A review of fructosyl-transferases from catalytic characteristics and structural features to reaction mechanisms and product specificity

Carbohydrate-active enzymes are accountable for the synthesis and degradation of glycosidic bonds among diverse carbohydrates. Fructosyl-transferases represent a subclass of these enzymes, employing sucrose as a substrate to generate fructooligosaccharides (FOS) and fructan polymers. This category primarily includes levansucrase (LS, EC 2.4.1.10), inulosucrase (IS, EC 2.4.1.9), and β-fructofuranosidase (Ffase, EC 3.2.1.26). These three enzymes possess a similar five-bladed β-propeller fold and employ an anomer-retaining reaction mechanism mediated by nucleophiles, transition state stabilizers, and general acids/bases. However, they exhibit distinct product profiles, characterized by variations in linkage specificity and molecular mass distribution. Consequently, this article comprehensively explores recent advancements in the catalytic characteristics, structural features, reaction mechanisms, and product specificity of levansucrase, inulosucrase, and β-fructofuranosidase (abbreviated as LS, IS, and Ffase, respectively). Furthermore, it discusses the potential for modifying catalytic properties and product specificity through structure-based design, which enables the rational production of custom fructan and FOS.

Investigating the Potential of Phenolic Compounds and Carbohydrates as Acceptor Substrates for Levansucrase-Catalyzed Transfructosylation Reaction

This study characterizes the acceptor specificity of levansucrases (LSs) from Gluconobacter oxydans (LS1), Vibrio natriegens (LS2), Novosphingobium aromaticivorans (LS3), and Paraburkholderia graminis (LS4) using sucrose as fructosyl donor and selected phenolic compounds and carbohydrates as acceptors. Overall, V. natriegens LS2 proved to be the best biocatalyst for the transfructosylation of phenolic compounds. More than one fructosyl unit could be attached to fructosylated phenolic compounds. The transfructosylation of epicatechin by P. graminis LS4 resulted in the most diversified products, with up to five fructosyl units transferred. In addition to the LS source, the acceptor specificity of LS towards phenolic compounds and their transfructosylation products were found to greatly depend on their chemical structure: the number of phenolic rings, the reactivity of hydroxyl groups and the presence of aliphatic chains or methoxy groups. Similarly, for carbohydrates, the transfructosylation yield was dependent on both the LS source and the acceptor type. The highest yield of fructosylated-trisaccharides was Erlose from the transfructosylation of maltose catalyzed by LS2, with production reaching 200 g/L. LS2 was more selective towards the transfructosylation of phenolic compounds and carbohydrates, while reactions catalyzed by LS1, LS3 and LS4 also produced fructooligosaccharides. This study shows the high potential for the application of LSs in the glycosylation of phenolic compounds and carbohydrates.

A strategy to reduce the byproduct glucose by simultaneously producing levan and single cell oil using an engineered Yarrowia lipolytica strain displaying levansucrase on the surface

Currently, levan is attracting attention due to its promising applications in the food and biomedical fields. Levansucrase synthesizes levan by polymerizing the fructosyl unit in sucrose. However, a large amount of the byproduct glucose is produced during this process. In this paper, an engineered oleaginous yeast (Yarrowia lipolytica) strain was constructed using a surface display plasmid containing the LevS gene of Gluconobacter sp. MP2116. The levansucrase activity of the engineered yeast strain reached 327.8 U/g of cell dry weight. The maximal levan concentration (58.9 g/l) was achieved within 156 h in the 5-liter fermentation. Over 81.2 % of the sucrose was enzymolyzed by the levansucrase, and the byproduct glucose was converted to 21.8 g/l biomass with an intracellular oil content of 25.5 % (w/w). The obtained oil was comprised of 91.3 % long-chain fatty acids (C16-C18). This study provides new insight for levan production and comprehensive utilization of the byproduct in levan biosynthesis.

Insights into the heterogeneity of levan polymers synthesized by levansucrase Bs-SacB from Bacillus subtilis 168

Levan is an enzymatically synthesized fructose polymer with widely reported structural heterogeneity depending on the producing levansucrase, the reaction conditions employed for its synthesis and the characterization techniques. We studied here the specific properties of levan produced by recombinant levansucrase from B. subtilis 168 (Bs-SacB), often characterized as a bimodal distribution, that is, a mixture of low and high molecular weight levan. We found significant differences between both levans in terms of the already reported molecular weight, size and morphology using different analytical methods. The low molecular weight levan consists of a non-uniform polymer ranging from 50 to 230 kDa, synthesized through a non-processive mechanism that can spontaneously form spherical nanoparticles in the reaction medium. In contrast, high molecular weight levan is a uniform polymer, most probably synthesized through a processive mechanism, with an average molecular weight of 30,750 kDa and a poorly defined nano-structure. This is the first report exploring differences in morphology between low and high molecular weight levans. Our findings demonstrate that only the low molecular weight levan forms spherical nanoparticles in the reaction medium and that high molecular weight levan is mainly composed of a 33,000 kDa fraction with a microgel behavior.

Similarities and differences of nano-sized levan synthesized by Bacillus haynesii at low and high temperatures: Characterization and bioactivity

Levan is a biopolymer with many different uses. Temperature is an important parameter in biopolymer synthesis. Herein, levan production was carried out from Bacillus haynesii, a thermophilic microorganism, in the temperature range of 4 °C-95 °C. The highest levan production was measured as 10.9 g/L at 37 °C. The synthesized samples were characterized by FTIR and NMR analysis. The particle size of the levan samples varied between 153 and 824.4 nm at different temperatures. In levan samples produced at high temperatures, the water absorption capacity is higher in accordance with the particle size. Irregularities were observed in the surface pores at temperatures of 60 °C and above. The highest emulsion capacity of 83.4 % was measured in the sample synthesized at 4 °C. The antioxidant activity of all levan samples synthesized at different temperatures was measured as 84 % on average. All synthesized levan samples showed antibacterial effect on pathogenic bacteria. In addition, levan synthesized at 45 °C showed the highest antimicrobial effect on E. coli ATCC 35218 with an inhibition zone of 21.3 ± 1.82 mm. Antimicrobial activity against yeast sample C. albicans, was measured only in levan samples synthesized at 80 °C, 90 °C, 95 °C temperatures. Levan synthesized from Bacillus haynesii at low and high temperatures showed differences in characterization and bioactivity.

Rational design of a self-assembly promoting fusion domain enhances high molecular weight levan synthesis by levansucrase SacB

The catalytic product of levansucrase from Bacillus subtilis (SacB) is mainly composed of 10 % high molecular weight levan (HMW, ~2000 kDa) and 90 % low molecular weight levan (LMW, ~7000 Da). In order to achieve efficient production of food hydrocolloid, high molecular weight levan (HMW), with the help of molecular dynamics simulation software, a protein self-assembly element, Dex-GBD, was found and fused with the C-terminus of SacB to construct a novel fusion enzyme, SacB-GBD. The product distribution of SacB-GBD was reversed compared with SacB, and the proportion of HMW in the total polysaccharide was significantly increased to >95 %. We then confirmed that the self-assembly was responsible for the reversal of the SacB-GBD product distribution by the simultaneous modulation of SacB-GBD particle size and product distribution by SDS. The hydrophobic effect may be the main driver of self-assembly as analyzed by molecular simulations and hydrophobicity determination. Our study provides an enzyme source for the industrial production of HMW and provides a new theoretical basis for guiding the molecular modification of levansucrase towards the size of the catalytic product.

Investigation of biocatalytic production of lactosucrose and fructooligosaccharides using levansucrases and dairy by-products as starting materials

Selected levansucrases (LSs) were investigated for their ability to catalyze the transfructosylation of lactose/sucrose into lactosucrose and fructooligosaccharides (FOSs). Additionally, dairy by-products, including whey permeate (WP) and milk permeate (MP), were assessed for their effectiveness as lactose sources. LSs from Gluconobacter oxydans (LS1), Vibrio natriegens (LS2), Novosphingobium aromaticivorans (LS3), and Burkholderia graminis (LS4) were utilized in three transfructosylation reactions that combined sucrose with either lactose, WP, or MP. All LSs demonstrated a higher transfructosylation activity than hydrolytic one, except for V. natriegens LS2 in the presence of sucrose and MP/sucrose. Furthermore, the bioconversion efficiency of lactose/sucrose into lactosucrose and FOSs exhibited varying time courses and end-product profiles. Both the acceptor specificity of LS and the thermodynamic equilibrium of its reaction modulated the end-product profile. V. natriegens LS2 resulted in the highest lactosucrose production of 328 and 251 g/L with lactose/sucrose and WP/sucrose, respectively. Our results revealed the potential of LS-catalyzed transfructosylation for the biocatalytic production of both lactosucrose and FOSs from abundant biomasses.

Simultaneous enzyme production, Levan-type FOS synthesis and sugar by-products elimination using a recombinant Pichia pastoris strain expressing a levansucrase-endolevanase fusion enzyme

BACKGROUND

Although Levan-type fructooligosaccharides (L-FOS) have been shown to exhibit prebiotic properties, no efficient methods for their large-scale production have been proposed. One alternative relies on the simultaneous levan synthesis from sucrose, followed by endolevanase hydrolysis. For this purpose, several options have been described, particularly through the synthesis of the corresponding enzymes in recombinant Escherichia coli. Major drawbacks still consist in the requirement of GRAS microorganisms for enzyme production, but mainly, the elimination of glucose and fructose, the reaction by-products.

RESULTS

The expression of a fusion enzyme between Bacillus licheniformis endolevanase (LevB1) and B. subtilis levansucrase (SacB) in Pichia pastoris cultures, coupled with the simultaneous synthesis of L-FOS from sucrose and the elimination of the residual monosaccharides, in a single one-pot process was developed. The proof of concept at 250 mL flask-level, resulted in 8.62 g of monosaccharide-free L-FOS and 12.83 gDCW of biomass, after 3 successive sucrose additions (30 g in total), that is a 28.7% yield (w L-FOS/w sucrose) over a period of 288 h. At a 1.5 L bioreactor-level, growth considerably increased and, after 59 h and two sucrose additions, 72.9 g of monosaccharide-free L-FOS and 22.77 gDCW of biomass were obtained from a total of 160 g of sucrose fed, corresponding to a 45.5% yield (w L-FOS/w sucrose), 1.6 higher than the flask system. The L-FOS obtained at flask-level had a DP lower than 20 fructose units, while at bioreactor-level smaller oligosaccharides were obtained, with a DP lower than 10, as a consequence of the lower endolevanase activity in the flask-level.

CONCLUSION

We demonstrate here in a novel system, that P. pastoris cultures can simultaneously be used as comprehensive system to produce the enzyme and the enzymatic L-FOS synthesis with growth sustained by sucrose by-products. This system may be now the center of an optimization strategy for an efficient production of glucose and fructose free L-FOS, to make them available for their application as prebiotics. Besides, P. pastoris biomass also constitutes an interesting source of unicellular protein.

Characterization of levansucrase produced by novel Bacillus siamensis and optimization of culture condition for levan biosynthesis

Levan has attracted interest due to the potential health benefits associated with its prebiotic, biological, and functional properties. However, the production of levan is expensive due to its high resource requirements. With the growing demand for levan, it is vital to determine suitable cultivation condition for its production and reduce costs accordingly. The present study characterized the enzyme levansucrase produced by a novel strain of Bacillus siamensis and optimized the conditions for the biosynthesis of levansucrase and levan. The crude levansucrase enzyme production by B. siamensis was induced at a specific temperature in a medium containing different concentrations of sucrose, fructose, and glucose to evaluate transfructosylation and hydrolysis activities. Crude levansucrase significantly increased transfructosylation relative to hydrolysis activity at 37 °C in a medium containing 20% (w/v) sucrose. Both transfructosylation and hydrolysis activities were inhibited in glucose and fructose containing medium. Purification and characterization of the levansucrase were performed by precipitating the enzyme with ammonium sulfate solution, purified anion-exchange chromatography, and analyzed by Sodium Dodecyl Sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The results showed the molecular weight of the enzyme to be approximately 30 kDa with specific activity at 15.95 U/mg, corresponding to a protein purification efficiency of 11.47 and a yield of 78.75%. The optimal culture condition for the purified-levansucrase activity for levan biosynthesis was obtained at 37 °C after 48 h, at pH 6.0 in 50 mM phosphate buffer and 20% (w/v) sucrose. The study demonstrated the optimized condition for levan biosynthesis utilizing the B. siamensis that can serve as a model for various commercial and industrial applications for efficient levan production.

Production of fructan synthesis/hydrolysis of endophytic bacteria involved in inulin production in Jerusalem artichoke

Endophytic bacteria refer to bacteria which promote plant growth via direct and indirect mechanisms. Three endophytic bacteria isolated from Jerusalem artichoke exhibited plant growth induction and inulin production. These bacteria had functions of fructan degradation and synthesis from inulinase and levansucrase, respectively. Rossellomorea aquimaris 3.13 and Priestia megaterium 3.5 obtained inulinase/levanase enzyme with inulin and levan as substrates; enzyme production showed the optimum conditions in 1% inulin medium of 35 °C, pH 7.0. Bacillus velezensis 5.18 and Priestia megaterium 3.5 had inulosucrase/levansucrase enzyme with sucrose as a major carbon source; the enzyme had optimum temperature and pH conditions of 30 °C and pH 7.0, respectively. A combination of carbon sources had effect on decreasing enzyme activity; in addition, co-inoculation of bacteria showed a slight difference in enzyme production compared with single inoculation. The inulosucrase/levansucrase was produced earlier in co-culture containing bacteria with inulinase activity. Plant fructan synthesis was involved in 1-SST and 1-FFT, while 1-FEH encoded inulin degradation; these genes were evaluated in Jerusalem artichoke inoculated with the endophytic bacteria to quantify gene expression level using qPCR. All genes expressed in low levels at early stage of growth, responding to all endophytic bacteria. Significantly, Bacillus velezensis 5.18 induced all genes of the plant at 65 days of inoculation; Rossellomorea aquimaris 3.13 induced 1-FFT while Priestia megaterium 3.5 induced 1-SST.

Biotransformation of sucrose rich Maple syrups into fructooligosaccharides, oligolevans and levans using levansucrase biocatalyst: Bioprocess optimization and prebiotic activity assessment

Maple syrup was investigated as a source to produce FOSs and β-(2-6)-linked-oligolevans/levans. The modulation of this biotransformation was achieved through the control of Maple syrup °Bx and reaction conditions. Reaction time was identified as the most influential factor for the oligolevans/FOSs production in Maple syrup 30°Bx reaction system as well as for the oligolevans/levans synthesis in the 66°Bx one. In the predictive model of oligolevans/levans production in Maple syrup 60°Bx, the interactive effect between levansucrase unit and reaction time was significant (p-value of 0.0008). The optimal conditions for oligolevans/FOSs production (109.20 g/L) in Maple syrup 30°Bx were 3.73 U/mL, pH 6.60 and 23.12 h; while 5 U/mL, pH 6.04 and 29.92 h were identified as the optimal conditions for oligolevans/levans production (147.09 g/L) in Maple syrup 66°Bx. As compared to inulin-type commercial FOSs, the fermentation of oligolevans/FOSs from Maple syrup led to a higher count of Lactobacillus acidophilus and Bifidobacterium lactis and resulted in a higher production of lactic acid. This study lays the foundation for the biotransformation of Maple syrups into functional prebiotic ingredients.

Rapid, real-time sucrase characterization: Showcasing the feasibility of a one-pot activity assay

Sucrases can modify numerous carbohydrates, and short-chain oligosaccharides produced by the unique transfructosylation activity of levansucrases are promising candidates for the growing sugar substitute market. These compounds could counteract the increasing number of diseases associated with the consumption of high-calorie sugars. Thus, there is great interest in the characterization of novel levansucrases. The commonly used method for sucrase activity determination is to quantify d-glucose released in the sucrose-splitting reaction. This is usually done in a discontinuous mode, i.e., several samples taken from the sucrase reaction are applied to a separately performed d-glucose determination (e.g., GOPOD assay). Employing the newly isolated levansucrase LevS_KK21 from Pseudomonas sp. KK21, the feasibility of a one-pot sucrase characterization was investigated by combining sucrase reaction and GOPOD-based d-glucose determination into a single, continuous assay (Real-time GOPOD). The enzyme was characterized with respect to kinetic parameters, ion dependency, pH value, and reaction temperature in a comparative approach employing Real-time GOPOD and HPLC. High data consistency for all investigated enzyme parameters demonstrated that current processes for sucrase characterization can be considerably accelerated by the continuous assay while maintaining data validity. However, the assay was not applicable at acidic pH, as decolorization of the quinoneimine dye formed during the GOPOD reaction was observed. Overall, the study presents valuable data on the potentials of real-time sucrase activity assessment for an accelerated discovery and characterization of interesting enzymes such as the hereby introduced levansucrase LevS_KK21. Progress in sucrase discovery will finally foster the development of health-promoting sucrose substitutes.

Evaluation of different bacterial honey isolates as probiotics and their efficient roles in cholesterol reduction

Continue to hypothesize that honey is a storehouse of beneficial bacteria, and the majority of these isolates are levansucrase producers. Accordingly, ten bacterial strains were isolated from different honey sources. Four honey isolates that had the highest levansucrase production and levan yield were identified by the partial sequencing of the 16S rRNA gene as Achromobacter sp. (10A), Bacillus paralicheniformis (2M), Bacillus subtilis (9A), and Bacillus paranthracis (13M). The cytotoxicity of the selected isolates showed negative blood hemolysis. Also, they are sensitive to the tested antibiotics (Amoxicillin + Flucloxacillin, Ampicillin, Gentamicin, Benzathine benzylpenicillin, Epicephin, Vancomycin, Amikacin, and Zinol). The isolates had strong alkaline stability (pHs 9, 11) and were resistant to severe acidic conditions (29-100 percent). The tested isolates recorded complete tolerance to both H₂O₂ and the bile salt (0.3% Oxgall powder) after 24 h incubation. The cell-free supernatant of the examined strains had antifungal activities against C. Albicans with varying degrees. Also, isolates 2M and 13M showed strong activities against S. aureus. The isolates showed strong adhesion and auto-aggregation capacity. Isolate 10A showed the highest antioxidant activity (91.45%) followed by 2M (47.37%). The isolates recorded different catalase and protease activity. All isolates produced cholesterol oxidase and lipase with different levels. Besides, the four isolates reduced LDL (low-density lipoprotein) to different significant values. The cholesterol-reducing ability varied not only for strains but also for the time of incubation. The previous results recommended these isolates be used safely in solving the LDL problem.

Synergistic enzyme cocktail between levansucrase and inulosucrase for superb levan-type fructooligosaccharide synthesis

Inulosucrase (ISC) and levansucrase (LSC) utilise sucrose and produce inulin- and levan-type fructans, respectively. This study aims to propose a new strategy to improve levan-type fructooligosaccharide (L-FOS) production. The effect of ISC/ LSC -mixed reaction was elucidated on L-FOS production. The presence of ISC in the LSC reaction significantly leads to the higher production of L-FOSs as the main products. Furthermore, the different ratios between ISC and LSC affected the distribution of L-FOSs. A greater amount of ISC compared to LSC promoted the synthesis of short-chain L-FOSs. Conversely, when LSC was increased, the synthesis of longer-chain L-FOSs was enhanced. The addition of trisaccharide mixtures obtained from either a single ISC or LSC reaction could enhance L-FOSs synthesis in the LSC reaction. Analysis of these trisaccharides revealed that most species of the oligosaccharides were similar, with 1-kestose being the major one. The supplement of only 1-kestose in the LSC reaction showed similar results to those of the reaction in the presence of trisaccharide mixtures. Moreover, the results were supported by molecular dynamics simulations. This work not only provides an improvement in L-FOS production but also revealed and supported some insights into the mechanism of fructansucrases.

Syntrophy of Crypthecodinium cohnii and immobilized Zymomonas mobilis for docosahexaenoic acid production from sucrose-containing substrates

Marine heterotrophic dinoflagellate Crypthecodinium cohnii is an aerobic oleaginous microorganism that accumulates intracellular lipid with high content of 4,7,10,13,16,19-docosahexaenoic acid (DHA), a polyunsaturated ω-3 (22:6) fatty acid with multiple health benefits. C. cohnii can grow on glucose and ethanol, but not on sucrose or fructose. For conversion of sucrose-containing renewables to C. cohnii DHA, we investigated a syntrophic process, involving immobilized cells of ethanologenic bacterium Zymomonas mobilis for fermenting sucrose to ethanol. The non-respiring, NADH dehydrogenase-deficient Z. mobilis strain Zm6-ndh, with high ethanol yield both under anaerobic and aerobic conditions, was taken as the genetic background for inactivation of levansucrase (sacB). SacB mutation eliminated the levan-forming activity on sucrose. The double mutant Zm6-ndh-sacB cells were immobilized in Ca alginate, and applied for syntrophic conversion of sucrose to DHA of C. cohnii, either taking the ethanol-containing fermentation medium from the immobilized Z. mobilis for feeding to the C. cohnii fed-batch culture, or directly coculturing the immobilized Zm6-ndh-sacB with C. cohnii on sucrose. Both modes of cultivation produced C. cohnii CCMP 316 biomass with DHA content around 2-3 % of cell dry weight, corresponding to previously reported results for this strain on glucose.

Improving the catalytic behaviors of Lactobacillus-derived fructansucrases by truncation strategies

Fructansucrases (FSs), including inulosucrase (IS) and levansucrase (LS), are the members of the Glycoside Hydrolase family 68 (GH68) enzymes. IS and LS catalyze the polymerization of the fructosyl moiety from sucrose to inulin- and levan-type fructans, respectively. Lactobacillus-derived FSs have relatively extended N- and C-terminal sequences. However, the functional roles of these sequences in their enzymatic properties and fructan biosynthesis remain largely unknown. Limosilactobacillus reuteri (basionym: Lactobacillus reuteri) 121 could produce both IS and LS, abbreviated as Lare121-IS and Lare121-LS, respectively. In this study, it was found that the terminal truncation displayed an obvious effect on their activities and the N-terminal truncated variants, Lare121-ISΔ177-701 and Lare121-LSΔ154-686, displayed the highest activities. Melting temperature (T_m) and the thermostability at 50 °C were measured to evaluate the stability of various truncated versions, revealing the different effects of N-terminal on the stability. The average molecular weight and polymerization degree of the fructans produced by different truncated variants did not change considerably, indicating that N-terminal truncation had low influence on fructan biosynthesis. In addition, it was found that N-terminal truncation could also improve the activity of other reported FSs from Lactobacillus species.

Optimization strategy of Bacillus subtilis MT453867 levansucrase and evaluation of levan role in pancreatic cancer treatment

Pancreatic cancer is the fourth most lethal cancer type worldwide. Due to multiple levan applications including anticancer activities, studies related to levansucrase production are of interest. To our knowledge, levan effect on pancreatic cancer cells has not been tested previously. In this work, among eighteen bacterial honey isolates, Bacillus subtilis MT453867 showed the highest levan yield (33 g/L) and levansucrase production (8.31 U/mL). One-factor-at-a-time technique increased levansucrase activity by 60% when MgSO₄ was eliminated. The addition of 60 g/L banana peels enhanced the enzyme activity (192 U/mL). Placket Burman design determined the media composition for maximum levan yield (54.8 g/L) and levansucrase production (505 U/mL). The identification of levan was confirmed by thin-layer chromatography, Fourier-Transform Infrared spectrometric analysis, ¹³C-nuclear-magnetic resonance, and ¹H-nuclear-magnetic resonance. Both crude and dialyzed levan completely inhibited the pancreatic cancer cell line at 100 ppm with no cytotoxicity on the normal retinal cell line. The LD₅₀ of crude levan was 4833 mg/kg body weight. Levan had strong antioxidant activity and significantly reduced the expression of CXCR4 and MCM7 genes in pancreatic cancer cells with significant DNA fragmentation. In conclusion, Bacillus subtilis MT453867 levan is a promising adjunct to pancreatic-anticancer agents with both anti-cancer and chemoprotective effects.

Enzymatic synthesis of phlorizin fructosides

Phlorizin is a low soluble dihydrochalcone with relevant pharmacological properties. In this study, enzymatic fructosylation was approached to enhance the water solubility of phlorizin, and consequently its bioavailability. Three enzymes were assayed for phlorizin fructosylation in aqueous reactions using sucrose as fructosyl donor. Levansucrase (EC 2.4.1.10) from Gluconacetobacter diazotrophicus (Gd_LsdA) was 6.5-fold more efficient than invertase (EC 3.2.1.26) from Rhodotorula mucilaginosa (Rh_Inv), while sucrose:sucrose 1-fructosyltransferase (EC 2.4.1.99) from Schedonorus arundinaceus (Sa_1-SST) failed to modify the non-sugar acceptor. Gd_LsdA synthesized series of phlorizin mono- di- and tri-fructosides with maximal conversion efficiency of 73 %. The three most abundant products were identified by ESI-MS and NMR analysis as β-D-fructofuranosyl-(2→6)-phlorizin (P1a), phlorizin-4'-O-β-D-fructofuranosyl-(2→6)-D-fructofuranoside (P2c) and phlorizin-4-O-monofructofuranoside (P1b), respectively. Purified P1a was 16 times (30.57 g L^-1 at 25 °C) more soluble in water than natural phlorizin (1.93 g L^-1 at 25 °C) and exhibited 44.56 % free radical scavenging activity. Gd_LsdA is an attractive candidate enzyme for the scaled synthesis of phlorizin fructosides in the absence of co-solvent.

actinidiae biovar 3, the causal agent of bacterial canker of kiwifruit, show different enzymatic properties

Bacterial canker disease caused by Pseudomonas syringae pv. actinidiae (Psa) biovar 3 involved all global interest since 2008. We have found that in Psa3 genome, similarly to other P. syringae, there are three putative genes, lscα, lscβ and lscγ, coding for levansucrases. These enzymes, breaking the sucrose moiety and releasing glucose can synthetize the fructose polymer levan, a hexopolysaccharide that is well known to be part of the survival strategies of many different bacteria. Considering lscα non-coding because of a premature stop codon, in the present work we cloned and expressed the two putatively functional levansucrases of Psa3, lscβ and lscγ, in E. coli and characterized their biochemical properties such as optimum of pH, temperature and ionic strength. Interestingly, we found completely different behaviour for both sucrose splitting activity and levan synthesis between the two proteins; lscγ polymerizes levan quickly at pH 5.0 while lscβ has great sucrose hydrolysis activity at pH 7.0. Moreover, we demonstrated that at least in vitro conditions, they are differentially expressed suggesting two distinct roles in the physiology of the bacterium.

Levan from Leuconostoc citreum BD1707: production optimization and changes in molecular weight distribution during cultivation

Background

Levan is a well-known homopolymer of fructose composed predominantly of β-(2, 6) fructofuranosyl linkages in the backbone with occasional β-(2, 1) linkages in the branch chains with varied applications. However, high production cost due to low yield of microbial levan has become a bottleneck for its practical applications. Furthermore, factors affecting the molecular mass of the synthesized levan by Leuconostoc spp. during prolonged cultivation is not fully elucidated.

Methods

The cultivation condition for Leuconostoc citreum BD1707 to synthesize levan was optimized by single-factor experiments and subsequently with response surface methodology (RSM). The average molecular weight (Mw) of levan synthesized by the strain L.citreum BD1707 under the optimized cultivation conditions was monitored by high-performance size exclusion chromatography (HPSEC). Finally, the enzyme with levan-degrading activity was determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE).

Results

The levan yield of BD1707 reached 34.86 g/L with a corresponding productivity of 7.47 g/L/d under the optimal cultivation conditions deduced by RSM, i.e., cultivation at 26 °C and 200 rpm for 112 h in tomato juice supplemented with 172 g/L sucrose with an initial pH value of 6.12. The Mw of levan reached a peak value of 2.320 × 10⁷ Da at 6 h of cultivation under the optimized cultivation conditions and then gradually decreased to 8.809 × 10⁶ Da after 120 h of cultivation.

Conclusion

The levan yield of the strain L.citreum BD1707 could be sufficiently enhanced via cultivation condition optimization. The decrease in molecular mass of the synthesized levan was attributed predominantly to the hydrolytic activity of levansucrase secreted by L.citreum BD1707 during cultivation, with an estimated Mw of 130 KD by SDS-PAGE, while the effect of acid hydrolysis could be nearly neglected.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12896-021-00673-y.

Insights into the pH-dependent, extracellular sucrose utilization and concomitant levan formation by Gluconobacter albidus TMW 2.1191

Many bacteria and archaea produce the polydisperse fructose polymer levan from sucrose upon biofilm formation via extracellular levansucrases (EC 2.4.1.10). We have investigated levansucrase-release and -activities as well as molecular size of the levan formed by the acetic acid bacterium Gluconobacter albidus TMW 2.1191 at varying environmental pH conditions to obtain insight in the ecological role of its constitutively expressed levansucrase and the produced levan. A buffer system was established enabling the recovery of levansucrase-containing supernatants from preincubated cell suspensions at pH 4.3-pH 5.7. The enzyme solutions were used to produce levans at different pH values and sucrose concentrations. Finally, the amounts and size distributions of the produced levans as well as the corresponding levansucrase activities were determined and correlated with each other. The data revealed that the levansucrase was released into the environment independently of its substrate sucrose, and that more levansucrase was released at pH ≥ 5.0. The glucose release and formation of high molecular weight levans (> 3.5 kDa) from 0.1 M initial sucrose was comparable between pH ~ 4.3-5.7 using equal amounts of released levansucrase. Hence, this type of levansucrase appears to be structurally adapted to changes in the extracellular pH and to exhibit a similar total activity over a wide acidic pH range, while it produced higher amounts of larger levan molecules at higher production pH and sucrose concentrations. These findings indicate the physiological adaptation of G. albidus TMW 2.1191 to efficient colonisation of sucrose-rich habitats via released levansucrases despite changing extracellular pH conditions in course of acid formation.

Implications of the mutation S164A on Bacillus subtilis levansucrase product specificity and insights into protein interactions acting upon levan synthesis

Mutation S164A largely affects the transfructosylation properties of Bacillus subtilis levansucrase (SacB). The variant uses acceptors such as glucose and short levans with an average molecular weight of 7.6 kDa more efficiently than SacB, leading to the enhanced synthesis of medium and high molecular weight polymer and a blasto-oligosaccharide series with a polymerization degree of 2-10. A 3-fold increase in blasto-oligosaccharides yield is provoked by the modified interplay between the variant and glucose. Despite its modified product specificity, protein-carbohydrate and protein-protein interactions are still a major factor affecting size and distribution of levan molecular weight. This study highlights the importance of critical factors such as protein concentration in the analysis of wild-type and mutagenized levansucrases. Docking experiments with the crystal structures of SacB and variant S164A - the latter obtained at a 2.6 Å resolution - identified unreported potential binding subsites for fructosyl moieties on the surface of both enzymes.

Computational design of oligosaccharide producing levansucrase from Bacillus licheniformis RN-01 to improve its thermostability for production of levan-type fructooligosaccharides from sucrose

Levansucrase catalyzes production of levan and levan-type fructooligosaccharides (LFOs) with potential applications in food and pharmaceutical industries such as prebiotics and anti-tumor agents. Previous study found that Y246S mutant of Bacillus licheniformis RN-01 levansucrase (oligosaccharide producing levansucrase, OPL) could effectively produce LFOs but its thermostability is limited at high temperature. In this study, molecular dynamics (MD) and computational protein design were used to create mutants with higher thermostability than OPL by rigidifying highly flexible residues on enzyme surface. MD results show that highly flexible residues suitable for design are K82, N83, D179, and Q308. Two approaches were employed to improve their interactions by allowing them to be amino acids that could potentially form favorable interactions with their neighboring residues or natural amino acids except G, P and C. Flexibilities of designed residues of K82H, N83R, Q308S and K82H/N83R mutants are lower than those of OPL. Experimental results show that characteristics and product patterns of designed mutants are relatively similar to those of OPL. K82H/N83R mutant has higher thermostability than OPL with 1.7-fold increase in t_1/2. Circular dichroism result suggests that designed mutations do not drastically affect secondary structures. This study shows how computational technique can engineer enzyme for thermostability improvement.

Computational design of Bacillus licheniformis RN-01 levansucrase for control of the chain length of levan-type fructooligosaccharides

Levansucrase (LS) from Gram-positive bacteria generally produces a large quantity of levan polymer, a polyfructose with glucose at the end (GF_n) but a small quantity of levan-type fructooligosaccharides (LFOs). The properties of levan and LFOs depend on their chain lengths, thereby determining their potential applications in food and pharmaceutical industries such as prebiotics and anti-tumor agents. Therefore, an ability to redesign and engineer the active site of levansucrase for synthesis of products with desired degree of polymerization (DP) is very beneficial. We employed computational protein design, docking and molecular dynamics to redesign and engineer the active site of Bacillus licheniformis RN-01 levansucrase for production of LFOs with DP up to five (GF₄), using two approaches: 1) blocking oligosaccharide binding track of GF₃-LS complex with large aromatic residues and 2) eliminating hydrogen bond interactions between terminal glucose of GF₄ and side chains of binding residues of GF₄-LS complex. The designed enzymes and their product patterns from these two approaches were experimentally characterized. The experimental results show that the first approach was successful in creating N251W and N251W/K372Y mutants that synthesized LFOs with DP up to five. This work illustrates how computer-aided approaches can offer novel opportunities to engineer enzymes for desired products.

Efficient production of levan using a recombinant yeast Saccharomyces cerevisiae hypersecreting a bacterial levansucrase

Levan is a fructose polymer with diverse applications in the food and medical industries. In this study, levansucrase from Rahnella aquatilis (RaLsrA) was hyper-secreted using a Saccharomyces cerevisiae protein secretion system. An optimal secretion signal, a translation fusion partner (TFP) containing an N-terminal 98 amino acid domain from a mitochondrial inner membrane protein, UTH1, was employed to secrete approximately 50 U/mL of bioactive RaLsrA into culture media with 63% secretion efficiency by fed-batch fermentation. Although the purified RaLsrA was useful for enzymatic conversion of high-molecular-weight levan of approximately 3.75 × 10⁶ Da, recombinant yeast secreting RaLsrA could produce levan more efficiently by microbial fermentation. In a 50-L scale fermenter, 76-g/L levan was directly converted from 191-g/L sucrose by recombinant yeast cells, attaining an 80% conversion yield and 3.17-g/L/h productivity. Thus, we developed a cost-effective and industrially applicable production system for food-grade levan.

Modulation of fructooligosaccharide chain length and insight into the product binding motif of Lactobacillus reuteri 121 inulosucrase

Inulosucrase (E.C. 2.4.1.9) is a bacterial fructosyltransferase that synthesizes inulin-type fructooligosaccharide, using sucrose as a substrate. We modulated the size of fructooligosaccharide synthesized by Lactobacillus reuteri 121 inulosucrase using rational designed mutagenesis. Nine residues: D478, D479, S482, R483, N543, W551, N555, N561 and D689, were changed based on the active site architecture and amino acids potentially interacting with saccharides. The selected residues were substituted with alanine to investigate the contribution of these residues to FOS chain length. Enzymatic activity assays demonstrated that the transglycosylation/hydrolysis ratios of D479A, R483A, N543A, W551A and N555A mutants were significantly different from that of the wild type. Almost all mutants, except D478A, synthesized oligosaccharides with different size distribution compared to that of wild type. Molecular docking further provides insights into the product binding motif of Lactobacillus reuteri 121 inulosucrase and strengthens an important role of amino acid residues at remote locations from the active site on the enzymatic activity and product specificity.

Preparation of a novel water-soluble gel from Erwinia amylovora levan

Less attention has been focused on the industrial applications of levan-type fructan than that of inulin. Levan-type fructan is a unique homopolysaccharide consisting of fructose residues with a β-(2, 6) linkage that possesses unique physiochemical properties such as low intrinsic viscosity. In this study, the recombinant levansucrase from Erwinia amylovora was used to efficiently produce levan from sucrose, and under optimised conditions, 195 g/L levan was produced from 500 g/L sucrose, with the highest conversion rate of 59%. The physicochemical properties of E. amylovora levan, such as surface morphology, thermal behaviour, rheology behaviour and texture analysis, were evaluated and compared with those of commercial gels, including xanthan, guar, carrageenan and Arabic gums. The produced E. amylovora levan showed a series of acceptable physicochemical properties, indicating a potential application for levan as a novel water-soluble micro gel. The conclusions of this study support the exploration of the use of more hydrogels in the food, medicinal and cosmetic industries.

Composition and metabolism of fecal microbiota from normal and overweight children are differentially affected by melibiose, raffinose and raffinose-derived fructans

The aim of the study was to investigate the metabolism of non-digestible oligo- and polysaccharides by fecal microbiota, using isothermal microcalorimetry. The five tested substrates were raffinose, melibiose, a mixture of oligo- and polysaccharides produced from raffinose by levansucrase, levan synthesized from raffinose, and levan from timothy grass. Two inocula were comprised of pooled fecal samples from overweight or normal-weight children, from healthy adult volunteers and a pure culture of Bacteroides thetaiotaomicron as a reference bacterium for colon microbiota. The growth was analyzed based on the heat evolution curves, and the production of organic acids and gases. Taxonomic profiles of the microbiota were assessed by 16S rDNA sequencing. Raffinose and melibiose promoted the growth of bifidobacteria in all fecal pools. Several pool-specific substrate-related responses to raffinose and melibiose were revealed. Lactate-producing bacteria (Streptococcus and Enterococcus) became enriched in the pool of overweight children resulting in lactic acid as the major fermentation product on short saccharides. Acetic and butyric acids were prevalent at fermentation in the normal-weight pool coinciding with the enrichment of Catenibacterium. In the adult pool, the specific promotion of Bacteroides and Lachnospiraceae by levans was disclosed. In the fecal pool of normal-weight children, levans stimulated the growth of Senegalimassilia and Lachnoclostridium and this particular pool also showed the highest maximum heat production rate at levan fermentation. Levans and raffinose-derived oligosaccharides, but not raffinose and melibiose were completely fermented by a pure culture of Bacteroides thetaiotaomicron. The main conclusion from the study is that fecal microbiota of normal and overweight children have different compositions and they respond in specific manners to non-digestible oligo- and polysaccharides: raffinose, melibiose, raffinose-derived oligosaccharides and levans. The potential of the tested saccharides to support a healthy balance of colon microbiota requires further studies.

Levan-type fructooligosaccharides synthesis by a levansucrase-endolevanase fusion enzyme (LevB1SacB)

We describe here the enzymatic production of levan type-fructooligosaccharides (L-FOS) with a DP from 2 to 10, through simultaneous synthesis and hydrolysis reactions. This was accomplished by LevB₁SacB, a new enzyme resulting from the fusion of SacB, a levansucrase from Bacillus subtilis and LevB₁, an endolevanase from B. licheniformis. In the fusion enzyme, SacB retains its catalytic behavior with a decrease in kcat from 164 to 108s^-1. LevB₁ in LevB₁SacB kinetic behavior improves considerably reaching saturation with levan and following Michaelis-Menten kinetics, quite differently from the previously reported first order kinetic behavior. We also report that LevB₁SacB or both enzymes (LevB₁ & SacB) at equimolar concentrations in simultaneous reactions result in an optimal, wide and diverse L-FOS profile, including 6-kestose, levanbiose and blastose among other L-FOS and 1-kestose, which accumulates as by-product of SacB levan synthesis. Yields of around 40% (w/w) were obtained from 600g/l sucrose with either LevB₁SacB or LevB₁ & SacB. The reaction was successfully scaled up to a stirred 2l bioreactor.

isolated from honey and honey bee

Five bacterial isolates from honey and bee gut were selected based on their high levansucrase activity and levan yield which were strongly positively correlated. All isolates showed good tolerance to temperature up to 70 °C, to NaCl up to 3 M and to 0.1% H₂O₂. They maintained over 59 and 64% survival at pH 9.0 and 2.0 respectively, but showed varying tolerance to 0.1% bile salts and pancreatic enzymes. Most isolates were susceptible to widely used antibiotics, but demonstrated diverse antimicrobial activity. Non hemolytic isolates were identified on the basis of 16S rRNA sequencing as Bacillus subtilis HMNig-2 and B. subtilis MENO2 with 97% homology. They exhibited promising probiotic characteristics and achieved highest levansucrase activity of 94.1 and 81.5 U/mL respectively. Both exhibited highest biofilm formation ability in static microtiter plate assay. Also, they achieved 34 and 26% adhesion respectively to Caco-2cells and had highest free radical scavenging activity of 30.8 and 26.2% respectively. The levans of the two isolates showed good antimicrobial activity against some pathogens and exhibited positive prebiotic effect (prebiotic index >1) with Lactobacillus casei and Lactobacillus reuteri. Results suggest a correlation between levansucrase production, levan yield and pre-probiotic activities of the studied strains.

Characterization of the levan produced by Paenibacillus bovis sp. nov BD3526 and its immunological activity

Paenibacillus bovis sp. nov BD3526 synthesizes a large amount of exopolysaccharides (EPSs) (36.25g/L) in a semi-defined chemical medium containing 20% (w/v) sucrose. The EPSs were extracted from the cultured broth by ethanol precipitation and purified via anion-exchange and gel permeation chromatography. The Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectra showed that the primary EPS fraction (F1) was a linear β (2→6)-linked levan. The peak molecular weight (Mp) of the levan exceeded 2.6×10(6)Da based on high-performance size-exclusion chromatography (HPSEC). The levan adopted a spherical conformation in aqueous solution as confirmed by transmission electron microscopy (TEM). The corresponding levansucrase was identified by SDS-PAGE analysis and in situ polymer synthesis. The in vitro assay demonstrated that the levan significantly stimulated the proliferation of spleen cells and induced the expression of TNF-α, indicating its potential as a natural immunomodulator.

Bacillus subtilis 168 levansucrase (SacB) activity affects average levan molecular weight

Levan is a fructan polymer that offers a variety of applications in the chemical, health, cosmetic and food industries. Most of the levan applications depend on levan molecular weight, which in turn depends on the source of the synthesizing enzyme and/or on reaction conditions. Here we demonstrate that in the particular case of levansucrase from Bacillus subtilis 168, enzyme concentration is also a factor defining the molecular weight levan distribution. While a bimodal distribution has been reported at the usual enzyme concentrations (1 U/ml equivalent to 0.1 μM levansucrase) we found that a low molecular weight normal distribution is solely obtained al high enzyme concentrations (>5 U/ml equivalent to 0.5 μM levansucrase) while a high normal molecular weight distribution is synthesized at low enzyme doses (0.1 U/ml equivalent to 0.01 μM of levansucrase).