Expression and characterisation of a pH and salt tolerant, thermostable and xylose tolerant recombinant GH43 β-xylosidase from Thermobifida halotolerans YIM 90462T for promoting hemicellulose degradation

Biochemical characterization of a xylose-tolerant GH43 β-xylosidase from Geobacillus thermodenitrificans

Hemicelluloses are the second most abundant polysaccharide in plant biomass, in which xylan is the main constituent. Aiming at the total degradation of xylan and the obtention of fermentable sugars, several enzymes acting synergistically are required, especially β-xylosidases. In this study, β-xylosidase from Geobacillus thermodenitrificans (GtXyl) was expressed in E. coli BL21 and characterized. The enzyme GtXyl has been grouped within the family of glycoside hydrolases 43 (GH43). Results showed that GtXyl obtained the highest activity at pH 5.0 and temperature of 60 °C. In the additive's tests, the enzyme remained stable in the presence of metal ions and EDTA, and showed high tolerance to xylose, with a relative activity of 55.4% at 400 mM. The enzyme also presented bifunctional activity of β-xylosidase and α-l-arabinofuranosidase, with the highest activity on the substrate p-nitrophenyl-β-d-xylopyranoside. The specific activity on p-nitrophenyl-β-d-xylopyranoside was 18.33 U mg-1 and catalytic efficiency of 20.21 mM-1 s-1, which is comparable to other β-xylosidases reported in the literature. Putting together, the GtXyl enzyme presented interesting biochemical characteristics that are desirable for the application in the enzymatic hydrolysis of plant biomass, such as activity at higher temperatures, high thermostability and stability to metal ions.

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.

Structural insights of a putative β-1,4-xylosidase (PsGH43F) of glycoside hydrolase family 43 from Pseudopedobacter saltans

Structural and conformational insights of a putative β-1,4-xylosidase (PsGH43F) of glycoside hydrolase family 43 from Pseudopedobacter saltans were investigated by computational and Circular Dichroism (CD) analyses. PsGH43F was cloned and expressed in E. coli BL21 (DE3) cells and the purified enzyme gave the size ~50 kDa on SDS-PAGE analysis. Multiple Sequence Alignment of PsGH43F sequence followed by superposition of modeled structure with homologous structures displayed the presence of three conserved catalytic amino acid residues, Asp33, Asp149 and Glu212. The secondary structure analysis by CD showed 2.72 % α-helix and 36.06 % β-strands. The homology modeled structure of PsGH43F displayed a 5-bladed β-propeller fold for catalytic module at N-terminal and a β-sandwich structure for CBM6 at the C-terminal. Ramachandran plot displayed 99.5 % of residues in the allowed regions. MD simulation of PsGH43F revealed the compactness and stability of the structure. Molecular docking studies of PsGH43F with xylo-oligosaccharides revealed its maximum binding affinity for xylobiose. MD simulation of PsGH43F-xylobiose complex confirmed the increased structural and conformational stability in presence of substrate. The Hydrodynamic diameter analysis of PsGH43F by DLS was in the range, 0.25-0.28 μm.

Biotechnological Aspects of Salt-Tolerant Xylanases: A Review

β-1,4-Xylan is the main component of hemicelluloses in land plant cell walls, whereas β-1,3-xylan is widely found in seaweed cell walls. Complete hydrolysis of xylan requires a series of synergistically acting xylanases. High-saline environments, such as saline-alkali lands and oceans, frequently occur in nature and are also involved in a broad range of various industrial processes. Thus, salt-tolerant xylanases may contribute to high-salt and marine food processing, aquatic feed production, industrial wastewater treatment, saline-alkali soil improvement, and global carbon cycle, with great commercial and environmental benefits. This review mainly introduces the definition, sources, classification, biochemical and molecular characteristics, adaptation mechanisms, and biotechnological applications of salt-tolerant xylanases. The scope of development for salt-tolerant xylanases is also discussed. It is anticipated that this review would serve as a reference for further development and utilization of salt-tolerant xylanases and other salt-tolerant enzymes.

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.