Characterization of a β-D-mannosidase from a marine gastropod, Aplysia kurodai

Crystal structure of a homotrimeric verrucomicrobial exo-β-1,4-mannosidase active in the hindgut of the wood-feeding termite Reticulitermes flavipes

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First structure of a glycoside hydrolase from a bacterial symbiont isolated from the digestive tract of the notorious termite pest Reticulitermes flavipes.

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First example of a GH5 glycoside hydrolase that features a GH42-type homotrimeric structure.

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High exo-type specificity for the terminal ®-1,4-mannosidic linkages in mannooligosaccharides and unsubstituted®-mannans.

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Verrucomicrobial gut symbiont with high potential for hemicellulose degradation.

The termite Reticulitermes flavipes causes extensive damage due to the high efficiency and broad specificity of the ligno- and hemicellulolytic enzyme systems produced by its symbionts. Thus, the R. flavipes gut microbiome is expected to constitute an excellent source of enzymes that can be used for the degradation and valorization of plant biomass. The symbiont Opitutaceae bacterium strain TAV5 belongs to the phylum Verrucomicrobia and thrives in the hindgut of R. flavipes. The sequence of the gene with the locus tag opit5_10225 in the Opitutaceae bacterium strain TAV5 genome has been classified as a member of glycoside hydrolase family 5 (GH5), and provisionally annotated as an endo-β-mannanase. We characterized biochemically and structurally the opit5_10225 gene product, and show that the enzyme, Op5Man5, is an exo-β-1,4-mannosidase [EC 3.2.1.25] that is highly specific for β-1,4-mannosidic bonds in mannooligosaccharides and ivory nut mannan. The structure of Op5Man5 was phased using electron cryo-microscopy and further determined and refined at 2.2 Å resolution using X-ray crystallography. Op5Man5 features a 200-kDa large homotrimer composed of three modular monomers. Despite insignificant sequence similarity, the structure of the monomer, and homotrimeric assembly are similar to that of the GH42-family β-galactosidases and the GH164-family exo-β-1,4-mannosidase Bs164 from Bacteroides salyersiae. To the best of our knowledge Op5Man5 is the first structure of a glycoside hydrolase from a bacterial symbiont isolated from the R. flavipes digestive tract, as well as the first example of a GH5 glycoside hydrolase with a GH42 β-galactosidase-type homotrimeric structure.

In vitro and in vivo characterization of genes involved in mannan degradation in Neurospora crassa

To better understand the roles of genes involved in mannan degradation in filamentous fungi, in this study we searched, identified, and characterized one putative GH5 endo-β-mannanase (GH5-7) and two putative GH2 mannan-degrading enzymes (GH2-1 and GH2-4) in Neurospora crassa. Real-time RT-PCR analyses showed that the expression levels of these genes were significantly up-regulated when the cells were grown in mannan-containing media where the induction level of gh5-7 was the highest. All three proteins were heterologously expressed and purified. GH5-7 displayed a substrate preference toward galactomannan by showing 10-times higher catalytic efficiency than to linear β-mannan. In contrast, GH2-1 preferred short manno-oligosaccharides or β-mannan as substrates. Compared to the wild type strain, the growth of Δgh5-7 and Δgh5-7Δgh2-4 mutants, but not Δgh2-1, Δgh2-4, and Δgh2-1Δgh2-4 mutants, was poor in the cultures containing glucomannan or galactomannan as the sole carbon source, suggesting that GH5-7 plays a critical role in the utilization of heteromannans in vivo. On the other hand, all the mutants showed significantly slow growth when grown in the medium containing linear β-mannan. Collectively, these results indicate that N. crassa can utilize glucomannan and galactomannan without GH2-1 and GH2-4, but efficient degradation of β-mannan requires a concerted action of three enzymes, GH5-7, GH2-1, and GH2-4.

Expression of a β-mannosidase from Paenibacillus polymyxa A-8 in Escherichia coli and characterization of the recombinant enzyme

Paenibacillus polymyxa A-8, which secretes β-mannosidase, was isolated from the soil sample under a pine tree located in the "Laoban" mountain region of Sichuan, China. The β-mannosidase gene (MANB) was isolated from P. polymyxa A-8, using primers according to the complete genome. The MANB (2,550 bp) encoding 849 amino acid residues was expressed in Escherichia coli. The specific activities of β-mannosidase produced by P. polymyxa A-8 and E. coli pET30a-MANB were 12 nkat/mg and 635 nkat/mg respectively. SDS-PAGE analysis indicated that the molecular mass of the recombinant MANB was approximately 96 kDa. The recombinant MANB was active between pH 7.0-8.5 with the maximum activity at pH 7.0. It had good pH stability and adaptability. The MANB had the optimal temperature of 35°C and was relatively stable at 35-40°C. In addition, the MANB activity was enhanced by K+, Ca2+, Mn2+, and Mg2+ and inhibited by Zn2+, Cu2+, and Hg2+.

High-level expression of a novel thermostable and mannose-tolerant β-mannosidase from Thermotoga thermarum DSM 5069 in Escherichia coli

Background

Mannan is one of the primary polysaccharides in hemicellulose and is widely distributed in plants. β-Mannosidase is an important constituent of the mannan-degrading enzyme system and it plays an important role in many industrial applications, such as food, feed and pulp/paper industries as well as the production of second generation bio-fuel. Therefore, the mannose-tolerant β-mannosidase with high catalytic efficiency for bioconversion of mannan has a great potential in the fields as above.

Results

A β-mannosidase gene (Tth man5) of 1,827 bp was cloned from the extremely thermophilic bacterium Thermotoga thermarum DSM 5069 that encodes a protein containing 608 amino acid residues, and was over-expressed in Escherichia coli BL21 (DE3). The results of phylogenetic analysis, amino acid alignment and biochemical properties indicate that the Tth Man5 is a novel β-mannosidase of glycoside hydrolase family 5. The optimal activity of the Tth Man5 β-mannosidase was obtained at pH 5.5 and 85°C and was stable over a pH range of 5.0 to 8.5 and exhibited 2 h half-life at 90°C. The kinetic parameters K_m and V_max values for p-nitrophenyl-β-D-mannopyranoside and 1,4-β-D-mannan were 4.36±0.5 mM and 227.27±1.59 μmol min^-1 mg^-1, 58.34±1.75 mg mL^-1 and 285.71±10.86 μmol min^-1 mg^-1, respectively. The k_cat/K_m values for p-nitrophenyl-β-D-mannopyranoside and 1,4-β-D-mannan were 441.35±0.04 mM^-1 s^-1 and 41.47±1.58 s^-1 mg^-1 mL, respectively. It displayed high tolerance to mannose, with a K_i value of approximately 900 mM.

Conclusions

This work provides a novel and useful β-mannosidase with high mannose tolerance, thermostability and catalytic efficiency, and these characteristics constitute a powerful tool for improving the enzymatic conversion of mannan through synergetic action with other mannan-degrading enzymes.