High level expression of a novel family 3 neutral β-xylosidase from Humicola insolens Y1 with high tolerance to D-xylose

Gene-trait matching among Bifidobacterium dentium strains reveals various glycan metabolism loci including a strain-specific fucosyllactose utilization cluster

In contrast to other human-associated bifidobacteria, Bifidobacterium dentium is commonly classified as an opportunistic pathogen as its presence in the oral cavity has been associated with the development of dental caries. While B. dentium is frequently isolated from the oral cavity of children with caries, recent microbiome investigations and preliminary genomic analyses have suggested that this species is also adapted to colonize the gastrointestinal tract. Understanding the genetic and metabolic adaptations that enable this flexible colonization ability is crucial to clarify its role in human health and disease. To assess B. dentium genomic diversity and metabolic potential, the current study presents analysis and characterization of 10 complete genome sequences from recently isolated B. dentium strains obtained from human fecal samples together with 48 publicly available genome sequences. We investigated genetic loci predicted to be involved in host interaction and carbohydrate utilization in this species by means of comparative genomics, pan-genome analysis, and gene-trait matching. These analyses identified gene clusters involved in the utilization of plant-derived glycans and, for the first time, revealed B. dentium strains capable of utilizing human milk oligosaccharides (HMOs) through a fucosyllactose utilization cluster homologous to the one found in several infant-derived bifidobacterial species. Moreover, additional investigations of strain-specific genetic features highlighted a taxon that is evolved to colonize multiple niches and to compete with other colonizers. These findings challenge the narrow classification of B. dentium as an opportunist and underscore its ecological versatility.

Research progress of β-xylosidase in green synthesis

β-Xylosidase, an important hydrolase, catalyzes the degradation of xylan and xylosides, demonstrating significant potential for applications in biomass conversion and green synthesis. In recent years, with the rise of green chemistry, research on β-xylosidase in sustainable chemical synthesis has garnered increasing attention. Lignocellulosic biomass, a readily available and sustainable natural resource, requires the involvement of β-xylosidase for the production of biofuels. This enzyme not only efficiently degrades the xylan components of plant cell walls to produce biofuels but also synthesizes high-value glycosides through transglycosylation reactions, providing an eco-friendly catalytic tool for green chemical synthesis. This review summarizes the structural characteristics and catalytic mechanisms of β-xylosidase, along with related techniques to enhance its catalytic performance, such as enzyme immobilization, enzyme fusion technology, genetic engineering, and enzyme synergy. It focuses on recent advancements in its green applications, including the production of active compounds, waste degradation, bioenergy development, pulp bleaching, and deinking of waste paper (as shown in Fig. 1). Additionally, in light of current research trends, this review offers insights into the future prospects and challenges of β-xylosidase in green synthesis, aiming to provide valuable references for related fields.

Structures, Biochemical Characteristics, and Functions of β-Xylosidases

The complete degradation of abundant xylan derived from plants requires the participation of β-xylosidases to produce the xylose which can be converted to xylitol, ethanol, and other valuable chemicals. Some phytochemicals can also be hydrolyzed by β-xylosidases into bioactive substances, such as ginsenosides, 10-deacetyltaxol, cycloastragenol, and anthocyanidins. On the contrary, some hydroxyl-containing substances such as alcohols, sugars, and phenols can be xylosylated by β-xylosidases into new chemicals such as alkyl xylosides, oligosaccharides, and xylosylated phenols. Thus, β-xylosidases shows great application prospects in food, brewing, and pharmaceutical industries. This review focuses on the molecular structures, biochemical properties, and bioactive substance transformation function of β-xylosidases derived from bacteria, fungi, actinomycetes, and metagenomes. The molecular mechanisms of β-xylosidases related to the properties and functions are also discussed. This review will serve as a reference for the engineering and application of β-xylosidases in food, brewing, and pharmaceutical industries.

Alicyclobacillus mali FL18 as a Novel Source of Glycosyl Hydrolases: Characterization of a New Thermophilic β-Xylosidase Tolerant to Monosaccharides

A thermo-acidophilic bacterium, Alicyclobacillus mali FL18, was isolated from a hot spring of Pisciarelli, near Naples, Italy; following genome analysis, a novel putative β-xylosidase, AmβXyl, belonging to the glycosyl hydrolase (GH) family 3 was identified. A synthetic gene was produced, cloned in pET-30a(+), and expressed in Escherichia coli BL21 (DE3) RIL. The purified recombinant protein, which showed a dimeric structure, had optimal catalytic activity at 80 °C and pH 5.6, exhibiting 60% of its activity after 2 h at 50 °C and displaying high stability (more than 80%) at pH 5.0-8.0 after 16 h. AmβXyl is mainly active on both para-nitrophenyl-β-D-xylopyranoside (K_M 0.52 mM, k_cat 1606 s^-1, and k_cat/K_M 3088.46 mM^-1·s^-1) and para-nitrophenyl-α-L-arabinofuranoside (K_M 10.56 mM, k_cat 2395.8 s^-1, and k_cat/K_M 226.87 mM^-1·s^-1). Thin-layer chromatography showed its ability to convert xylooligomers (xylobiose and xylotriose) into xylose, confirming that AmβXyl is a true β-xylosidase. Furthermore, no inhibitory effect on enzymatic activity by metal ions, detergents, or EDTA was observed except for 5 mM Cu²⁺. AmβXyl showed an excellent tolerance to organic solvents; in particular, the enzyme increased its activity at high concentrations (30%) of organic solvents such as ethanol, methanol, and DMSO. Lastly, the enzyme showed not only a good tolerance to inhibition by xylose, arabinose, and glucose, but was activated by 0.75 M xylose and up to 1.5 M by both arabinose and glucose. The high tolerance to organic solvents and monosaccharides together with other characteristics reported above suggests that AmβXyl may have several applications in many industrial fields.

Biochemical properties of cellulolytic and xylanolytic enzymes from Sporotrichum thermophile and their utility in bioethanol production using rice straw

Production of cellulolytic and xylanolytic enzymes by Sporotrichum thermophile was enhanced using response surface methodology in solid-state fermentation (SSF) using wheat straw and cotton oil cake. Cellulolytic and xylanolytic enzymes were partially purified by ammonium sulfate precipitation followed by ion exchange and gel filtration chromatographic techniques. Xylanase of S. thermophile is neutral xylanase displaying optimal activity at 60 °C with K_m and V_max values of 0.2 mg/mL and 238.05 µmole/min, respectively. All cellulases produced by the thermophilic mold showed optimal activity at pH 5.0 and 60 °C with K_m values of 0.312 mg/mL, 0.113 mg/mL, and 0.285 mM for carboxymethyl cellulase (CMCase), filter paper cellulase (FPase), and β-glucosidase, respectively and while V_max values were 181.81, 138.88, and 66.67 µmole/min, respectively. The presence of various metal ions (Ca²⁺ and Co²⁺), chemical reagent (glutaraldehyde), and surfactants (Tween 80 and Triton X-100) significantly improved the activities of all enzymes. All the enzymes showed high storage stability under low temperature (-20 and 4 °C) conditions. Cellulolytic and xylanolytic enzymes resulted in enhanced liberation of reducing sugars (356.34 mg/g) by hydrolyzing both cellulosic and hemicellulosic fractions of ammonia-pretreated rice straw as compared to other pretreatment methods used in the study. Fermentation of enzymatic hydrolysate resulted in the formation of 28.88 and 27.18 g/L of bioethanol in separate hydrolysis and fermentation (SHF) process by Saccharomyces cerevisiae and Pichia stipitis, respectively. Therefore, cellulolytic and xylanolytic enzymes of S. thermophile exhibited ideal properties of biocatalysts useful in the saccharification of cellulosic and hemicellulosic fractions of rice straw for the production of bioethanol.

β-Xylosidases: Structural Diversity, Catalytic Mechanism, and Inhibition by Monosaccharides

Xylan, a prominent component of cellulosic biomass, has a high potential for degradation into reducing sugars, and subsequent conversion into bioethanol. This process requires a range of xylanolytic enzymes. Among them, β-xylosidases are crucial, because they hydrolyze more glycosidic bonds than any of the other xylanolytic enzymes. They also enhance the efficiency of the process by degrading xylooligosaccharides, which are potent inhibitors of other hemicellulose-/xylan-converting enzymes. On the other hand, the β-xylosidase itself is also inhibited by monosaccharides that may be generated in high concentrations during the saccharification process. Structurally, β-xylosidases are diverse enzymes with different substrate specificities and enzyme mechanisms. Here, we review the structural diversity and catalytic mechanisms of β-xylosidases, and discuss their inhibition by monosaccharides.

Cloning and characterization of the β-xylosidase from Dictyoglomus turgidum for high efficient biotransformation of 10-deacetyl-7-xylosltaxol

With the aim of finding an extracellular biocatalyst that can efficiently remove the C-7 xylose group from 10-deacetyl-7-xylosltaxol, a Dictyoglomus turgidum β-xylosidase was cloned and expressed in Escherichia coli BL21 (DE3). The molecular mass of purified Dt-Xyl3 was approximately 84 kDa. The recombinant Dt-Xyl3 was most active at pH 5.0 and 75 °C, retaining 88% activity at 65 °C for 1 h, and displaying excellent stability over pH 4.0-7.5 for 24 h. In terms of kinetic parameters, the K_m and V_max values for pNPX were 0.8316 mM and 5.0178 μmol/mL·min, respectively. Moreover, Dt-Xyl3 was activated by Mn²⁺ and Ba²⁺ and inhibited by Cu²⁺, Ni⁺ and Al³⁺. In particular, it displayed high tolerance to salts with 60.8% activity in 20% (w/v) NaCl. Ethanol and methanol at 5-15% showed little effect on the enzymatic activity. Dt-Xyl3 demonstrated multifunctional activities followed by pNPX, pNPAraf and pNPG and had a high selectivity for cleaving the outer xylose moieties of 10-deacetyl-7-xylosltaxol with K_cat/K_m 110.87 s^-1/mM, which produced 10-deacetyl-taxol to semi-synthesize paclitaxel. Under the optimized conditions (60 °C, pH 4.5, enzyme dosage of 0.5 U/mL), 1 g of 10-deacetyl-7-xylosltaxol was transformed to its corresponding aglycone 10-deacetyl-taxol within 30 min, with a molar conversion of 98%. This is the first report that Dictyoglomus turgidum can produce extracellular GH3 β-xylosidase with highly specific activity for 10-deacetyl-7-xylosltaxol biotransformation, thus leading to the application of β-xylosidase Dt-Xyl3 as a biocatalyst in biopharmaceutics.

Production, characteristics, and biotechnological applications of microbial xylanases

Microbial xylanases have gathered great attention due to their biotechnological potential at industrial scale for many processes. A variety of lignocellulosic materials, such as sugarcane bagasse, rice straw, rice bran, wheat straw, wheat bran, corn cob, and ragi bran, are used for xylanase production which also solved the great issue of solid waste management. Both solid-state and submerged fermentation have been used for xylanase production controlled by various physical and nutritional parameters. Majority of xylanases have optimum pH in the range of 4.0-9.0 with optimum temperature at 30-60 °C. For biochemical, molecular studies and also for successful application in industries, purification and characterization of xylanase have been carried out using various appropriate techniques. Cloning and genetic engineering are used for commercial-level production of xylanase, to meet specific economic viability and industrial needs. Microbial xylanases are used in various biotechnological applications like biofuel production, pulp and paper industry, baking and brewing industry, food and feed industry, and deinking of waste paper. This review describes production, characteristics, and biotechnological applications of microbial xylanases.

A Novel CreA-Mediated Regulation Mechanism of Cellulase Expression in the Thermophilic Fungus Humicola insolens

The thermophilic fungus Humicola insolens produces cellulolytic enzymes that are of great scientific and commercial interest; however, few reports have focused on its cellulase expression regulation mechanism. In this study, we constructed a creA gene (carbon catabolite repressor gene) disruption mutant strain of H. insolens that exhibited a reduced radial growth rate and stouter hyphae compared to the wild-type (WT) strain. The creA disruption mutant also expressed elevated pNPCase (cellobiohydrolase activities), pNPGase (β-glucosidase activities), and xylanase levels in non-inducing fermentation with glucose. Unlike other fungi, the H. insolenscreA disruption mutant displayed lower FPase (filter paper activity), CMCase (carboxymethyl cellulose activity), pNPCase, and pNPGase activity than observed in the WT strain when fermentation was induced using Avicel, whereas its xylanase activity was higher than that of the parental strain. These results indicate that CreA acts as a crucial regulator of hyphal growth and is part of a unique cellulase expression regulation mechanism in H. insolens. These findings provide a new perspective to improve the understanding of carbon catabolite repression regulation mechanisms in cellulase expression, and enrich the knowledge of metabolism diversity and molecular regulation of carbon metabolism in thermophilic fungi.

Characterization of a novel salt-, xylose- and alkali-tolerant GH43 bifunctional β-xylosidase/α-l-arabinofuranosidase from the gut bacterial genome

A GH43 bifunctional β-xylosidase encoding gene (XylRBM26) was cloned from Massilia sp. RBM26 and successfully expressed in Escherichia coli. Recombinant XylRBM26 exhibited β-xylosidase and α-l-arabinofuranosidase activities. When 4-nitrophenyl-β-d-xylopyranoside was used as a substrate, the enzyme reached optimal activity at pH 6.5 and 50°C and remained stable at pH 5.0-10.0. Purified XylRBM26 presented good salt tolerance and retained 96.6% activity in 3.5 M NaCl and 77.9% initial activity even in 4.0 M NaCl. In addition, it exhibited high tolerance to xylose with Ki value of 500 mM. This study was the first to identify and characterize NaCl-tolerant β-xylosidase/α-l-arabinofuranosidase from the gut microbiota. The enzyme's salt, xylose, and alkali stability and resistance to various chemicals make it a potential biocatalyst for the saccharification of lignocellulose, the food industry, and industrial processes conducted in sea water.

Redefining Humicola sensu stricto and related genera in the Chaetomiaceae

The traditional concept of the genus Humicola includes species that produce pigmented, thick-walled and single-celled spores laterally or terminally on hyphae or minimally differentiated conidiophores. More than 50 species have been described in the genus. Species commonly occur in soil, indoor environments, and compost habitats. The taxonomy of Humicola and morphologically similar genera is poorly understood in modern terms. Based on a four-locus phylogeny, the morphological concept of Humicola proved to be polyphyletic. The type of Humicola, H. fuscoatra, belongs to the Chaetomiaceae. In the Chaetomiaceae, species producing humicola-like thick-walled spores are distributed among four lineages: Humicola sensu stricto, Mycothermus, Staphylotrichum, and Trichocladium. In our revised concept of Humicola, asexual and sexually reproducing species both occur. The re-defined Humicola contains 24 species (seven new and thirteen new combinations), which are described and illustrated in this study. The species in this genus produce conidia that are lateral, intercalary or terminal on/in hyphae, and conidiophores are not formed or are minimally developed (micronematous). The ascospores of sexual Humicola species are limoniform to quadrangular in face view and bilaterally flattened with one apical germ pore. Seven species are accepted in Staphylotrichum (four new species, one new combination). Thick-walled conidia of Staphylotrichum species usually arise either from hyphae (micronematous) or from apically branched, seta-like conidiophores (macronematous). The sexual morph represented by Staphylotrichum longicolleum (= Chaetomium longicolleum) produces ascomata with long necks composed of a fused basal part of the terminal hairs, and ascospores that are broad limoniform to nearly globose, bilaterally flattened, with an apical germ pore. The Trichocladium lineage has a high morphological diversity in both asexual and sexual structures. Phylogenetic analysis revealed four subclades in this lineage. However, these subclades are genetically closely related, and no distinctive phenotypic characters are linked to any of them. Fourteen species are accepted in Trichocladium, including one new species, twelve new combinations. The type species of Gilmaniella, G. humicola, belongs to the polyphyletic family Lasiosphaeriaceae (Sordariales), but G. macrospora phylogenetically belongs to Trichocladium. The thermophilic genus Mycothermus and the type species My. thermophilum are validated, and one new Mycothermus species is described. Phylogenetic analyses show that Remersonia, another thermophilic genus, is sister to Mycothermus and two species are known, including one new species. Thermomyces verrucosus produces humicola-like conidia and is transferred to Botryotrichum based on phylogenetic affinities. This study is a first attempt to establish an inclusive modern classification of Humicola and humicola-like genera of the Chaetomiaceae. More research is needed to determine the phylogenetic relationships of “humicola”-like species outside the Chaetomiaceae.

Production and Characteristics of a Novel Xylose- and Alkali-tolerant GH 43 β-xylosidase from Penicillium oxalicum for Promoting Hemicellulose Degradation

β-xylosidase is a pivotal enzyme for complete degradation of xylan in hemicelluloses of lignocelluloses, and the xylose- and alkali-tolerant β-xylosidase with high catalytic activity is very attractive for promoting enzymatic hydrolysis of alkaline-pretreated lignocellulose. In this study, a novel intracellular glycoside hydrolase family 43 β-xylosidase gene (xyl43) from Penicillium oxalicum 114-2 was successfully high-level overexpressed in Pichia pastoris, and the secreted enzyme was characterized. The β-xylosidase Xyl43 exhibited great pH stability and high catalytic activity in the range of pH 6.0 to 8.0, and high tolerance to xylose with the Ki value of 28.09 mM. The Xyl43 could effectively promote enzymatic degradation of different source of xylan and hemicellulose contained in alkaline-pretreated corn stover, and high conversion of xylan to xylose could be obtained.

Molecular cloning and comparative sequence analysis of fungal β-Xylosidases

Commercial scale degradation of hemicelluloses into easily accessible sugar residues is practically crucial in industrial as well as biochemical processes. Xylanolytic enzymes have a great number of possible applications in many biotechnological processes and therefore, these enzymes are continuously attracting the attention of scientists. Due to this fact, different β-Xylosidases have been isolated, purified and characterized from several bacteria and fungi. Microorganisms in this respect have gained much momentum for production of these significant biocatalysts with remarkable features. It is difficult to propagate microorganisms for efficient and cost-competitive production of β-Xylosidase from hemicelluloses due to expensive conditions of fermentation. The screening of new organisms with an enhanced production of β-Xylosidases has been made possible with the help of recombinant DNA technology. β-Xylosidase genes haven been cloned and expressed on large scale in both homologous and heterologous hosts with the advent of genetic engineering. Therefore, we have reviewed the literature regarding cloning of β-Xylosidase genes into various hosts for their heterologous production along with sequence similarities among different β-Xylosidases. The study provides insight into the current status of cloning, expression and sequence analysis of β-Xylosidases for industrial applications.

Enzymatic fine-tuning for 2-(6-hydroxynaphthyl) β-D-xylopyranoside synthesis catalyzed by the recombinant β-xylosidase BxTW1 from Talaromyces amestolkiae

Background

Glycosides are compounds displaying crucial biological roles and plenty of applications. Traditionally, these molecules have been chemically obtained, but its efficient production is limited by the lack of regio- and stereo-selectivity of the chemical synthesis. As an interesting alternative, glycosidases are able to catalyze the formation of glycosides in a process considered green and highly selective. In this study, we report the expression and characterization of a fungal β-xylosidase in Pichia pastoris. The transglycosylation potential of the enzyme was evaluated and its applicability in the synthesis of a selective anti-proliferative compound demonstrated.

Results

The β-xylosidase BxTW1 from the ascomycete fungus Talaromyces amestolkiae was cloned and expressed in Pichia pastoris GS115. The yeast secreted 8 U/mL of β-xylosidase that was purified by a single step of cation-exchange chromatography. rBxTW1 in its active form is an N-glycosylated dimer of about 200 kDa. The enzyme was biochemically characterized displaying a K_m and k_cat against p-nitrophenyl-β-d-xylopyranoside of 0.20 mM and 69.3 s⁻¹ respectively, and its maximal activity was achieved at pH 3 and 60 °C. The glycan component of rBxTW1 was also analyzed in order to interpret the observed loss of stability and maximum velocity when compared with the native enzyme. A rapid screening of aglycone specificity was performed, revealing a remarkable high number of potential transxylosylation acceptors for rBxTW1. Based on this analysis, the enzyme was successfully tested in the synthesis of 2-(6-hydroxynaphthyl) β-d-xylopyranoside, a well-known selective anti-proliferative compound, enzymatically obtained for the first time. The application of response surface methodology, following a Box-Behnken design, enhanced this production by eightfold, fitting the reaction conditions into a multiparametric model. The naphthyl derivative was purified and its identity confirmed by NMR.

Conclusions

A β-xylosidase from T. amestolkiae was produced in P. pastoris and purified. The final yields were much higher than those attained for the native protein, although some loss of stability and maximum velocity was observed. rBxTW1 displayed remarkable acceptor versatility in transxylosylation, catalyzing the synthesis of a selective antiproliferative compound, 2-(6-hydroxynaphthyl) β-d-xylopyranoside. These results evidence the interest of rBxTW1 for transxylosylation of relevant products with biotechnological interest.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0568-6) contains supplementary material, which is available to authorized users.

The use of T-DNA insertional mutagenesis to improve cellulase production by the thermophilic fungus Humicola insolens Y1

Humicola insolens is an excellent producer of pH-neutral active, thermostable cellulases that find many industrial applications. In the present study, we developed an efficient Agrobacterium tumefaciens-mediated transformation system for H. insolens. We transformed plasmids carrying the promoter of the glyceraldehyde-3-phosphate dehydrogenase gene of H. insolens driving the transcription of genes encoding neomycin phosphotransferase, hygromycin B phosphotransferase, and enhanced green fluorescent protein. We optimized transformation efficiency to obtain over 300 transformants/10⁶ conidia. T-DNA insertional mutagenesis was employed to generate an H. insolens mutant library, and we isolated a transformant termed T4 with enhanced cellulase and hemicellulase activities. The FPase, endoglucanase, cellobiohydrolase, β-glucosidase, and xylanase activities of T4, measured at the end of fermentation, were 60%, 440%, 320%, 41%, and 81% higher than those of the wild-type strain, respectively. We isolated the sequences flanking the T-DNA insertions and thus identified new genes potentially involved in cellulase and hemicellulase production. Our results show that it is feasible to use T-DNA insertional mutagenesis to identify novel candidate genes involved in cellulase production. This will be valuable when genetic improvement programs seeking to enhance cellulase production are planned, and will also allow us to gain a better understanding of the genetics of the thermophilic fungus H. insolens.