Functional characterization of a recombinant thermostable xylanase from Pichia pastoris: A hybrid enzyme being suitable for xylooligosaccharides production

Heteroxylan hydrolysis by a recombinant cellulase-free GH10 xylanase from the alkaliphilic bacterium Halalkalibacterium halodurans C-125

The search for affordable enzymes with exceptional characteristics is fundamental to overcoming industrial and environmental constraints. In this study, a recombinant GH10 xylanase (Xyn10-HB) from the extremely alkaliphilic bacterium Halalkalibacterium halodurans C-125 cultivated at pH 10 was cloned and expressed in E. coli BL21(DE3). Removal of the signal peptide improved the expression, and an overall activity of 8 U/mL was obtained in the cell-free supernatant. The molecular weight of purified Xyn10-HB was estimated to be 42.6 kDa by SDS-PAGE. The enzyme was active across a wide pH range (5-10) with optimal activity recorded at pH 8.5 and 60 °C. It also presented good stability with a half-life of 3 h under these conditions. Substrate specificity studies showed that Xyn10-HB is a cellulase-free enzyme that conventionally hydrolyse birchwood and oat spelts xylans (Apparent Km of 0.46 mg/mL and 0.54 mg/mL, respectively). HPLC analysis showed that both xylans hydrolysis produced xylooligosaccharides (XOS) with a degree of polymerization (DP) ranging from 2 to 9. The conversion yield was 77% after 24 h with xylobiose and xylotriose as the main end-reaction products. When assayed on alkali-extracted wheat straw heteroxylan, the Xyn10-HB produced active XOS with antioxidant activity determined by the DPPH radical scavenging method (IC50 of 0.54 mg/mL after 4 h). Owing to its various characteristics, Xyn10-HB xylanase is a promising candidate for multiple biotechnological applications.

Enhancement of catalytic performance of a metagenome-derived thermophilic oligosaccharide-specific xylanase by binding module removal and random mutagenesis

Xylo-oligosaccharide (XO) is a promising pre-biotic with applications in food, feed and healthcare products. XO can be produced by enzymatic digestion of xylan with xylanase. In this study, we aimed to improve the biochemical properties relevant to catalysis and kinetics of X11, a thermophilic glycosyl hydrolase (GH) family 11 endo-β-1,4-xylanase derived from a metagenomic library isolated from sugarcane bagasse, under high-temperature conditions preferred for XO synthesis. Removal of a carbohydrate-binding module (X11C) resulted in 6.5 fold greater catalytic efficiency. X11C was further improved by a Pro71Thr mutation in the X11P variant obtained from a random mutagenesis library, which exhibited 15.9 fold greater catalytic efficiency compared with wild-type X11 under the enzyme's optimal conditions of 80°C and pH 6.0. Homology modeling suggested that the improved performance of X11P could be attributed to formation of an extra H-bond between Thr71 and Ser75, which stabilizes the key catalytic residue Glu180 at the active pocket and β-sheet layers and agrees with the respective increase in melting temperature (T_m) where X11P >X11C >X11 as determined by differential scanning fluorimetry. The X11P variant was tested for hydrolysis of beechwood xylan, which showed X6 as the major product followed by X3 and X4 XOs. The highest yield of 5.5 g total XOs product/mg enzyme was observed for X11P, equivalent to 3.7 fold higher than that of wild-type with XO production of >800 mg/g xylan. The X11P enzyme could be developed as a thermophilic biocatalyst for XO synthesis in biorefineries.

GH-10 and GH-11 Endo-1,4-β-xylanase enzymes from Kitasatospora sp. produce xylose and xylooligosaccharides from sugarcane bagasse with no xylose inhibition

A novel strategy for the low-cost, high-yield co-production of xylose and xylooligosaccharides together with no xylose inhibition was developed using a novel heterologous expression of XYN10Ks_480 endo-1,4-β-xylanase with a ricin-type β-trefoil type of domain and XYN11Ks_480 endo-1,4-β-xylanase with a CBM 2 superfamily from the Kitasatospora sp in an actinomycetes expression system. Xylose is the main building block for hemicellulose xylan. Our findings demonstrated high levels of expression and catalytic activity for XYN10Ks_480 during hydrolysis of the extracted xylan of bagasse, and three types of xylan-based substrates were used to produce xylose and xylooligosaccharides. However, hydrolysis by XYN11Ks_480 produced xylooligosaccharides without xylose formation. This study demonstrated how integrating sodium hypochlorite-extracted xylan and enzymatic hydrolysis could provide an alternative strategy for the generation of XOS from lignocellulosic material.

Lignocellulose degrading extremozymes produced by Pichia pastoris: current status and future prospects

In this review article, extremophilic lignocellulosic enzymes with special interest on xylanases, β-mannanases, laccases and finally cellulases, namely, endoglucanases, exoglucanases and β-glucosidases produced by Pichia pastoris are reviewed for the first time. Recombinant lignocellulosic extremozymes are discussed from the perspectives of their potential application areas; characteristics of recombinant and native enzymes; the effects of P. pastoris expression system on recombinant extremozymes; and their expression levels and applied strategies to increase the enzyme expression yield. Further, effects of enzyme domains on activity and stability, protein engineering via molecular dynamics simulation and computational prediction, and site-directed mutagenesis and amino acid modifications done are also focused. Superior enzyme characteristics and improved stability due to the proper post-translational modifications and better protein folding performed by P. pastoris make this host favourable for extremozyme production. Especially, glycosylation contributes to the structure, function and stability of enzymes, as generally glycosylated enzymes produced by P. pastoris exhibit better thermostability than non-glycosylated enzymes. However, there has been limited study on enzyme engineering to improve catalytic efficiency and stability of lignocellulosic enzymes. Thus, in the future, studies should focus on protein engineering to improve stability and catalytic efficiency via computational modelling, mutations, domain replacements and fusion enzyme technology. Also metagenomic data need to be used more extensively to produce novel enzymes with extreme characteristics and stability.

Cloning, over-expression and characterization of a thermo-tolerant xylanase from Thermotoga thermarum

The xyn10B gene, encoding the endo-1,4-β-xylanase Xyn10B from Thermotoga thermarum, was cloned and expressed in Escherichia coli. The ORF of the xyn10B was 1,095 bp and encoded to mature peptide of 344 amino acids with a calculated MW of 40,531 Da. The recombinant xylanase was optimally active at 80 °C, pH 6.0 and retained approx. 60 % of its activity after 2 h at 75 °C. Apparent K m , k cat and k cat /K m values of the xylanase for beechwood xylan were 1.8 mg ml(-1), 520 s(-1) and 289 ml mg(-1) s(-1), respectively. The end products of the hydrolysis of beechwood xylan were mainly oligosaccharides but without xylose after 2 h hydrolysis.

A novel highly thermostable xylanase stimulated by Ca2+ from Thermotoga thermarum: cloning, expression and characterization

BACKGROUND

Xylanase is an important component of hemicellulase enzyme system. Since it plays an important role in the hydrolysis of hemicellulose into xylooligosaccharides (XOs), high thermostable xylanase has been the focus of much recent attention as powerful enzyme as well as in the field of biomass utilization.

RESULTS

A xylanase gene (xyn10A) with 3,474 bp was cloned from the extremely thermophilic bacterium Thermotoga thermarum that encodes a protein containing 1,158 amino acid residues. Based on amino acid sequence homology, hydrophobic cluster and three dimensional structure analyses, it was attested that the xylanase belongs to the glycoside hydrolase (GH) families 10 with five carbohydrate binding domains. When the xylanase gene was cloned and expressed in Escherichia coli BL21 (DE3), the specific enzyme activity of xylanase produced by the recombinant strain was up to 145.8 U mg-1. The xylanase was optimally active at 95°C, pH 7.0. In addition, it exhibited high thermostability over broad range of pH 4.0-8.5 and temperature 55-90°C upon the addition of 5 mM Ca2+. Confirmed by Ion Chromatography System (ICS) analysis, the end products of the hydrolysis of beechwood xylan were xylose, xylobiose, xylotriose, xylotetraose, xylopentaose and xylohexaose.

CONCLUSIONS

The xylanase from T. thermarum is one of the hyperthermophilic xylanases that exhibits high thermostability, and thus, is a suitable candidate for generating XOs from cellulosic materials such as agricultural and forestry residues for the uses as prebiotics and precursors for further preparation of furfural and other chemicals.

Biotechnological approaches for the production of prebiotics and their potential applications

Worldwide interest in prebiotics have been increasing extensively both as food ingredients and pharmacological supplements, since they have beneficial properties for human health. Prebiotics not only stimulate the growth of healthy bacteria such as bifidobacteria and lactobacilli in the gut but also increase the resistance towards pathogens. In addition to this, they also act as dietary fiber, an energy source for intestinal cells after converting to short-chain fatty acids, a stimulator of immune systems, sugar replacer etc. Moreover, due to heat resistant properties, they are able to maintain their intact form during the baking process and allow them to be incorporated into every day food products. Thus, they can be interesting and useful ingredients in the development of novel functional foods. This review provides comprehensive information about the different biotechnological techniques employed in the production of prebiotics and their potential applications in different areas.