Enhanced exo-inulinase activity and stability by fusion of an inulin-binding module

Deletion of the Loop Linking Two Domains of Exo-Inulinase InuAMN8 Diminished the Enzymatic Thermo-Halo-Alcohol Tolerance

Inulin is the rich water-soluble storage polysaccharide after starch in nature, and utilization of inulin through hydrolysis of exo-inulinases has attracted much attention. Thermo-halo-alcohol tolerance is essential for exo-inulinase applications, while no report reveals the molecular basis involved in halo-alcohol tolerance of exo-inulinases via experimental data. In this study, two loops of exo-inulinase InuAMN8, including the loop built with ³⁶⁰GHVRLGPQP³⁶⁸ linking domains of Glyco_hydro_32N and Glyco_hydro_32C and another loop built with ¹⁶⁹GGAG¹⁷² in the catalytic domain, were deleted to generate mutants MutG360Δ9 and MutG169Δ4, respectively. After heterologous expression, purification, and dialysis, InuAMN8, MutG169Δ4, and MutG360Δ9 showed half-lives of 144, 151, and 7 min at 50°C, respectively. InuAMN8 and MutG169Δ4 were very stable, while MutG360Δ9 showed a half-life of approximately 60 min in 5.0% (w/v) NaCl, and they showed half-lives of approximately 60 min in 25.0, 25.0, and 5.0% (w/v) ethanol, respectively. Structural analysis indicated that two cation-π bonds, which contributed to thermal properties of InuAMN8 at high temperatures, broke in MutG360Δ9. Four basic amino acid residues were exposed to the structural surface of MutG360Δ9 and formed positive and neutral electrostatic potential that caused detrimental effects on halo-alcohol tolerance. The study may provide a better understanding of the loop-function relationships that are involved in thermo-halo-alcohol adaptation of enzymes in extreme environment.

C-Terminal Bacterial Immunoglobulin-like Domain of κ-Carrageenase Serves as a Multifunctional Module to Promote κ-Carrageenan Hydrolysis

κ-Carrageenase is an important component for κ-carrageenan oligosaccharide production. Generally, noncatalytic domains are appended to carbohydrate-active domains and potentiate catalytic activity. However, studies devoted to κ-carrageenase are relatively few. Here, a C-terminal bacterial immunoglobulin-like domain (Big_2) was identified in κ-carrageenase (PpCgk) from Pseudoalteromonas porphyrae. Biochemical characterization of native PpCgk and its two truncations, PpCgkCD (catalytic domain) and PpBig_2 (Big_2 domain), revealed that the specific activity, catalytic efficiency (k_cat/K_m(app)), specific κ-carrageenan-binding capacity, and thermostability of PpCgk were significantly higher than those of PpCgkCD, suggesting that the noncatalytic PpBig_2 domain is a multifunctional module and essential for maintaining the activity and thermostability of PpCgk. Furthermore, it was found that the mode of action of PpCgk was more processive on both the dissolved and gelled substrates than that of PpCgkCD, indicating that PpBig_2 contributes to the processivity of PpCgk. Interestingly, PpBig_2 can be used as an independent module to enhance the hydrolysis of κ-carrageenan through its disruptive function. In addition, sequence analysis suggests that Big_2 domains are highly conserved in bacterial κ-carrageenases, implying the universality of their noncatalytic functions. These findings reveal the multifunctional role of the noncatalytic PpBig_2 and will guide future functional analyses and biotechnology applications of Big_2 domains.

Improving low-temperature activity and thermostability of exo-inulinase InuAGN25 on the basis of increasing rigidity of the terminus and flexibility of the catalytic domain

Enzymes displaying high activity at low temperatures and good thermostability are attracting attention in many studies. However, improving low-temperature activity along with the thermostability of enzymes remains challenging. In this study, the mutant Mut8S, including eight sites (N61E, K156R, P236E, T243K, D268E, T277D, Q390K, and R409D) mutated from the exo-inulinase InuAGN25, was designed on the basis of increasing the number of salt bridges through comparison between the low-temperature-active InuAGN25 and thermophilic exo-inulinases. The recombinant Mut8S, which was expressed in Escherichia coli, was digested by human rhinovirus 3 C protease to remove the amino acid fusion sequence at N-terminus, producing RfsMut8S. Compared with wild-type RfsMInuAGN25, the mutant RfsMut8S showed (1) lower root mean square deviation values, (2) lower root mean square fluctuation (RMSF) values of residues in six regions of the N and C termini but higher RMSF values in five regions of the catalytic pocket, (3) higher activity at 0-40°C, and (4) better thermostability at 50°C. This study proposes a way to increase low-temperature activity along with a thermostability improvement of exo-inulinase on the basis of increasing the rigidity of the terminus and the flexibility of the catalytic domain. These findings may prove useful in formulating rational designs for increasing the thermal performance of enzymes.

Removal of N-terminal tail changes the thermostability of the low-temperature-active exo-inulinase InuAGN25

Exo-inulinases are members of the glycoside hydrolase family 32 and function by hydrolyzing inulin into fructose with yields up to 90–95%. The N-terminal tail contributes to enzyme thermotolerance, which plays an important role in enzyme applications. However, the role of N-terminal amino acid residues in the thermal performance and structural properties of exo-inulinases remains to be elucidated. In this study, three and six residues of the N-terminus starting from Gln23 of the exo-inulinase InuAGN25 were deleted and expressed in Escherichia coli. After digestion with human rhinovirus 3 C protease to remove the N-terminal amino acid fusion sequence that may affect the thermolability of enzymes, wild-type RfsMInuAGN25 and its mutants RfsMutNGln23Δ3 and RfsMutNGln23Δ6 were produced. Compared with RfsMInuAGN25, thermostability of RfsMutNGln23Δ3 was enhanced while that of RfsMutNGln23Δ6 was slightly reduced. Compared with the N-terminal structures of RfsMInuAGN25 and RfsMutNGln23Δ6, RfsMutNGln23Δ3 had a higher content of (1) the helix structure, (2) salt bridges (three of which were organized in a network), (3) cation–π interactions (one of which anchored the N-terminal tail). These structural properties may account for the improved thermostability of RfsMutNGln23Δ3. The study provides a better understanding of the N-terminus–function relationships that are useful for rational design of thermostability of exo-inulinases.

Efficient Immobilization of Bacterial GH Family 46 Chitosanase by Carbohydrate-Binding Module Fusion for the Controllable Preparation of Chitooligosaccharides

Chitooligosaccharide has been reported to possess diverse bioactivities. The development of novel strategies for obtaining optimum degree of polymerization (DP) chitooligosaccharides has become increasingly important. In this study, two glycoside hydrolase family 46 chitosanases were studied for immobilization on curdlan (insoluble β-1,3-glucan) using a novel carbohydrate binding module (CBM) family 56 domain from a β-1,3-glucanase. The CBM56 domain provided a spontaneous and specific sorption of the fusion proteins onto a curdlan carrier, and two fusion enzymes showed increased enzyme stability in comparison with native enzymes. Furthermore, a continuous packed-bed reactor was constructed with chitosanase immobilized on a curdlan carrier to control the enzymatic hydrolysis of chitosan. Three chitooligosaccharide products with different molecular weights were prepared in optimized reaction conditions. This study provides a novel CBM tag for the stabilization and immobilization of enzymes. The controllable hydrolysis strategy offers potential for the industrial-scale preparation of chitooligosaccharides with different desired DPs.

A new engineered endo-inulinase with improved activity and thermostability: Application in the production of prebiotic fructo-oligosaccharides from inulin

To construct a high-performance engineered endo-inulinase for fructo-oligosaccharides (FOS) production from inulin, an inulin binding module (IBM) was fused into either N- or C-terminal of an endo-inulinase. After heterologous expression, purification and characterization, the C-terminal fusion one (Eninu-IBM) with better activity, thermostability and inulin binding ability was employed for high-temperature in situ inulin hydrolysis in a 10-L fermentor. During this process, Eninu-IBM was first efficiently produced by the yeast cells at 28 °C for 96 h, and subsequently 1600 g unsterilized inulin per liter fermentation liquor was directly supplemented into the bioreactor for FOS production at 60 °C for 2 h. Finally, high purity of FOS (91.4%) were obtained with FOS titer, yield and productivity of 717.3 g/L, 0.912 g_FOS/g_Inulin and 358.6 g/L/h, respectively. The in vitro prebiotic assay indicated that the final FOS products with main polymerization degrees of 3-5 were preferably fermented by beneficial bifidobacteria and lactobacilli.

Biocatalytic strategies for the production of high fructose syrup from inulin

The consumption of natural and low calorie sugars has increased enormously from the past few decades. To fulfil the demands, the production of healthy sweeteners as an alternative to sucrose has recently received considerable interest. Fructose is the most health beneficial and safest sugar amongst them. It is generally recognised as safe (GRAS) and has become an important food ingredient due its sweetening and various health promising functional properties. Commercially, high fructose syrup is prepared from starch by multienzymatic process. Single-step enzymatic hydrolysis of inulin using inulinase has emerged as an alternate to the conventional approach to reduce complexity, time and cost. The present review, outlines the enzymatic strategies used for the preparation of high fructose syrup from inulin/inulin-rich plant materials in batch and continuous systems, and its conclusions.

Thermostability enhancement of chitosanase CsnA by fusion a family 5 carbohydrate-binding module

OBJECTIVE

To determine the effects of carbohydrate-binding modules (CBMs) on the thermostability and catalytic efficiency of chitosanase CsnA.

RESULTS

Three CBMs (BgCBM5, PfCBM32-2 and AoCBM35) were engineered at the C-terminus of chitosanase CsnA to create hybrid enzymes CsnA-CBM5, CsnA-CBM32 and CsnA-CBM35. K _m values of all the hybrid enzymes were lower than that of the wild type (WT) enzyme; however, only CsnA-CBM5 had an elevated specific activity and catalytic efficiency. The fusion of BgCBM5 enhanced the thermostability of the enzyme, which exhibited a 8.9 °C higher T₅₀ and a 2.9 °C higher T_m than the WT. Secondary structural analysis indicated that appending BgCBM5 at the C-terminus considerably changed the secondary structure content.

CONCLUSIONS

The fusion of BgCBM5 improved the thermal stability of CsnA, and the obtained hybrid enzyme (CsnA-CBM5) is a useful candidate for industrial application.

High lactic acid and fructose production via Mn2+-mediated conversion of inulin by Lactobacillus paracasei

Lactobacillus paracasei DSM 23505 is able to produce high amounts of lactic acid (LA) by simultaneous saccharification and fermentation (SSF) of inulin. Aiming to obtain the highest possible amounts of LA and fructose, the present study is devoted to evaluate the impact of bivalent metal ions on the process of inulin conversion. It was shown that Mn2+ strongly increases the activity of the purified key enzyme β-fructosidase. In vivo, batch fermentation kinetics revealed that the high Mn2+ concentrations accelerated inulin hydrolysis by raise of the inulinase activity, and increased sugars conversion to LA through enhancement of the whole glycolytic flux. The highest LA concentration and yield were reached by addition of 15 mM Mn2+-151 g/L (corresponding to 40% increase) and 0.83 g/g, respectively. However, the relative quantification by real-time reverse transcription assay showed that the presence of Mn2+ decreases the expression levels of fosE gene encoding β-fructosidase. Contrariwise, the full exclusion of metal ions resulted in fosE gene expression enhancement, blocked fructose transport, and hindered fructose conversion thus leading to huge fructose accumulation. During fed-batch with optimized medium and fermentation parameters, the fructose content reached 35.9% (w/v), achieving yield of 467 g fructose from 675 g inulin containing chicory flour powder (0.69 g/g). LA received in course of the batch fermentation and fructose gained by the fed-batch are the highest amounts ever obtained from inulin, thus disclosing the key role of Mn2+ as a powerful tool to guide inulin conversion to targeted bio-chemicals.