Single amino acid residue changes in subsite -1 of inulosucrase from Lactobacillus reuteri 121 strongly influence the size of products synthesized

Effect of loop-grafting on the activity, stability and regioselectivity of Priestia megaterium levansucrase using two inulosucrases as loop donors

Active-site loops of enzymes modulate activity, stability, regioselectivity and substrate specificity. This study examines the impact of loop transplantation on Priestia megaterium levansucrase Pm-SacB, which produces levan-type, β-(2,6)-linked fructooligosaccharides. Inulosucrases from Lactobacillus johnsonii (Lj-InuJ) and Halalkalicoccus jeotgali (Hje-Inu), which synthesize β-(2,1)-linked inulin fructans, served as donors of eight active-site loops. Most Lj-InuJ-based loop-grafting enzymes produce smaller oligosaccharides than Pm-SacB, and variants carrying a Lj-InuJ loop 3, whether alone or in combination with grafted-loops 7 and 8, exhibit inulin-type oligosaccharides 1-kestose and 1-nystose among their products. Pm-SacB does not synthesize inulin-type oligosaccharides other than 1-kestose but can elongate them via β-(2,6)-linkages. A construct containing Lj-InuJ loops 3, 7 and 8, along with the mutations S372N, K373R and H423Y (variant Lj-Loop3-7-8**), has impaired 6-kestose and levan synthesis, accumulates 14-fold more 1-kestose than Pm-SacB and outperforms other variants in the synthesis of 1-nystose. The enzyme exhibits similar thermal stability to Pm-SacB, but its catalytic efficiency is sevenfold lower. The product profile of Lj-Loop3-7-8** could not be replicated by mutating reported levan- and inulin-binding residues in Pm-SacB, suggesting that modifying neighboring residues, in addition to those involved in FOS-binding may be necessary to gain access to levansucrases with unique properties.

Exploring the role of the residues into catalytic cavity of inulosucrase from Leuconostoc citreum CW28

Inulosucrases are enzymes capable of synthesizing inulin polymers using sucrose as the main substrate. The enzymatic activity relies on the catalytic triad within the active site and residues responsible for substrate recognition and orientation, termed carbohydrate-binding subsites. This study investigates the role of specific residues within the catalytic cavity of a truncated version of IslA4 in enzymatic catalysis. Mutants at residues S425, L499, A602, R618, F619, Y676, Y692, and R696 were constructed and characterized. Characterization results, and in silico structural comparison with other fructansucrases, reveal these residues' functional significance in catalysis. Residue S425 belongs to subsite -1; residues R618 and Y692 are part of subsite +1, and residue R696 belongs to subsites +1 and +2. Residues L499 and A602 are support residues; the former favors the formation of the fructosyl-enzyme intermediate, while the latter stabilizes the acid/base catalyst during catalysis. Residues Y676 and F619 may participate in stabilizing residues at -1/+1 subsites. This study represents the first comprehensive exploration of the structural determinants essential for enzymatic function in the inulosucrase of Leuconostoc citreum, and proposes the identity of residues involved in the -1 to +2 subsites.

Production, effects, and applications of fructans with various molecular weights

Fructan, a widespread functional polysaccharide, has been used in the food, pharmaceutical, cosmetic, and material production fields because of its versatile physicochemical properties and biological activities. Inulin from plants and levan from microorganisms are two of the most extensively studied fructans. Fructans from different plants or microorganisms have inconsistent molecular weights, and the molecular weight of fructan affects its properties, functions, and applications. Recently, increasing attention has been paid to the production and application of fructans having various molecular weights, and biotechnological processes have been explored to produce tailor-made fructans from sucrose. This review encompasses the introduction of extraction, enzymatic transformation, and fermentation production processes for fructans with diverse molecular weights. Notably, it highlights the enzymes involved in fructan biosynthesis and underscores their physiological effects, with a special emphasis on their prebiotic properties. Moreover, the applications of fructans with varying molecular weights are also emphasized.

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.

Energy- and evolution-based design of inulosucrase for enhanced thermostability and inulin production

Inulosucrase from Lactobacillus reuteri 121 (LrInu) exhibits promise in the synthesis of prebiotic inulin and fructooligosaccharides. However, for its use in industry, LrInu's thermostability is a crucial consideration. In this study, the computational program FireProt was used to predict the thermostable variants of LrInu. Using rational criteria, nine variants were selected for protein expression and characterization. The G237P variant was determined to be the greatest designed candidate due to its greatly enhanced stability and activity in comparison to the wild-type enzyme. The optimum temperature of G237P increased from 50 to 60°C, with an over 5-fold increase in the half-life. Spectroscopy studies revealed that the G237P mutation could prevent the structural change in LrInu caused by heat or urea treatment. Molecular dynamics (MD) simulations showed that the enhanced thermostability of the G237P variant resulted from an increase in structural rigidity and the number of native contacts within the protein molecule. In addition, G237P variant synthesizes inulin with greater efficiency than WT. KEY POINTS: • Thermostable inulosucrase variant(s) were designed by Fireprot server. • G237P variant showed significantly improved thermostability compared to the wild type. • Inulin is synthesized more efficiently by G237P variant.

High-yield production and purification of prebiotic inulin-type fructooligosaccharides

Due to the health-promoting effects and functional properties of inulin-type fructooligosaccharides (I-FOS), the global market for I-FOS is constantly growing. Hence, there is a continuing demand for new, efficient biotechnological approaches for I-FOS production. In this work, crude inulosucrase InuGB-V3 from Lactobacillus gasseri DSM 20604 was used to synthesize I-FOS from sucrose. Supplementation with 1 mM CaCl2, a pH of 3.5–5.5, and an incubation temperature of 40 °C were found to be optimal production parameters at which crude inulosucrase showed high conversion rates, low sucrose hydrolysis, and excellent stability over 4 days. The optimal process conditions were employed in cell-free bioconversion reactions. By elevating the substrate concentration from 570 to 800 g L−1, the I-FOS concentration and the synthesis of products with a low degree of polymerization (DP) could be increased, while sucrose hydrolysis was decreased. Bioconversion of 800 g L−1 sucrose for 20 h resulted in an I-FOS-rich syrup with an I-FOS concentration of 401 ± 7 g L−1 and an I-FOS purity of 53 ± 1% [w/w]. I-FOS with a DP of 3–11 were synthesized, with 1,1-kestotetraose (DP4) being the predominant transfructosylation product. The high-calorie sugars glucose, sucrose, and fructose were removed from the generated I-FOS-rich syrup using activated charcoal. Thus, 81 ± 5% of the initially applied I-FOS were recovered with a purity of 89 ± 1%.

Characterization of a novel fructosyltransferase InuCA from Lactobacillus crispatus that attaches to the cell surface by electrostatic interaction

Fructosyltransferases (FTases), a group of carbohydrate-active enzymes, synthesize fructooligosaccharides (FOS) and fructans, which are promising prebiotics for human health. Here we originally identified a novel FTase InuCA from L. crispatus, a dominant species in the vaginal microbiotas of human. InuCA was characterized by a shortest C-terminus and the highest isoelectric point among the reported Lactobacillus FTases. InuCA was an inulosucrase and produced a serial of FOS using sucrose as substrate at a moderate temperature. Surprisingly, the C-terminal deletion mutant synthesized oligosaccharides with fructosyl chain longer than that of the wild type, suggesting that the C-terminal part blocked the binding of long-chain receptor. Moreover, InuCA bound to the cell surface by electrostatic interaction, which was dependent on the environmental pH and represented a distinctive binding mode in FTases. The catalytic and structural properties of InuCA will be contributed to the FTases engineering and the knowledge of the adaptation of L. crispatus in the vaginal environment. Importance L. crispatus is one of the most important species in human vaginal microbiotas and its persistence is strongly negatively correlated with the vaginal diseases. Our research reveals that a novel inulosucrase InuCA is present in L. cirspatus. InuCA keeps the ability to synthesize prebiotic fructo-oligosaccharides, although it lacks a large part of the C-terminal region compared to other FTases. Remarkably, the short C-terminus of InuCA blocks the transfructosylation activity for producing oligosaccharides with longer chain, which is meaningful to the directional modification of FTases and the oligosaccharide products. Besides the catalytic activity, InuCA is anchored on the cell surface dependent on the environmental pH and may be also involved in the adhesion of L. crispatus to the vaginal epithelial cells. Since L. crispatus plays an essential role in the normal vaginal micro-ecosystem, the described work will be helpful to elucidate the functional genes and colonization mechanism of the dominant species.

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

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

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.

The molecular basis of the nonprocessive elongation mechanism in levansucrases

Levansucrases (LSs) synthesize levan, a β2-6-linked fructose polymer, by successively transferring the fructosyl moiety from sucrose to a growing acceptor molecule. Elucidation of the levan polymerization mechanism is important for using LSs in the production of size-defined products for application in the food and pharmaceutical industries. For a deeper understanding of the levan synthesis reaction, we determined the crystallographic structure of Bacillus subtilis LS (SacB) in complex with a levan-type fructooligosaccharide and utilized site-directed mutagenesis to identify residues involved in substrate binding. The presence of a levanhexaose molecule in the central catalytic cavity allowed us to identify five substrate-binding subsites (−1, +1, +2, +3, and +4). Mutants affecting residues belonging to the identified acceptor subsites showed similar substrate affinity (Km) values to the wildtype (WT) Km value but had a lower turnover number and transfructosylation/hydrolysis ratio. Of importance, compared with the WT, the variants progressively yielded smaller-sized low-molecular-weight levans, as the affected subsites that were closer to the catalytic site, but without affecting their ability to synthesized high-molecular-weight levans. Furthermore, an additional oligosaccharide-binding site 20 Å away from the catalytic pocket was identified, and its potential participation in the elongation mechanism is discussed. Our results clarify, for the first time, the interaction of the enzyme with an acceptor/product oligosaccharide and elucidate the molecular basis of the nonprocessive levan elongation mechanism of LSs.

Investigating the Product Profiles and Structural Relationships of New Levansucrases with Conventional and Non-Conventional Substrates

The synthesis of complex oligosaccharides is desired for their potential as prebiotics, and their role in the pharmaceutical and food industry. Levansucrase (LS, EC 2.4.1.10), a fructosyl-transferase, can catalyze the synthesis of these compounds. LS acquires a fructosyl residue from a donor molecule and performs a non-Lenoir transfer to an acceptor molecule, via β-(2→6)-glycosidic linkages. Genome mining was used to uncover new LS enzymes with increased transfructosylating activity and wider acceptor promiscuity, with an initial screening revealing five LS enzymes. The product profiles and activities of these enzymes were examined after their incubation with sucrose. Alternate acceptor molecules were also incubated with the enzymes to study their consumption. LSs from Gluconobacter oxydans and Novosphingobium aromaticivorans synthesized fructooligosaccharides (FOSs) with up to 13 units in length. Alignment of their amino acid sequences and substrate docking with homology models identified structural elements causing differences in their product spectra. Raffinose, over sucrose, was the preferred donor molecule for the LS from Vibrio natriegens, N. aromaticivorans, and Paraburkolderia graminis. The LSs examined were found to have wide acceptor promiscuity, utilizing monosaccharides, disaccharides, and two alcohols to a high degree.

Rational re-design of Lactobacillus reuteri 121 inulosucrase for product chain length control

Fructooligosaccharides (FOSs) are well-known prebiotics that are widely used in the food, beverage and pharmaceutical industries. Inulosucrase (E.C. 2.4.1.9) can potentially be used to synthesise FOSs from sucrose. In this study, inulosucrase from Lactobacillus reuteri 121 was engineered by site-directed mutagenesis to change the FOS chain length. Three variants (R483F, R483Y and R483W) were designed, and their binding free energies with 1,1,1-kestopentaose (GF4) were calculated with the Rosetta software. R483F and R483Y were predicted to bind with GF4 better than the wild type, suggesting that these engineered enzymes should be able to effectively extend GF4 by one residue and produce a greater quantity of GF5 than the wild type. MALDI-TOF MS analysis showed that R483F, R483Y and R483W variants could synthesise shorter chain FOSs with a degree of polymerization (DP) up to 11, 10, and 10, respectively, while wild type produced longer FOSs and in polymeric form. Although the decrease in catalytic activity and the increase of hydrolysis/transglycosylation activity ratio was observed, the variants could effectively synthesise FOSs with the yield up to 73% of substrate. Quantitative analysis demonstrated that these variants produced a larger quantity of GF5 than wild type, which was in good agreement with the predicted binding free energy results. Our findings demonstrate the success of using aromatic amino acid residues, at position D418, to block the oligosaccharide binding track of inulosucrase in controlling product chain length.

Comparison of the Levansucrase from the epiphyte Erwinia tasmaniensis vs its homologue from the phytopathogen Erwinia amylovora

Erwinia tasmaniensis is an epiphytic bacterium related to the plant pathogen Erwinia amylovora, the etiological agent of fire blight. In this study the levansucrase from E. tasmaniensis (EtLsc) has been compared with the homologous enzyme from E. amylovora (EaLsc). We characterized the enzymatic activity and compared the products profile of both enzymes by High Performance Anion Exchange Chromatography coupled with Pulsed Amperometric Detector (HPAEC-PAD). Moreover we determined the crystal structure of EtLsc to understand the structural peculiarity causing the different product profiles of the two homologues. EtLsc exhibits increased efficiency in the production of FOS, resulting in a better catalyst for biotechnological synthesis than EaLsc. Based on our results, we propose that the role of this enzyme in the life cycle of the two bacteria is most likely related to survival, rather than linked to pathogenicity in E. amylovora.

Product-oriented chemical surface modification of a levansucrase (SacB) via an ene-type reaction

Carbohydrate processing enzymes are sophisticated tools of living systems that have evolved to execute specific reactions on sugars. Here we present for the first time the site-selective chemical modification of exposed tyrosine residues in SacB, a levansucrase from Bacillus megaterium (Bm-LS) for enzyme engineering purposes via an ene-type reaction. Bm-LS is unable to sustain the synthesis of high molecular weight (HMW) levan (a fructose polymer) due to protein-oligosaccharide dissociation events occurring at an early stage during polymer elongation. We switched the catalyst from levan-like oligosaccharide synthesis to the efficient production of a HMW fructan polymer through the covalent addition of a flexible chemical side-chain that fluctuates over the central binding cavity of the enzyme preventing premature oligosaccharide disengagement.

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.

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.

Asparagine 42 of the conserved endo-inulinase INU2 motif WMNDPN from Aspergillus ficuum plays a role in activity specificity

Endo-inulinase INU2 from Aspergillus ficuum belongs to glycosidase hydrolase family 32 (GH32) that degrades inulin into fructo oligosaccharides consisting mainly of inulotriose and inulotetraose. The 3D structure of INU2 was recently obtained (Pouyez et al., 2012, Biochimie, 94, 2423–2430). An enlarged cavity compared to exo-inulinase formed by the conserved motif W-M(I)-N-D(E)-P-N-G, the so-called loop 1 and the loop 4, was identified. In the present study we have characterized the importance of 12 residues situated around the enlarged cavity. These residues were mutated by site-directed mutagenesis. Comparative activity analysis was done by plate, spectrophotometric and thin-layer chromatography assay. Most of the mutants were less active than the wild-type enzyme. Most interestingly, mutant N42G differed in the size distribution of the FOS synthesized.

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Endo-inulinase INU2 degrades inulin into fructo oligosaccharides.

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12 residues around the catalytic pockets of INU2 enzyme were determined.

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These residues were mutated to either a G or A residue.

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The activity has been tested by plate, spectrophotometric and TLC assays.

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One mutation, N42G, which changes the specificity of activity, has been identified.

Crystal structure of inulosucrase from Lactobacillus: insights into the substrate specificity and product specificity of GH68 fructansucrases

Fructansucrases (FSs) catalyze a transfructosylation reaction with sucrose as substrate to produce fructo-oligosaccharides and fructan polymers that contain either β-2,1 glycosidic linkages (inulin) or β-2,6 linkages (levan). Levan-synthesizing FSs (levansucrases) have been most extensively investigated, while detailed information on inulosucrases is limited. Importantly, the molecular basis of the different product specificities of levansucrases and inulosucrases is poorly understood. We have elucidated the three-dimensional structure of a truncated active bacterial GH68 inulosucrase, InuJ of Lactobacillus johnsonii NCC533 (residues 145-708), in its apo form, with a bound substrate (sucrose), and with a transfructosylation product. The sucrose binding pocket and the sucrose binding mode are virtually identical with those of GH68 levansucrases, confirming that both enzyme types use the same fully conserved structural framework for the binding and cleavage of the donor substrate sucrose in the active site. The binding mode of the first transfructosylation product 1-kestose (Fru-β(2-1)-Fru-α(2-1)-Glc, where Fru=fructose and Glc=glucose) in subsites -1 to +2 shows for the first time how inulin-type fructo-oligosaccharide bind in GH68 FS and how an inulin-type linkage can be formed. Surprisingly, observed interactions with the sugar in subsites +1 and +2 are provided by residues that are also present in levansucrases. The binding mode of 1-kestose and the presence of a more distant sucrose binding site suggest that residues beyond the +2 subsite, in particular residues from the nonconserved 1B-1C loop, determine product linkage type specificity in GH68 FSs.

Single amino acid residue changes in subsite -1 of levansucrase from Zymomonas mobilis 10232 strongly influence the enzyme activities and products

The -1 subsite of bacterial fructansucrases (FSs) (levansucrases and inulosucrases) plays an important role in the substrate recognition, binding and catalysis. Three residues (for example W47, W118 and R193, Zymomonas mobilis levansucrase numbering) at the -1 subsite are completely conserved among FSs. Site-directed mutational analysis showed that the substitutions of the three strictly conserved amino acid residues, W47N, W47H, W118N, W118H, R193K and R193H, significantly decreased enzyme activities and synthesis rates of levan, while the size of the synthesized oligosaccharides had been influenced. These experimental results, combined with 3D structure modeling, lead to our proposal that a single amino acid residue change in subsite -1 of levansucrase can influence change to the size and polarity of the sucrose binding pocket with a concomitant change to substrate binding and catalysis, and thus having an overall influence on the enzyme activities and products.

Enzymes in food processing: a condensed overview on strategies for better biocatalysts

Food and feed is possibly the area where processing anchored in biological agents has the deepest roots. Despite this, process improvement or design and implementation of novel approaches has been consistently performed, and more so in recent years, where significant advances in enzyme engineering and biocatalyst design have fastened the pace of such developments. This paper aims to provide an updated and succinct overview on the applications of enzymes in the food sector, and of progresses made, namely, within the scope of tapping for more efficient biocatalysts, through screening, structural modification, and immobilization of enzymes. Targeted improvements aim at enzymes with enhanced thermal and operational stability, improved specific activity, modification of pH-activity profiles, and increased product specificity, among others. This has been mostly achieved through protein engineering and enzyme immobilization, along with improvements in screening. The latter has been considerably improved due to the implementation of high-throughput techniques, and due to developments in protein expression and microbial cell culture. Expanding screening to relatively unexplored environments (marine, temperature extreme environments) has also contributed to the identification and development of more efficient biocatalysts. Technological aspects are considered, but economic aspects are also briefly addressed.

Inulin and levan synthesis by probiotic Lactobacillus gasseri strains: characterization of three novel fructansucrase enzymes and their fructan products

Fructansucrase enzymes polymerize the fructose moiety of sucrose into levan or inulin fructans, with beta(2-6) and beta(2-1) linkages, respectively. Here, we report an evaluation of fructan synthesis in three Lactobacillus gasseri strains, identification of the fructansucrase-encoding genes and characterization of the recombinant proteins and fructan (oligosaccharide) products. High-performance anion-exchange chromatography and nuclear magnetic resonance analysis of the fructo-oligosaccharides (FOS) and polymers produced by the L. gasseri strains and the recombinant enzymes revealed that, in situ, L. gasseri strains DSM 20604 and 20077 synthesize inulin (and oligosaccharides) and levan products, respectively. L. gasseri DSM 20604 is only the second Lactobacillus strain shown to produce inulin polymer and FOS in situ, and is unique in its distribution of FOS synthesized, ranging from DP2 to DP13. The probiotic bacterium L. gasseri DSM 20243 did not produce any fructan, although we identified a fructansucrase-encoding gene in its genome sequence. Further studies showed that this L. gasseri DSM 20243 gene was prematurely terminated by a stop codon. Exchanging the stop codon for a glutamine codon resulted in a recombinant enzyme producing inulin and FOS. The three recombinant fructansucrase enzymes characterized from three different L. gasseri strains have very similar primary protein structures, yet synthesize different fructan products. An interesting feature of the L. gasseri strains is that they were unable to ferment raffinose, whereas their respective recombinant enzymes converted raffinose into fructan and FOS.

The probiotic Lactobacillus johnsonii NCC 533 produces high-molecular-mass inulin from sucrose by using an inulosucrase enzyme

Fructansucrase enzymes polymerize the fructose moiety of sucrose into levan or inulin fructans, with beta(2-6) and beta(2-1) linkages, respectively. The probiotic bacterium Lactobacillus johnsonii strain NCC 533 possesses a single fructansucrase gene (open reading frame AAS08734) annotated as a putative levansucrase precursor. However, (13)C nuclear magnetic resonance (NMR) analysis of the fructan product synthesized in situ revealed that this is of the inulin type. The ftf gene of L. johnsonii was cloned and expressed to elucidate its exact identity. The purified L. johnsonii protein was characterized as an inulosucrase enzyme, producing inulin from sucrose, as identified by (13)C NMR analysis. Thin-layer chromatographic analysis of the reaction products showed that InuJ synthesized, besides the inulin polymer, a broad range of fructose oligosaccharides. Maximum InuJ enzyme activity was observed in a pH range of 4.5 to 7.0, decreasing sharply at pH 7.5. InuJ exhibited the highest enzyme activity at 55 degrees C, with a drastic decrease at 60 degrees C. Calcium ions were found to have an important effect on enzyme activity and stability. Kinetic analysis showed that the transfructosylation reaction of the InuJ enzyme does not obey Michaelis-Menten kinetics. The non-Michaelian behavior of InuJ may be attributed to the oligosaccharides that were initially formed in the reaction and which may act as better acceptors than the growing polymer chain. This is only the second example of the isolation and characterization of an inulosucrase enzyme and its inulin (oligosaccharide) product from a Lactobacillus strain. Furthermore, this is the first Lactobacillus strain shown to produce inulin polymer in situ.

In vitro synthesis of artificial polysaccharides by glycosidases and glycosynthases

Artificial polysaccharides produced by in vitro enzymatic synthesis are new biomaterials with defined structures that either mimic natural polysaccharides or have unnatural structures and functionalities. This review summarizes recent developments in the in vitro polysaccharide synthesis by endo-glycosidases, grouped in two major strategies: (a) native retaining endo-glycosidases under kinetically controlled conditions (transglycosylation with activated glycosyl donors), and (b) glycosynthases, engineered glycosidases devoid of hydrolase activity but with high transglycosylation activity. Polysaccharides are obtained by enzymatic polymerization of simple glycosyl donors by repetitive condensation. This approach not only provides a powerful methodology to produce polysaccharides with defined structures and morphologies as novel biomaterials, but is also a valuable tool to analyze the mechanisms of polymerization and packing to acquire high-order molecular assemblies.