Cloning and characterization of thermostable endoglucanase (Cel8Y) from the hyperthermophilic Aquifex aeolicus VF5

Overview of lignocellulolytic enzyme systems with special reference to valorization of lignocellulosic biomass

Lignocellulosic biomass, one of the most valuable natural resources, is abundantly present on earth. Being a renewable feedstock, it harbors a great potential to be exploited as a raw material, to produce various value-added products. Lignocellulolytic microorganisms hold a unique position regarding the valorization of lignocellulosic biomass as they contain efficient enzyme systems capable of degrading this biomass. The ubiquitous nature of these microorganisms and their survival under extreme conditions have enabled their use as an effective producer of lignocellulolytic enzymes with improved biochemical features crucial to industrial bioconversion processes. These enzymes can prove to be an exquisite tool when it comes to the eco-friendly manufacturing of value-added products using waste material. This review focuses on highlighting the significance of lignocellulosic biomass, microbial sources of lignocellulolytic enzymes and their use in the formation of useful products.

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.

Cellulases from Thermophiles Found by Metagenomics

Cellulases are a heterogeneous group of enzymes that synergistically catalyze the hydrolysis of cellulose, the major component of plant biomass. Such reaction has biotechnological applications in a broad spectrum of industries, where they can provide a more sustainable model of production. As a prerequisite for their implementation, these enzymes need to be able to operate in the conditions the industrial process requires. Thus, cellulases retrieved from extremophiles, and more specifically those of thermophiles, are likely to be more appropriate for industrial needs in which high temperatures are involved. Metagenomics, the study of genes and gene products from the whole community genomic DNA present in an environmental sample, is a powerful tool for bioprospecting in search of novel enzymes. In this review, we describe the cellulolytic systems, we summarize their biotechnological applications, and we discuss the strategies adopted in the field of metagenomics for the discovery of new cellulases, focusing on those of thermophilic microorganisms.

Expression and biochemical characterization of a multifunctional glycosidase from the thermophilic Bacillus licheniformis SR01

A gene (gkdA) (741 bp) encoding a putative protein of 247 amino acids was cloned from the Bacillus licheniformis SR01. The protein was expressed in Escherichia coli BL21 with a molecular mass estimated by SDS-PAGE of approximately 28.03 kDa and showed a calculating isoelectric point (pI) of 6.42. Structure analysis and function identification showed that the enzyme was a multifunctional glycosidase. Its specific activity was 0.013 U/μg. The recombinant glycosidase showed a maximum activity at 50°C and pH 7.0. It was very stable below 90°C and may have heat activation at higher temperatures. The relative residual activity was still more than 80% after 120 min at pH 5.0-10.0. The enzyme activity was inhibited by Cu²⁺, Fe²⁺, Ca²⁺, Mg²⁺, Co²⁺, Li⁺, SDS and EDTA, activated by Ca²⁺, and not affected by Mn²⁺ and K⁺. Under simulated stomach, and in vitro intestine, conditions, the enzyme retained 80%, and more than 100%, activity, respectively, after incubation for 90 min. The excellent properties of this enzyme, specifically its thermal stability and multifunctional abilities, give it potential application in the field of feed processing and other high-temperature processing industries.

Expression, purification and characterization of an endoglucanase from Serratia proteamaculans CDBB-1961, isolated from the gut of Dendroctonus adjunctus (Coleoptera: Scolytinae)

Serratia proteamaculans CDBB-1961, a gut symbiont from the roundheaded pine beetle Dendroctonus adjunctus, displayed strong cellulolytic activity on agar-plates with carboxymethyl cellulose (CMC) as carbon source. Automatic genome annotation of S. proteamaculans made possible the identification of a single endoglucanase encoding gene, designated spr cel8A. The predicted protein, named Spr Cel8A shows high similarity (59–94 %) to endo-1,4-β-d-glucanases (EC 3.2.1.4) from the glycoside hydrolase family 8 (GH8). The gene spr cel8A has an ORF of 1113 bp, encoding a 371 amino acid residue protein (41.2 kDa) with a signal peptide of 23 amino acid residues. Expression of the gene spr cel8A in Escherichia coli yields a mature recombinant endoglucanase 39 kDa. Cel8A displayed optimal activity at pH 7.0 and 40 °C, with a specific activity of 0.85 U/mg. The enzyme was stable at pH from 4 to 8.5, retaining nearly 40–80 % of its original activity, and exhibited a half-life of 8 days at 40 °C. The K_m and V_max values for Spr Cel8A were 6.87 mg/ml and 3.5 μmol/min/mg of protein, respectively, using CMC as substrate. The final principle products of Spr Cel8A-mediated hydrolysis of CMC were cellobiose, cello oligosaccharides and a small amount of glucose, suggesting that Spr Cel8A is an endo-β-1,4-glucanase manifesting exo-activity. This is the first report regarding the functional biochemical and molecular characterization of an endoglucanase from S. proteamaculans, found in the gut-associated bacteria community of Dendroctonus bark beetles. These results contribute to improved understanding of the functional role played by this bacterium as a symbiont of bark beetles.

Proteomic analysis of temperature dependent extracellular proteins from Aspergillus fumigatus grown under solid-state culture condition

Fungal species of the genus Aspergillus are filamentous ubiquitous saprophytes that play a major role in lignocellulosic biomass recycling and also are considered as cell factories for the production of organic acids, pharmaceuticals, and industrially important enzymes. Analysis of extracellular secreted biomass degrading enzymes using complex lignocellulosic biomass as a substrate by solid-state fermentation could be a more practical approach to evaluate application of the enzymes for lignocellulosic biorefinery. This study isolated a fungal strain from compost, identified as Aspergillus fumigatus, and further analyzed it for lignocellulolytic enzymes at different temperatures using label free quantitative proteomics. The profile of secretome composition discovered cellulases, hemicellulases, lignin degrading proteins, peptidases and proteases, and transport and hypothetical proteins; while protein abundances and further their hierarchical clustering analysis revealed temperature dependent expression of these enzymes during solid-state fermentation of sawdust. The enzyme activities and protein abundances as determined by exponentially modified protein abundance index (emPAI) indicated the maximum activities at the range of 40-50 °C, demonstrating the thermophilic nature of the isolate A. fumigatus LF9. Characterization of the thermostability of secretome suggested the potential of the isolated fungal strain in the production of thermophilic biomass degrading enzymes for industrial application.

Improved lignocellulose conversion to biofuels with thermophilic bacteria and thermostable enzymes

Second-generation feedstock, especially nonfood lignocellulosic biomass is a potential source for biofuel production. Cost-intensive physical, chemical, biological pretreatment operations and slow enzymatic hydrolysis make the overall process of lignocellulosic conversion into biofuels less economical than available fossil fuels. Lignocellulose conversions carried out at ≤ 50 °C have several limitations. Therefore, this review focuses on the importance of thermophilic bacteria and thermostable enzymes to overcome the limitations of existing lignocellulosic biomass conversion processes. The influence of high temperatures on various existing lignocellulose conversion processes and those that are under development, including separate hydrolysis and fermentation, simultaneous saccharification and fermentation, and extremophilic consolidated bioprocess are also discussed.

Thermostable enzymes as biocatalysts in the biofuel industry

Lignocellulose is the most abundant carbohydrate source in nature and represents an ideal renewable energy source. Thermostable enzymes that hydrolyze lignocellulose to its component sugars have significant advantages for improving the conversion rate of biomass over their mesophilic counterparts. We review here the recent literature on the development and use of thermostable enzymes for the depolymerization of lignocellulosic feedstocks for biofuel production. Furthermore, we discuss the protein structure, mechanisms of thermostability, and specific strategies that can be used to improve the thermal stability of lignocellulosic biocatalysts.

Cyclic AMP regulates the biosynthesis of cellobiohydrolase in Cellulomonas flavigena growing in sugar cane bagasse

Cellulomonas flavigena produces a battery of cellulase components that act concertedly to degrade cellulose. The addition of cAMP to repressed C. flavigena cultures released catabolic repression, while addition of cAMP to induced C. flavigena cultures led to a cellobiohydrolase hyperproduction. Exogenous cAMP showed positive regulation on cellobiohydrolase production in C. flavigena grown on sugar cane bagasse. A C. flavigena cellobiohydrolase gene was cloned (named celA), which coded for a 71- kDa enzyme. Upstream, a repressor celR1, identified as a 38 kDa protein, was monitored by use of polyclonal antibodies.

Conservation of RecG activity from pathogens to hyperthermophiles

Maintaining the integrity of the genome is essential for the survival of all organisms. RecG helicase plays an important part in this process in Escherichia coli, promoting recombination and DNA repair, and providing ways to rescue stalled replication forks by way of a Holliday junction intermediate. We purified RecG proteins from three other species: two Gram-positive mesophiles, Bacillus subtilis and Streptococcus pneumoniae, and one extreme thermophile, Aquifex aeolicus. All three proteins bind and unwind replication fork and Holliday junction DNA molecules with efficiencies similar to the E. coli protein. Proteins from the Gram-positive species promote DNA repair in E. coli, indicating either that RecG acts alone or that any necessary protein-protein interactions are conserved. The S. pneumoniae RecG reduces plasmid copy number when expressed in E. coli, indicating that like the E. coli protein it unwinds plasmid R loop structures used to prime replication. This effect is not seen with B. subtilis RecG; the protein either lacks R loop unwinding activity or is compromised by having insufficient ATP. The A. aeolicus protein unwinds DNA well at 60 degrees C but is less efficient at 37 degrees C, explaining its inability to function in E. coli at this temperature. The N-terminal extension present in this protein was investigated and found to be dispensable for activity and thermo-stability. The results presented suggest that the role of RecG in DNA replication and repair is likely to be conserved throughout all bacteria, which underlines the importance of this protein in genome duplication and cell survival.

Activity enhancement of Cel5Z from Pectobacterium chrysanthemi PY35 by removing C-terminal region

The phytopathogenic bacterium Pectobacteium chrysanthemi PY35 secretes Cel5Z endoglucanase belonging to the glycoside hydrolase family 5 of EC 3.2.1.4. The mutation of cel5Z::Omega gene was constructed by cloning the 2.0-kb SmaI fragment containing the streptomycin/spectinomycin-resistance gene of pHP45(Omega) into the BalI site of pPY100. The insertion of Omega fragment generated a new stop codon, removing the Ser/Thr-rich linker region and the cellulose binding domain (CBD) in the C-terminal region of cel5Z gene. By subsequent subcloning from this 4.9-kb fragment (pPY1001), a 1.0-kb (pPY1002) fragment was obtained and designated as cel5Z::Omega. The cel5Z::Omega gene had an open reading frame (ORF) of 1011 bp, encoding 336 amino acids, starting with an ATG codon and ending with a new TGA stop codon. The molecular mass of the Cel5Z::Omega protein in E. coli transformant appeared to be 32 kDa by SDS-PAGE analysis in the presence of carboxymethyl-cellulose (CMC). The Cel5Z::Omega protein hydrolyzed CMC with 1.7-fold higher activity than the intact Cel5Z cellulase.