Improvement of alkalophilicity of an alkaline xylanase Xyn11A-LC from Bacillus sp. SN5 by random mutation and Glu135 saturation mutagenesis

Expression in Pichia pastoris of Thermostable Endo-1,4-β-xylanase from the Actinobacterium Nocardiopsis halotolerans: Properties and Use for Saccharification of Xylan-Containing Products

A gene encoding a polysaccharide-degrading enzyme was cloned from the genome of the bacterium Nocardiopsis halotolerans. Analysis of the amino acid sequence of the protein showed the presence of the catalytic domain of the endo-1,4-β-xylanases of the GH11 family. The gene was amplified by PCR and ligated into the pPic9m vector. A recombinant producer based on Pichia pastoria was obtained. The production of the enzyme, which we called NhX1, was carried out in a 10 L fermenter. Enzyme production was 10.4 g/L with an activity of 927 U/mL. Purification of NhX1 was carried out using Ni-NTA affinity chromatography. The purified enzyme catalyzed the hydrolysis of xylan but not other polysaccharides. Endo-1,4-β-xylanase NhX1 showed maximum activity and stability at pH 6.0-7.0. The enzyme showed high thermal stability, remaining active at 90 °C for 20 min. With beechwood xylan, the enzyme showed Km 2.16 mg/mL and Vmax 96.3 U/mg. The products of xylan hydrolysis under the action of NhX1 were xylobiose, xylotriose, xylopentaose, and xylohexaose. Endo-1,4-β-xylanase NhX1 effectively saccharified xylan-containing products used for the production of animal feed. The xylanase described herein is a thermostable enzyme with biotechnological potential produced in large quantities by P. pastoria.

Isolation of Saccharibacillus WB17 strain from wheat bran phyllosphere and genomic insight into the cellulolytic and hemicellulolytic complex of the Saccharibacillus genus

The microorganisms living on the phyllosphere (the aerial part of the plants) are in contact with the lignocellulosic plant cell wall and might have a lignocellulolytic potential. We isolated a Saccharibacillus strain (Saccharibacillus WB17) from wheat bran phyllosphere and its cellulolytic and hemicellulolytic potential was investigated during growth onto wheat bran. Five other type strains from that genus selected from databases were also cultivated onto wheat bran and glucose. Studying the chemical composition of wheat bran residues by FTIR after growth of the six strains showed an important attack of the stretching C-O vibrations assigned to polysaccharides for all the strains, whereas the C = O bond/esterified carboxyl groups were not impacted. The genomic content of the strains showed that they harbored several CAZymes (comprised between 196 and 276) and possessed four of the fifth modules reflecting the presence of a high diversity of enzymes families. Xylanase and amylase activities were the most active enzymes with values reaching more than 4746 ± 1400 mIU/mg protein for the xylanase activity in case of Saccharibacillus deserti KCTC 33693 T and 452 ± 110 mIU/mg protein for the amylase activity of Saccharibacillus WB17. The total enzymatic activities obtained was not correlated to the total abundance of CAZyme along that genus. The Saccharibacillus strains harbor also some promising proteins in the GH30 and GH109 modules with potential arabinofuranosidase and oxidoreductase activities. Overall, the genus Saccharibacillus and more specifically the Saccharibacillus WB17 strain represent biological tools of interest for further biotechnological applications.

Engineering mesophilic GH11 xylanase from Cellulomonas flavigena by rational design of N-terminus substitution

Xylanase, a glycoside hydrolase, is widely used in the food, papermaking, and textile industries; however, most xylanases are inactive at high temperatures. In this study, a xylanase gene, CFXyl3, was cloned from Cellulomonas flavigena and expressed in Escherichia coli BL21 (DE3). To improve the thermostability of xylanase, four hybrid xylanases with enhanced thermostability (designated EcsXyl1–4) were engineered from CFXyl3, guided by primary and 3D structure analyses. The optimal temperature of CFXyl3 was improved by replacing its N-terminus with the corresponding area of SyXyn11P, a xylanase that belongs to the hyperthermostable GH11 family. The optimal temperatures of the hybrid xylanases EcsXyl1–4 were 60, 60, 65, and 85°C, respectively. The optimal temperature of EcsXyl4 was 30 C higher than that of CFXyl3 (55°C) and its melting temperature was 34.5°C higher than that of CFXyl3. After the hydrolysis of beechwood xylan, the main hydrolysates were xylotetraose, xylotriose, and xylobiose; thus, these hybrid xylanases could be applied to prebiotic xylooligosaccharide manufacturing.

Extremophilic Prokaryotic Endoxylanases: Diversity, Applicability, and Molecular Insights

Extremophilic endoxylanases grabbed attention in recent years due to their applicability under harsh conditions of several industrial processes. Thermophilic, alkaliphilic, and acidophilic endoxylanases found their employability in bio-bleaching of paper pulp, bioconversion of lignocellulosic biomass into xylooligosaccharides, bioethanol production, and improving the nutritious value of bread and other bakery products. Xylanases obtained from extremophilic bacteria and archaea are considered better than fungal sources for several reasons. For example, enzymatic activity under broad pH and temperature range, low molecular weight, cellulase-free activity, and longer stability under extreme conditions of prokaryotic derived xylanases make them a good choice. In addition, a short life span, easy cultivation/harvesting methods, higher yield, and rapid DNA manipulations of bacterial and archaeal cells further reduces the overall cost of the product. This review focuses on the diversity of prokaryotic endoxylanases, their characteristics, and their functional attributes. Besides, the molecular mechanisms of their extreme behavior have also been presented here.

Phylogenetic, functional and structural characterization of a GH10 xylanase active at extreme conditions of temperature and alkalinity

Endoxylanases active under extreme conditions of temperature and alkalinity can replace the use of highly pollutant chemicals in the pulp and paper industry. Searching for enzymes with these properties, we carried out a comprehensive bioinformatics study of the GH10 family. The phylogenetic analysis allowed the construction of a radial cladogram in which protein sequences putatively ascribed as thermophilic and alkaliphilic appeared grouped in a well-defined region of the cladogram, designated TAK Cluster. One among five TAK sequences selected for experimental analysis (Xyn11) showed extraordinary xylanolytic activity under simultaneous conditions of high temperature (90 °C) and alkalinity (pH 10.5). Addition of a carbohydrate binding domain (CBM2) at the C-terminus of the protein sequence further improved the activity of the enzyme at high pH. Xyn11 structure, which has been solved at 1.8 Å resolution by X-ray crystallography, reveals an unusually high number of hydrophobic, ionic and hydrogen bond atomic interactions that could account for the enzyme’s extremophilic nature.

Enzymatic characterization of a novel thermostable and alkaline tolerant GH10 xylanase and activity improvement by multiple rational mutagenesis strategies

Thermo-alkaline xylanases are widely applied in paper pulping industry. In this study, a novel thermostable and alkaline tolerant GH10 xylanase (Xyn30Y5) gene from alkaliphilic Bacillus sp. 30Y5 was cloned and the surface-layer homology (SLH) domains truncated enzyme (Xyn30Y5-SLH) was expressed in Escherichia coli. The purified Xyn30Y5-SLH was most active at 70 °C and pH 7.0 and showed the highest specific activity of 349.4 U mg^-1. It retained more than 90% activity between pH 6.0 to 9.5 and was stable at pH 6.0-10.0. To improve the activity, 47 mutants were designed based on eight rational strategies and 21 mutants showed higher activity. By combinatorial mutagenesis, the best mutant 3B demonstrated specific activity of 1016.8 U mg^-1 with a doubled catalytic efficiency (k_cat/K_m) and RA60_1/2h value, accompanied by optimal pH shift to 8.0. The molecular dynamics simulation analysis indicated that the increase of flexibility of α5 helix and loop7 located near to the catalytic residues is likely responsible for its activity improvement. And the decrease of flexibility of the most unstable regions is vital for the thermostablity improvement. This work provided not only a novel thermostable and alkaline tolerant xylanase with industrial application potential but also an effective mutagenesis strategy for xylanase activity improvement.

Studying the Role of a Single Mutation of a Family 11 Glycoside Hydrolase Using High-Resolution X-ray Crystallography

XynII is a family 11 glycoside hydrolase that uses the retaining mechanism for catalysis. In the active site, E177 works as the acid/base and E86 works as the nucleophile. Mutating an uncharged residue (N44) to an acidic residue (D) near E177 decreases the enzyme's optimal pH by ~ 1.0 unit. D44 was previously suggested to be a second proton carrier for catalysis. To test this hypothesis, we abolished the activity of E177 by mutating it to be Q, and mutated N44 to be D or E. These double mutants have dramatically decreased activities. Our high-resolution crystallographic structures and the microscopic pK_a calculations show that D44 has similar position and pK_a value during catalysis, indicating that D44 changes electrostatics around E177, which makes it prone to rotate as the acid/base in acidic conditions, thus decreases the pH optimum. Our results could be helpful to design enzymes with different pH optimum.

Recent Development of Extremophilic Bacteria and Their Application in Biorefinery

The biorefining technology for biofuels and chemicals from lignocellulosic biomass has made great progress in the world. However, mobilization of laboratory research toward industrial setup needs to meet a series of criteria, including the selection of appropriate pretreatment technology, breakthrough in enzyme screening, pathway optimization, and production technology, etc. Extremophiles play an important role in biorefinery by providing novel metabolic pathways and catalytically stable/robust enzymes that are able to act as biocatalysts under harsh industrial conditions on their own. This review summarizes the potential application of thermophilic, psychrophilic alkaliphilic, acidophilic, and halophilic bacteria and extremozymes in the pretreatment, saccharification, fermentation, and lignin valorization process. Besides, the latest studies on the engineering bacteria of extremophiles using metabolic engineering and synthetic biology technologies for high-efficiency biofuel production are also introduced. Furthermore, this review explores the comprehensive application potential of extremophiles and extremozymes in biorefinery, which is partly due to their specificity and efficiency, and points out the necessity of accelerating the commercialization of extremozymes.

Novel oseltamivir-resistant mutations distant from the active site of influenza B neuraminidase

We performed a neuraminidase sequence analysis of thirty-two pediatric patients with influenza B who visited Teikyo University Hospital from January 2016 to March 2017, and found oseltamivir-resistant samples belonging to the Yamagata and Victoria lineages. Comparison with the neuraminidase sequence of oseltamivir-susceptible B/Brisbane/60/2008 revealed 5 common amino acid substitutions in many of these samples. According to the binding free energy calculation, the N340D and E358K substitutions reduced the affinity of oseltamivir to neuraminidase. Unexpectedly, these substitutions were located distant from the oseltamivir-binding site in neuraminidase. According to the molecular dynamics simulations, the N340D substitution rearranged complicated hydrogen bond networks in an extensive surface region of neuraminidase. The E358K substitution extensively altered the electrostatic potential map of the overall neuraminidase structure. Through these novel mechanisms, the N340D and E358K substitutions indirectly influenced the affinity reduction. These results may be useful for designing drugs for the treatment of oseltamivir-resistant virus infections.Communicated by Ramaswamy H. Sarma.

Improving the catalytic efficiency of thermostable Geobacillus stearothermophilus xylanase XT6 by single-amino acid substitution

Directed evolution using error-prone polymerase chain reaction was employed in the current study to enhance the catalytic efficiency of a thermostable Geobacillus stearothermophilus xylanase XT6 parent. High-throughput screening identified two variants with enhanced activity. Sequencing analysis revealed the presence of a single-amino acid substitution (P209L or V161L) in each variant. The maximum activity of mutant V161L and P209L was at 85°C and 70°C, respectively. Both mutants exhibited maximum activity at pH 7. The thermal and alkaline tolerance of mutant V161L only were markedly improved. The two mutants were more resistant to ethanol inhibition than the parent. Substrate specificity of the two mutants was shifted from beechwood xylan to birchwood xylan. The potential of the two mutants to hydrolyze rice straw and sugarcane bagasse increased. Both turnover number (kcat) and catalytic efficiency (kcat/kM) increased 12.2- and 5.7-folds for variant P209L and 13- and 6.5-folds for variant V161L, respectively, towards birchwood xylan. Based on the previously published crystal structure of extracellular G. stearothermophilus xylanase XT6, V161L and P209L mutation locate on βα-loops. Conformational changes of the respective loops could potentiate the loop swinging, product release and consequently result in enhancement of the catalytic performance.

Increasing xylanase activity of Bacillus subtilis by atmospheric pressure plasma jet for biomass hydrolysis

Xylanase producing bacteria, Bacillus subtilis, was bombarded by an atmospheric pressure plasma jet (APPJ) and screened for higher catalytic activity. The bacteria were bombarded with plasma of argon or helium with energy of 120 W for a duration of 1-5 min. A mutant with higher xylanase activity was observed under argon plasma treatment at 1 min on media containing xylan as substrate. Subsequently, the xylanase gene from the mutant was sequenced and named MxynA. Sequence analysis revealed only a single missense mutation on the MxynA gene causing amino acid substitution from threonine to serine at position 162 (T162S) within the xylanase protein of the mutant. Consequently, MxynA was subcloned into expression vector, pETDuet-1 under T7 promoter and expressed in E. coli BL21 (DE3). The optimum temperature and pH of MxynA and its parent expressed in E. coli, named CxynA were 60 °C and pH 5, respectively. Moreover, MxynA showed higher xylanase activity approximately 4 fold higher than that of the control upon a wide range of pH and temperature conditions. From kinetic parameters analysis, the mutant showed higher enzyme turnover (k cat) than the control. The hydrolysis ability of the MxynA enzyme on lignocellulosic wastes, such as rice straw, corncob and para grass was investigated using the released reducing sugar as an indicator. The MxynA enzyme showed a greater amount of reducing sugar released from all lignocellulosic wastes other than the control, particularly from para grass. This study demonstrated that the T162S mutation possibly improved the catalytic efficiency of MxynA.

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

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

Spiked Genes: A Method to Introduce Random Point Nucleotide Mutations Evenly throughout an Entire Gene Using a Complete Set of Spiked Oligonucleotides for the Assembly

In vitro mutagenesis methods have revolutionized biological research and the biotechnology industry. In this study, we describe a mutagenesis method based on synthesizing a gene using a complete set of forward and reverse spiked oligonucleotides that have been modified to introduce a low ratio of mutant nucleotides at each position. This novel mutagenesis scheme named "Spiked Genes" yields a library of clones with an enhanced mutation distribution due to its unbiased nucleotide incorporation. Using the far-red fluorescent protein emKate as a model, we demonstrated that Spiked Genes yields richer libraries than those obtained via enzymatic methods. We obtained a library without bias toward any nucleotide or base pair and with even mutations, transitions, and transversion frequencies. Compared with enzymatic methods, the proposed synthetic approach for the creation of gene libraries represents an improved strategy for screening protein variants and does not require a starting template.