Production of prebiotic enriched maple syrup through enzymatic conversion of sucrose into fructo-oligosaccharides

Maple syrup, a popular natural sweetener has a high content of sucrose, whose consumption is linked to different health issues such as obesity and diabetes. Hence, within this paper, the conversion of sucrose to prebiotics (fructo-oligosaccharides, FOS) was proposed as a promising approach to obtaining a healthier, value-added product. Enzymatic conversion was optimized with respect to key experimental factors, and thereafter derived immobilized preparation of fructosyltransferase (FTase) from Pectinex® Ultra SP-L (FTase-epoxy Purolite, 255 IU/g support) was successfully utilized to produce novel functional product in ten consecutive reaction cycles. The product, obtained under optimal conditions (60 °C, 7.65 IU/mL, 12 h), resulted in 56.0% FOS, 16.7% sucrose, and 27.3% monosaccharides of total carbohydrates, leading to a 1.6-fold reduction in caloric content. The obtained products` prebiotic potential toward the probiotic strain Lactobacillus plantarum 299v was demonstrated. The changes in physico-chemical and sensorial characteristics were esteemed as negligible.

Innovative application of a novel and thermostable inulin fructotransferase from Arthrobacter sp. ISL-85 to fructan inulin in burdock root to improve nutrition

Inulin fructotransferase converts prebiotic polysaccharide inulin to difructose anhydride III, known for its numerous beneficial physiological effects. While previous studies focused on using inulin extracts under optimal conditions, this study delves into the enzyme's behavior when dealing with more complex food materials, inulin-rich burdock root, which possesses greater nutritional value but may influence the enzymatic reaction. An inulin fructotransferase from Arthrobacter sp. ISL-85 was identified and characterized, which has the highest activity of 783 U mg-1 at pH 6.5 and 65 °C and remains stable even up to 80 °C. When applied to inulin-rich burdock root (pH 4.7) at 80 °C for 2 h, the enzyme yielded 4.1 g of difructose anhydride III, concurrently increasing fructo-oligosaccharides. This study demonstrates the potential of this enzyme as a valuable tool for efficiently processing inulin within whole food materials under high temperatures. Such an approach could pave the way for enhancing nutrition and promoting health benefits.

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.

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.

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.

Complete secretion of recombinant Bacillus subtilis levansucrase in Pichia pastoris for production of high molecular weight levan

Three signal peptides from α-mating factor (α-MF), inulinase (INU) and native levansucrase (LS) were compared for secretion efficiency of Bacillus subtilis levansucrase SacB-T305A in Pichia pastoris GS115. The first complete secretion of bacterial levansucrase in yeasts under methanol induction was achieved while using α-MF signal. The secreted recombinant Lev(α-MF) proved to be glycosylated by combination of NanoLC-MS/MS and Endo H digestion. Interestingly, glycosylation not only improved significantly the polymerase thermostability, but also reversed the products profiles to favor synthesis of high molecular weight (HMW) levan which accounted for approximately 73 % to total levan-type polysaccharides. It indicated for the first time that the glycosylation of recombinant B. subtilis levansucrase affected significantly the products molecular weight distribution. It also provided a promising enzymatic way to effectively product HMW levan from sucrose resources.

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.

Production of Human ABC Transporters and Oligosaccharyltransferase Complexes for Structural Studies

Structural studies of membrane proteins require high-quality samples. The target proteins should not only be pure and homogeneous but should also be active and allow the capture of a functionally relevant state. Here we present optimized methods for the expression and purification of human ABC transporters and oligosaccharyltransferase (OST) complexes that can be used for high-resolution structure determination using single-particle cryo-electron microscopy (cryo-EM). The protocols are based on the generation of stable cell lines that enable tetracycline-inducible expression of the target proteins. For the multidrug exporter ABCB1, we describe a protocol for reconstitution into nanodiscs and evaluation of the ATPase activity in the presence of drugs. For human OST, we describe a strategy for the purification of OST-A and OST-B complexes, including techniques to evaluate their integrity and activity using in vitro glycosylation assays. These protocols can be adapted for the production of other human ABC transporters and multimeric membrane protein complexes.

NDP-rhamnose biosynthesis and rhamnosyltransferases: building diverse glycoconjugates in nature

Rhamnose is an important 6-deoxy sugar present in many natural products, glycoproteins, and structural polysaccharides. Whilst predominantly found as the l-enantiomer, instances of d-rhamnose are also found in nature, particularly in the Pseudomonads bacteria. Interestingly, rhamnose is notably absent from humans and other animals, which poses unique opportunities for drug discovery targeted towards rhamnose utilizing enzymes from pathogenic bacteria. Whilst the biosynthesis of nucleotide-activated rhamnose (NDP-rhamnose) is well studied, the study of rhamnosyltransferases that synthesize rhamnose-containing glycoconjugates is the current focus amongst the scientific community. In this review, we describe where rhamnose has been found in nature, as well as what is known about TDP-β-l-rhamnose, UDP-β-l-rhamnose, and GDP-α-d-rhamnose biosynthesis. We then focus on examples of rhamnosyltransferases that have been characterized using both in vivo and in vitro approaches from plants and bacteria, highlighting enzymes where 3D structures have been obtained. The ongoing study of rhamnose and rhamnosyltransferases, in particular in pathogenic organisms, is important to inform future drug discovery projects and vaccine development.

High-resolution method for isocratic HPLC analysis of inulin-type fructooligosaccharides

In recent decades, strategies to improve human health by modulating the gut microbiota have developed rapidly. One of the most prominent is the use of prebiotics, which can lead to a higher abundance of health-promoting microorganisms in the gut. Currently, oligosaccharides dominate the prebiotic sector due to their ability to promote the growth and activity of probiotic bacteria selectively. Extensive efforts are made to develop effective production strategies for the synthesis of prebiotic oligosaccharides, including the use of microbial enzymes. Within the genus Lactobacillus, several inulosucrases have been identified, which are suitable for the synthesis of prebiotic inulin-type fructooligosaccharides (inulin-FOS). In this study, a truncated version of the inulosucrase from Lactobacillus gasseri DSM 20604 was used for the efficient synthesis of inulin-FOS. Product titers of 146.2 ± 7.4 g inulin-FOSL^-1 were achieved by the catalytic activity of the purified recombinant protein InuGB-V3. A time and resource-saving HPLC method for rapid analysis of inulin-FOS in isocratic mode was developed and optimized, allowing baseline separated analysis of inulin-FOS up to a degree of polymerization (DP) of five in less than six minutes. Long-chain inulin-FOS with a DP of 17 can be analyzed in under 45 min. The developed method offers the advantages of isocratic HPLC analysis, such as low flow rates, high sensitivity, and the use of a simple, inexpensive chromatographic setup. Furthermore, it provides high-resolution separation of long-chain inulin-FOS, which can usually only be achieved with gradient systems.

Two-State Exchange Dynamics in Membrane-Embedded Oligosaccharyltransferase Observed in Real-Time by High-Speed AFM

Oligosaccharyltransferase (OST) is a membrane-bound enzyme that catalyzes the transfer of oligosaccharide chains from lipid-linked oligosaccharides (LLO) to asparagine residues in polypeptide chains. Using high-speed atomic force microscopy (AFM), we investigated the dynamic properties of OST molecules embedded in biomembranes. An archaeal single-subunit OST protein was immobilized on a mica support via biotin-avidin interactions and reconstituted in a lipid bilayer. The distance between the top of the protein molecule and the upper surface of the lipid bilayer was monitored in real-time. The height of the extramembranous part exhibited a two-step variation with a difference of 1.8 nm. The high and low states are designated as state 1 and state 2, respectively. The transition processes between the two states fit well to single exponential functions, suggesting that the observed dynamic exchange is an intrinsic property of the archaeal OST protein. The two sets of cross peaks in the NMR spectra of the protein supported the conformational changes between the two states in detergent-solubilized conditions. Considering the height values measured in the AFM measurements, state 1 is closer to the crystal structure, and state 2 has a more compact form. Subsequent AFM experiments indicated that the binding of the sugar donor LLO decreased the structural fluctuation and shifted the equilibrium almost completely to state 1. This dynamic behavior is likely necessary for efficient catalytic turnover. Presumably, state 2 facilitates the immediate release of the bulky glycosylated polypeptide product, thus allowing OST to quickly prepare for the next catalytic cycle.

1H, 13C, 15N resonance assignments and secondary structure of yeast oligosaccharyltransferase subunit Ost4 and its functionally important mutant Ost4V23D

Asparagine-linked glycosylation is an essential and highly conserved protein modification reaction that occurs in the endoplasmic reticulum of cells during protein synthesis at the ribosome. In the central reaction, a pre-assembled high-mannose sugar is transferred from a lipid-linked donor substrate to the side-chain of an asparagine residue in an -N-X-T/S- sequence (where X is any residue except proline). This reaction is carried by a membrane-bound multi-subunit enzyme complex, oligosaccharyltransferase (OST). In humans, genetic defects in OST lead to a group of rare metabolic diseases collectively known as Congenital Disorders of Glycosylation. Certain mutations are lethal for all organisms. In yeast, the OST is composed of nine non-identical protein subunits. The functional enzyme complex contains eight subunits with either Ost3 or Ost6 at any given time. Ost4, an unusually small protein, plays a very important role in the stabilization of the OST complex. It bridges the catalytic subunit Stt3 with Ost3 (or Ost6) in the Stt3-Ost4-Ost3 (or Ost6) sub-complex. Mutation of any residue from M18-I24 in the trans-membrane helix of yeast Ost4 negatively impacts N-linked glycosylation and the growth of yeast. Indeed, mutation of valine23 to an aspartate impairs OST function in vivo resulting in a lethal phenotype in yeast. To understand the structural mechanism of Ost4 in the stabilization of the enzyme complex, we have initiated a detailed investigation of Ost4 and its functionally important mutant, Ost4V23D. Here, we report the backbone ¹H, ¹³C, and ¹⁵N resonance assignments for Ost4 and Ost4V23D in dodecylphosphocholine micelles.

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.

Levansucrase from Bacillus amyloliquefaciens KK9 and Its Y237S Variant Producing the High Bioactive Levan-Type Fructooligosaccharides

Levan-typed fructooligosaccharide (LFOS), a β-2,6 linked oligofructose, displays the potential application as a prebiotic and therapeutic dietary supplement. In the present study, LFOS was synthesized using levansucrase from Bacillus amyloliquefaciens KK9 (LsKK9). The wild-type LsKK9 was cloned and expressed in E. coli, and purified by cation exchanger chromatography. Additionally, Y237S variant of LsKK9 was constructed based on sequence alignment and structural analysis to enhance the LFOS production. High-performance anion-exchange chromatography coupled with pulsed amperometric detection (HPAEC-PAD) analysis indicated that Y237S variant efficiently produced a higher amount of short-chain LFOS than wild type. Also, the concentration of enzyme and sucrose in the reactions was optimized. Finally, prebiotic activity assay demonstrated that LFOS produced by Y237S variant had higher prebiotic activity than that of the wild-type enzyme, making the variant enzyme attractive for food biotechnology.

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.

Enzymatic Process Yielding a Diversity of Inulin-Type Microbial Fructooligosaccharides

The specificity of fructooligosaccharides as prebiotics depends on their size and structure, which in turn depend on their origin or the synthesis procedure. In this work we describe the application of an inulosucrase (IslA) from Leuconostoc citreum CW28 to produce high molecular weight inulin from sucrose alongside a commercial endoinulinase (Novozym 960) produced by Aspergillus niger for a simultaneous or sequential reaction to synthesize fructooligosaccharides (FOS). The simultaneous reaction resulted in a higher substrate conversion and a wide diversity of FOS when compared to the sequential reaction. A shotgun MS analysis of the commercial endoinulinase preparation surprisingly revealed an additional enzymatic activity: a fructosyltransferase, responsible for the synthesis of FOS from sucrose. Consequentially, the range of FOS obtained in reactions combining inulosucrase from Ln. citreum with the fructosyltransferase and endoinulinase from A. niger with sucrose as substrate may be extended and regulated.

Synthesis of Isorhamnetin-3-O-Rhamnoside by a Three-Enzyme (Rhamnosyltransferase, Glycine Max Sucrose Synthase, UDP-Rhamnose Synthase) Cascade Using a UDP-Rhamnose Regeneration System

Isorhamnetin-3-O-rhamnoside was synthesized by a highly efficient three-enzyme (rhamnosyltransferase, glycine max sucrose synthase and uridine diphosphate (UDP)-rhamnose synthase) cascade using a UDP-rhamnose regeneration system. The rhamnosyltransferase gene (78D1) from Arabidopsis thaliana was cloned, expressed, and characterized in Escherichia coli. The optimal activity was at pH 7.0 and 45 °C. The enzyme was stable over the pH range of 6.5 to 8.5 and had a 1.5-h half-life at 45 °C. The Vmax and Km for isorhamnetin were 0.646 U/mg and 181 μM, respectively. The optimal pH and temperature for synergistic catalysis were 7.5 and 25 °C, and the optimal concentration of substrates were assayed, respectively. The highest titer of isorhamnetin-3-O-rhamnoside production reached 231 mg/L with a corresponding molar conversion of 100%. Isorhamnetin-3-O-rhamnoside was purified and the cytotoxicity against HepG2, MCF-7, and A549 cells were evaluated. Therefore, an efficient method for isorhamnetin-3-O-rhamnoside production described herein could be widely used for the rhamnosylation of flavonoids.

Crystal structures of rhamnosyltransferase UGT89C1 from Arabidopsis thaliana reveal the molecular basis of sugar donor specificity for UDP-β-l-rhamnose and rhamnosylation mechanism

Glycosylation is a key modification for most molecules including plant natural products, for example, flavonoids and isoflavonoids, and can enhance the bioactivity and bioavailability of the natural products. The crystal structure of plant rhamnosyltransferase UGT89C1 from Arabidopsis thaliana was determined, and the structures of UGT89C1 in complexes with UDP-β-l-rhamnose and acceptor quercetin revealed the detailed interactions between the enzyme and its substrates. Structural and mutational analysis indicated that Asp356, His357, Pro147 and Ile148 are key residues for sugar donor recognition and specificity for UDP-β-l-rhamnose. The mutant H357Q exhibited activity with both UDP-β-l-rhamnose and UDP-glucose. Structural comparison and mutagenesis confirmed that His21 is a key residue as the catalytic base and the only catalytic residue involved in catalysis independently as UGT89C1 lacks the other catalytic Asp that is highly conserved in other reported UGTs and forms a hydrogen bond with the catalytic base His. Ser124 is located in the corresponding position of the catalytic Asp in other UGTs and is not able to form a hydrogen bond with His21. Mutagenesis further showed that Ser124 may not be important in its catalysis, suggesting that His21 and acceptor may form an acceptor-His dyad and UGT89C1 utilizes a catalytic dyad in catalysis instead of catalytic triad. The information of structure and mutagenesis provides structural insights into rhamnosyltransferase substrate specificity and rhamnosylation mechanism.

Molecular dynamics provides insight into how N251A and N251Y mutations in the active site of Bacillus licheniformis RN-01 levansucrase disrupt production of long-chain levan

Produced by levansucrase, levan and levan oligosaccharides (GF_n) have potential applications in food and pharmaceutical industries such as prebiotics, anti-tumor and anti-inflammatory agents. Previous study reported that Bacillus licheniformis RN-01 levansucrase could produce levan oligosaccharides and long-chain levan. However, its N251A and N251Y mutants could effectively produce short-chain oligosaccharides upto GF_3, but they could not produce long-chain levan. We hypothesized that these mutations probably reduced GF₃ binding affinity in levansucrase active site that contains fructosyl-Asp93 intermediate and caused GF₃ to be in an unfavorable orientation for transfructosylation; therefore, levansucrase could not effectively extend GF₃ by one fructosyl residue to produce GF₄ and subsequently long-chain levan. However, these mutations probably did not significantly reduce binding affinity or drastically change orientation of GF₂; therefore, levansucrase could still extend GF₂ to produce GF₃. Using this hypothesis, we employed molecular dynamics to investigate effects of these mutations on GF₂/GF₃ binding in levansucrase active site. Our results reasonably support this hypothesis as N251A and N251Y mutations did not significantly reduce GF₂ binding affinity, as calculated by MM-GBSA technique and hydrogen bond occupations, or drastically change orientation of GF₂ in levansucrase active site, as measured by distance between atoms necessary for transfructosylation. However, these mutations drastically decreased GF₃ binding affinity and caused GF₃ to be in an unfavorable orientation for transfructosylation. Furthermore, the free energy decomposition and hydrogen bond occupation results suggest the importance of Arg255 in GF₂/GF₃ binding in levansucrase active site. This study provides important and novel insight into the effects of N251A and N251Y mutations on GF₂/GF₃ binding in levansucrase active site and how they may disrupt production of long-chain levan. This knowledge could be beneficial in designing levansucrase to efficiently produce levan oligosaccharides with desired length.

Enzymatic Synthesis and Structural Characterization of Theanderose through Transfructosylation Reaction Catalyzed by Levansucrase from Bacillus subtilis CECT 39

This work addresses the high-yield and fast enzymatic production of theanderose, a naturally occurring carbohydrate, also known as isomaltosucrose, whose chemical structure determined by NMR is α-d-glucopyranosyl-(1 → 6)-α-d-glucopyranosyl-(1 → 2)-β-d-fructofuranose. The ability of isomaltose to act as an acceptor in the Bacillus subtilis CECT 39 levansucrase-catalyzed transfructosylation reaction to efficiently produce theanderose in the presence of sucrose as a donor is described by using four different sucrose:isomaltose concentration ratios. The maximum theanderose concentration ranged from 122.4 to 130.4 g L^-1, was obtained after only 1 h and at a moderate temperature (37 °C), leading to high productivity (109.7-130.4 g L^-1h^-1) and yield (up to 37.3%) values. The enzymatic synthesis was highly regiospecific, since no other detectable acceptor reaction products were formed. The development of efficient and cost-effective procedures for the biosynthesis of unexplored but appealing oligosaccharides as potential sweeteners, such as theanderose, could help to expand its potential applications which are currently limited by their low availability.

Improving the Catalytic Behavior of DFA I-Forming Inulin Fructotransferase from Streptomyces davawensis with Site-Directed Mutagenesis

Previously, a α-d-fructofuranose-β-d-fructofuranose 1,2':2,1'-dianhydride (DFA I)-forming inulin fructotransferase (IFTase), namely, SdIFTase, was identified. The enzyme does not show high performances. In this work, to improve catalytic behavior including activity and thermostability, the enzyme was modified using site-directed mutagenesis on the basis of structure. The mutated residues were divided into three groups. Those in group I are located at central tunnel including G236, A257, G281, T313, and A314S. The group II contains residues at the inner edge of substrate binding pocket including I80, while group III at the outer edge includes G121 and T122. The thermostability was reflected by the melting temperature (T_m) determined by Nano DSC. Finally, the T_m values of G236S/G281S/A257S/T313S/A314S in group I and G121A/T122L in group III were enhanced by 3.2 and 4.5 °C, and the relative activities were enhanced to 140.5% and 148.7%, respectively. The method in this work may be applicable to other DFA I-forming IFTases.

Enzymatic Production of Melibiose from Raffinose by the Levansucrase from Leuconostoc mesenteroides B-512 FMC

Melibiose, which is an important reducing disaccharide formed by α-1,6 linkage between galactose and glucose, has been proven to have beneficial applications in both medicine and agriculture. In this study, a characterized levansucrase from Leuconostoc mesenteroides B-512 FMC was further used to study the bioproduction of melibiose from raffinose. The reaction conditions were optimized for melibiose synthesis. The optimal pH, temperature, substrate concentration, ratio of substrates, and units of enzymes were determined as pH 6.0, 45 °C, 210 g/L, 1:1 (210 g/L:210 g/L), and 5 U/mL, respectively. The transfructosylation product of raffinose was determined to be melibiose by FTIR and NMR. A high raffinose concentration was found to strongly favor the production of melibiose. When 210 g/L raffinose and 210 g/L lactose were catalyzed using 5 U/mL purified levansucrase at pH 6.0 and 45 °C, the maximal yield of melibiose was 88 g/L.

Expression and Characterization of Levansucrase from Clostridium acetobutylicum

The Clostridium acetobutylicum gene Ca-SacB encoding levansucrase was cloned and expressed in Escherichia coli. Ca-SacB is composed of 1287 bp and encodes 428 amino acid residues, which could convert 150 mmol/L sucrose to levan with the liberation of glucose. The optimum pH and temperature of this enzyme for levan formation were pH 6 and 60 °C, respectively. Levansucrase activity of Ca-SacB was completely abolished by 5 mmol/L Ag⁺ and Hg²⁺. The K_m and V_max values for levansucrase were calculated to be 64 mmol/L and 190 μmol/min/mg, respectively. Interestingly, Ca-SacB was found to have high product specificity, and no fructooligosaccharide was identified in the product, indicating that Ca-SacB may be valuable for industrial production of levan. In addition, Ca-SacB is the first characterized levansucrase isolated from an anaerobic bacterium, which should be valuable for exploring new enzyme resources and deepening the understanding of the catalytic mechanisms of levansucrases.

Synthesis of raffinose by transfructosylation using recombinant levansucrase from Clostridium arbusti SL206

BACKGROUND

Raffinose, a functional trisaccharide of α-d-galactopyranosyl-(1 → 6)-α-d-glucopyranosyl-(1 → 2)-β-d-fructofuranoside, is a prebiotic that shows promise for use as a food ingredient.

RESULTS

In this study, the production of raffinose from melibiose and sucrose was studied using whole recombinant Escherichia coli cells harboring the levansucrase from Clostridium arbusti SL206. The reaction conditions were optimized for raffinose synthesis. The optimal pH, temperature and washed cell concentration were pH 6.5 (sodium phosphate buffer, 50 mmol L^-1 ), 55 °C and 3% (w/v), respectively. High substrate concentrations, which led to low water activity and thus reduced levansucrase hydrolysis activity, strongly favored the production of raffinose through the fructosyl transfer reaction. Additionally, high concentrations of excess acceptor and donor glycosides favored raffinose production. When 30% (w/v) sucrose and 30% (w/v) melibiose were catalyzed using 3% (w/v) whole cells at pH 6.5 (sodium phosphate buffer, 50 mmol L^-1 ) and 55 °C, the highest raffinose yield was 222 g L^-1 after a 6 h reaction. The conversion ratio from each substrate to raffinose was 50%.

CONCLUSION

Raffinose could be effectively produced with melibiose as an acceptor and with sucrose as a fructosyl donor by whole recombinant E. coli cells harboring C. arbusti levansucrase. The yield from E. coli was significantly higher than those of the previously reported Bacillus subtilis levansucrase and fungal α-galactosidases. © 2016 Society of Chemical Industry.

Probing the Role of Two Critical Residues in Inulin Fructotransferase (DFA III-Producing) Thermostability from Arthrobacter sp. 161MFSha2.1

Inulin fructotransferase (IFTase) is an important enzyme that produces di-d-fructofuranose 1,2':2,3' dianhydride (DAF III), which is beneficial for human health. Present investigations mainly focus on screening and characterizing IFTase, including catalytic efficiency and thermostability, which are two important factors for enzymatic industrial applications. However, few reports aimed to improve these two characteristics based on the structure of IFTase. In this work, a structural model of IFTase (DFA III-producing) from Arthrobacter sp. 161MFSha2.1 was constructed through homology modeling. Analysis of this model reveals that two residues, Ser-309 and Ser-333, may play key roles in the structural stability. Therefore, the functions of the two residues were probed by site-directed mutagenesis combined with the Nano-DSC method and assays for residual activity. In contrast to other mutations, single mutation of serine 309 (or serine 333) to threonine did not decrease the enzymatic stability, whereas double mutation (serine 309 and serine 333 to threonine) can enhance thermostability (by approximately 5 °C). The probable mechanisms for this enhancement were investigated.

Tofu Whey Permeate Is an Efficient Source To Enzymatically Produce Prebiotic Fructooligosaccharides and Novel Fructosylated α-Galactosides

This work addresses a novel and efficient bioconversion method for the utilization of tofu whey permeate (TWP), an important byproduct from the soybean industry, as a precursor of high value-added ingredients such as prebiotic fructooligosaccharides and novel fructosylated α-galactosides. This process is based on the high capacity of the commercial enzyme preparation Pectinex Ultra SP-L to transfructosylate the main carbohydrates present in TWP as sucrose, raffinose, and stachyose to produce up to a maximum of 164.2 g L(-1) (equivalent to 57% with respect to initial sucrose, raffinose, and stachyose contents in TWP) of fructooligosaccharides and fructosylated α-galactosides in a balanced proportion. Raffinose- and stachyose-derived oligosaccharides were formed by elongation from the nonreducing terminal fructose residue up to three fructosyl groups bound by β-(2→1) linkages. These results could provide new findings on the valorization and upgrading of the management of TWP and an alternative use of raw material for the production of FOS and derivatives.

A novel N-terminal region of the membrane β-hexosyltransferase: its role in secretion of soluble protein by Pichia pastoris

The β-hexosyltransferase (BHT) from Sporobolomyces singularis is a membrane-bound enzyme that catalyses transgalactosylation reactions to synthesize galacto-oligosaccharides (GOSs). To increase the secretion of the active soluble version of this protein, we examined the uncharacterized novel N-terminal region (amino acids 1-110), which included two predicted endogenous structural domains. The first domain (amino acids 1-22) may act as a classical leader while a non-classical signal was located within the remaining region (amino acids 23-110). A functional analysis of these domains was performed by evaluating the amounts of the rBHT forms secreted by recombinant P. pastoris strains carrying combinations of the predicted structural domains and the α mating factor (MFα) from Saccharomyces cerevisiae as positive control. Upon replacement of the leader domain (amino acids 1-22) by MFα (MFα-rBht(23-594)), protein secretion increased and activity of both soluble and membrane-bound enzymes was improved 53- and 14-fold, respectively. Leader interference was demonstrated when MFα preceded the putative classical rBHT(1-22) leader (amino acids 1-22), explaining the limited secretion of soluble protein by P. pastoris (GS115 : : MFα-rBht(1-594)). To validate the role of the N-terminal domains in promoting protein secretion, we tested the domains using a non-secreted protein, the anti-β-galactosidase single-chain variable antibody fragment scFv13R4. The recombinants carrying chimeras of the N-terminal 1-110 regions of rBHT preceding scFv13R4 correlated with the secretion strength of soluble protein observed with the rBHT recombinants. Finally, soluble bioactive HIS-tagged and non-tagged rBHT (purified to homogeneity) obtained from the most efficient recombinants (GS115 : : MFα-rBht(23-594)-HIS and GS115 : : MFα-rBht(23-594)) showed comparable activity rates of GOS generation.

Efficient Biosynthesis of Lactosucrose from Sucrose and Lactose by the Purified Recombinant Levansucrase from Leuconostoc mesenteroides B-512 FMC

Lactosucrose, a rare trisaccharide formed from sucrose and lactose by enzymatic transglycosylation, is a type of indigestible carbohydrate with a good prebiotic effect. In this study, lactosucrose biosynthesis was efficiently carried out by a purified levansucrase from Leuconostoc mesenteroides B-512. The target gene was cloned and expressed in Escherichia coli, and the recombinant enzyme was purified to homogeneity by nickel affinity and gel filtration chromatography. The effects of pH, temperature, substrate concentration, substrate ratio, and enzyme amount on lactosucrose biosynthesis were studied in detail, and the optimized conditions were determined to be pH 6.5, 50 °C, 27% (W/V) sucrose, 27% (W/V) lactose, and 5 U mL(-1) of the purified recombinant enzyme. Under the optimized reaction conditions, the maximal lactosucrose yield reached 224 g L(-1) after reaction for 1 h. Therefore, L. mesenteroides levansucrase could be considered a potential candidate for future industrial production of lactosucrose.

[The synthesis of P1-[11-(anthracen-9-ylmethoxy)undecyl]-P2(2-Acetamido-2-deoxy-α-D-glucopyranosyl) diphosphate and the study of its acceptor properties in the enzymic reaction catalyzed by D-rhamnosyltransferase from Pseudomonas aeruginosa]

P1-[11-(Anthracen-9-ylmethoxy)undecyl]-P2-(2-acetamido-2-deoxy-α-D-glucopyranosyl) diphosphate, a fluorescent derivative of undecyl diphosphate 2-acetamido-2-deoxyglucose, was chemically synthesized. The ability of the compound to serve as acceptor substrate of D-rhamnose residue in the enzymatic reaction catalyzed by D-rhamnosyltransferase from Pseudomonas aeruginosa PAO1 was demonstrated.

Identification of a recombinant inulin fructotransferase (difructose dianhydride III forming) from Arthrobacter sp. 161MFSha2.1 with high specific activity and remarkable thermostability

Difructose dianhydride III (DFA III) is a functional carbohydrate produced from inulin by inulin fructotransferase (IFTase, EC 4.2.2.18). In this work, an IFTase gene from Arthrobacter sp. 161MFSha2.1 was cloned and expressed in Escherachia coli. The recombinant enzyme was purified by metal affinity chromatography. It showed significant inulin hydrolysis activity, and the produced main product from inulin was determined as DFA III by nuclear magnetic resonance analysis. The molecular mass of the purified protein was calculated to be 43 and 125 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration, respectively, suggesting the native enzyme might be a homotrimer. The recombinant enzyme showed maximal activity as 2391 units/mg at pH 6.5 and 55 °C. It displayed the highest thermostability among previously reported IFTases (DFA III forming) and was stable up to 80 °C for 4 h of incubation. The smallest substrate was determined as nystose. The conversion ratio of inulin to DFA III reached 81% when 100 g/L inulin was catalyzed by 80 nM recombinant enzyme for 20 min at pH 6.5 and 55 °C. All of these data indicated that the IFTase (DFA III forming) from Arthrobacter sp. 161MFSha2.1 had great potential for industrial DFA III production.

[Application of levansucrase in levan synthesis--a review]

Levan is a fructan mainly linked by beta-(2,6)-glycosidic bonds with some beta-(2,1)-linked branch chains. Some microbial levan exhibit biological activities such as antitumor, antidiabetic and immunostimulating activities. hypolipidemic effect, and function as prebiotics, which has a wide and potential application in the pharmaceutical and food industry. Because of low extraction yields from microbial fermentation and a very complex process for chemical synthesis of levan, enzymatic synthesis of levan has attracted tremendous interest. Levansucrase (EC 2. 4. 1. 10), a beta-propeller protein belonging to the glycoside hydrolase family 68 (GH68) with reaction mechanism of non-Leloir glycosyltransferase, catalyzes the synthesis of levan by transferring the fructosyl group of non-activated sucrose into the fructan chain. The molecular structure and regulation of gene expression of some microbial levansucrases have been elucidated. Meanwhile, the enzymatic synthesis of levan by levansucrase is widely studied. In this review, catalytic mechanism of levansucrase, molecular structure and regulation of gene expression of some microbial levansucrases, and the application of levansucrases in enzymatic synthesis of levan were summarized.

Extending synthetic routes for oligosaccharides by enzyme, substrate and reaction engineering

The integration of all relevant tools for bioreaction engineering has been a recent challenge. This approach should notably favor the production of oligo- and polysaccharides, which is highly complex due to the requirements of regio- and stereoselectivity. Oligosaccharides (OS) and polysaccharides (PS) have found many interests in the fields of food, pharmaceuticals, and cosmetics due to different specific properties. Food, sweeteners, and food ingredients represent important sectors where OS are used in major amounts. Increasing attention has been devoted to the sophisticated roles of OS and glycosylated compounds, at cell or membrane surfaces, and their function, e.g., in infection and cancer proliferation. The challenge for synthesis is obvious, and convenient approaches using cheap and readily available substrates and enzymes will be discussed. We report on new routes for the synthesis of oligosaccharides (OS), with emphasis on enzymatic reactions, since they offer unique properties, proceeding highly regio- and stereoselective in water solution, and providing for high yields in general.

Transcript profiling of fructan biosynthetic pathway genes reveals association of a specific fructosyltransferase isoform with the high sugar trait in Lolium perenne

Lolium perenne cultivars with elevated levels of fructans in leaf blades (high sugar-content grasses) have been developed to improve animal nutrition and reduce adverse environmental impacts of pastoral agricultural systems. Expression of the high sugar trait can vary substantially depending on genotype×environment (G×E) interactions. We grew three potential high sugar-content and a control cultivar in three temperature regimes and quantified water soluble carbohydrates (WSCs) and the expression of all functionally characterised L. perenne fructan pathway genes in leaf tissues. We also analysed the distribution, expression and sequence variation of two specific isoforms of Lp6G-FFT (fructan: fructan 6G-fructosyltransferase). Our study confirmed a significant G×E interaction affecting the accumulation of fructans in the high sugar-content cultivar AberDart, which accumulated higher levels of high DP (degree of polymerisation) fructans in blades compared to the control cultivar only when grown at 20°C (day)/10°C (night) temperatures. The cultivar Expo on the other hand accumulated significantly higher levels of high DP fructans in blades independent of temperature. Fructan levels in pseudostems were higher than in blades, and they increased markedly with decreasing temperature, but there was no consistent effect of cultivar in this tissue. The expression of the high sugar trait was generally positively correlated with transcript levels of fructosyltransferases. Presence and expression of only one of the two known 6G-FFT isoforms was positively correlated with high fructan biosynthesis, while the second isoform was associated with low fructan concentrations and positively correlated with fructan exohydrolase gene expression. The presence of distinct 6G-FFT sequence variants appears to be associated with the capacity of high sugar-content grasses to accumulate higher fructan levels particularly at warmer temperatures. These findings might be exploited for the selection and breeding of 'warm-effective' high sugar-content grasses to overcome some of the limitations of current high sugar-content ryegrass cultivars.

Linkage mapping, molecular cloning and functional analysis of soybean gene Fg2 encoding flavonol 3-O-glucoside (1 → 6) rhamnosyltransferase

There are substantial genotypic differences in the levels of flavonol glycosides (FGs) in soybean leaves. The first objective of this study was to identify and locate genes responsible for FG biosynthesis in the soybean genome. The second objective was to clone and verify the function of these candidate genes. Recombinant inbred lines (RILs) were developed by crossing the Kitakomachi and Koganejiro cultivars. The FGs were separated by high performance liquid chromatography (HPLC) and identified. The FGs of Koganejiro had rhamnose at the 6″-position of the glucose or galactose bound to the 3-position of kaempferol, whereas FGs of Kitakomachi were devoid of rhamnose. Among the 94 RILs, 53 RILs had HPLC peaks classified as Koganejiro type, and 41 RILs had peaks classified as Kitakomachi type. The segregation fitted a 1:1 ratio, suggesting that a single gene controls FG composition. SSR analysis, linkage mapping and genome database survey revealed a candidate gene in the molecular linkage group O (chromosome 10). The coding region of the gene from Koganejiro, designated as GmF3G6″Rt-a, is 1,392 bp long and encodes 464 amino acids, whereas the gene of Kitakomachi, GmF3G6″Rt-b, has a two-base deletion resulting in a truncated polypeptide consisting of 314 amino acids. The recombinant GmF3G6″Rt-a protein converted kaempferol 3-O-glucoside to kaempferol 3-O-rutinoside and utilized 3-O-glucosylated/galactosylated flavonols and UDP-rhamnose as substrates. GmF3G6″Rt-b protein had no activity. These results indicate that GmF3G6″Rt encodes a flavonol 3-O-glucoside (1 → 6) rhamnosyltransferase and it probably corresponds to the Fg2 gene. GmF3G6″Rt was designated as UGT79A6 by the UGT Nomenclature Committee.

[Expression analyses of BcUGT3 and BcUGT6, and their in vitro expression in Escherichia coli]

The tissue-specific and MeJA-induced transcriptional levels of BcUGT3 and BcUGT6 in Bupleurum chinense were analyzed in the present study. The transcriptional levels of BcUGT3 in root, leaf, flower and fruit were similar and they all were higher than those in stem. The transcriptional level of BcUGT6 was the highest in leaf and the lowest in flower among in all tested tissues. With non-treated adventitious roots as control, BcUGT6's transcriptional levels were elevated to nearly 2 folds for 2 h, 8 h, 24 h, 2 d and 4 d in MeJA-treated adventitious roots of B. chinense. It showed that the transcriptional level of BcUGT6 was slightly affected by MeJA. While, BcUGT3's transcriptional levels were gradually elevated, and till 4 d after MeJA treatment, the expression level was about 7 folds than that of non-treated control. Using pET-28a (+), the expressions of two genes was investigated. Induced by IPTG, the target proteins were expressed in E. coli and then purified. All the results obtained in the present study will be helpful for follow-up bio-function analysis of BcUGT3 and BcUGT6.

Biomolecular characterization of the levansucrase of Erwinia amylovora, a promising biocatalyst for the synthesis of fructooligosaccharides

Erwinia amylovora is a plant pathogen that affects Rosaceae, such as apple and pear. In E. amylovora the fructans, produced by the action of a levansucrase (EaLsc), play a role in virulence and biofilm formation. Fructans are bioactive compounds, displaying health-promoting properties in their own right. Their use as food and feed supplements is increasing. In this study, we investigated the biomolecular properties of EaLsc using HPAEC-PAD, MALDI-TOF MS, and spectrophotometric assays. The enzyme, which was heterologously expressed in Escherichia coli in high yield, was shown to produce mainly fructooligosaccharides (FOSs) with a degree of polymerization between 3 and 6. The kinetic properties of EaLsc were similar to those of other phylogenetically related Gram-negative bacteria, but the good yield of FOSs, the product spectrum, and the straightforward production of the enzyme suggest that EaLsc is an interesting biocatalyst for future studies aimed at producing tailor-made fructans.

Tailor-made fructan synthesis in plants: a review

Fructan, a fructose polymer, is produced by many bacteria and plants. Fructan is used as carbohydrate reserve, and in bacteria also as protective outside layer. Chicory is a commercial fructan producing crop. The disadvantage of this crop is its fructan breakdown before harvest. Studies using genetically modification showed that fructan biosynthesis is difficult to steer in chicory. Alternatives for production of tailor-made fructan, fructan with a desired polymer length and linkage type, are originally non-fructan-accumulating plants expressing introduced fructosyltransferase genes. The usage of bacterial fructosyltransferases hindered plant performance, whereas plant-derived fructan genes can successfully be used for this purpose. The polymer length distribution and the yield are dependent on the origin of the fructan genes and the availability of sucrose in the host. Limitations seen in chicory for the production of tailor-made fructan are lacking in putative new platform crops like sugar beet and sugarcane and rice.

Glutaraldehyde-mediated protein immobilization

In this chapter, we describe different approaches for the utilization of glutaraldehyde in protein immobilization. First, we focus on the covalent attachment of proteins to glutaraldehyde-activated matrixes. We describe conditions for the synthesis of such supports and provide an example of the immobilization and stabilization of fructosyltransferase. We also describe how glutaraldehyde may be used for the cross-linking of protein-protein aggregates and protein adsorbed onto amino-activated matrixes. In these cases, glutaraldehyde bridges either two lysine groups from different proteic molecules or a lysine from the protein structure and an amine group from the support. Examples of cross-linking are given for the immobilization of DAAO on different amino-activated supports.

Engineering, conjugation, and immunogenicity assessment of Escherichia coli O121 O antigen for its potential use as a typhoid vaccine component

State-of-the-art production technologies for conjugate vaccines are complex, multi-step processes. An alternative approach to produce glycoconjugates is based on the bacterial N-linked protein glycosylation system first described in Campylobacter jejuni. The C. jejuni N-glycosylation system has been successfully transferred into Escherichia coli, enabling in vivo production of customized recombinant glycoproteins. However, some antigenic bacterial cell surface polysaccharides, like the Vi antigen of Salmonella enterica serovar Typhi, have not been reported to be accessible to the bacterial oligosaccharyltransferase PglB, hence hamper development of novel conjugate vaccines against typhoid fever. In this report, Vi-like polysaccharide structures that can be transferred by PglB were evaluated as typhoid vaccine components. A polysaccharide fulfilling these requirements was found in Escherichia coli serovar O121. Inactivation of the E. coli O121 O antigen cluster encoded gene wbqG resulted in expression of O polysaccharides reactive with antibodies raised against the Vi antigen. The structure of the recombinantly expressed mutant O polysaccharide was elucidated using a novel HPLC and mass spectrometry based method for purified undecaprenyl pyrophosphate (Und-PP) linked glycans, and the presence of epitopes also found in the Vi antigen was confirmed. The mutant O antigen structure was transferred to acceptor proteins using the bacterial N-glycosylation system, and immunogenicity of the resulting conjugates was evaluated in mice. The conjugate-induced antibodies reacted in an enzyme-linked immunosorbent assay with E. coli O121 LPS. One animal developed a significant rise in serum immunoglobulin anti-Vi titer upon immunization.

Glycocodon theory--the first table of glycocodons

Hydrophobic cellular membranes separate cells from an environment that is generally based on water. Therefore, it is not surprising that hydrophilic glycans and glycoproteins are exposed on the lipidic surface of membranes and that the glycocalyx has evolved in all basic cell types. During the evolution of multicellular life, the surface exposed protein-glycan interactions were taken as the origin of the language of cell-cell communication. The bioinformatics analysis presented here reveals that the amino acid triplets, the glycocodons, can be deduced for each glycan letter (monosaccharide). This theory proposes to distinguish between the "sugar code" (the sugar sequence) and the "glycocode" (evolutionary selected amino acids recognising the mono-sugar). Similarly to genetic code, original glycocodons are related to G, A, V, and D amino acids. Modern glycocodons can be deduced from GAVD-glycocodons using hydropathic similarity. In general, the amino acid triplets can be assembled from one dipeptide that is specific to a monosaccharide plus a polar amino acid. This theory may shed a different light on the reason for WWD conservation in the active sites of oligosaccharyltransferases and for GGQ in the active sites of ribosomes.

Crystal structure of 6-SST/6-SFT from Pachysandra terminalis, a plant fructan biosynthesizing enzyme in complex with its acceptor substrate 6-kestose

Fructans play important roles as reserve carbohydrates and stress protectants in plants, and additionally serve as prebiotics with emerging antioxidant properties. Various fructan types are synthesized by an array of plant fructosyltransferases belonging to family 32 of the glycoside hydrolases (GH32), clustering together with GH68 in Clan-J. Here, the 3D structure of a plant fructosyltransferase from a native source, the Pachysandra terminalis 6-SST/6-SFT (Pt6-SST/6-SFT), is reported. In addition to its 1-SST (1-kestose-forming) and hydrolytic side activities, the enzyme uses sucrose to create graminan- and levan-type fructans, which are probably associated with cold tolerance in this species. Furthermore, a Pt6-SST/6-SFT complex with 6-kestose was generated, representing a genuine acceptor binding modus at the +1, +2 and +3 subsites in the active site. The enzyme shows a unique configuration in the vicinity of its active site, including a unique D/Q couple located at the +1 subsite that plays a dual role in donor and acceptor substrate binding. Furthermore, it shows a unique orientation of some hydrophobic residues, probably contributing to its specific functionality. A model is presented showing formation of a β(2-6) fructosyl linkage on 6-kestose to create 6,6-nystose, a mechanism that differs from the creation of a β(2-1) fructosyl linkage on sucrose to produce 1-kestose. The structures shed light on the evolution of plant fructosyltransferases from their vacuolar invertase ancestors, and contribute to further understanding of the complex structure-function relationships within plant GH32 members.

Design of chimeric levansucrases with improved transglycosylation activity

Fructansucrases (FSs), including levansucrases and inulosucrases, are enzymes that synthesize fructose polymers from sucrose by the direct transfer of the fructosyl moiety to a growing polymer chain. These enzymes, particularly the single domain fructansucrases, also possess an important hydrolytic activity, which may account for as much as 70 to 80% of substrate conversion, depending on reaction conditions. Here, we report the construction of four chimeric levansucrases from SacB, a single domain levansucrase produced by Bacillus subtilis. Based on observations derived from the effect of domain deletion in both multidomain fructansucrases and glucansucrases, we attached different extensions to SacB. These extensions included the transitional domain and complete C-terminal domain of Leuconostoc citreum inulosucrase (IslA), Leuconostoc mesenteroides levansucrase (LevC), and a L. mesenteroides glucansucrase (DsrP). It was found that in some cases the hydrolytic activity was reduced to less than 10% of substrate conversion; however, all of the constructs were as stable as SacB. This shift in enzyme specificity was observed even when the SacB catalytic domain was extended only with the transitional region found in multidomain FSs. Specific kinetic analysis revealed that this change in specificity of the SacB chimeric constructs was derived from a 5-fold increase in the transfructosylation k(cat) and not from a reduction of the hydrolytic k(cat), which remained constant.

A novel and simple method of production and biophysical characterization of a mini-membrane protein, Ost4p: a subunit of yeast oligosaccharyl transferase

Asparagine-linked glycosylation is an essential and highly conserved protein modification reaction. In eukaryotes, oligosaccharyl transferase (OT), a multi-subunit membrane-associated enzyme complex, catalyzes this reaction in newly synthesized proteins. In Saccharomyces cerevisiae, OT consists of nine nonidentical membrane proteins. Ost4p, the smallest subunit, bridges the catalytic subunit Stt3p with Ost3p. Mutation of transmembrane residues 18-24 in Ost4p has negative effect on OT activity, disrupts the Stt3p-Ost4p-Ost3p complex, results in temperature-sensitive phenotype, and hypoglycosylation. Heterologous expression and purification of integral membrane proteins are the bottleneck in membrane protein research. The authors report the cloning, successful overexpression and purification of recombinant Ost4p with a novel but simple method producing milligram quantities of pure protein. GB1 protein was found to be the most suitable tag for the large scale production of Ost4p. The cleavage of Ost4p conveniently leaves GB1 protein in solution eliminating further purification. The precipitated pure Ost4p is reconstituted in appropriate membrane mimetic. The recombinant protein is highly helical as indicated by the far-UV CD spectrum. The well-dispersed heteronuclear single quantum coherence spectrum indicates that this minimembrane protein is well-folded. The successful production of pure recombinant Ost4p with a novel yet simple method may have important ramification for the production of other membrane proteins.

Polysaccharide synthesis of the levansucrase SacB from Bacillus megaterium is controlled by distinct surface motifs

Despite the widespread biological function of carbohydrates, the polysaccharide synthesis mechanisms of glycosyltransferases remain largely unexplored. Bacterial levansucrases (glycoside hydrolase family 68) synthesize high molecular weight, β-(2,6)-linked levan from sucrose by transfer of fructosyl units. The kinetic and biochemical characterization of Bacillus megaterium levansucrase SacB variants Y247A, Y247W, N252A, D257A, and K373A reveal novel surface motifs remote from the sucrose binding site with distinct influence on the polysaccharide product spectrum. The wild type activity (k(cat)) and substrate affinity (K(m)) are maintained. The structures of the SacB variants reveal clearly distinguishable subsites for polysaccharide synthesis as well as an intact active site architecture. These results lead to a new understanding of polysaccharide synthesis mechanisms. The identified surface motifs are discussed in the context of related glycosyltransferases.

Inhibition of Streptococcus mutans biofilm formation by Streptococcus salivarius FruA

The oral microbial flora consists of many beneficial species of bacteria that are associated with a healthy condition and control the progression of oral disease. Cooperative interactions between oral streptococci and the pathogens play important roles in the development of dental biofilms in the oral cavity. To determine the roles of oral streptococci in multispecies biofilm development and the effects of the streptococci in biofilm formation, the active substances inhibiting Streptococcus mutans biofilm formation were purified from Streptococcus salivarius ATCC 9759 and HT9R culture supernatants using ion exchange and gel filtration chromatography. Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry analysis was performed, and the results were compared to databases. The S. salivarius HT9R genome sequence was determined and used to indentify candidate proteins for inhibition. The candidates inhibiting biofilms were identified as S. salivarius fructosyltransferase (FTF) and exo-beta-d-fructosidase (FruA). The activity of the inhibitors was elevated in the presence of sucrose, and the inhibitory effects were dependent on the sucrose concentration in the biofilm formation assay medium. Purified and commercial FruA from Aspergillus niger (31.6% identity and 59.6% similarity to the amino acid sequence of FruA from S. salivarius HT9R) completely inhibited S. mutans GS-5 biofilm formation on saliva-coated polystyrene and hydroxyapatite surfaces. Inhibition was induced by decreasing polysaccharide production, which is dependent on sucrose digestion rather than fructan digestion. The data indicate that S. salivarius produces large quantities of FruA and that FruA alone may play an important role in multispecies microbial interactions for sucrose-dependent biofilm formation in the oral cavity.

Purification and characterization of inulin fructotransferase (DFA III-forming) from Arthrobacter aurescens SK 8.001

The soil bacterium Arthrobacter aurescens SK 8.001 produces inulin fructotransferase (IFTase), and liquid chromatography-mass spectrometry (LC-MS) and carbon-13 nuclear magnetic resonance (13C NMR) analysis demonstrated that the main product of the enzyme was difructose anhydride III (DFA III). The IFTase was purified by ethanol precipitation, DEAE Sepharose Fast Flow, and Superdex 200 10/300 GL gel chromatography. Its molecular mass was estimated to be 40 kDa by SDS-PAGE and 35 kDa by gel filtration. The enzyme showed maximum activity at pH 5.5 and 60-70 °C, and retained 86.5% of its initial activity after incubation at 60 °C for 4 h. Chemical modification results suggested that a tryptophan residue is essential to enzyme activity. The N-terminal amino acid sequence was determined as AEGAKASPLNSPNVYDVT. The kinetic values, Km and Vmax, were estimated to be 0.52 mM and 0.3 μmol/ml min. Nystose was observed to be the smallest substrate for the produced IFTase. This IFTase provides a promising way to utilize inulin for the production of DFA III.

Unexpected presence of graminan- and levan-type fructans in the evergreen frost-hardy eudicot Pachysandra terminalis (Buxaceae): purification, cloning, and functional analysis of a 6-SST/6-SFT enzyme

About 15% of flowering plants accumulate fructans. Inulin-type fructans with β(2,1) fructosyl linkages typically accumulate in the core eudicot families (e.g. Asteraceae), while levan-type fructans with β(2,6) linkages and branched, graminan-type fructans with mixed linkages predominate in monocot families. Here, we describe the unexpected finding that graminan- and levan-type fructans, as typically occurring in wheat (Triticum aestivum) and barley (Hordeum vulgare), also accumulate in Pachysandra terminalis, an evergreen, frost-hardy basal eudicot species. Part of the complex graminan- and levan-type fructans as accumulating in vivo can be produced in vitro by a sucrose:fructan 6-fructosyltransferase (6-SFT) enzyme with inherent sucrose:sucrose 1-fructosyltransferase (1-SST) and fructan 6-exohydrolase side activities. This enzyme produces a series of cereal-like graminan- and levan-type fructans from sucrose as a single substrate. The 6-SST/6-SFT enzyme was fully purified by classic column chromatography. In-gel trypsin digestion led to reverse transcription-polymerase chain reaction-based cDNA cloning. The functionality of the 6-SST/6-SFT cDNA was demonstrated after heterologous expression in Pichia pastoris. Both the recombinant and native enzymes showed rather similar substrate specificity characteristics, including peculiar temperature-dependent inherent 1-SST and fructan 6-exohydrolase side activities. The finding that cereal-type fructans accumulate in a basal eudicot species further confirms the polyphyletic origin of fructan biosynthesis in nature. Our data suggest that the fructan syndrome in P. terminalis can be considered as a recent evolutionary event. Putative connections between abiotic stress and fructans are discussed.

[Characterization of the functional domain of STT3a of oligosaccharyltransferase from Dunaliella salina]

To investigate the function of STT3a gene in salt adaptation and flagellar regeneration of Dunaliella salina (D. salina), a pair of degenerate primers was designed according to conserved homologous amino acid sequences of VCVFTA and DVDYVL of STT3a from Chlamydomonas, Arabidopsis thaliana and other organisms. A cDNA sequence of 1 650 bp encoding a whole functional domain of STT3a was amplified from D. salina by RT-PCR and 3' Rapid Amplification of cDNA Ends (RACE), which shared homology with Chlamydomonas (48%), Arabidopsis thaliana (50%), Homo sapiens (46%), etc. Real-time fluorescence quantitative PCR (real-time Q-PCR) demonstrated that the STT3a mRNAs from D. salina were induced by increased concentration of NaCl, and increased to 11-fold higher by 3.5 mol/L NaCl than that by 1.5 mol/L NaCl (P < 0.01). Also, STT3a mRNA of D. salina maintained at a higher level in the process of flagellar regeneration with than without experiencing deflagellar treatment. In conclusion, the findings of this study demonstrate that the high expression of the STT3a gene enhances the capability of salt adaptation and flagellar regeneration in D. salina.