides. This work characterizes the biological functions of expansin in the gelling and degradation of algal polysaccharides. In this study, the bacterial expansin BpEX from Bacillus pumilus was fused with the dockerin module of a cellulosome system for assembly with agarolytic complexes. The assembly of chimeric expansin caused an indicative enhancement in agarase activity. The enzymatic activities on agar substrate and natural biomass were 3.7-fold and 3.3-fold higher respectively than that of agarase as a single enzyme. To validate the effect on the agar degradation, the regulation potential of parameters related to gel rheology by bacterial expansin was experimentally investigated to indicate that the bacterial expansin lowered the gelling temperature and viscosity of agar. Thus, these results demonstrated the possibility of advancing more efficient strategies for utilizing agar as oligo sugar source in the biorefinery field that uses marine biomass as feedstocks.

Metabolic Design of Corynebacterium glutamicum for Production of l-Cysteine with Consideration of Sulfur-Supplemented Animal Feed

l-Cysteine is a valuable sulfur-containing amino acid widely used as a nutrition supplement in industrial food production, agriculture, and animal feed. However, this amino acid is mostly produced by acid hydrolysis and extraction from human or animal hairs. In this study, we constructed recombinant Corynebacterium glutamicum strains that overexpress combinatorial genes for l-cysteine production. The aims of this work were to investigate the effect of the combined overexpression of serine acetyltransferase (CysE), O-acetylserine sulfhydrylase (CysK), and the transcriptional regulator CysR on l-cysteine production. The CysR-overexpressing strain accumulated approximately 2.7-fold more intracellular sulfide than the control strain (empty pMT-tac vector). Moreover, in the resulting CysEKR recombinant strain, combinatorial overexpression of genes involved in l-cysteine production successfully enhanced its production by approximately 3.0-fold relative to that in the control strain. This study demonstrates a biotechnological model for the production of animal feed supplements such as l-cysteine using metabolically engineered C. glutamicum.

Cellulosomes: bacterial nanomachines for dismantling plant polysaccharides

Cellulosomes are multienzyme complexes that are produced by anaerobic cellulolytic bacteria for the degradation of lignocellulosic biomass. They comprise a complex of scaffoldin, which is the structural subunit, and various enzymatic subunits. The intersubunit interactions in these multienzyme complexes are mediated by cohesin and dockerin modules. Cellulosome-producing bacteria have been isolated from a large variety of environments, which reflects their prevalence and the importance of this microbial enzymatic strategy. In a given species, cellulosomes exhibit intrinsic heterogeneity, and between species there is a broad diversity in the composition and configuration of cellulosomes. With the development of modern technologies, such as genomics and proteomics, the full protein content of cellulosomes and their expression levels can now be assessed and the regulatory mechanisms identified. Owing to their highly efficient organization and hydrolytic activity, cellulosomes hold immense potential for application in the degradation of biomass and are the focus of much effort to engineer an ideal microorganism for the conversion of lignocellulose to valuable products, such as biofuels.

Design of nanoscale enzyme complexes based on various scaffolding materials for biomass conversion and immobilization

The utilization of scaffolds for enzyme immobilization involves advanced bionanotechnology applications in biorefinery fields, which can be achieved by optimizing the function of various enzymes. This review presents various current scaffolding techniques based on proteins, microbes and nanomaterials for enzyme immobilization, as well as the impact of these techniques on the biorefinery of lignocellulosic materials. Among them, architectural scaffolds have applied to useful strategies for protein engineering to improve the performance of immobilized enzymes in several industrial and research fields. In complexed enzyme systems that have critical roles in carbon metabolism, scaffolding proteins assemble different proteins in relatively durable configurations and facilitate collaborative protein interactions and functions. Additionally, a microbial strain, combined with designer enzyme complexes, can be applied to the immobilizing scaffold because the in vivo immobilizing technique has several benefits in enzymatic reaction systems related to both synthetic biology and metabolic engineering. Furthermore, with the advent of nanotechnology, nanomaterials possessing ideal physicochemical characteristics, such as mass transfer resistance, specific surface area and efficient enzyme loading, can be applied as novel and interesting scaffolds for enzyme immobilization. Intelligent application of various scaffolds to couple with nanoscale engineering tools and metabolic engineering technology may offer particular benefits in research.

Nanoscale Engineering of Designer Cellulosomes

Biocatalysts showcase the upper limit obtainable for high-speed molecular processing and transformation. Efforts to engineer functionality in synthetic nanostructured materials are guided by the increasing knowledge of evolving architectures, which enable controlled molecular motion and precise molecular recognition. The cellulosome is a biological nanomachine, which, as a fundamental component of the plant-digestion machinery from bacterial cells, has a key potential role in the successful development of environmentally-friendly processes to produce biofuels and fine chemicals from the breakdown of biomass waste. Here, the progress toward so-called "designer cellulosomes", which provide an elegant alternative to enzyme cocktails for lignocellulose breakdown, is reviewed. Particular attention is paid to rational design via computational modeling coupled with nanoscale characterization and engineering tools. Remaining challenges and potential routes to industrial application are put forward.

Efficient biological conversion of carbon monoxide (CO) to carbon dioxide (CO2) and for utilization in bioplastic production by Ralstonia eutropha through the display of an enzyme complex on the cell surface

An enzyme complex for biological conversion of CO to CO2 was anchored on the cell surface of the CO2-utilizing Ralstonia eutropha and successfully resulted in a 3.3-fold increase in conversion efficiency. These results suggest that this complexed system may be a promising strategy for CO2 utilization as a biological tool for the production of bioplastics.

Fungal-mediated consolidated bioprocessing: the potential of Fusarium oxysporum for the lignocellulosic ethanol industry

Microbial bioprocessing of lignocellulose to bioethanol still poses challenges in terms of substrate catabolism. The most important challenge is to overcome substrate recalcitrance and to thus reduce the number of steps needed to biorefine lignocellulose. Conventionally, conversion involves chemical pretreatment of lignocellulose, followed by hydrolysis of biomass to monomer sugars that are subsequently fermented into bioethanol. Consolidated bioprocessing (CBP) has been suggested as an efficient and economical method of manufacturing bioethanol from lignocellulose. CBP integrates the hydrolysis and fermentation steps into a single process, thereby significantly reducing the amount of steps in the biorefining process. Filamentous fungi are remarkable organisms that are naturally specialised in deconstructing plant biomass and thus they have tremendous potential as components of CBP. The fungus Fusarium oxysporum has potential for CBP of lignocellulose to bioethanol. Here we discuss the complexity and potential of CBP, the bottlenecks in the process, and the potential influence of fungal genetic diversity, substrate complexity and new technologies on the efficacy of CPB of lignocellulose, with a focus on F. oxysporum.

Toward Sustainable Amino Acid Production

Because the global amino acid production industry has been growing steadily and is expected to grow even more in the future, efficient production by fermentation is of great importance from economic and sustainability viewpoints. Many systems biology technologies, such as genome breeding, omics analysis, metabolic flux analysis, and metabolic simulation, have been employed for the improvement of amino acid-producing strains of bacteria. Synthetic biological approaches have recently been applied to strain development. It is also important to use sustainable carbon sources, such as glycerol or pyrolytic sugars from cellulosic biomass, instead of conventional carbon sources, such as glucose or sucrose, which can be used as food. Furthermore, reduction of sub-raw substrates has been shown to lead to reduction of environmental burdens and cost. Recently, a new fermentation system for glutamate production under acidic pH was developed to decrease the amount of one sub-raw material, ammonium, for maintenance of culture pH. At the same time, the utilization of fermentation coproducts, such as cells, ammonium sulfate, and fermentation broth, is a useful approach to decrease waste. In this chapter, further perspectives for future amino acid fermentation from one-carbon compounds are described.

5-Aminolevulinic acid production in engineered Corynebacterium glutamicum via C5 biosynthesis pathway

ALA (5-aminolevulinic acid) is an important intermediate in the synthesis of tetrapyrroles and the use of ALA has been gradually increasing in many fields, including medicine and agriculture. In this study, improved biological production of ALA in Corynebacterium glutamicum was achieved by overexpressing glutamate-initiated C5 pathway. For this purpose, copies of the glutamyl t-RNA reductase HemA from several bacteria were mutated by site-directed mutagenesis of which a HemA version from Salmonella typhimurium exhibited the highest ALA production. Cultivation of the HemA-expressing strain produced approximately 204 mg/L of ALA, while co-expression with HemL (glutamate-1-semialdehyde aminotransferase) increased ALA concentration to 457 mg/L, representing 11.6- and 25.9-fold increases over the control strain (17 mg/L of ALA). Further effects of metabolic perturbation were investigated, leading to penicillin addition that further improves ALA production to 584 mg/L. In an optimized flask fermentation, engineered C. glutamicum strains expressing the HemA and hemAL operon produced up to 1.1 and 2.2g/L ALA, respectively, under glutamate-producing conditions. The final yields represent 10.7- and 22.0-fold increases over the control strain (0.1g/L of ALA). From these findings, ALA biosynthesis from glucose was successfully demonstrated and this study is the first to report ALA overproduction in C. glutamicum via metabolic engineering.

Synthetic scaffold based on a cohesin–dockerin interaction for improved production of 2,3-butanediol in Saccharomyces cerevisiae

Substrate channeling is a process of transferring an intermediate from one enzyme to the next enzyme without diffusion into the bulk phase, thereby leading to an enhanced reaction rate. Here, we newly designed substrate channeling modules in Saccharomyces cerevisiae based on a high affinity interaction between dockerin and cohesin domains, which is a key process in the formation of cellulosome structure. Synthetic scaffolds containing two, three, or seven cohesin domains were constructed, and the assembly of dockerin-tagged proteins onto the scaffolds was confirmed by pull-down assay and bimolecular fluorescent complementation (BiFC) assay in vivo. This system was applied to produce 2,3-butanediol in S. cerevisiae by using dockerin-tagged AlsS, AlsD, and Bdh1 enzymes, resulting in a gradual increase in 2,3-butanediol production depending on the number of cohesin domains in the scaffold.

Signal amplification by a self-assembled biosensor system designed on the principle of dockerin-cohesin interactions in a cellulosome complex

To construct a self-assembled biosensor with signal amplification, a cellulosome system, comprising type I and type II dockerin-cohesin interactions with different specificities, from the anaerobic Clostridia bacterium was applied. The self-assembled biosensor was highly sensitive and achieved 128.1-fold increase in detection levels compared to the control.

Bi-functional cellulases complexes displayed on the cell surface of Corynebacterium glutamicum increase hydrolysis of lignocelluloses at elevated temperature

Introducing cellulases into Corynebacterium glutamicum leads to the direct degradation of lignocellulosic materials for energy sources. In this study, a cellulase complex containing two cellulolytic enzymes, endoglucanase E (CelE) and β-glucosidase A (BglA), was established to completely degrade cellulose to glucose. The cellulases complexes were displayed on the cell surface of C. glutamicum by using the mechanosensitive channel (Msc) to anchor enzymes in the cytoplasmic membrane. As confirmed by comparison enzyme activities in the cell pellet fraction and supernatant and dual color based immunofluorescence microscopy, the cellulolytic enzymes was successfully associated with the cell surface of C. glutamicum. The displayed cellulases complexes had a synergic effect on the direct conversion of biomass to reducing sugars leading to 3.1- to 6.0-fold increase compared to the conversion by the secreted cellulases complexes. In addition, the displayed cellulases complexes increased the residual activities of cCelE and cBglA at 70°C from 28.3% and 24.3% in the secreted form to 65.1% and 82.8%, respectively. The display of cellulases complexes on the cell surface of C. glutamicum enhances the polysaccharide equivalent and the direct saccharification of low cost biomass via the action of multi-thermostable enzyme complexes.

Challenges and advances in the heterologous expression of cellulolytic enzymes: a review

Second generation biofuel development is increasingly reliant on the recombinant expression of cellulases. Designing or identifying successful expression systems is thus of preeminent importance to industrial progress in the field. Recombinant production of cellulases has been performed using a wide range of expression systems in bacteria, yeasts and plants. In a number of these systems, particularly when using bacteria and plants, significant challenges have been experienced in expressing full-length proteins or proteins at high yield. Further difficulties have been encountered in designing recombinant systems for surface-display of cellulases and for use in consolidated bioprocessing in bacteria and yeast. For establishing cellulase expression in plants, various strategies are utilized to overcome problems, such as the auto-hydrolysis of developing plant cell walls. In this review, we investigate the major challenges, as well as the major advances made to date in the recombinant expression of cellulases across the commonly used bacterial, plant and yeast systems. We review some of the critical aspects to be considered for industrial-scale cellulase production.

Cellulosome-based, Clostridium-derived multi-functional enzyme complexes for advanced biotechnology tool development: advances and applications

The cellulosome is one of nature's most elegant and elaborate nanomachines and a key biological and biotechnological macromolecule that can be used as a multi-functional protein complex tool. Each protein module in the cellulosome system is potentially useful in an advanced biotechnology application. The high-affinity interactions between the cohesin and dockerin domains can be used in protein-based biosensors to improve both sensitivity and selectivity. The scaffolding protein includes a carbohydrate-binding module (CBM) that attaches strongly to cellulose substrates and facilitates the purification of proteins fused with the dockerin module through a one-step CBM purification method. Although the surface layer homology (SLH) domain of CbpA is not present in other strains, replacement of the cell surface anchoring domain allows a foreign protein to be displayed on the surface of other strains. The development of a hydrolysis enzyme complex is a useful strategy for consolidated bioprocessing (CBP), enabling microorganisms with biomass hydrolysis activity. Thus, the development of various configurations of multi-functional protein complexes for use as tools in whole-cell biocatalyst systems has drawn considerable attention as an attractive strategy for bioprocess applications. This review provides a detailed summary of the current achievements in Clostridium-derived multi-functional complex development and the impact of these complexes in various areas of biotechnology.

Metabolic engineering of industrial platform microorganisms for biorefinery applications--optimization of substrate spectrum and process robustness by rational and evolutive strategies

Bio-based production promises a sustainable route to myriads of chemicals, materials and fuels. With regard to eco-efficiency, its future success strongly depends on a next level of bio-processes using raw materials beyond glucose. Such renewables, i.e., polymers, complex substrate mixtures and diluted waste streams, often cannot be metabolized naturally by the producing organisms. This particularly holds for well-known microorganisms from the traditional sugar-based biotechnology, including Escherichia coli, Corynebacterium glutamicum and Saccharomyces cerevisiae which have been engineered successfully to produce a broad range of products from glucose. In order to make full use of their production potential within the bio-refinery value chain, they have to be adapted to various feed-stocks of interest. This review focuses on the strategies to be applied for this purpose which combine rational and evolutive approaches. Hereby, the three industrial platform microorganisms, E. coli, C. glutamicum and S. cerevisiae are highlighted due to their particular importance.

A review of enzymes and microbes for lignocellulosic biorefinery and the possibility of their application to consolidated bioprocessing technology

The biorefinery manufacturing process for producing chemicals and liquid fuels from biomass is a promising approach for securing energy and resources. To establish cost-effective fermentation of lignocellulosic biomass, the consolidation of sacccharification and fermentation processes is a desirable strategy, but requires the development of microorganisms capable of cellulose/hemicellulose hydrolysis and target chemical production. Such an endeavor requires a large number of prerequisites to be realized, including engineering microbial strains with high cellulolytic activity, high product yield, productivities, and titers, ability to use many carbon sources, and resistance to toxic compounds released during the pretreatment of lignocellulosic biomass. Researchers have focused on either engineering naturally cellulolytic microorganisms to improve product-related properties or modifying non-cellulolytic organisms with high product yields to become cellulolytic. This article reviews recent advances in the development of microorganisms for the production of renewable chemicals and advanced biofuels, as well as ethanol, from lignocellulosic materials through consolidated bioprocessing.

Next generation biofuel engineering in prokaryotes

Next-generation biofuels must be compatible with current transportation infrastructure and be derived from environmentally sustainable resources that do not compete with food crops. Many bacterial species have unique properties advantageous to the production of such next-generation fuels. However, no single species possesses all characteristics necessary to make high quantities of fuels from plant waste or CO2. Species containing a subset of the desired characteristics are used as starting points for engineering organisms with all desired attributes. Metabolic engineering of model organisms has yielded high titer production of advanced fuels, including alcohols, isoprenoids, and fatty acid derivatives. Technical developments now allow engineering of native fuel producers, as well as lignocellulolytic and autotrophic bacteria, for the production of biofuels. Continued research on multiple fronts is required to engineer organisms for truly sustainable and economical biofuel production.

Functional heterologous expression of an engineered full length CipA from Clostridium thermocellum in Thermoanaerobacterium saccharolyticum

BACKGROUND

Cellulose is highly recalcitrant and thus requires a specialized suite of enzymes to solubilize it into fermentable sugars. In C. thermocellum, these extracellular enzymes are present as a highly active multi-component system known as the cellulosome. This study explores the expression of a critical C. thermocellum cellulosomal component in T. saccharolyticum as a step toward creating a thermophilic bacterium capable of consolidated bioprocessing by employing heterologously expressed cellulosomes.

RESULTS

We developed an inducible promoter system based on the native T. saccharolyticum xynA promoter, which was shown to be induced by xylan and xylose. The promoter was used to express the cellulosomal component cipA*, an engineered form of the wild-type cipA from C. thermocellum. Expression and localization to the supernatant were both verified for CipA*. When a ΔcipA mutant C. thermocellum strain was cultured with a CipA*-expressing T. saccharolyticum strain, hydrolysis and fermentation of 10 grams per liter SigmaCell 101, a highly crystalline cellulose, were observed. This trans-species complementation of a cipA deletion demonstrated the ability for CipA* to assemble a functional cellulosome.

CONCLUSION

This study is the first example of an engineered thermophile heterologously expressing a structural component of a cellulosome. To achieve this goal we developed and tested an inducible promoter for controlled expression in T. saccharolyticum as well as a synthetic cipA. In addition, we demonstrate a high degree of hydrolysis (up to 93%) on microcrystalline cellulose.

Development of microorganisms for cellulose-biofuel consolidated bioprocessings: metabolic engineers' tricks

Cellulose waste biomass is the most abundant and attractive substrate for "biorefinery strategies" that are aimed to produce high-value products (e.g. solvents, fuels, building blocks) by economically and environmentally sustainable fermentation processes. However, cellulose is highly recalcitrant to biodegradation and its conversion by biotechnological strategies currently requires economically inefficient multistep industrial processes. The need for dedicated cellulase production continues to be a major constraint to cost-effective processing of cellulosic biomass. Research efforts have been aimed at developing recombinant microorganisms with suitable characteristics for single step biomass fermentation (consolidated bioprocessing, CBP). Two paradigms have been applied for such, so far unsuccessful, attempts: a) "native cellulolytic strategies", aimed at conferring high-value product properties to natural cellulolytic microorganisms; b) "recombinant cellulolytic strategies", aimed to confer cellulolytic ability to microorganisms exhibiting high product yields and titers. By starting from the description of natural enzyme systems for plant biomass degradation and natural metabolic pathways for some of the most valuable product (i.e. butanol, ethanol, and hydrogen) biosynthesis, this review describes state-of-the-art bottlenecks and solutions for the development of recombinant microbial strains for cellulosic biofuel CBP by metabolic engineering. Complexed cellulases (i.e. cellulosomes) benefit from stronger proximity effects and show enhanced synergy on insoluble substrates (i.e. crystalline cellulose) with respect to free enzymes. For this reason, special attention was held on strategies involving cellulosome/designer cellulosome-bearing recombinant microorganisms.