Improving activity of minicellulosomes by integration of intra- and intermolecular synergies

Enhancement of the catalytic performance of UIO-66 for the CO2 synthesis of cyclic carbonate using natural nanomaterials as a carrier

One of the environmentally friendly methods is the intelligent utilization of natural one-dimensional nanomaterials as carriers to improve the CO2 catalytic performance of MOF materials. This paper reports an efficient composite catalyst preparation using a cheap and readily available magnesium-aluminosilicate nanometer, attapulgite (ATP), as a carrier for MOF materials. Due to its Lewis acidic site and unique alkaline pore structure, ATP exhibits excellent catalytic activity in the coupling reaction of CO2 with epoxy compounds, and its regular one-dimensional nanorod shape has tremendous potential as a carrier compared to other natural minerals. Given the diversity of MOF material types and structures, the design of this UIO-66/ATP nanocomposite catalyst provides both a new pathway for CO2 capture and conversion and a developmental space for the synthesis of such nanocomposites.

Enzyme synergy for plant cell wall polysaccharide degradation

Valorizing plant cell wall, marine and algal polysaccharides is of utmost importance for the development of the circular bioeconomy. This is because polysaccharides are by far the most abundant organic molecules found in nature with complex chemical structures that require a large set of enzymes for their degradation. Microorganisms produce polysaccharide-specific enzymes that act in synergy when performing hydrolysis. Although discovered since decades enzyme synergy is still poorly understood at the molecular level and thus it is difficult to harness and optimize. In the last few years, more attention has been given to improve and characterize enzyme synergy for polysaccharide valorization. In this review, we summarize literature to provide an overview of the different type of synergy involving carbohydrate modifying enzymes and the recent advances in the field exemplified by plant cell-wall degradation.

Current challenges in designer cellulosome engineering

Designer cellulosomes (DCs) are engineered multi-enzyme complexes, comprising carbohydrate-active enzymes attached to a common backbone, the scaffoldin, via high-affinity cohesin-dockerin interactions. The use of DCs in the degradation of renewable biomass polymers is a promising approach for biorefineries. Indeed, DCs have shown significant hydrolytic activities due to the enhanced enzyme-substrate proximity and inter-enzyme synergies, but technical hurdles in DC engineering have hindered further progress towards industrial application. The challenge in DC engineering lies in the large diversity of possible building blocks and architectures, resulting in a multivariate and immense design space. Simultaneously, the precise DC composition affects many relevant parameters such as activity, stability, and manufacturability. Since protein engineers face a lack of high-throughput approaches to explore this vast design space, DC engineering may result in an unsatisfying outcome. This review provides a roadmap to guide researchers through the process of DC engineering. Each step, starting from concept to evaluation, is described and provided with its challenges, along with possible solutions, both for DCs that are assembled in vitro or are displayed on the yeast cell surface. KEY POINTS: • Construction of designer cellulosomes is a multi-step process. • Designer cellulosome research deals with multivariate construction challenges. • Boosting designer cellulosome efficiency requires exploring a vast design space.

Effect of multimodularity and spatial organization of glycoside hydrolases on catalysis

The wide diversity among the carbohydrate-active enzymes (CAZymes) reflects the equally broad versatility in terms of composition and chemicals bonds found in the plant cell wall polymers on which they are active. This diversity is also expressed through the various strategies developed to circumvent the recalcitrance of these substrates to biological degradation. Glycoside hydrolases (GHs) are the most abundant of the CAZymes and are expressed as isolated catalytic modules or in association with carbohydrate-binding module (CBM), acting in synergism within complex arrays of enzymes. This multimodularity can be even more complex. The cellulosome presents a scaffold protein immobilized to the outer membrane of some microorganisms on which enzymes are grafted to prevent their dispersion and increase catalytic synergism. In polysaccharide utilization loci (PUL), GHs are also distributed across the membranes of some bacteria to co-ordinate the deconstruction of polysaccharides and the internalization of metabolizable carbohydrates. Although the study and characterization of these enzymatic activities need to take into account the entirety of this complex organization-in particular because of the dynamics involved in it-technical problems limit the present study to isolated enzymes. However, these enzymatic complexes also have a spatiotemporal organization, whose still neglected aspect must be considered. In the present review, the different levels of multimodularity that can occur in GHs will be reviewed, from its simplest forms to the most complex. In addition, attempts to characterize or study the effect on catalytic activity of the spatial organization within GHs will be addressed.

Combinatorial assembly and optimisation of designer cellulosomes: a galactomannan case study

BACKGROUND

Designer cellulosomes are self-assembled chimeric enzyme complexes that can be used to improve lignocellulosic biomass degradation. They are composed of a synthetic multimodular backbone protein, termed the scaffoldin, and a range of different chimeric docking enzymes that degrade polysaccharides. Over the years, several functional designer cellulosomes have been constructed. Since many parameters influence the efficiency of these multi-enzyme complexes, there is a need to optimise designer cellulosome architecture by testing combinatorial arrangements of docking enzyme and scaffoldin variants. However, the modular cloning procedures are tedious and cumbersome.

RESULTS

VersaTile is a combinatorial DNA assembly method, allowing the rapid construction and thus comparison of a range of modular proteins. Here, we present the extension of the VersaTile platform to facilitate the construction of designer cellulosomes. We have constructed a tile repository, composed of dockerins, cohesins, linkers, tags and enzymatically active modules. The developed toolbox allows us to efficiently create and optimise designer cellulosomes at an unprecedented speed. As a proof of concept, a trivalent designer cellulosome able to degrade the specific hemicellulose substrate, galactomannan, was constructed and optimised. The main factors influencing cellulosome efficiency were found to be the selected dockerins and linkers and the docking enzyme ratio on the scaffoldin. The optimised designer cellulosome was able to hydrolyse the galactomannan polysaccharide and release mannose and galactose monomers.

CONCLUSION

We have eliminated one of the main technical hurdles in the designer cellulosome field and anticipate the VersaTile platform to be a starting point in the development of more elaborate multi-enzyme complexes.

Enzyme systems of thermophilic anaerobic bacteria for lignocellulosic biomass conversion

Economic production of lignocellulose degrading enzymes for biofuel industries is of considerable interest to the biotechnology community. While these enzymes are widely distributed in fungi, their industrial production from other sources, particularly by thermophilic anaerobic bacteria (growth T_opt ≥ 60 °C), is an emerging field. Thermophilic anaerobic bacteria produce a large number of lignocellulolytic enzymes having unique structural features and employ different schemes for biomass degradation, which can be classified into four systems namely; 'free enzyme system', 'cell anchored enzymes', 'complex cellulosome system', and 'multifunctional multimodular enzyme system'. Such enzymes exhibit high specific activity and have a natural ability to withstand harsh bioprocessing conditions. However, achieving a higher production of these thermostable enzymes at current bioprocessing targets is challenging. In this review, the research opportunities for these distinct enzyme systems in the biofuel industry and the associated technological challenges are discussed. The current status of research findings is highlighted along with a detailed description of the categorization of the different enzyme production schemes. It is anticipated that high temperature-based bioprocessing will become an integral part of sustainable bioenergy production in the near future.

Succession of Gut Microbial Structure in Twin Giant Pandas During the Dietary Change Stage and Its Role in Polysaccharide Metabolism

Adaptation to a bamboo diet is an essential process for giant panda growth, and gut microbes play an important role in the digestion of the polysaccharides in bamboo. The dietary transition in giant panda cubs is particularly complex, but it is an ideal period in which to study the effects of gut microbes on polysaccharide use because their main food changes from milk to bamboo (together with some bamboo shoot and coarse pastry). Here, we used 16S rDNA and internal transcribed spacer 1 (ITS1) DNA sequencing and metagenomic sequencing analysis to investigate the succession of the gut microbial structure in feces sampled from twin giant panda cubs during the completely dietary transition and determine the abundances of polysaccharide-metabolizing genes and their corresponding microbes to better understand the degradation of bamboo polysaccharides. Successive changes in the gut microbial diversity and structure were apparent in the growth of pandas during dietary shift process. Microbial diversity increased after the introduction of supplementary foods and then varied in a complex way for 1.5–2 years as bamboo and complex food components were introduced. They then stabilized after 2 years, when the cubs consumed a specialized bamboo diet. The microbes had more potential to metabolize the cellulose in bamboo than the hemicellulose, providing genes encoding cellulase systems corresponding to glycoside hydrolases (GHs; such as GH1, GH3, GH5, GH8, GH9, GH74, and GH94). The cellulose-metabolizing species (or genes) of gut bacteria was more abundant than that of gut fungi. Although cellulose-metabolizing species did not predominate in the gut bacterial community, microbial interactions allowed the giant pandas to achieve the necessary dietary shift and ultimately adapt to a bamboo diet.

Changing surface grafting density has an effect on the activity of immobilized xylanase towards natural polysaccharides

Enzymes are involved in various types of biological processes. In many cases, they are part of multi-component machineries where enzymes are localized in close proximity to each-other. In such situations, it is still not clear whether inter-enzyme spacing actually plays a role or if the colocalization of complementary activities is sufficient to explain the efficiency of the system. Here, we focus on the effect of spatial proximity when identical enzymes are immobilized onto a surface. By using an innovative grafting procedure based on the use of two engineered protein fragments, Jo and In, we produce model systems in which enzymes are immobilized at surface densities that can be controlled precisely. The enzyme used is a xylanase that participates to the hydrolysis of plant cell wall polymers. By using a small chromogenic substrate, we first show that the intrinsic activity of the enzymes is fully preserved upon immobilization and does not depend on surface density. However, when using beechwood xylan, a naturally occurring polysaccharide, as substrate, we find that the enzymatic efficiency decreases by 10–60% with the density of grafting. This unexpected result is probably explained through steric hindrance effects at the nanoscale that hinder proper interaction between the enzymes and the polymer. A second effect of enzyme immobilization at high densities is the clear tendency for the system to release preferentially shorter oligosaccharides from beechwood xylan as compared to enzymes in solution.

Development and characterization of stable anaerobic thermophilic methanogenic microbiomes fermenting switchgrass at decreasing residence times

BACKGROUND

Anaerobic fermentation of lignocellulose occurs in both natural and managed environments, and is an essential part of the carbon cycle as well as a promising route to sustainable production of fuels and chemicals. Lignocellulose solubilization by mixed microbiomes is important in these contexts.

RESULTS

Here, we report the development of stable switchgrass-fermenting enrichment cultures maintained at various residence times and moderately high (55 °C) temperatures. Anaerobic microbiomes derived from a digester inoculum were incubated at 55 °C and fed semi-continuously with medium containing 30 g/L mid-season harvested switchgrass to achieve residence times (RT) of 20, 10, 5, and 3.3 days. Stable, time-invariant cellulolytic methanogenic cultures with minimal accumulation of organic acids were achieved for all RTs. Fractional carbohydrate solubilization was 0.711, 0.654, 0.581 and 0.538 at RT = 20, 10, 5 and 3.3 days, respectively, and glucan solubilization was proportional to xylan solubilization at all RTs. The rate of solubilization was described well by the equation r = k(C - C₀f_r), where C represents the concentration of unutilized carbohydrate, C₀ is the concentration of carbohydrate (cellulose and hemicellulose) entering the bioreactor and f_r is the extrapolated fraction of entering carbohydrate that is recalcitrant at infinite residence time. The 3.3 day RT is among the shortest RT reported for stable thermophilic, methanogenic digestion of a lignocellulosic feedstock. 16S rDNA phylotyping and metagenomic analyses were conducted to characterize the effect of RT on community dynamics and to infer functional roles in the switchgrass to biogas conversion to the various microbial taxa. Firmicutes were the dominant phylum, increasing in relative abundance from 54 to 96% as RT decreased. A Clostridium clariflavum strain with genetic markers for xylose metabolism was the most abundant lignocellulose-solubilizing bacterium. A Thermotogae (Defluviitoga tunisiensis) was the most abundant bacterium in switchgrass digesters at RT = 20 days but decreased in abundance at lower RTs as did multiple Chloroflexi. Synergistetes and Euryarchaeota were present at roughly constant levels over the range of RTs examined.

CONCLUSIONS

A system was developed in which stable methanogenic steady-states were readily obtained with a particulate biomass feedstock, mid-season switchgrass, at laboratory (1 L) scale. Characterization of the extent and rate of carbohydrate solubilization in combination with 16S rDNA and metagenomic sequencing provides a multi-dimensional view of performance, species composition, glycoside hydrolases, and metabolic function with varying residence time. These results provide a point of reference and guidance for future studies and organism development efforts involving defined cultures.

Correlation Between Size and Activity Enhancement of Recombinantly Assembled Cellulosomes

As multienzyme complexes, cellulosomes hydrolyze cellulosic biomass with high efficiency, which is believed to be attributed to either one or both factors: (1) synergy among the catalytic and substrate-binding entities and (2) the large size of cellulosome complexes. Although the former factor has been extensively documented, the correlation between size and specific activity of cellulosomes is still elusive to date. In this study, primary and secondary scaffoldins with 1, 3, or 5 copies of type I/II cohesin domains were recombinantly synthesized and various cellulosomes carrying 1, 3, 5, 9, 15, or 25 molecules of cellulase mixtures of family 5, 9, and 48 glycoside hydrolases were assembled. In addition, the assembled complex was annexed to cellulose with the aid of a family 3a carbohydrate-binding module (CBM3a). Measuring cellulolytic hydrolysis activities of assembled cellulosomes on crystalline Avicel revealed that higher degree of cellulosome complexity resulted in more efficient cellulose hydrolysis with plateaued synergic effects after the cellulosome size reaches certain degree.

Function, distribution, and annotation of characterized cellulases, xylanases, and chitinases from CAZy

The enzymatic deconstruction of structural polysaccharides, which relies on the production of specific glycoside hydrolases (GHs), is an essential process across environments. Over the past few decades, researchers studying the diversity and evolution of these enzymes have isolated and biochemically characterized thousands of these proteins. The carbohydrate-active enzymes database (CAZy) lists these proteins and provides some metadata. Here, the sequences and metadata of characterized sequences derived from GH families associated with the deconstruction of cellulose, xylan, and chitin were collected and discussed. First, although few polyspecific enzymes are identified, characterized GH families are mostly monospecific. Next, the taxonomic distribution of characterized GH mirrors the distribution of identified sequences in sequenced genomes. This provides a rationale for connecting the identification of GH sequences to specific reactions or lineages. Finally, we tested the annotation of the characterized GHs using HMM scan and the protein families database (Pfam). The vast majority of GHs targeting cellulose, xylan, and chitin can be identified using this publicly accessible approach.

Combining in Vitro and in Silico Single-Molecule Force Spectroscopy to Characterize and Tune Cellulosomal Scaffoldin Mechanics

Cellulosomes are polyprotein machineries that efficiently degrade cellulosic material. Crucial to their function are scaffolds consisting of highly homologous cohesin domains, which serve a dual role by coordinating a multiplicity of enzymes as well as anchoring the microbe to its substrate. Here we combined two approaches to elucidate the mechanical properties of the main scaffold ScaA of Acetivibrio cellulolyticus. A newly developed parallelized one-pot in vitro transcription-translation and protein pull-down protocol enabled high-throughput atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS) measurements of all cohesins from ScaA with a single cantilever, thus promising improved relative force comparability. Albeit very similar in sequence, the hanging cohesins showed considerably lower unfolding forces than the bridging cohesins, which are subjected to force when the microbe is anchored to its substrate. Additionally, all-atom steered molecular dynamics (SMD) simulations on homology models offered insight into the process of cohesin unfolding under force. Based on the differences among the individual force propagation pathways and their associated correlation communities, we designed mutants to tune the mechanical stability of the weakest hanging cohesin. The proposed mutants were tested in a second high-throughput AFM SMFS experiment revealing that in one case a single alanine to glycine point mutation suffices to more than double the mechanical stability. In summary, we have successfully characterized the force induced unfolding behavior of all cohesins from the scaffoldin ScaA, as well as revealed how small changes in sequence can have large effects on force resilience in cohesin domains. Our strategy provides an efficient way to test and improve the mechanical integrity of protein domains in general.

Modular Organization of the Thermobifida fusca Exoglucanase Cel6B Impacts Cellulose Hydrolysis and Designer Cellulosome Efficiency

Cellulose deconstruction can be achieved by three distinct enzymatic paradigms: free enzymes, multifunctional enzymes, and self-assembled, multi-enzyme complexes (cellulosomes). To study their comparative efficiency, the simple and efficient cellulolytic system of the aerobic bacterium, Thermobifida fusca, is developed as an enzymatic model. In previous studies, most of its cellulases are successfully converted to the cellulosomal mode and exhibited high cellulolytic activities, except for Cel6B, a key exoglucanase of the T. fusca enzymatic system. Here, the impact of the modular organization of Cel6B on enzymatic activity is investigated. The position of the cellulose-binding module (CBM), its family and linker segment are shown to affect activity. Surprisingly, exchange of the native family-2 CBM to family-3 generates an increase in Cel6B activity on cellulosic substrates. Conversion of Cel6B to the cellulosomal mode by fusing a cohesin to the catalytic module enables formation of divalent enzyme complexes with dockerin-bearing enzymes. The resultant pseudo-cellulosomes, containing Cel6B combined with endoglucanase Cel5A, exhibits enhanced enzymatic activity, compared to mixtures of wild-type enzymes or bifunctional enzymes, unlike similar pseudo-cellulosomes containing endoglucanase Cel6A or proccessive endoglucanase Cel9A. Insight into the different enzymatic paradigms benefits ongoing development of efficient cellulolytic systems for conversion of plant-derived biomass into valuable sugars.

NOVELTY STATEMENT

The protein engineering of the modular arrangement of a key exoglucanase from a highly cellulolytic bacterium, Thermobifida fusca, served to explore and compare three major enzymatic paradigms for cellulose degradation. This approach revealed highly active chimaeric forms of the exoglucanase that act in synergy together with a potent endoglucanase in bifunctional enzymes or divalent pseudo-cellulosome-like complexes. Such engineered enzymes could be further integrated into larger enzymatic complexes, thereby providing a significant step forward towards conversion of the entire T. fusca free cellulolytic system into the cellulosomal modex and the enhanced conversion of cellulosic biomass into soluble sugars.

Adaptor Scaffoldins: An Original Strategy for Extended Designer Cellulosomes, Inspired from Nature

Designer cellulosomes consist of chimeric cohesin-bearing scaffoldins for the controlled incorporation of recombinant dockerin-containing enzymes. The largest designer cellulosome reported to date is a chimeric scaffoldin that contains 6 cohesins. This scaffoldin represented a technical limit of sorts, since adding another cohesin proved problematic, owing to resultant low expression levels, instability (cleavage) of the scaffoldin polypeptide, and limited numbers of available cohesin-dockerin specificities—the hallmark of designer cellulosomes. Nevertheless, increasing the number of enzymes integrated into designer cellulosomes is critical, in order to further enhance degradation of plant cell wall material. Adaptor scaffoldins comprise an intermediate type of scaffoldin that can both incorporate various enzymes and attach to an additional scaffoldin. Using this strategy, we constructed an efficient form of adaptor scaffoldin that possesses three type I cohesins for enzyme integration, a single type II dockerin for interaction with an additional scaffoldin, and a carbohydrate-binding module for targeting to the cellulosic substrate. In parallel, we designed a hexavalent scaffoldin capable of connecting to the adaptor scaffoldin by the incorporation of an appropriate type II cohesin. The resultant extended designer cellulosome comprised 8 recombinant enzymes—4 xylanases and 4 cellulases—thereby representing a potent enzymatic cocktail for solubilization of natural lignocellulosic substrates. The contribution of the adaptor scaffoldin clearly demonstrated that proximity between the two scaffoldins and their composite set of enzymes is crucial for optimized degradation. After 72 h of incubation, the performance of the extended designer cellulosome was determined to be approximately 70% compared to that of native cellulosomes.

Plant cell wall residues represent a major source of renewable biomass for the production of biofuels such as ethanol via breakdown to soluble sugars. The natural microbial degradation process, however, is inefficient for achieving cost-effective processes in the conversion of plant-derived biomass to biofuels, either from dedicated crops or human-generated cellulosic wastes. The accumulation of the latter is considered a major environmental pollutant. The development of designer cellulosome nanodevices for enhanced plant cell wall degradation thus has major impacts in the fields of environmental pollution, bioenergy production, and biotechnology in general. The findings reported in this article comprise a true breakthrough in our capacity to produce extended designer cellulosomes via synthetic biology means, thus enabling the assembly of higher-order complexes that can supersede the number of enzymes included in a single multienzyme complex.

Dramatic performance of Clostridium thermocellum explained by its wide range of cellulase modalities

Clostridium thermocellum is the most efficient microorganism for solubilizing lignocellulosic biomass known to date. Its high cellulose digestion capability is attributed to efficient cellulases consisting of both a free-enzyme system and a tethered cellulosomal system wherein carbohydrate active enzymes (CAZymes) are organized by primary and secondary scaffoldin proteins to generate large protein complexes attached to the bacterial cell wall. This study demonstrates that C. thermocellum also uses a type of cellulosomal system not bound to the bacterial cell wall, called the "cell-free" cellulosomal system. The cell-free cellulosome complex can be seen as a "long range cellulosome" because it can diffuse away from the cell and degrade polysaccharide substrates remotely from the bacterial cell. The contribution of these two types of cellulosomal systems in C. thermocellum was elucidated by characterization of mutants with different combinations of scaffoldin gene deletions. The primary scaffoldin, CipA, was found to play the most important role in cellulose degradation by C. thermocellum, whereas the secondary scaffoldins have less important roles. Additionally, the distinct and efficient mode of action of the C. thermocellum exoproteome, wherein the cellulosomes splay or divide biomass particles, changes when either the primary or secondary scaffolds are removed, showing that the intact wild-type cellulosomal system is necessary for this essential mode of action. This new transcriptional and proteomic evidence shows that a functional primary scaffoldin plays a more important role compared to secondary scaffoldins in the proper regulation of CAZyme genes, cellodextrin transport, and other cellular functions.

Natural diversity of cellulases, xylanases, and chitinases in bacteria

BACKGROUND

Glycoside hydrolases (GH) targeting cellulose, xylan, and chitin are common in the bacterial genomes that have been sequenced. Little is known, however, about the architecture of multi-domain and multi-activity glycoside hydrolases. In these enzymes, combined catalytic domains act synergistically and thus display overall improved catalytic efficiency, making these proteins of high interest for the biofuel technology industry.

RESULTS

Here, we identify the domain organization in 40,946 proteins targeting cellulose, xylan, and chitin derived from 11,953 sequenced bacterial genomes. These bacteria are known to be capable, or to have the potential, to degrade polysaccharides, or are newly identified potential degraders (e.g., Actinospica, Hamadaea, Cystobacter, and Microbispora). Most of the proteins we identified contain a single catalytic domain that is frequently associated with an accessory non-catalytic domain. Regarding multi-domain proteins, we found that many bacterial strains have unique GH protein architectures and that the overall protein organization is not conserved across most genera. We identified 217 multi-activity proteins with at least two GH domains for cellulose, xylan, and chitin. Of these proteins, 211 have GH domains targeting similar or associated substrates (i.e., cellulose and xylan), whereas only six proteins target both cellulose and chitin. Fifty-two percent of multi-activity GHs are hetero-GHs. Finally, GH6, -10, -44 and -48 domains were mostly C-terminal; GH9, -11, -12, and -18 were mostly N-terminal; and GH5 domains were either N- or C-terminal.

CONCLUSION

We identified 40,946 multi-domain/multi-activity proteins targeting cellulase, chitinase, and xylanase in bacterial genomes and proposed new candidate lineages and protein architectures for carbohydrate processing that may play a role in biofuel production.

HaloTag mediated artificial cellulosome assembly on a rolling circle amplification DNA template for efficient cellulose hydrolysis

We report here the generation of artificial cellulosomes onto a DNA scaffold using the self-labeling HaloTag for DNA conjugation. Rolling circle amplification multiplexing templates were used to increase the complexity of this system with higher efficiency observed.

The contribution of cellulosomal scaffoldins to cellulose hydrolysis by Clostridium thermocellum analyzed by using thermotargetrons

BACKGROUND

Clostridium thermocellum is a thermophilic anaerobic bacterium that degrades cellulose by using a highly effective cellulosome, a macromolecular complex consisting of multiple cellulose degrading enzymes organized and attached to the cell surface by non-catalytic scaffoldins. However, due largely to lack of efficient methods for genetic manipulation of C. thermocellum, it is still unclear how the different scaffoldins and their functional modules contribute to cellulose hydrolysis.

RESULTS

We constructed C. thermocellum mutants with the primary scaffoldin CipA (cellulosome-integrating protein A) truncated at different positions or lacking four different secondary scaffoldins by using a newly developed thermotargetron system, and we analyzed cellulose hydrolysis, cellulosome formation, and cellulose binding of the mutants. A CipA truncation that deletes six type I cohesin modules, which bind cellulolytic enzymes, decreased cellulose hydrolysis rates by 46%, and slightly longer truncations that also delete the carbohydrate binding module decreased rates by 89 to 92%, indicating strong cellulosome-substrate synergy. By contrast, a small CipA truncation that deletes only the C-terminal type II dockerin (XDocII) module detached cellulosomes from the cells, but decreased cellulose hydrolysis rates by only 9%, suggesting a relatively small contribution of cellulosome-cell synergy. Disruptants lacking any of four different secondary scaffoldins (OlpB, 7CohII, Orf2p, or SdbA) showed moderately decreased cellulose hydrolysis rates, suggesting additive contributions. Surprisingly, the CipA-ΔXDocII mutant, which lacks cell-associated polycellulosomes, adheres to cellulose almost as strongly as wild-type cells, revealing an alternate, previously unknown cellulose-binding mechanism.

CONCLUSIONS

Our results emphasize the important role of cellulosome-substrate synergy in cellulose degradation, demonstrate a contribution of secondary scaffoldins, and suggest a previously unknown, non-cellulosomal system for binding insoluble cellulose. Our findings provide new insights into cellulosome function and impact genetic engineering of microorganisms to enhance bioconversions of cellulose substrates.

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.

A synthetic biology approach for evaluating the functional contribution of designer cellulosome components to deconstruction of cellulosic substrates

BACKGROUND

Select cellulolytic bacteria produce multi-enzymatic cellulosome complexes that bind to the plant cell wall and catalyze its efficient degradation. The multi-modular interconnecting cellulosomal subunits comprise dockerin-containing enzymes that bind cohesively to cohesin-containing scaffoldins. The organization of the modules into functional polypeptides is achieved by intermodular linkers of different lengths and composition, which provide flexibility to the complex and determine its overall architecture.

RESULTS

Using a synthetic biology approach, we systematically investigated the spatial organization of the scaffoldin subunit and its effect on cellulose hydrolysis by designing a combinatorial library of recombinant trivalent designer scaffoldins, which contain a carbohydrate-binding module (CBM) and 3 divergent cohesin modules. The positions of the individual modules were shuffled into 24 different arrangements of chimaeric scaffoldins. This basic set was further extended into three sub-sets for each arrangement with intermodular linkers ranging from zero (no linkers), 5 (short linkers) and native linkers of 27-35 amino acids (long linkers). Of the 72 possible scaffoldins, 56 were successfully cloned and 45 of them expressed, representing 14 full sets of chimaeric scaffoldins. The resultant 42-component scaffoldin library was used to assemble designer cellulosomes, comprising three model C. thermocellum cellulases. Activities were examined using Avicel as a pure microcrystalline cellulose substrate and pretreated cellulose-enriched wheat straw as a model substrate derived from a native source. All scaffoldin combinations yielded active trivalent designer cellulosome assemblies on both substrates that exceeded the levels of the free enzyme systems. A preferred modular arrangement for the trivalent designer scaffoldin was not observed for the three enzymes used in this study, indicating that they could be integrated at any position in the designer cellulosome without significant effect on cellulose-degrading activity. Designer cellulosomes assembled with the long-linker scaffoldins achieved higher levels of activity, compared to those assembled with short-and no-linker scaffoldins.

CONCLUSIONS

The results demonstrate the robustness of the cellulosome system. Long intermodular scaffoldin linkers are preferable, thus leading to enhanced degradation of cellulosic substrates, presumably due to the increased flexibility and spatial positioning of the attached enzymes in the complex. These findings provide a general basis for improved designer cellulosome systems as a platform for bioethanol production.