Insights into hydrolysis versus transfructosylation: Mutagenesis studies of a novel levansucrase from Brenneria sp. EniD312

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.

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.

Identification of a Thermostable Levansucrase from Pseudomonas orientalis That Allows Unique Product Specificity at Different Temperatures

The biological production of levan by levansucrase (LS, EC 2.4.1.10) has aroused great interest in the past few years. Previously, we identified a thermostable levansucrase from Celerinatantimonas diazotrophica (Cedi-LS). A novel thermostable LS from Pseudomonas orientalis (Psor-LS) was successfully screened using the Cedi-LS template. The Psor-LS showed maximum activity at 65 °C, much higher than the other LSs. However, these two thermostable LSs showed significantly different product specificity. When the temperature was decreased from 65 to 35 °C, Cedi-LS tended to produce high-molecular-weight (HMW) levan. By contrast, Psor-LS prefers to generate fructooligosaccharides (FOSs, DP ≤ 16) rather than HMW levan under the same conditions. Notably, at 65 °C, Psor-LS would produce HMW levan with an average Mw of 1.4 × 106 Da, indicating that a high temperature might favor the accumulation of HMW levan. In summary, this study allows a thermostable LS suitable for HMW levan and levan-type FOSs production simultaneously.

Successful Manipulation of the Product Spectrum of the Erwinia amylovora Levansucrase by Modifying the Residues around loop1, Loop 3, and Loop 4

Levansucrase (LS, EC 2.4.1.10) catalyzes the synthesis of levan by successively transferring the fructosyl moiety from sucrose to an elongated fructan chain. Although the product distribution of LS from Erwinia amylovora (Ea-LS) was studied under different sucrose concentrations, the effect of residues on the product formation is yet unknown. The first levanhexaose-complexed structure of LS from Bacillus subtilis (Bs-SacB) provided information on the oligosaccharide binding sites (OB sites), from +1 to +4 subsites. Since Ea-LS would efficiently produce fructooligosaccharides, a substitution mutation of OB sites in Bs-SacB and the corresponding residues of Ea-LS were conducted to investigate how these mutants would influence the product distribution. As a result, a series of mutants with different product spectrum were obtained. Notably, the mutants of G98E, V151F, and N200T around loop 1, loop 3, and loop 4 all showed a significant increase in both the molecular mass and the yield of high-molecular-mass levan, suggesting that the product profile of Ea-LS was significantly modified.

Crystal Structure of Levansucrase from the Gram-Negative Bacterium Brenneria Provides Insights into Its Product Size Specificity

Microbial levansucrases (LSs, EC 2.4.1.10) have been widely studied for the synthesis of β-(2,6)-fructans (levan) from sucrose. LSs synthesize levan-type fructo-oligosaccharides, high-molecular-mass levan polymer or combinations of both. Here, we report crystal structures of LS from the G^--bacterium Brenneria sp. EniD 312 (Brs-LS) in its apo form, as well as of two mutants (A154S, H327A) targeting positions known to affect LS reaction specificity. In addition, we report a structure of Brs-LS complexed with sucrose, the first crystal structure of a G^--LS with a bound substrate. The overall structure of Brs-LS is similar to that of G^-- and G⁺-LSs, with the nucleophile (D68), transition stabilizer (D225), and a general acid/base (E309) in its active site. The H327A mutant lacks an essential interaction with glucosyl moieties of bound substrates in subsite +1, explaining the observed smaller products synthesized by this mutant. The A154S mutation affects the hydrogen-bond network around the transition stabilizing residue (D225) and the nucleophile (D68), and may affect the affinity of the enzyme for sucrose such that it becomes less effective in transfructosylation. Taken together, this study provides novel insights into the roles of structural elements and residues in the product specificity of LSs.

Crystal structure of an inulosucrase from Halalkalicoccus jeotgali B3T, a halophilic archaeal strain

Several archaea harbor genes that code for fructosyltransferase (FTF) enzymes. These enzymes have not been characterized yet at structure-function level, but are of extreme interest in view of their potential role in the synthesis of novel compounds for food, nutrition, and pharmaceutical applications. In this study, 3D structure of an inulin-type fructan producing enzyme, inulosucrase (InuHj), from the archaeon Halalkalicoccus jeotgali was resolved in its apo form and with bound substrate (sucrose) molecule and first transglycosylation product (1-kestose). This is the first crystal structure of an FTF from halophilic archaea. Its overall five-bladed β-propeller fold is conserved with previously reported FTFs, but also shows some unique features. The InuHj structure is closer to those of Gram-negative bacteria, with exceptions such as residue E266, which is conserved in FTFs of Gram-positive bacteria and has possible role in fructan polymer synthesis in these bacteria as compared to fructooligosaccharide (FOS) production by FTFs of Gram-negative bacteria. Highly negative electrostatic surface potential of InuHj, due to a large amount of acidic residues, likely contributes to its halophilicity. The complex of InuHj with 1-kestose indicates that the residues D287 in the 4B-4C loop, Y330 in 4D-5A, and D361 in the unique α2 helix may interact with longer FOSs and facilitate the binding of longer FOS chains during synthesis. The outcome of this work will provide targets for future structure-function studies of FTF enzymes, particularly those from archaea.

Cloning and Expression of Levansucrase Gene of Bacillus velezensis BM-2 and Enzymatic Synthesis of Levan

Levan is a versatile and valuable fructose homopolymer, and a few bacterial strains have been found to produce levan. Although levan products have numerous specific functions, their application and promotion were limited by the production capacity and production cost. Bacillus velezensis BM-2 is a levan-synthesizing strain, but its levan production is too low to apply. In this study, the levansucrase gene of B. velezensis BM-2 was cloned to plasmid pET-32a-Acma-zz, and the recombinant plasmids were transferred to Escherichia coli BL21. A transformed clone was selected to express and secrete the fusion enzymes with an Acma-tag efficiently. The expressed products were further purified by a self-developed separating material called bacterial enhancer matrix (BEM) particles. The purification efficiency was 93.4%, with a specific activity of 16.589 U/mL protein. The enzymatic reaction results indicated that the optimal reaction temperature is 50 °C, the optimal pH of the acetate buffer is 5.6, and the buffer system greatly influenced the enzyme activity. The enzyme activity was enhanced to 130% in the presence of 5 mM Ca2+, K+, Zn2+, and Mn2+, whereas it was almost abolished in the case of Cu2+ and Fe3+. The values of Km, kcat, and kcat/Km were 17.41 mM, 376.83 s−1, and 21.64 mM−1s−1, respectively. The enzyme amount of 20 U/g sucrose was added to the system containing 400 g/L sucrose, and the levan products with a concentration of 120 g/L reached after an incubation of 18 h, which was 8 times that of the yield before optimization. The results of molecular docking analysis indicated that the Asp86 might act as a nucleophilic catalytic residue for sucrose, Arg246 and Asp247 act as transition state stabilizer of transfructosylation, and Glu340 and Arg306 were recognized as general acid donors. They formed the catalytic-groups triad. The unique properties and catalytic activity of the levansucrase suggest that it deserves further research and might have good industrial application prospects.

A comprehensive study on levan nanoparticles formation: Kinetics and self-assembly modeling

Levan nanoparticles formation is a complicated phenomenon involving simultaneously polymeric reaction kinetics and nanoparticles self-assembly theory. These phenomena are studied in this work with experimental and computational methodologies. Specifically, the effect of different parameters on levan kinetics and nanoparticles production in a cell-free system environment have been studied. Results point out that 37 °C is the best temperature for synthesizing levan as well as the existence of a substrate inhibition effect for polymeric reaction. This work also highlights that raffinose can be used for producing and that an increase on the ratio enzyme-substrate increases the velocity of conversion. However, the previous experimental conditions did not produce an important effect on self-assembly formed levan nanoparticles (always 110 nm) as long as the required levan concentration (CAC) for nanoparticles reorganization is achieved. To have a better understanding of these results, a model was developed to explain numerically levan kinetics and nanoparticle self-assembly. This model was built by taking into account enzyme poisoning effect (also demonstrated experimentally) and a diffusion limited cluster model for the aggregation phenomenon. Simulation results fit properly experimental data and catalytic parameters as well as predicting accurately the value of CAC for producing its reorganization into nanoparticles by self-assembly.

A close look on the effect of polyethylene glycol on the levansucrase thermostability: a case study of Brenneria sp. levansucrase

BACKGROUND

To increase the low residual activity of levansucrase during long-time processing, an enhancement of its weak thermostability is needed. Here, the effect of metal ions and polyethylene glycol (PEG) on the thermostability of levansucrase from Brenneria sp. EniD312 were studied and evaluated. The residual activity was determined and the protein structure was evaluated by circular dichroism spectrum, fluorescence intensity (FI), and surface hydrophobicity (S₀ ).

RESULTS

As a result of incubation with 10% (w/v) PEG 4000, the enzyme activity was increased by 1.24-fold. After incubation with 5% PEG 4000 for 6 h, the residual activity at 35 and 45 °C was decreased to 55% and 60% of the initial activity, with an increase of 1.2- and 3.3-fold than the wild-type enzyme. Furthermore, the random coil content of enzyme was decreased from 53% of the wild-type enzyme to 33.9% of the PEG pre-incubated enzyme. Additionally, the FI was maximally increased and the S₀ was decreased from 117 to 69.

CONCLUSION

All of these results suggested that after incubation with PEG 4000, the secondary and tertiary structure of wild-type enzyme could be greatly maintained and then its thermostability could be increased. This study was the first report on the enhancement of levansucrase thermostability by PEG incubation and might be a good guideline to other researches on levansucrase. © 2019 Society of Chemical Industry.

Improving Thermostability and Catalytic Behavior of l-Rhamnose Isomerase from Caldicellulosiruptor obsidiansis OB47 toward d-Allulose by Site-Directed Mutagenesis

d-Allose, a rare sugar, is an ideal table-sugar substitute and has many advantageous physiological functions. l-Rhamnose isomerase (l-RI) is an important d-allose-producing enzyme, but it exhibits comparatively low catalytic activity on d-allulose. In this study, an array of hydrophobic residues located within β1-α1-loop were solely or collectively replaced with polar amino acids by site-directed mutagenesis. A group of mutants was designed to weaken the hydrophobic environment and strengthen the catalytic behavior on d-allulose. Compared with that of the wild-type enzyme, the relative activities of the V48N/G59N/I63N and V48N/G59N/I63N/F335S mutants toward d-allulose were increased by 105.6 and 134.1%, respectively. Another group of mutants was designed to enhance thermostability. Finally, the t_1/2 values of mutant S81A were increased by 7.7 and 1.1 h at 70 and 80 °C, respectively. These results revealed that site-directed mutagenesis is efficient for improving thermostability and catalytic behavior toward d-allulose.

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.