Bifunctional recombinant cellulase-xylanase (rBhcell-xyl) from the polyextremophilic bacterium Bacillus halodurans TSLV1 and its utility in valorization of renewable agro-residues

TtCel7A: A Native Thermophilic Bifunctional Cellulose/Xylanase Exogluclanase from the Thermophilic Biomass-Degrading Fungus Thielavia terrestris Co3Bag1, and Its Application in Enzymatic Hydrolysis of Agroindustrial Derivatives

The biomass-degrading thermophilic ascomycete fungus Thielavia terrestris Co3Bag1 produces TtCel7A, a native bifunctional cellulase/xylanase GH7 family. The purified TtCel7A, with an estimated molecular weight of 71 kDa, was biochemically characterized. TtCel7A displayed an optimal pH of 5.5 for both activities and an optimal temperature of 60 and 50 °C for cellulolytic and xylanolytic activities, respectively. The half-lives determined for cellulase activity were 140, 106, and 41 min at 50, 60, and 70 °C, respectively, whereas the half-lives observed for xylanase activity were 24, 10, and 1.4 h at 50, 60, and 70 °C, respectively. The K_M and V_max values were 3.12 mg/mL and 50 U/mg for cellulase activity and 0.17 mg/mL and 42.75 U/mg for xylanase activity. Circular dichroism analysis suggests changes in the secondary structure of TtCel7A in the presence of CMC as the substrate, whereas no modifications were observed with beechwood xylan. TtCel7A displayed the excellent capability to hydrolyze CMC, beechwood xylan, and complex substrates such as oat bran, wheat bran, and sugarcane bagasse, with glucose and cellobiose being the main products released; also, slightly less endo cellulase and xylanase activities were observed. Thus, suggesting TtCel7A has an exo- and endomode of action. Based on the characteristics of the enzyme, it might be considered a good candidate for industrial applications.

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.

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.

Thermostable cellulose saccharifying microbial enzymes: Characteristics, recent advances and biotechnological applications

Cellulases play a promising role in the bioconversion of renewable lignocellulosic biomass into fermentable sugars which are subsequently fermented to biofuels and other value-added chemicals. Besides biofuel industries, they are also in huge demand in textile, detergent, and paper and pulp industries. Low titres of cellulase production and processing are the main issues that contribute to high enzyme cost. The success of ethanol-based biorefinery depends on high production titres and the catalytic efficiency of cellulases functional at elevated temperatures with acid/alkali tolerance and the low cost. In view of their wider application in various industrial processes, stable cellulases that are active at elevated temperatures in the acidic-alkaline pH ranges, and organic solvents and salt tolerance would be useful. This review provides a recent update on the advances made in thermostable cellulases. Developments in their sources, characteristics and mechanisms are updated. Various methods such as rational design, directed evolution, synthetic & system biology and immobilization techniques adopted in evolving cellulases with ameliorated thermostability and characteristics are also discussed. The wide range of applications of thermostable cellulases in various industrial sectors is described.

Insights into the Catalytic Mechanism of a Novel XynA and Structure-Based Engineering for Improving Bifunctional Activities

Xylan and cellulose are the two major constituents of numerous types of lignocellulose. The bifunctional enzyme that exhibits xylanase/cellulase activity has attracted a great deal of attention in biofuel production. Previously, a thermostable GH10 family enzyme (XynA) from Bacillus sp. KW1 was found to degrade both xylan and cellulose. To improve bifunctional activity on the basis of structure, we first determined the crystal structure of XynA at 2.3 Å. Via molecular docking and activity assays, we revealed that Gln250 and His252 were indispensable to bifunctionality, because they could interact with two conserved catalytic residues, Glu182 and Glu280, while bringing the substrate close to the activity pocket. Then we used a structure-based engineering strategy to improve xylanase/cellulase activity. Although no mutants with increased bifunctional activity were obtained after much screening, we found the answer in the N-terminal 36-amino acid truncation of XynA. The activities of XynA_ΔN36 toward beechwood xylan, wheat arabinoxylan, filter paper, and barley β-glucan were significantly increased by 0.47-, 0.53-, 2.46-, and 1.04-fold, respectively. Furthermore, upon application, this truncation released more reducing sugars than the wild type in the degradation of pretreated corn stover and sugar cane bagasse. These results showed the detailed molecular mechanism of the GH10 family bifunctional endoxylanase/cellulase. The basis of these catalytic performances and the screened XynA_ΔN36 provide clues for the further use of XynA in industrial applications.

In-silico discovery of bifunctional enzymes with enhanced lignocellulose hydrolysis from microbiota big data

Due to the importance of using lignocellulosic biomass, it is always important to find an effective novel enzyme or enzyme cocktail or fusion enzymes. Identification of bifunctional enzymes through a metagenomic approach is an efficient method for converting agricultural residues and a beneficial way to reduce the cost of enzyme cocktail and fusion enzyme production. In this study, a novel stable bifunctional cellulase/xylanase, PersiCelXyn1 was identified from the rumen microbiota by the multi-stage in-silico screening pipeline and computationally assisted methodology. The enzyme exhibited the optimal activity at pH 5 and 50°C. Analyzing the enzyme activity at extreme temperature, pH, long-term storage, and presence of inhibitors and metal ions, confirmed the stability of the bifunctional enzyme under harsh conditions. Hydrolysis of the rice straw by PersiCelXyn1 showed its capability to degrade both cellulose and hemicellulose polymers. Also, the enzyme improved the degradation of various biomass substrates after 168 h of hydrolysis. Our results demonstrated the power of the multi-stage in-silico screening to identify bifunctional enzymes from metagenomic big data for effective bioconversion of lignocellulosic biomass.

Multifunctional cellulases are potent, versatile tools for a renewable bioeconomy

Enzyme performance is critical to the future bioeconomy based on renewable plant materials. Plant biomass can be efficiently hydrolyzed by multifunctional cellulases (MFCs) into sugars suitable for conversion into fuels and chemicals, and MFCs fall into three functional categories. Recent work revealed MFCs with broad substrate specificity, dual exo-activity/endo-activity on cellulose, and intramolecular synergy, among other novel characteristics. Binding modules and accessory catalytic domains amplify MFC and xylanase activity in a wide variety of ways, and processive endoglucanases achieve autosynergy on cellulose. Multidomain MFCs from Caldicellulosiruptor are heat-tolerant, adaptable to variable cellulose crystallinity, and may provide interchangeable scaffolds for recombinant design. Further studies of MFC properties and their reactivity with plant biomass are recommended for increasing biorefinery yields.

Progress in Ameliorating Beneficial Characteristics of Microbial Cellulases by Genetic Engineering Approaches for Cellulose Saccharification

Lignocellulosic biomass is a renewable and sustainable energy source. Cellulases are the enzymes that cleave β-1, 4-glycosidic linkages in cellulose to liberate sugars that can be fermented to ethanol, butanol, and other products. Low enzyme activity and yield, and thermostability are, however, some of the limitations posing hurdles in saccharification of lignocellulosic residues. Recent advancements in synthetic and systems biology have generated immense interest in metabolic and genetic engineering that has led to the development of sustainable technology for saccharification of lignocellulosics in the last couple of decades. There have been several attempts in applying genetic engineering in the production of a repertoire of cellulases at a low cost with a high biomass saccharification. A diverse range of cellulases are produced by different microbes, some of which are being engineered to evolve robust cellulases. This review summarizes various successful genetic engineering strategies employed for improving cellulase kinetics and cellulolytic efficiency.

Deep Hypersaline Anoxic Basins as Untapped Reservoir of Polyextremophilic Prokaryotes of Biotechnological Interest

Deep-sea hypersaline anoxic basins (DHABs) are considered to be among the most extreme ecosystems on our planet, allowing only the life of polyextremophilic organisms. DHABs’ prokaryotes exhibit extraordinary metabolic capabilities, representing a hot topic for microbiologists and biotechnologists. These are a source of enzymes and new secondary metabolites with valuable applications in different biotechnological fields. Here, we review the current knowledge on prokaryotic diversity in DHABs, highlighting the biotechnological applications of identified taxa and isolated species. The discovery of new species and molecules from these ecosystems is expanding our understanding of life limits and is expected to have a strong impact on biotechnological applications.

Characterization of two thermophilic cellulases from Talaromyces leycettanus JCM12802 and their synergistic action on cellulose hydrolysis

Talaromyces leycettanus JCM12802 is a great producer of thermophilic glycoside hydrolases (GHs). In this study, two cellulases (TlCel5A and TlCel6A) belonging to GH5 and GH6 respectively were expressed in Pichia pastoris and functionally characterized. The enzymes had acidic and thermophilic properties, showing optimal activities at pH 3.5–4.5 and 75–80°C, and retained stable at temperatures up to 60°C and over a broad pH range of 2.0−8.0. TlCel5A and TlCel6A acted against several cellulose substrates with varied activities (3,101.1 vs. 92.9 U/mg to barley β-glucan, 3,905.6 U/mg vs. 109.0 U/mg to lichenan, and 840.3 and 0.09 U/mg to CMC-Na). When using Avicel, phosphoric acid swollen cellulose (PASC) or steam-exploded corn straw (SECS) as the substrate, combination of TlCel5A and TlCel6A showed significant synergistic action, releasing more reduced sugars (1.08–2.87 mM) than the individual enzymes. These two cellulases may represent potential enzyme additives for the efficient biomass conversion and bioethanol production.

A novel thermostable GH10 xylanase with activities on a wide variety of cellulosic substrates from a xylanolytic Bacillus strain exhibiting significant synergy with commercial Celluclast 1.5 L in pretreated corn stover hydrolysis

BACKGROUND

Cellulose and hemicellulose are the two largest components in lignocellulosic biomass. Enzymes with activities towards cellulose and xylan have attracted great interest in the bioconversion of lignocellulosic biomass, since they have potential in improving the hydrolytic performance and reducing the enzyme costs. Exploring glycoside hydrolases (GHs) with good thermostability and activities on xylan and cellulose would be beneficial to the industrial production of biofuels and bio-based chemicals.

RESULTS

A novel GH10 enzyme (XynA) identified from a xylanolytic strain Bacillus sp. KW1 was cloned and expressed. Its optimal pH and temperature were determined to be pH 6.0 and 65 °C. Stability analyses revealed that XynA was stable over a broad pH range (pH 6.0-11.0) after being incubated at 25 °C for 24 h. Moreover, XynA retained over 95% activity after heat treatment at 60 °C for 60 h, and its half-lives at 65 °C and 70 °C were about 12 h and 1.5 h, respectively. More importantly, in terms of substrate specificity, XynA exhibits hydrolytic activities towards xylans, microcrystalline cellulose (filter paper and Avicel), carboxymethyl cellulose (CMC), cellobiose, p-nitrophenyl-β-d-cellobioside (pNPC), and p-nitrophenyl-β-d-glucopyranoside (pNPG). Furthermore, the addition of XynA into commercial cellulase in the hydrolysis of pretreated corn stover resulted in remarkable increases (the relative increases may up to 90%) in the release of reducing sugars. Finally, it is worth mentioning that XynA only shows high amino acid sequence identity (88%) with rXynAHJ14, a GH10 xylanase with no activity on CMC. The similarities with other characterized GH10 enzymes, including xylanases and bifunctional xylanase/cellulase enzymes, are no more than 30%.

CONCLUSIONS

XynA is a novel thermostable GH10 xylanase with a wide substrate spectrum. It displays good stability in a broad range of pH and high temperatures, and exhibits activities towards xylans and a wide variety of cellulosic substrates, which are not found in other GH10 enzymes. The enzyme also has high capacity in saccharification of pretreated corn stover. These characteristics make XynA a good candidate not only for assisting cellulase in lignocellulosic biomass hydrolysis, but also for the research on structure-function relationship of bifunctional xylanase/cellulase.

A bifunctional cellulase-xylanase of a new Chryseobacterium strain isolated from the dung of a straw-fed cattle

A new cellulolytic strain of Chryseobacterium genus was screened from the dung of a cattle fed with cereal straw. A putative cellulase gene (cbGH5) belonging to glycoside hydrolase family 5 subfamily 46 (GH5_46) was identified and cloned by degenerate PCR plus genome walking. The CbGH5 protein was overexpressed in Pichia pastoris, purified and characterized. It is the first bifunctional cellulase-xylanase reported in GH5_46 as well as in Chryseobacterium genus. The enzyme showed an endoglucanase activity on carboxymethylcellulose of 3237 μmol min-1  mg-1 at pH 9, 90 °C and a xylanase activity on birchwood xylan of 1793 μmol min-1  mg-1 at pH 8, 90 °C. The activity level and thermophilicity are in the front rank of all the known cellulases and xylanases. Core hydrophobicity had a positive effect on the thermophilicity of this enzyme. When similar quantity of enzymatic activity units was applied on the straws of wheat, rice, corn and oilseed rape, CbGH5 could obtain 3.5-5.0× glucose and 1.2-1.8× xylose than a mixed commercial cellulase plus xylanase of Novozymes. When applied on spent mushroom substrates made from the four straws, CbGH5 could obtain 9.2-15.7× glucose and 3.5-4.3× xylose than the mixed Novozymes cellulase+xylanase. The results suggest that CbGH5 could be a promising candidate for industrial lignocellulosic biomass conversion.

Characterization of cellulase from Aspergillus tubingensis NKBP-55 for generation of fermentable sugars from agricultural residues

The aim of this work was to characterize cellulase from Aspergillus tubingensis NKBP-55 for generation of fermentable sugars from agricultural residues. The strain produced high titres of cellulase (750 U/gds) on copra meal in solid state fermentation (SSF). The enzyme preparation also showed hemicellulolytic activities (U/gds) viz. endo-mannanase (1023), endo-xylanase (167), β-glucosidase (72) and α-galactosidase (54). Zymography revealed presence of six cellulases, six mannanases and one β-glucosidase. It effectively degraded sugarcane bagasse (SCB) and rice straw (RS) releasing xylose, glucose and cellobiose. One cellulase (Cat 1, Mr ∼65 kDa) was purified and characterized. It retained more than 50% activity at 70 °C after 150 mins and its activity was enhanced in the presence of Mn²⁺ ions (130%) and β-mercaptoethanol (140%). FTIR and ¹³C CP/MAS NMR analysis of the enzyme treated SCB and RS revealed degradation of cellulose and hemicellulose, while ¹H and ¹³C liquid state NMR experiments confirmed release of glucose.