Crystal structures of Aspergillus japonicus fructosyltransferase complex with donor/acceptor substrates reveal complete subsites in the active site for catalysis

Multiscale QM/MM Simulations Identify the Roles of Asp239 and 1-OH···Nucleophile in Transition State Stabilization in Arabidopsis thaliana Cell-Wall Invertase 1

Arabidopsis thaliana cell-wall invertase 1 (AtCWIN1), a key enzyme in sucrose metabolism in plants, catalyzes the hydrolysis of sucrose into fructose and glucose. AtCWIN1 belongs to the glycoside hydrolase GH-J clan, where two carboxylate residues (Asp23 and Glu203 in AtCWIN1) are well documented as a nucleophile and an acid/base catalyst. However, details at the atomic level about the role of neighboring residues and enzyme-substrate interactions during catalysis are not fully understood. Here, quantum mechanical/molecular mechanical (QM/MM) free-energy simulations were carried out to clarify the origin of the observed decreased rates in Asp239Ala, Asp239Asn, and Asp239Phe in AtCWIN1 compared to the wild type and delineate the role of Asp239 in catalysis. The glycosylation and deglycosylation steps were considered in both wild type and mutants. Deglycosylation is predicted to be the rate-determining step in the reaction, with a calculated overall free-energy barrier of 15.9 kcal/mol, consistent with the experimental barrier (15.3 kcal/mol). During the reaction, the -1 furanosyl ring underwent a conformational change corresponding to 3E ↔ [E2]⧧ ↔ 1E according to the nomenclature of saccharide structures along the full catalytic reaction. Asp239 was found to stabilize not only the transition state but also the fructosyl-enzyme intermediate, which explains findings from previous structural and mutagenesis experiments. The 1-OH···nucleophile interaction has been found to provide an important contribution to the transition state stabilization, with a contribution of ∼7 kcal/mol, and affected glycosylation more significantly than deglycosylation. This study provides molecular insights that improve the current understanding of sucrose binding and hydrolysis in members of clan GH-J, which may benefit protein engineering research. Finally, a rationale on the sucrose inhibitor configuration in chicory 1-FEH IIa, proposed a long time ago in the literature, is also provided based on the QM/MM calculations.

Structural insight into the hydrolase and synthase activities of an alkaline α-galactosidase from Arabidopsis from complexes with substrate/product

The alkaline α-galactosidase AtAkαGal3 from Arabidopsis thaliana catalyzes the hydrolysis of α-D-galactose from galacto-oligosaccharides under alkaline conditions. A phylogenetic analysis based on sequence alignment classifies AtAkαGal3 as more closely related to the raffinose family of oligosaccharide (RFO) synthases than to the acidic α-galactosidases. Here, thin-layer chromatography is used to demonstrate that AtAkαGal3 exhibits a dual function and is capable of synthesizing stachyose using raffinose, instead of galactinol, as the galactose donor. Crystal structures of complexes of AtAkαGal3 and its D383A mutant with various substrates and products, including galactose, galactinol, raffinose, stachyose and sucrose, are reported as the first representative structures of an alkaline α-galactosidase. The structure of AtAkαGal3 comprises three domains: an N-terminal domain with 13 antiparallel β-strands, a catalytic domain with an (α/β)₈-barrel fold and a C-terminal domain composed of β-sheets that form two Greek-key motifs. The WW box of the N-terminal domain, which comprises the conserved residues FRSK₇₅XW₇₇W₇₈ in the RFO synthases, contributes Trp77 and Trp78 to the +1 subsite to contribute to the substrate-binding ability together with the (α/β)₈ barrel of the catalytic domain. The C-terminal domain is presumably involved in structural stability. Structures of the D383A mutant in complex with various substrates and products, especially the natural substrate/product stachyose, reveal four complete subsites (-1 to +3) at the catalytic site. A functional loop (residues 329-352) that exists in the alkaline α-galactosidase AtAkαGal3 and possibly in RFO synthases, but not in acidic α-galactosidases, stabilizes the stachyose at the +2 and +3 subsites and extends the catalytic pocket for the transferase mechanism. Considering the similarities in amino-acid sequence, catalytic domain and activity between alkaline α-galactosidases and RFO synthases, the structure of AtAkαGal3 might also serve a model for the study of RFO synthases, structures of which are lacking.

Insights into the Structure of the Highly Glycosylated Ffase from Rhodotorula dairenensis Enhance Its Biotechnological Potential

Rhodotorula dairenensis β-fructofuranosidase is a highly glycosylated enzyme with broad substrate specificity that catalyzes the synthesis of 6-kestose and a mixture of the three series of fructooligosaccharides (FOS), fructosylating a variety of carbohydrates and other molecules as alditols. We report here its three-dimensional structure, showing the expected bimodular arrangement and also a unique long elongation at its N-terminus containing extensive O-glycosylation sites that form a peculiar arrangement with a protruding loop within the dimer. This region is not required for activity but could provide a molecular tool to target the dimeric protein to its receptor cellular compartment in the yeast. A truncated inactivated form was used to obtain complexes with fructose, sucrose and raffinose, and a Bis-Tris molecule was trapped, mimicking a putative acceptor substrate. The crystal structure of the complexes reveals the major traits of the active site, with Asn387 controlling the substrate binding mode. Relevant residues were selected for mutagenesis, the variants being biochemically characterized through their hydrolytic and transfructosylating activity. All changes decrease the hydrolytic efficiency against sucrose, proving their key role in the activity. Moreover, some of the generated variants exhibit redesigned transfructosylating specificity, which may be used for biotechnological purposes to produce novel fructosyl-derivatives.

Significant Improvement of Both Catalytic Efficiency and Stability of Fructosyltransferase from Aspergillus niger by Structure-Guided Engineering of Key Residues in the Conserved Sequence of the Catalytic Domain

Fructosyltransferase is a key enzyme in fructo-oligosaccharide production, while the highly demanding conditions of industrial processes may reduce its stability and activity. This study employs sequence alignment and structural analysis to target three potential residues (Gln38, Ile39, and Cys43) around the active center of FruSG from Aspergillus niger, and mutants with greatly improved activity and stability were obtained through site-directed mutagenesis. The K_m values of C43N and Q38Y were, respectively, reduced to 60.8 and 93.1% compared to those of WT. Meanwhile, the k_cat of C43N was increased by 21.2-fold compared to that of WT. These imply that both the affinity and catalytic efficiency of C43N were significantly enhanced compared to WT. The Glide docking score of sucrose inside C43N was calculated to be -5.980, which was lower than that of WT (-4.887). What is more, the proposed general acid/base catalyst Glu273 with a lower pK_a value of C43N calculated by PROPKA might contribute to an easier catalytic reaction compared to that of WT. The thermal stability and pH stability of the mutant C43N were significantly enhanced compared to those of WT, and more hydrogen bonds formed during molecular dynamics simulations might contribute to the improved stability of C43N.

Current status of microbes involved in the degradation of pharmaceutical and personal care products (PPCPs) pollutants in the aquatic ecosystem

Contamination of aquatic systems with pharmaceuticals, personal care products, steroid hormones, and agrochemicals has been an immense problem for the earth's ecosystem and health impacts. The environmental issues of well-known persistence pollutants, their metabolites, and other micro-pollutants in diverse aquatic systems around the world were collated and exposed in this review assessment. Waste Water Treatment Plant (WWTP) influents and effluents, as well as industrial, hospital, and residential effluents, include detectable concentrations of known and undiscovered persistence pollutants and metabolites. These components have been found in surface water, groundwater, drinking water, and natural water reservoirs receiving treated and untreated effluents. Several studies have found that these persistence pollutants, and also similar recalcitrant pollutants, are hazardous to a variety of non-targeted creatures in the environment. In human and animals, they can also have severe and persistent harmful consequences. Because these pollutants are harmful to aquatic organisms, microbial degradation of these persistence pollutants had the least efficiency. Fortunately, only a few wild and Genetically Modified (GMOs) microbial species have the ability to degrade these PPCPs contaminants. Hence, researchers have been studying the degradation competence of microbial communities in persistence pollutants of Pharmaceutical and Personal Care Products (PPCPs) and respective metabolites for decades, as well as possible degradation processes in various aquatic systems. As a result, this review provides comprehensive information about environmental issues and the degradation of PPCPs and their metabolites, as well as other micro-pollutants, in aquatic systems.

Functionalized Cu-based metal oxide nanoparticles with enhanced Cd+2 adsorption capacity and their ecotoxicity assessment by molecular docking

In the present study, synthesis of eco-friendly Cu-based metal oxides nanoparticles [CuO, Cu₂O, and CuO&Cu₂O nanoparticles (NPs)] without and with functionalization with Diethylene glycol (DEG) has been demonstrated. The synthesized NPs were screened for their ability to adsorb multiple heavy metal ions from an aqueous solution. Based on the maximum Cadmium (Cd⁺²) ion adsorption capacity, functionalized Cu₂O (fCu₂O) NPs were selected for the detailed characterization and batch studies. The average size of fCu₂O NPs was found to be 57.4 ± 6.14 nm in comparison to NPs without capping (72.6 ± 5.19 nm). The experimental parameters viz. contact time, initial pH, and initial concentration were optimized, and the obtained results were interpreted using standard isotherms and kinetic models. The maximum Cd⁺² adsorption on fCu₂O NPs was observed at initial solution pH 7. The adsorption of Cd⁺² was found to be decreased at acidic pH due to the protonation of functional groups present on the NPs surface. A maximum Cd⁺² adsorption capacity of 204 ± 6.2 mg g^-1 was obtained from the Langmuir adsorption isotherm. The crystal structure of NPs was prepared and docked with the protein targets of selected soil microbes in order to determine their ecotoxicity. The obtained results showed that NPs exhibited low affinity towards protein targets in comparison to the standard used. It suggests that NPs have less impact on the functionality of soil microbes and are thus safe for their disposal into the soil micro-environment.

Engineering a heterologously expressed fructosyltransferase from Aspergillus oryzae N74 in Komagataella phaffii (Pichia pastoris) for kestose production

Fructo-oligosaccharides (FOS) are one of the most well-studied and commercialized prebiotics. FOS can be obtained either by controlled hydrolysis of inulin or by sucrose transfructosylation. FOS produced from sucrose are typically classified as short-chain FOS (scFOS), of which the best known are 1-kestotriose (GF₂), 1,1-kestotetraose (GF₃), and 1,1,1-kestopentaose (GF₄), produced by fructosyltransferases (FTases) or β-fructofuranosidases. In previous work, FOS production was studied using the Aspergillus oryzae N74 strain, its ftase gene was heterologously expressed in Komagataella phaffii (Pichia pastoris), and the enzyme's tertiary structure modeled. More recently, residues that may be involved in protein-substrate interactions were predicted. In this study, the aim was to experimentally validate previous in silico results by independently producing recombinant wild-type A. oryzae N74 FTase and three single-point mutations in Komagataella phaffii (Pichia pastoris). The R163A mutation virtually abolished the transfructosylating activity, indicating a requirement for the positively charged arginine residue in the catalytic domain D. In contrast, transfructosylating activity was improved by introducing the mutations V242E or F254H, with V242E resulting in higher production of GF₂ without affecting that of GF₃. Interestingly, initial sucrose concentration, reaction temperature and the presence of metal cofactors did not affect the enhanced activity of mutant V242E. Overall, these results shed light on the mechanism of transfructosylation of the FTase from A. oryzae and expand considerations regarding the design of biotechnological processes for specific FOS production.

Crystal Structure of a Sucrose-6-phosphate Hydrolase from Lactobacillus gasseri with Potential Applications in Fructan Production and the Food Industry

Fructooligosaccharides (FOSs) are polymers of fructose with a prebiotic activity because of their production and fermentation by bacteria that inhabit the gastrointestinal tract and are widely used in the industry and new functional foods. Lactobacillus gasseri stands out as an important homofermentative microorganism related to FOS production, and its potential applications in the industry are undeniable. In this study, we report the production and characterization of a sucrose-6-phosphate hydrolase from L. gasseri belonging to the GH32 family. Apo-LgAs32 and LgAs32 complexed with β-d-fructose structures were determined at a resolution of 1.94 and 1.84 Å, respectively. The production of FOS, fructans, 1-kestose, and nystose by the recombinant LgAs32, using sucrose as a substrate, shown in this study is very promising. When compared to its homologous enzyme from Lactobacillus reuteri, the production of 1-kestose by LgAs32 is increased; thus, LgAs32 can be considered as an alternative in fructan production and other industrial applications.

Production, properties, and applications of fructosyltransferase: a current appraisal

BACKGROUND

Fructosyltransferases (FTases) are drawing increasing attention due to their application in prebiotic fructooligosaccharide (FOS) generation. FTases have been reported to occur in a variety of microorganisms but are predominantly found in filamentous fungi. These are employed at the industrial scale for generating FOS which make the key ingredient in functional food supplements and nutraceuticals due to their bifidogenic and various other health-promoting properties.

SCOPE AND APPROACH

This review is aimed to discuss recent developments made in the area of FTase production, characterization, and application in order to present a comprehensive account of their present status to the reader. Structural features, catalytic mechanisms, and FTase improvement strategies have also been discussed in order to provide insight into these aspects.

KEY FINDINGS AND CONCLUSIONS

Although FTases occur in several plants and microorganisms, fungal FTases are being exploited commercially for industrial-scale FOS generation. Several fungal FTases have been characterized and heterologously expressed. However, considerable scope exists for improved production and application of FTases for cost-effective production of prebiotic FOS.HIGHLIGHTSFructosyltrasferase (FTase) is a key enzyme in fructo-oligosaccharide (FOS) generationDevelopments in the production, properties, and functional aspects of FTasesMolecular modification and immobilization strategies for improved FOS generationFructosyltransferases are innovation hotspots in the food and nutraceutical industries.

Continuous production of fructooligosaccharides by recycling of the thermal-stable β-fructofuranosidase produced by Aspergillus niger

OBJECTIVE

To achieve continuous production of fructooligosaccharides (FOS) by recycling of the mycelial cells containing the thermal-stable β-fructofuranosidase in Aspergillus niger without immobilization.

RESULTS

The thermal-stable β-fructofuranosidase FopA-V1 was successfully expressed in A. niger ATCC 20611 under the control of the constitutive promoter PgpdA. The engineered A. niger strain FV1-11 produced the β-fructofuranosidase with improved thermostability, which remained 91.2% of initial activity at 50 °C for 30 h. Then its mycelial β-fructofuranosidase was recycled for the synthesis of FOS. It was found that the enzyme still had 79.3% of initial activity after being reused for six consecutive cycles, whereas only 62.3% β-fructofuranosidase activity was detected in the parental strain ATCC 20611. Meanwhile, the FOS yield of FV1-11 after six consecutive cycles reached 57.1% (w/w), but only 51.0% FOS yield was detected in ATCC 20611.

CONCLUSIONS

The thermal-stable β-fructofuranosidase produced by A. niger can be recycled to achieve continuous synthesis of FOS with high efficiency, providing a powerful and economical strategy for the industrial production of FOS.

Innovations in CAZyme gene diversity and its modification for biorefinery applications

For sustainable growth, concept of biorefineries as recourse to the "fossil derived" energy source is important. Here, the Carbohydrate Active enZymes (CAZymes) play decisive role in generation of biofuels and related sugar-based products utilizing lignocellulose as a carbon source. Given their industrial significance, extensive studies on the evolution of CAZymes have been carried out. Various bacterial and fungal organisms have been scrutinized for the development of CAZymes, where advance techniques for strain enhancement such as CRISPR and analysis of specific expression systems have been deployed. Specific Omic-based techniques along with protein engineering have been adopted to unearth novel CAZymes and improve applicability of existing enzymes. In-Silico computational research and functional annotation of new CAZymes to synergy experiments are being carried out to devise cocktails of enzymes for use in biorefineries. Thus, with the establishment of these technologies, increased diversity of CAZymes with broad span of functions and applications is seen.

Recent advances in β-galactosidase and fructosyltransferase immobilization technology

The highly demanding conditions of industrial processes may lower the stability and affect the activity of enzymes used as biocatalysts. Enzyme immobilization emerged as an approach to promote stabilization and easy removal of enzymes for their reusability. The aim of this review is to go through the principal immobilization strategies addressed to achieve optimal industrial processes with special care on those reported for two types of enzymes: β-galactosidases and fructosyltransferases. The main methods used to immobilize these two enzymes are adsorption, entrapment, covalent coupling and cross-linking or aggregation (no support is used), all of them having pros and cons. Regarding the support, it should be cost-effective, assure the reusability and an easy recovery of the enzyme, increasing its stability and durability. The discussion provided showed that the type of enzyme, its origin, its purity, together with the type of immobilization method and the support will affect the performance during the enzymatic synthesis. Enzymes' immobilization involves interdisciplinary knowledge including enzymology, nanotechnology, molecular dynamics, cellular physiology and process design. The increasing availability of facilities has opened a variety of possibilities to define strategies to optimize the activity and re-usability of β-galactosidases and fructosyltransferases, but there is still great place for innovative developments.

Cytosolic/Plastid Glyceraldehyde-3-Phosphate Dehydrogenase Is a Negative Regulator of Strawberry Fruit Ripening

Cytosolic glyceraldehyde-3-phosphate dehydrogenase (GAPC) and plastid glyceraldehyde-3-phosphate dehydrogenase (GAPCp) are key enzymes in glycolysis. Besides their catalytic function, GAPC/GAPCp participates in the regulation of plant stress response and growth and development. However, the involvement of GAPC/GAPCp in the regulation of fruit ripening is unclear. In this study, FaGAPC2 and FaGAPCp1 in strawberries were isolated and analyzed. FaGAPC2 and FaGAPCp1 transcripts showed high transcript levels in the fruit. Transient overexpression of FaGAPC2 and FaGAPCp1 delayed fruit ripening, whereas RNA interference promoted fruit ripening and affected fruit anthocyanins and sucrose levels. Change in the expression patterns of FaGAPC2 and FaGAPCp1 also influenced the expression of several glycolysis-related and ripening-related genes such as CEL1, CEL2, SS, ANS, MYB5, NCED1, ABI1, ALDO, PK, and G6PDH, and H₂O₂ level and reduced glutathione (GSH)/glutathione disulfide (GSSG) redox potential. Meanwhile, metabolomics experiments showed that transient overexpression of FaGAPCp1 resulted in a decrease in anthocyanins, flavonoids, organic acid, amino acids, and their derivatives. In addition, abscisic acid (ABA) and sucrose treatment induced the production of large amounts of H₂O₂ and inhibited the expression of FaGAPC2/FaGAPCp1 in strawberry fruit. These results revealed that FaGAPC2/FaGAPCp1 is a negative regulator of ABA and sucrose mediated fruit ripening which can be regulated by oxidative stress.

Purification and biochemical characteristics of a novel fructosyltransferase with a high FOS transfructosylation activity from Aspergillus oryzae S719

Fructooligosaccharides (FOS) have widely used for the manufacture of low-calorie and functional foods, because they can inhibit intestinal pathogenic microorganism growth and increase the absorption of Ca²⁺ and Mg²⁺. In this study, the novel fructosyltransferase (FTase) from Aspergillus oryzae strain S719 was successfully purified and characterized. The specific activity of the final purified material was 4200 mg^-1 with purification ratio of 66 times and yield of 26%. The molecular weight of FTase of A. oryzae S719 was around 95 kDa by SDS-PAGE, which was identified as a type of FTase by Mass Spectrometry (MS). The purified FTase had optimum temperature and pH of 55 °C and 6.0, respectively. The FTase showed to be stable with more than 80% of its original activity at room temperature after 12 h and maintaining activity above 90% at pH 4.0-11.0. The Km and kcat values of the FTase were 310 mmol L^-1 and 2.0 × 10³ min^-1, respectively. The FTase was activated by 5 mmol L^-1 Mg²⁺ and 10 mmol L^-1 Na⁺ (relative activity of 116 and 114%, respectively), indicating that the enzyme was Mg²⁺ and Na⁺ dependent. About 64% of FOS was obtained by the purified FTase under 500 g L^-1 sucrose within 4 h of reaction time, which was the shortest reaction time to be reported regarding the purified enzyme production of FOS. Together, these results indicated that the FTase of A. oryzae S719 is an excellent candidate for the industrial production of FOS.

Two-Step Production of Neofructo-Oligosaccharides Using Immobilized Heterologous Aspergillus terreus 1F-Fructosyltransferase Expressed in Kluyveromyces lactis and Native Xanthophyllomyces dendrorhous G6-Fructosyltransferase

Fructo-oligosaccharides (FOS) are prebiotic low-calorie sweeteners that are synthesized by the transfer of fructose units from sucrose by enzymes known as fructosyltransferases. If these enzymes generate β-(2,6) glycosidic bonds, the resulting oligosaccharides belong to the neoseries (neoFOS). Here, we characterized the properties of three different fructosyltransferases using a design of experiments approach based on response surface methodology with a D-optimal design. The reaction time, pH, temperature, and substrate concentration were used as parameters to predict three responses: The total enzyme activity, the concentration of neoFOS and the neoFOS yield relative to the initial concentration of sucrose. We also conducted immobilization studies to establish a cascade reaction for neoFOS production with two different fructosyltransferases, achieving a total FOS yield of 47.02 ± 3.02%. The resulting FOS mixture included 53.07 ± 1.66 mM neonystose (neo-GF3) and 20.8 ± 1.91 mM neo-GF4.

In Silico insights on enhancing thermostability and activity of a plant Fructosyltransferase from Pachysandra terminalis via introduction of disulfide bridges

Drawbacks of industrially-used fructosyltransferases (FTs) such as low optimum temperature and low fructooligosaccharides (FOS) yield necessitates the search for engineered FTs that are highly thermostable and active. With the availability of the first plant FT crystal structure from Pachysandra terminalis (PDB ID: 3UGH), computer-aided protein engineering of plant FT is now feasible. To obtain insights on the effect of specific mutations i.e. disulfide bridge introduction, wild-type and mutant FTs were subjected to a 15 μs Martini Coarse-grained Molecular Dynamics (CGMD) simulations at 303 K and 334 K. We report here the five mutants, M31C-Q49C, L144C-S193C, P34C-W300C, S219C-L226C and V470C-S498C with enhanced thermostabilities and/or activities relative to the wild type. Interestingly, M31C-Q49C, which is located within the catalytic-carrying blade of the catalytic domain, has an activity enhancement at both temperatures. At 334 K, three mutations, L144C-S193C, P34C-W300C and V470C-S498C, achieved thermostability relative to the wild type. Intriguingly, both activity and stability enhancement exhibited only at 334 K can be achieved provided that the mutation is located either on the catalytic-carrying residue blade of the catalytic domain or on the non-catalytic domain. Our results suggest that V470C-S498C and L144C-S193C are promising mutants and that domain-specific approach may be exploited to customize enzyme properties.

Cloning, expression and characterization of a novel fructosyltransferase from Aspergillus niger and its application in the synthesis of fructooligosaccharides

Fructosyltransferases have been used in the industrial production of fructooligosaccharides (FOS).

Rapid evaluation of 1-kestose producing β-fructofuranosidases from Aspergillus species and enhancement of 1-kestose production using a PgsA surface-display system

1-Kestose is a key prebiotic fructooligosaccharide (FOS) sugar. Some β-fructofuranosidases (FFases) have high transfructosylation activity, which is useful for manufacturing FOS. Therefore, obtaining FFases that produce 1-kestose efficiently is important. Here, we established a rapid FFase evaluation method using Escherichia coli that display different FFases fused to a PgsA anchor protein from Bacillus subtilis. E. coli cell suspensions expressing the PgsA-FFase fusion efficiently produce FOS from sucrose. Using this screening technique, we found that the E. coli transformant expressing Aspergillus kawachii FFase (AkFFase) produced a larger amount of 1-kestose than those expressing FFases from A. oryzae and A. terreus. Saturation mutagenesis of AkFFase was performed, and the mutant G85W was obtained. The E. coli transformant expressing AkFFase G85W markedly increased production of 1-kestose. Our results indicate that the surface display technique using PgsA is useful for screening of FFases, and AkFFase G85W is likely to be suitable for 1-kestose production.

Abbreviations: AkFFase: Aspergillus kawachii FFase; AoFFase: Aspergillus oryzae FFase; AtFFase: Aspergillus terreus FFase; FFase: β-fructofuranosidase; FOS: fructooligosaccharide; fructosylnystose: 1F-β-fructofuranosylnystose

D181A Site-Mutagenesis Enhances Both the Hydrolyzing and Transfructosylating Activities of BmSUC1, a Novel β-Fructofuranosidase in the Silkworm Bombyx mori

β-fructofuranosidase (β-FFase) belongs to the glycosyl-hydrolase family 32 (GH32), which can catalyze both the release of β-fructose from β-d-fructofuranoside substrates to hydrolyze sucrose and the synthesis of short-chain fructooligosaccharide (FOS). BmSuc1 has been cloned and identified from the silkworm Bombyx mori as a first animal type of β-FFase encoding gene. It was hypothesized that BmSUC1 plays an important role in the silkworm-mulberry adaptation system. However, there is little information about the enzymatic core sites of BmSUC1. In this study, we mutated three amino acid residues (D63, D181, and E234) that represent important conserved motifs for β-FFase activity in GH32 to alanine respectively by using site-directed mutagenesis. Recombinant proteins of three mutants and wild type BmSUC1 were obtained by using a Bac-to-Bac/BmNPV expression system and BmN cells. Enzymatic activity, kinetic properties, and substrate specificity of the four proteins were analyzed. High Performance Liquid Chromatography (HPLC) was used to compare the hydrolyzing and transfructosylating activities between D181A and wtBmSUC1. Our results revealed that the D63A and E234A mutations lost activity, suggesting that D63 and E234 are key amino acid residues for BmSUC1 to function as an enzyme. The D181A mutation significantly enhanced both hydrolyzing and transfructosylating activities of BmSUC1, indicating that D181 may not be directly involved in catalyzation. The results provide insight into the chemical catalyzation mechanism of BmSUC1 in B. mori. Up-regulated transfructosylating activity of BmSUC1 could provide new ideas for using B. mori β-FFase to produce functional FOS.

Data characterizing the energetics of enzyme-catalyzed hydrolysis and transglycosylation reactions by DFT cluster model calculations

The data presented in this paper are related to the research article entitled “QM/MM modeling of the hydrolysis and transfructosylation reactions of fructosyltransferase from Aspergillus japonicas, an enzyme that produces prebiotic fructooligosaccharide” (Jitonnom et al., 2018) [1]. This paper presents the procedure and data for characterizing the whole relative energy profiles of hydrolysis and transglycosylation reactions whose elementary steps differ in chemical composition. The data also reflects the choices of the QM cluster model, the functional/basis set method and the equations in determining the reaction energetics.

Crystal structure of a β-fructofuranosidase with high transfructosylation activity from Aspergillus kawachii

β-Fructofuranosidases belonging to glycoside hydrolase family (GH) 32 are enzymes that hydrolyze sucrose. Some GH32 enzymes also catalyze transfructosylation to produce fructooligosaccharides. We found that Aspergillus kawachii IFO 4308 β-fructofuranosidase (AkFFase) produces fructooligosaccharides, mainly 1-kestose, from sucrose. We determined the crystal structure of AkFFase. AkFFase is composed of an N-terminal small component, a β-propeller catalytic domain, an α-helical linker, and a C-terminal β-sandwich, similar to other GH32 enzymes. AkFFase forms a dimer, and the dimerization pattern is different from those of other oligomeric GH32 enzymes. The complex structure of AkFFase with fructose unexpectedly showed that fructose binds both subsites −1 and +1, despite the fact that the catalytic residues were not mutated. Fructose at subsite +1 interacts with Ile146 and Glu296 of AkFFase via direct hydrogen bonds.

Expression and Activity Analysis of Fructosyltransferase from Aspergillus oryzae

The fructosyltransferase gene was isolated and cloned from Aspergillus oryzae. The gene was 1368 bp, which encoded a protein of 455 amino acids. To analyze the activity of the expressed fructosyltransferase, the pET32a-fructosyltransferase recombined plasmid was transformed into Escherichia coli BL21. The fructosyltransferase gene was successfully expressed by Isopropyl-β-d-thiogalactoside (IPTG) induction. The molecular weight of the expression protein was about 45 kDa. The optimal conditions of protein expression were 25 °C, 0.1 mM IPTG, and 8 h of inducing time. The optimal concentration of urea dealing with inclusion body was 2.5 M. The expressed protein exhibited a strong fructosyl transfer activity. These results showed that the expressed fructosyltransferas owned transferase activity, and could catalyze the synthesis of sucrose-6-acetate.

Characterization of a novel low-temperature-active, alkaline and sucrose-tolerant invertase

A glycoside hydrolase family 32 invertase from Bacillus sp. HJ14 was expressed in Escherichia coli. The purified recombinant enzyme (rInvHJ14) showed typical biochemical properties of low-temperature-active and alkaline enzymes: (i) rInvHJ14 was active and stable in the range of pH 7.0–9.5 with an apparent pH optimum of 8.0; (ii) rInvHJ14 was most active but not stable at 30–32.5 °C, with 19.7, 48.2 and 82.1% of its maximum activity when assayed at 0, 10 and 20 °C, respectively, and the E_a, ΔG^* (30 °C), K_m (30 °C) and k_cat (30 °C) values for hydrolysis of sucrose by rInvHJ14 was 47.6 kJ mol⁻¹, 57.6 kJ mol⁻¹, 62.9 mM and 746.2 s⁻¹, respectively. The enzyme also showed strong sucrose tolerance. rInvHJ14 preserved approximately 50% of its highest activity in the presence of 2045.0 mM sucrose. Furthermore, potential factors for low-temperature-active and alkaline adaptations of rInvHJ14 were presumed. Compared with more thermostable homologs, rInvHJ14 has a higher frequency of glycine residues and a longer loop but a lower frequency of proline residues (especially in a loop) in the catalytic domain. The catalytic pockets of acid invertases were almost negatively charged while that of alkaline rInvHJ14 was mostly positively charged.

Highly Efficient Synthesis of Fructooligosaccharides by Extracellular Fructooligosaccharide-Producing Enzymes and Immobilized Cells of Aspergillus aculeatus M105 and Purification and Biochemical Characterization of a Fructosyltransferase from the Fungus

In this work, Aspergillus aculeatus M105 was obtained to produce high extracellular fructooligosaccharide-producing enzyme activity. The maximum yields of fructooligosaccharides produced by its extracellular enzymes and immobilized cells were 67.54 and 65.47% (w/w), respectively. A fructosyltransferase (FTase), AaFT32A, was purified from M105. The optimal pH and temperature of AaFT32A were pH 5.0-6.0 and 65 °C, respectively. The Km, Vmax, and kcat values for the transfructosylating activity of AaFT32A were 2267 mM, 1347 μmol/min/mg protein, and 1550.2 s(-1), respectively, and those values for the hydrolytic activity of AaFT32A were 6.10 mM, 32.44 μmol/min/mg protein, and 37.3 s(-1), respectively. The sequence of AaFT32A deduced from the cloned gene shared 99.4% identity with a FTase from Aspergillus japonicus CB05 and a fructofuranosidase from Aspergillus niger and 96.5% identity with a FTase (Aspacl_37092) from A. aculeatus ATCC 16872. The fungal strain and its FTase may have potential applications in the prebiotics industry.

Heterologous expression of Aspergillus terreus fructosyltransferase in Kluyveromyces lactis

Fructo-oligosaccharides are prebiotic and hypocaloric sweeteners that are usually extracted from chicory. They can also be produced from sucrose using fructosyltransferases, but the only commercial enzyme suitable for this purpose is Pectinex Ultra, which is produced with Aspergillus aculeatus. Here we used the yeast Kluyveromyces lactis to express a secreted recombinant fructosyltransferase from the inulin-producing fungus Aspergillus terreus. A synthetic codon-optimised version of the putative β-fructofuranosidase ATEG 04996 (XP 001214174.1) from A. terreus NIH2624 was secreted as a functional protein into the extracellular medium. At 60°C, the purified A. terreus enzyme generated the same pattern of oligosaccharides as Pectinex Ultra, but at lower temperatures it also produced oligomers with up to seven units. We achieved activities of up to 986.4U/mL in high-level expression experiments, which is better than previous reports of optimised Aspergillus spp. fermentations.

Structural Analysis of β-Fructofuranosidase from Xanthophyllomyces dendrorhous Reveals Unique Features and the Crucial Role of N-Glycosylation in Oligomerization and Activity

Xanthophyllomyces dendrorhousβ-fructofuranosidase (XdINV)is a highly glycosylated dimeric enzyme that hydrolyzes sucrose and releases fructose from various fructooligosaccharides (FOS) and fructans. It also catalyzes the synthesis of FOS, prebiotics that stimulate the growth of beneficial bacteria in human gut. In contrast to most fructosylating enzymes, XdINV produces neo-FOS, which makes it an interesting biotechnology target. We present here its three-dimensional structure, which shows the expected bimodular arrangement and also a long extension of its C terminus that together with anN-linked glycan mediate the formation of an unusual dimer. The two active sites of the dimer are connected by a long crevice, which might indicate its potential ability to accommodate branched fructans. This arrangement could be representative of a group of GH32 yeast enzymes having the traits observed in XdINV. The inactive D80A mutant was used to obtain complexes with relevant substrates and products, with their crystals structures showing at least four binding subsites at each active site. Moreover, two different positions are observed from subsite +2 depending on the substrate, and thus, a flexible loop (Glu-334-His-343) is essential in binding sucrose and β(2-1)-linked oligosaccharides. Conversely, β(2-6) and neo-type substrates are accommodated mainly by stacking to Trp-105, explaining the production of neokestose and the efficient fructosylating activity of XdINV on α-glucosides. The role of relevant residues has been investigated by mutagenesis and kinetics measurements, and a model for the transfructosylating reaction has been proposed. The plasticity of its active site makes XdINV a valuable and flexible biocatalyst to produce novel bioconjugates.

Semirational Directed Evolution of Loop Regions in Aspergillus japonicus β-Fructofuranosidase for Improved Fructooligosaccharide Production

The Aspergillus japonicus β-fructofuranosidase catalyzes the industrially important biotransformation of sucrose to fructooligosaccharides. Operating at high substrate loading and temperatures between 50 and 60°C, the enzyme activity is negatively influenced by glucose product inhibition and thermal instability. To address these limitations, the solvent-exposed loop regions of the β-fructofuranosidase were engineered using a combined crystal structure- and evolutionary-guided approach. This semirational approach yielded a functionally enriched first-round library of 36 single-amino-acid-substitution variants with 58% retaining activity, and of these, 71% displayed improved activities compared to the parent. The substitutions yielding the five most improved variants subsequently were exhaustively combined and evaluated. A four-substitution combination variant was identified as the most improved and reduced the time to completion of an efficient industrial-like reaction by 22%. Characterization of the top five combination variants by isothermal denaturation assays indicated that these variants displayed improved thermostability, with the most thermostable variant displaying a 5.7°C increased melting temperature. The variants displayed uniquely altered, concentration-dependent substrate and product binding as determined by differential scanning fluorimetry. The altered catalytic activity was evidenced by increased specific activities of all five variants, with the most improved variant doubling that of the parent. Variant homology modeling and computational analyses were used to rationalize the effects of amino acid changes lacking direct interaction with substrates. Data indicated that targeting substitutions to loop regions resulted in improved enzyme thermostability, specific activity, and relief from product inhibition.

Synthesis of Fructooligosaccharides by IslA4, a truncated inulosucrase from Leuconostoc citreum

Background

IslA4 is a truncated single domain protein derived from the inulosucrase IslA, which is a multidomain fructosyltransferase produced by Leuconostoc citreum. IslA4 can synthesize high molecular weight inulin from sucrose, with a residual sucrose hydrolytic activity. IslA4 has been reported to retain the product specificity of the multidomain enzyme.

Results

Screening experiments to evaluate the influence of the reactions conditions, especially the sucrose and enzyme concentrations, on IslA4 product specificity revealed that high sucrose concentrations shifted the specificity of the reaction towards fructooligosaccharides (FOS) synthesis, which almost eliminated inulin synthesis and led to a considerable reduction in sucrose hydrolysis. Reactions with low IslA4 activity and a high sucrose activity allowed for high levels of FOS synthesis, where 70% sucrose was used for transfer reactions, with 65% corresponding to transfructosylation for the synthesis of FOS.

Conclusions

Domain truncation together with the selection of the appropriate reaction conditions resulted in the synthesis of various FOS, which were produced as the main transferase products of inulosucrase (IslA4). These results therefore demonstrate that bacterial fructosyltransferase could be used for the synthesis of inulin-type FOS.

Purification, cloning, characterization, and N-glycosylation analysis of a novel β-fructosidase from Aspergillus oryzae FS4 synthesizing levan- and neolevan-type fructooligosaccharides

β-Fructosidases are a widespread group of enzymes that catalyze the hydrolysis of terminal fructosyl units from various substrates. These enzymes also exhibit transglycosylation activity when they function with high concentrations of sucrose, which is used to synthesize fructooligosaccharides (FOS) in the food industry. A β-fructosidase (BfrA) with high transglycosylation activity was purified from Aspergillus oryzae FS4 as a monomeric glycoprotein. Compared with the most extensively studied Aspergillus spp. fructosidases that synthesize inulin-type β-(2-1)-linked FOS, BfrA has unique transfructosylating property of synthesizing levan- and neolevan-type β-(2-6)-linked FOS. The coding sequence (bfrAFS4, 1.86 kb) of BfrA was amplified and expressed in Escherichia coli and Pichia pastoris. Both native and recombinant proteins showed transfructosylation and hydrolyzation activities with broad substrate specificity. These proteins could hydrolyze the following linkages: Glc α-1, 2-β Fru; Glc α-1, 3-α Fru; and Glc α-1, 5-β Fru. Compared with the unglycosylated E. coli-expressed BfrA (E.BfrA), the N-glycosylated native (N.BfrA) and the P. pastoris-expressed BfrA (P.BfrA) were highly stable at a wide pH range (pH 4 to 11), and significantly more thermostable at temperatures up to 50°C with a maximum activity at 55°C. Using sucrose as substrate, the Km and kcat values for total activity were 37.19±5.28 mM and 1.0016±0.039×104 s-1 for N.BfrA. Moreover, 10 of 13 putative N-glycosylation sites were glycosylated on N.BfrA, and N-glycosylation was essential for enzyme thermal stability and optima activity. Thus, BfrA has demonstrated as a well-characterized A. oryzae fructosidase with unique transfructosylating capability of synthesizing levan- and neolevan-type FOS.

Synthesis of novel bioactive lactose-derived oligosaccharides by microbial glycoside hydrolases

Prebiotic oligosaccharides are increasingly demanded within the Food Science domain because of the interesting healthy properties that these compounds may induce to the organism, thanks to their beneficial intestinal microbiota growth promotion ability. In this regard, the development of new efficient, convenient and affordable methods to obtain this class of compounds might expand even further their use as functional ingredients. This review presents an overview on the most recent interesting approaches to synthesize lactose-derived oligosaccharides with potential prebiotic activity paying special focus on the microbial glycoside hydrolases that can be effectively employed to obtain these prebiotic compounds. The most notable advantages of using lactose-derived carbohydrates such as lactosucrose, galactooligosaccharides from lactulose, lactulosucrose and 2-α-glucosyl-lactose are also described and commented.