Fungal β-mannanases: Mannan hydrolysis, heterologous production and biotechnological applications

Directed modification of the Aspergillus usamii β-mannanase to improve its substrate affinity by in silico design and site-directed mutagenesis

β-Mannanases (EC 3.2.1.78) can catalyze the cleavage of internal β-1,4-d-mannosidic linkages of mannan backbones, and they have found applications in food, feed, pulp and paper, oil, pharmaceutical and textile industries. Suitable amino acid substitution can promote access to the substrate-binding groove and maintain the substrate therein, which probably improves the substrate affinity and, thus, increases catalytic efficiency of the enzyme. In this study, to improve the substrate affinity of AuMan5A, a glycoside hydrolase (GH) family 5 β-mannanase from Aspergillus usamii, had its directed modification conducted by in silico design, and followed by site-directed mutagenesis. The mutant genes, Auman5AY111F and Auman5AY115F, were constructed by megaprimer PCR, respectively. Then, Auman5A and its mutant genes were expressed in Pichia pastoris GS115 successfully. The specific activities of purified recombinant β-mannanases (reAuMan5A, reAuMan5AY111F and reAuMan5AY115F) towards locust bean gum were 152.5, 199.6 and 218.9 U mg−1, respectively. The two mutants were found to be similar to reAuMan5A regarding temperature and pH characteristics. Nevertheless, the K  m values of reAuMan5AY111F and reAuMan5AY115F, towards guar gum, decreased to 2.95 ± 0.22 and 2.39 ± 0.33 mg ml−1 from 4.49 ± 0.07 mg ml−1 of reAuMan5A, which would make reAuMan5AY111F and reAuMan5AY115F promising candidates for industrial processes. Structural analysis showed that the two mutants increased their affinity by decreasing the steric conflicts with those more complicated substrates. The results suggested that subtle conformational modification in the substrate-binding groove could substantially alter the substrate affinity of AuMan5A. This study laid a solid foundation for the directed modification of substrate affinities of β-mannanases and other enzymes.

Modular hyperthermostable bacterial endo-β-1,4-mannanase: molecular shape, flexibility and temperature-dependent conformational changes

Endo-β-1,4-mannanase from Thermotoga petrophila (TpMan) is a hyperthermostable enzyme that catalyzes the hydrolysis of β-1,4-mannoside linkages in various mannan-containing polysaccharides. A recent study reported that TpMan is composed of a GH5 catalytic domain joined by a linker to a carbohydrate-binding domain. However, at this moment, there is no three-dimensional structure determined for TpMan. Little is known about the conformation of the TpMan as well as the role of the length and flexibility of the linker on the spatial arrangement of the constitutive domains. In this study, we report the first structural characterization of the entire TpMan by small-angle X-ray scattering combined with the three-dimensional structures of the individual domains in order to shed light on the low-resolution model, overall dimensions, and flexibility of this modular enzyme at different temperatures. The results are consistent with a linker with a compact structure and that occupies a small volume with respect to its large number of amino acids. Furthermore, at 20°C the results are consistent with a model where TpMan is a molecule composed of three distinct domains and that presents some level of molecular flexibility in solution. Even though the full enzyme has some degree of molecular flexibility, there might be a preferable conformation, which could be described by the rigid-body modeling procedure. Finally, the results indicate that TpMan undergoes a temperature-driven transition between conformational states without a significant disruption of its secondary structure. Our results suggest that the linker can optimize the geometry between the other two domains with respect to the substrate at high temperatures. These studies should provide a useful basis for future biophysical studies of entire TpMan.

High yield production of sugars from deproteinated palm kernel cake under microwave irradiation via dilute sulfuric acid hydrolysis

Recent years, great interest has been devoted to the conversion of biomass-derived carbohydrate into sugars, such as glucose, mannose and fructose. These are important versatile intermediate products that are easily processed into high value-added biofuels. In this work, microwave-assisted dilute sulfuric acid hydrolysis of deproteinated palm kernel cake (DPKC) was systematically studied using Response Surface Methodology. The highest mannose yield (92.11%) was achieved at the optimized condition of 148°C, 0.75N H2SO4, 10min 31s and substrate to solvent (SS) ratio (w/v) of 1:49.69. Besides that, total fermentable sugars yield (77.11%), was obtained at 170°C, 0.181N H2SO4, 6min 6s and SS ratio (w/v) of 1:40. Ridge analysis was employed to further verify the optimum conditions. Thus, this work provides fundamental data of the practical use of DPKC as low cost, high yield and environmental-friendly material for the production of mannose and other sugars.

Fusing a carbohydrate-binding module into the Aspergillus usamii β-mannanase to improve its thermostability and cellulose-binding capacity by in silico design

The AuMan5A, an acidophilic glycoside hydrolase (GH) family 5 β-mannanase derived from Aspergillus usamii YL-01-78, consists of an only catalytic domain (CD). To perfect enzymatic properties of the AuMan5A, a family 1 carbohydrate-binding module (CBM) of the Trichoderma reesei cellobiohydrolase I (TrCBH I), having the lowest binding free energy with cellobiose, was selected by in silico design, and fused into its C-terminus forming a fusion β-mannanase, designated as AuMan5A-CBM. Then, its encoding gene, Auman5A-cbm, was constructed as it was designed theoretically, and expressed in Pichia pastoris GS115. SDS-PAGE analysis displayed that both recombinant AuMan5A-CBM (reAuMan5A-CBM) and AuMan5A (reAuMan5A) were secreted into the cultured media with apparent molecular masses of 57.3 and 49.8 kDa, respectively. The temperature optimum of the reAuMan5A-CBM was 75°C, being 5°C higher than that of the reAuMan5A. They were stable at temperatures of 68 and 60°C, respectively. Compared with reAuMan5A, the reAuMan5A-CBM showed an obvious decrease in K_m and a slight alteration in V_max. In addition, the fusion of a CBM of the TrCBH I into the AuMan5A contributed to its cellulose-binding capacity.

Hydrolysis of konjac glucomannan by Trichoderma reesei mannanase and endoglucanases Cel7B and Cel5A for the production of glucomannooligosaccharides

In this paper we describe the enzymatic hydrolysis of konjac glucomannan for the production of glucomannooligosaccharides using purified Trichoderma reesei mannanase, endoglucanases EGI (Tr Cel7b) and EGII (Tr Cel5a). Hydrolysis with each of the three enzymes produced a different pattern of oligosaccharides. Mannanase was the most selective of the three enzymes in the hydrolysis of konjac mannan and over 99% of the formed oligosaccharides had mannose as their reducing end pyranosyl unit. Tr Cel5A hydrolysate shared similarities with mannanase and Tr Cel7B hydrolysates and the enzyme had the lowest substrate specificity of the studied enzymes. The hydrolysate of Tr Cel7B contained a series of oligosaccharides with non-reducing end mannose (M) and reducing end glucose (G) (MG, MMG, MMMG, and MMMMG). These oligosaccharides were isolated from the hydrolysate by size exclusion chromatography in relatively high purity (86-95%) and total yield (23% of substrate). The isolated oligosaccharides were characterized using acid hydrolysis and HPAEC-PAD (carbohydrate composition), HPLC-RI and HPAEC-MS (to determine the DP of purified oligosaccharides), enzymatic hydrolysis (determination of non-reducing end carbohydrate) and NMR (both 1D and 2D, to verify structure and purity of purified compounds). Hydrolysis of konjac mannan with a specific enzyme, such as T. reesei Cel7B or mannanase, followed by fractionation with SEC offers the possibility to produce glucomannooligosaccharides with defined structure. The isolated oligosaccharides can be utilised as analytical standards, for determination of bioactivity of oligosaccharides with defined structure or as substrates for defining substrate specificity of novel carbohydrate hydrolyzing enzymes.

Gene cloning and enzymatic characterization of an alkali-tolerant endo-1,4-β-mannanase from Rhizomucor miehei

An endo-1,4-β-mannanase gene (RmMan5A) was cloned from the thermophilic fungus Rhizomucor miehei for the first time and expressed in Escherichia coli . The gene had an open reading frame of 1330 bp encoding 378 amino acids and contained four introns. It displayed the highest amino acid sequence identity (42%) with the endo-1,4-β-mannanases from glycoside hydrolase family 5. The purified enzyme was a monomer of 43 kDa. RmMan5A displayed maximum activity at 55 °C and an optimal pH of 7.0. It was thermostable up to 55 °C and alkali-tolerant, displaying excellent stability over a broad pH range of 4.0-10.0, when incubated for 30 min without substrate. The enzyme displayed the highest specificity for locust bean gum (K(m) = 3.78 mg mL⁻¹), followed by guar gum (K(m) = 7.75 mg mL⁻¹) and konjac powder (K(m) = 22.7 mg mL⁻¹). RmMan5A hydrolyzed locust bean gum and konjac powder yielding mannobiose, mannotriose, and a mixture of various mannose-linked oligosaccharides. It was confirmed to be a true endo-acting β-1,4-mannanase, which showed requirement of four mannose residues for hydrolysis, and was also capable of catalyzing transglycosylation reactions. These properties make RmMan5A highly useful in the food/feed, paper and pulp, and detergent industries.

Softening-up mannan-rich cell walls

The softening and degradation of the cell wall (CW), often mannan enriched, is involved in several processes during development of higher plants, such as meristematic growth, fruit ripening, programmed cell death, and endosperm rupture upon germination. Mannans are also the predominant hemicellulosic CW polymers in many genera of green algae. The endosperm CWs of dry seeds often contain mannan polymers, sometimes in the form of galactomannans (Gal-mannans). The endo-β-mannanases (MANs) that catalyse the random hydrolysis of the β-linkage in the mannan backbone are one of the main hydrolytic enzymes involved in the loosening and remodelling of CWs. In germinating seeds, the softening of the endosperm seed CWs facilitates the emergence of the elongating radicle. Hydrolysis and mobilization of endosperm Gal-mannans by MANs also provides a source of nutrients for early seedling growth, since Gal-mannan, besides its structural role, serves as a storage polysaccharide. Therefore, the role of mannans and of their hydrolytic enzymes is decisive in the life cycle of seeds. This review updates and discusses the significance of mannans and MANs in seeds and explores the increasing biotechnological potential of MAN enzymes.

Gene cloning, expression, and biochemical characterization of an alkali-tolerant β-mannanase from Humicola insolens Y1

In this article, we firstly report a highly alkali-tolerant fungal β-mannanase from Humicola insolens Y1. The full-length cDNA of the β-mannanase, designated as man5A, has an open reading frame of 1,233 bp that encodes a 411-amino acid polypeptide (Man5A) with a calculated molecular mass of 42.3 kDa. The deduced sequence of Man5A comprises a putative 20-residue signal peptide and a catalytic domain belonging to glycoside hydrolase family 5, and displays 61–85% identities with hypothetical proteins and 32–39% with experimentally verified fungal β-mannanases. Purified recombinant Man5A produced by Pichia pastoris has a specific activity of 1,122 U mg−1 and exhibits optimal activity at pH 5.5 and 70°C. Distinct from other reported fungal β-mannanases, Man5A is highly alkali tolerant, exhibiting 45 and 36% of the maximal activity at pH 8.0 and 9.0, respectively, and more than 10% activity even at pH 10.0. Moreover, Man5A has excellent pH stability at pH 5.0–12.0 and is highly thermostable at 50°C. The higher frequency of alkaline amino acids (Arg and Lys), greater pKa values of the catalytic residues, and more positively charged residues on the surface of Man5A might be the causes. Man5A has strong resistance to various neutral and alkaline proteases, retaining more than 97% of the activity after proteolytic treatment for 1 h. The superior characteristics of Man5A make it more advantageous for the application in the kraft pulp industry.

A review of lignocellulose bioconversion using enzymatic hydrolysis and synergistic cooperation between enzymes--factors affecting enzymes, conversion and synergy

Lignocellulose is a complex substrate which requires a variety of enzymes, acting in synergy, for its complete hydrolysis. These synergistic interactions between different enzymes have been investigated in order to design optimal combinations and ratios of enzymes for different lignocellulosic substrates that have been subjected to different pretreatments. This review examines the enzymes required to degrade various components of lignocellulose and the impact of pretreatments on the lignocellulose components and the enzymes required for degradation. Many factors affect the enzymes and the optimisation of the hydrolysis process, such as enzyme ratios, substrate loadings, enzyme loadings, inhibitors, adsorption and surfactants. Consideration is also given to the calculation of degrees of synergy and yield. A model is further proposed for the optimisation of enzyme combinations based on a selection of individual or commercial enzyme mixtures. The main area for further study is the effect of and interaction between different hemicellulases on complex substrates.

Cloning and functional expression of an acidophilic β-mannanase gene (Anman5A) from Aspergillus niger LW-1 in Pichia pastoris

A cDNA fragment of the Anman5A, a gene that encodes an acidophilic β-mannanase of Aspergillus niger LW-1 (abbreviated as AnMan5A), was cloned and functionally expressed in Pichia pastoris . Homology alignment of amino acid sequences verified that the AnMan5A belongs to the glycoside hydrolase (GH) family 5. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) assay demonstrated that the recombinant AnMan5A (reAnMan5A), a N-glycosylated protein with an apparent molecular weight of 52.0 kDa, was secreted into the medium. The highest reAnMan5A activity expressed by one P. pastoris transformant, labeled as GSAnMan4-12, reached 29.0 units/mL. The purified reAnMan5A displayed the highest activity at pH 3.5 and 70 °C. It was stable at a pH range of 3.0-7.0 and at a temperature of 60 °C or below. Its activity was not significantly affected by an array of metal ions and ethylenediaminetetraacetic acid (EDTA). The K(m) and V(max) of the reAnMan5A, toward locust bean gum, were 1.10 mg/mL and 266.7 units/mg, respectively.

Expression of Trichoderma reesei β-mannanase in tobacco chloroplasts and its utilization in lignocellulosic woody biomass hydrolysis

Lignocellulosic ethanol offers a promising alternative to conventional fossil fuels. One among the major limitations in the lignocellulosic biomass hydrolysis is unavailability of efficient and environmentally biomass degrading technologies. Plant-based production of these enzymes on large scale offers a cost-effective solution. Cellulases, hemicellulases including mannanases and other accessory enzymes are required for conversion of lignocellulosic biomass into fermentable sugars. β-mannanase catalyzes endo-hydrolysis of the mannan backbone, a major constituent of woody biomass. In this study, the man1 gene encoding β-mannanase was isolated from Trichoderma reesei and expressed via the chloroplast genome. PCR and Southern hybridization analysis confirmed site-specific transgene integration into the tobacco chloroplast genomes and homoplasmy. Transplastomic plants were fertile and set viable seeds. Germination of seeds in the selection medium showed inheritance of transgenes into the progeny without any Mendelian segregation. Expression of endo-β-mannanase for the first time in plants facilitated its characterization for use in enhanced lignocellulosic biomass hydrolysis. Gel diffusion assay for endo-β-mannanase showed the zone of clearance confirming functionality of chloroplast-derived mannanase. Endo-β-mannanase expression levels reached up to 25 units per gram of leaf (fresh weight). Chloroplast-derived mannanase had higher temperature stability (40 °C to 70 °C) and wider pH optima (pH 3.0 to 7.0) than E.coli enzyme extracts. Plant crude extracts showed 6-7 fold higher enzyme activity than E.coli extracts due to the formation of disulfide bonds in chloroplasts, thereby facilitating their direct utilization in enzyme cocktails without any purification. Chloroplast-derived mannanase when added to the enzyme cocktail containing a combination of different plant-derived enzymes yielded 20% more glucose equivalents from pinewood than the cocktail without mannanase. Our results demonstrate that chloroplast-derived mannanase is an important component of enzymatic cocktail for woody biomass hydrolysis and should provide a cost-effective solution for its diverse applications in the biofuel, paper, oil, pharmaceutical, coffee and detergent industries.

Cloning and characterization of a modular GH5 β-1,4-mannanase with high specific activity from the fibrolytic bacterium Cellulosimicrobium sp. strain HY-13

The gene (1272-bp) encoding a β-1,4-mannanase from a gut bacterium of Eisenia fetida, Cellulosimicrobium sp. strain HY-13 was cloned and expressed in Escherichia coli. The recombinant β-1,4-mannanase (rManH) was approximately 44.0 kDa and has a catalytic GH5 domain that is 65% identical to that of the Micromonospora sp. β-1,4-mannosidase. The enzyme exhibited the highest catalytic activity toward mannans at 50 °C and pH 6.0. rManH displayed a high specific activity of 14,711 and 8498 IU mg⁻¹ towards ivory nut mannan and locust bean gum, respectively; however it could not degrade the structurally unrelated polysaccharides, mannobiose, or p-nitrophenyl sugar derivatives. rManH was strongly bound to ivory nut mannan, Avicel, chitosan, and chitin but did not attach to curdlan, insoluble oat spelt xylan, lignin, or poly(3-hydroxybutyrate). The superior biocatalytic properties of rManH suggest that the enzyme can be exploited as an effective additive in the animal feed industry.

A thermotolerant and cold-active mannan endo-1,4-β-mannosidase from Aspergillus niger CBS 513.88: Constitutive overexpression and high-density fermentation in Pichia pastoris

The mannan endo-1,4-β-mannosidase gene man26A from Aspergillus niger CBS 513.88 was optimized according to the codon usage bias in Pichia pastoris and synthesized by splicing overlap extension PCR. It was successfully expressed in P. pastoris using constitutive expression vector pGAPzαA. The recombinant endo-beta-1,4-mannanase could work in an extremely board temperature range and over 30% relative activity were retained in the temperature range of 5-60°C. The optimal pH value and temperature for activity were 5.0 and 45°C, respectively. It was highly thermotolerant with a half-life time of 15min at 90°C. A novel fed-batch strategy was developed successfully for high cell-density fermentation and mannanase activity reached 5069U/mL after cultivation for 56h in 50L fermenter. The broad working temperature range, high thermotolerance and efficient expression made this enzyme possible to be applied in food, animal feed and the production of biofuels.