Enhancement of debitterness, water-solubility, and neuroprotective effects of naringin by transglucosylation

Naringin found in citrus fruits is a flavanone glycoside with numerous biological activities. However, the bitterness, low water-solubility, and low bioavailability of naringin are the main issues limiting its use in the pharmaceutical and nutraceutical industries. Herein, a glucansucrase from isolated Leuconostoc citreum NY87 was used for trans-α-glucosylattion of naringin by using sucrose as substrate. Two naringin glucosides (O-α-D-glucosyl-(1'''' → 6″) naringin (compound 1) and 4'-O-α-D-glucosyl naringin (compound 2)) were purified and determined their structures by nuclear magnetic resonance. The optimization condition for the synthesis of compound 1 was obtained at 10 mM naringin, 200 mM sucrose, and 337.5 mU/mL at 28 °C for 24 h by response surface methodology method. Compound 1 and compound 2 showed 1896- and 3272 times higher water solubility than naringin. Furthermore, the bitterness via the human bitter taste receptor TAS2R39 displayed that compound 1 was reduced 2.9 times bitterness compared with naringin, while compound 2 did not express bitterness at 1 mM. Both compounds expressed higher neuroprotective effects than naringin on human neuroblastoma SH-SY5Y cells treated with 5 mM scopolamine based on cell viability and cortisol content. Compound 1 reduced acetylcholinesterase activity more than naringin and compound 2. These results indicate that naringin glucosides could be utilized as functional material in the nutraceutical and pharmaceutical industries. KEY POINTS: • A novel O-α-D-glucosyl-(1 → 6) naringin was synthesized using glucansucrase from L. citreum NY87. • Naringin glucosides improved water-solubility and neuroprotective effects on SH-SY5Y cells. • Naringin glucosides showed a decrease in bitterness on bitter taste receptor 39.

Synthesis and biological characterization of low-calorie Schisandra chinensis syrup

Schisandra chinensis (Omija) is a well-known medicinal plant in East Asia. In this study, Omija oligosaccharide syrup was prepared from sucrose with Omija fruit extract using two glucansucrases of Leuconostoc mesenteroides B-512F/KM and L. mesenteroides B-1355CF10/KM. The degree of polymerization of Omija oligosaccharide syrup was ranged from 2 - 13 by MALDI-TOF-MS analysis. Compared to the Omija syrup, the Omija oligosaccharide syrup reduced 61% calories based on the enzymatic gravimetric method. It also reduced up to 96% insoluble glucan formation from sucrose by mutansucrase of Streptococcus mutans at 500 mg/mL. Additionally, it has 1.78-fold higher oxygen radical absorbance capacity value compared to Omija syrup. Using electronic tongue sensor system, Omija oligosaccharide syrup showed decreased sourness, astringency, and saltiness compared to Omija syrup. Thus, Omija oligosaccharides can be used as functional sweetener in nutraceutical industries.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10068-022-01061-8.

Synthesis of ellagic acid glucoside using glucansucrase from Leuconostoc and characterization of this glucoside as a functional neuroprotective agent

Ellagic acid glucoside was synthesized via transglucosylation using sucrose and glucansucrase derived from Leuconostoc mesenteroides B-512 FMCM. After such enzymatic synthesis, the product was purified by 50% ethyl acetate fraction and C18 column chromatography. Modification of ellagic acid glucoside was verified by LC–MS/MS at m/z 485.1 (M + Na)⁻ and m/z 531.1 (M + 3Na)⁻. The yield of ellagic acid glucoside was 69% (3.47 mM) by response surface methodology using 150 mM sucrose, 300 mU/mL glucansucrase, and 5 mM ellagic acid. The synthesized ellagic acid glucoside showed improved water solubility, up to 58% higher brain nerve cell (SH-SY5Y) protective effect, threefold higher cortisol reducing effect, and fourfold stronger inhibitory effect on acetylcholinesterase (AChE) than ellagic acid. These results indicate that ellagic acid glucoside could be used as a neuroprotective agent.

Inhibitory potential of EGCG on Streptococcus mutans biofilm: A new approach to prevent Cariogenesis

Dental caries is a common cause for tooth loss and Streptococcus mutans is identified as the etiologic pathogen. This study evaluates the inhibitory potential of Epigallocatechin gallate (EGCG) on S.mutans glucansucrase enzyme and its biofilm. Glucansucrase binding and the inhibitory potential of EGCG was validated using AutoDock tool and enzyme inhibitory assay. Biofilm inhibitory potential was also confirmed using Scanning Electron Microscopic (SEM) analysis in human tooth samples. Molecular docking revealed that EGCG interacted with GLU 515 and TRP 517 amino acids and binds to glucansucrase. SEM analysis revealed inhibition of S.mutans biofilm by various concentrations of EGCG on surfaces of tooth samples. Bioinformatics and biological assays confirmed that EGCG potentially binds to the S. mutans glucansucrase and inhibits its enzymatic activity. Enzymatic inhibition of glucansucrase attenuated biofilm formation potential of S. mutans on tooth surface. Thus, we conclude that EGCG inhibitory potential of S. mutans biofilm on the tooth surface is a novel approach in prevention of dental caries.

Characterisation of insoluble α-1,3-/α-1,6 mixed linkage glucan produced in addition to soluble α-1,6-linked dextran by glucansucrase (DEX-N) from Leuconostoc citreum ABK-1

Glucansucrases catalyse the formation of glucans from sucrose. The glucansucrase-encoding gene from Leuconostoc citreum ABK-1, dex-N, was successfully cloned and expressed in E. coli BL21 Star (DE3). DEX-N produces 2 types of glucans: soluble (S-dextran) and insoluble (I-glucan) glucans. The S-dextran was determined to be ca. 10 kDa in size and contained >90% α-1,6 linkages; along with its water solubility, this is similar to commercial dextran. On the other hand, I-glucan was water-insoluble, harbouring a block-wise pattern of α-1,3 and α-1,6 linkages in its structure. Notably, the FTIR and powder X-ray diffraction pattern of I-glucan exhibited a combination of features found in α-1,6-linked dextran and α-1,3-linked mutan. Although both I-glucan and mutan are insoluble glucans, their physical characteristics are notably dissimilar.

Enzymatic synthesis and biological characterization of a novel mangiferin glucoside

Mangiferin, a major constituent of Mangifera indica L., has attracted substantial attention due to its anti-oxidant, anti-diabetic, anti-inflammatory, and anti-microbial activities. However, its poor solubility in water limits its use in food and pharmaceutical industries. In this study, novel mangiferin-(1→6)-α-d-glucopyranoside (Mg-G1) was enzymatically synthesized from mangiferin and sucrose using glucansucrase from Leuconostoc mesenteroides B-512F/KM, and optimized using response surface methodology. The water solubility of Mg-G1 was found to be 824.7 mM, which is more than 2300-fold higher than that of mangiferin. Mg-G1 also showed DPPH radical scavenging activity and superoxide dismutase (SOD)-like scavenging activity, which were 4.77- and 3.71-fold higher than that of mangiferin, respectively. Mg-G1 displayed inhibitory activity against human intestinal maltase and COX-2. Thus, the novel glucosylated mangiferin may be used as an ingredient in functional food and pharmaceutical application.

Preparation of gentiobiose-derived oligosaccharides by glucansucrase E81 and determination of prebiotic and immune-modulatory functions

Gentiobiose-derived oligosaccharides were synthesized by the acceptor reaction of glucansucrase E81 obtained from Lactobacillus reuteri E81 with sucrose and gentiobiose as donor-acceptor sugars, respectively. The reaction products were monitored by TLC analysis and gentiobiose-derived oligosaccharides up to DP 8 were formed during the acceptor reaction as determined by ESI-MS/MS analysis. The glycosylation of the gentiobiose with α-(1 → 6) linkages and α-(1 → 3) linkages was shown by ¹H and ¹³C NMR analysis confirming the structure of these gentiobiose-derived oligosaccharides. The in vitro prebiotic function of the oligosaccharides was determined in which probiotic strains were stimulated whereas no growth was observed in pathogen strains. Gentiobiose-derived oligosaccharides showed immune-modulatory functions in vitro and triggered the production of IL-4, IL12 and TNF-α cytokines in HT29 cells in a dose dependent manner. This study showed the production and functional characterisation of gentiobiose-derived oligosaccharides establishing a promising avenue for future applications.

Biochemical characterization of a GH70 protein from Lactobacillus kunkeei DSM 12361 with two catalytic domains involving branching sucrase activity

The fructophilic bacterium Lactobacillus kunkeei has promising applications as probiotics promoting the health of both honey bees and humans. Here, we report the synthesis of a highly branched dextran by L. kunkeei DSM 12361 and biochemical characterization of a GH70 enzyme (GtfZ). Sequence analysis revealed that GtfZ harbors two separate catalytic cores (CD1 and CD2), predicted to have glucansucrase and branching sucrase specificity, respectively. GtfZ-CD1 was not characterized biochemically due to its unsuccessful expression. With only sucrose as substrate, GtfZ-CD2 was found to mainly catalyze sucrose hydrolysis and leucrose synthesis. When dextran was available as acceptor substrate, GtfZ-CD2 displayed an efficient transglycosidase activity with sucrose as donor substrate. Kinetic analysis showed that the GtfZ-CD2-catalyzed transglycosylation reaction follows a Ping Pong Bi Bi mechanism, indicating the in-turn binding of donor and acceptor substrates in the active site. Structural characterization of the products revealed that GtfZ-CD2 catalyzes the synthesis of single glucosyl (α1 → 3) linked branches onto dextran, resulting in the production of highly branched comb-like α-glucan products. These (α1 → 3) branches can be formed on adjacent positions, as shown when isomaltotriose was used as acceptor substrate. Homology modeling of the GtfZ-CD1 and GtfZ-CD2 protein structure strongly suggests that amino acid differences in conserved motifs II, III, and IV in the catalytic domain contribute to product specificity. Our present study highlights the ability of beneficial lactic acid bacteria to produce structurally complex α-glucans and provides novel insights into the molecular mechanism of an (α1 → 3) branching sucrase.

Synthesis of glucooligosaccharides with prebiotic potential by glucansucrase URE 13-300 acceptor reactions with maltose, raffinose and lactose

In the present work, we report an efficient synthesis of glucooligosaccharides (GOSs) with prebiotic potential by novel glucansucrase URE 13-300 from Leuconostoc mesenteroides URE 13 strain. The highest total yield of GOSs with degree of polymerization (DP) from 3 to 6 was obtained with maltose as an acceptor and maltose/sucrose (M/S) ratio 1-136 g/L. An efficient modulation of GOSs composition is achieved by varying the M/S ratio. At M/S = 1, 2, 4 and 7 the content of DP3 products gradually increase from 54.50 to 91.70%. When the M/S ratio was decreased the synthesis of DP>3 GOSs is predominant and reaches 75.60% (M/S = 0.25). In addition, the maltose derived GOSs with DP>3, as well as raffinose and lactose glucosylation products have a branched structure which is prerequisite for increased prebiotic potential. The synthesized GOSs were efficiently metabolized by probiotic strains of Lb. plantarum S26, Lb. brevis S27 and Lb. sakei S16, and the calculated values of specific growth rate (μ) were nearly identical to this on glucose media, when maltose derived GOSs were used as a carbohydrate source. Strain specific features were observed in the utilization of the synthesized GOSs, as well as in the production of lactic acid and acetic acid.

Biotechnological potential of novel glycoside hydrolase family 70 enzymes synthesizing α-glucans from starch and sucrose

Transglucosidases belonging to the glycoside hydrolase (GH) family 70 are promising enzymatic tools for the synthesis of α-glucans with defined structures from renewable sucrose and starch substrates. Depending on the GH70 enzyme specificity, α-glucans with different structures and physicochemical properties are produced, which have found diverse (potential) commercial applications, e.g. in food, health and as biomaterials. Originally, the GH70 family was established only for glucansucrase enzymes of lactic acid bacteria that catalyze the synthesis of α-glucan polymers from sucrose. In recent years, we have identified 3 novel subfamilies of GH70 enzymes (designated GtfB, GtfC and GtfD), inactive on sucrose but converting starch/maltodextrin substrates into novel α-glucans. These novel starch-acting enzymes considerably enlarge the panel of α-glucans that can be produced. They also represent very interesting evolutionary intermediates between sucrose-acting GH70 glucansucrases and starch-acting GH13 α-amylases. Here we provide an overview of the repertoire of GH70 enzymes currently available with focus on these novel starch-acting GH70 enzymes and their biotechnological potential. Moreover, we discuss key developments in the understanding of structure-function relationships of GH70 enzymes in the light of available three-dimensional structures, and the protein engineering strategies that were recently applied to expand their natural product specificities.

A dextran with unique rheological properties produced by the dextransucrase from Oenococcus kitaharae DSM 17330

A gene encoding a novel dextransucrase was identified in the genome of Oenococcus kitaharae DSM17330 and cloned into E. coli. With a kcat of 691s^-1 and a half-life time of 111h at 30°C, the resulting recombinant enzyme -named DSR-OK- stands as one of the most efficient and stable dextransucrase characterized to date. From sucrose, this enzyme catalyzes the synthesis of a quasi linear dextran with a molar mass higher than 1×10⁹g·mol^-1 that presents uncommon rheological properties such as a higher viscosity than that of the most industrially used dextran from L. mesenteroides NRRL-B-512F, a yield stress that was never described before for any type of dextran, as well as a gel-like structure. All these properties open the way to a vast array of new applications in health, food/feed, bulk or fine chemicals fields.

Catechol glucosides act as donor/acceptor substrates of glucansucrase enzymes of Lactobacillus reuteri

Previously, we have shown that the glucansucrase GtfA-ΔN enzyme of Lactobacillus reuteri 121, incubated with sucrose, efficiently glucosylated catechol and we structurally characterized catechol glucosides with up to five glucosyl units attached (te Poele et al. in Bioconjug Chem 27:937-946, 2016). In the present study, we observed that upon prolonged incubation of GtfA-ΔN with 50 mM catechol and 1000 mM sucrose, all catechol had become completely glucosylated and then started to reappear. Following depletion of sucrose, this glucansucrase GtfA-ΔN used both α-D-Glcp-catechol and α-D-Glcp-(1→4)-α-D-Glcp-catechol as donor substrates and transferred a glucose unit to other catechol glycoside molecules or to sugar oligomers. In the absence of sucrose, GtfA-ΔN used α-D-Glcp-catechol both as donor and acceptor substrate to synthesize catechol glucosides with 2 to 10 glucose units attached and formed gluco-oligosaccharides up to a degree of polymerization of 4. Also two other glucansucrases tested, Gtf180-ΔN from L. reuteri 180 and GtfML1-ΔN from L. reuteri ML1, used α-D-Glcp-catechol and di-glucosyl-catechol as donor/acceptor substrate to synthesize both catechol glucosides and gluco-oligosaccharides. With sucrose as donor substrate, the three glucansucrase enzymes also efficiently glucosylated the phenolic compounds pyrogallol, resorcinol, and ethyl gallate; also these mono-glucosides were used as donor/acceptor substrates.

Complete genome sequence of Leuconostoc garlicum KCCM 43211 producing exopolysaccharide

Leuconostoc garlicum KCCM 43211 isolated from traditional Korean fermented food is an intensive producer of exopolysaccharide (EPS). Here we report the first complete genome sequence of L. garlicum KCCM 43211. The genome sequence displayed that this strain contains genes involved in production of EPS possibly composed of glucose monomers. An uncharacterized EPS from the L. garlicum KCCM 43211 strains was also produced during fermentation in the sucrose medium. The MALDI-TOF results displayed the typical mass spectrometry pattern of dextran. This uncharacterized EPS may have use in commercial prebiotics, food additives, and medical purposes. The complete genome sequence of L. garlicum KCCM 43211 will provide valuable information for strain engineering based on the genetic information.

Isomelezitose formation by glucansucrases

Several glucansucrases were surveyed for their ability to produce isomelezitose, a trisaccharide with the structure α-D-glucopyranosyl (1 → 6) β-D-fructofuranosyl (2 ↔ 1) α-D-glucopyranoside. Nearly all strains tested, with one exception, produced at least trace levels of isomelezitose. Yields were low but significant, ranging from less than 1% to approximately 5% based on sucrose. This trisaccharide may arise in either of two ways: glucopyranosyl transfer to the 6^Fru-OH position of sucrose, or to the anomeric OH position of isomaltulose. This study indicates that isomelezitose formation may be a general phenomenon of many glucansucrase reactions.

Stevia Glycosides: Chemical and Enzymatic Modifications of Their Carbohydrate Moieties to Improve the Sweet-Tasting Quality

Stevia glycosides, extracted from the leaves of the plant Stevia rebaudiana Bertoni, display an amazing high degree of sweetness. As processed plant products, they are considered as excellent bio-alternatives for sucrose and artificial sweeteners. Being noncaloric and having beneficial properties for human health, they are the subject of an increasing number of studies for applications in food and pharmacy. However, one of the main obstacles for the successful commercialization of Stevia sweeteners, especially in food, is their slight bitter aftertaste and astringency. These undesirable properties may be reduced or eliminated by modifying the carbohydrate moieties of the steviol glycosides. A promising procedure is to subject steviol glycosides to enzymatic glycosylation, thereby introducing additional monosaccharide residues into the molecules. Depending on the number and positions of the monosaccharide units, the taste quality and sweetness potency of the compounds will vary. Many studies have been performed already, and this review summarizes the structures of native steviol glycosides and the recent data of modifications of the carbohydrate moieties that have been published to provide an overview of the current progress.

Characterization of the glucansucrase GTF180 W1065 mutant enzymes producing polysaccharides and oligosaccharides with altered linkage composition

Exopolysaccharides produced by lactic acid bacteria are extensively used for food applications. Glucansucrase enzymes of lactic acid bacteria use sucrose to catalyze the synthesis of α-glucans with different linkage compositions, size and physico-chemical properties. Crystallographic studies of GTF180-ΔN show that at the acceptor binding sites +1 and +2, residue W1065 provides stacking interactions to the glucosyl moiety. However, the detailed functional roles of W1065 have not been elucidated. We performed random mutagenesis targeting residue W1065 of GTF180-ΔN, resulting in the generation of 10 mutant enzymes that were characterized regarding activity and product specificity. Characterization of mutant enzymes showed that residue W1065 is critical for the activity of GTF180-ΔN. Using sucrose, and sucrose (donor) plus maltose (acceptor) as substrates, the mutant enzymes synthesized polysaccharides and oligosaccharides with changed linkage composition. The stacking interaction of an aromatic residue at position 1065 is essential for polysaccharide synthesis.

Structure-function relationships of family GH70 glucansucrase and 4,6-α-glucanotransferase enzymes, and their evolutionary relationships with family GH13 enzymes

Lactic acid bacteria (LAB) are known to produce large amounts of α-glucan exopolysaccharides. Family GH70 glucansucrase (GS) enzymes catalyze the synthesis of these α-glucans from sucrose. The elucidation of the crystal structures of representative GS enzymes has advanced our understanding of their reaction mechanism, especially structural features determining their linkage specificity. In addition, with the increase of genome sequencing, more and more GS enzymes are identified and characterized. Together, such knowledge may promote the synthesis of α-glucans with desired structures and properties from sucrose. In the meantime, two new GH70 subfamilies (GTFB- and GTFC-like) have been identified as 4,6-α-glucanotransferases (4,6-α-GTs) that represent novel evolutionary intermediates between the family GH13 and "classical GH70 enzymes". These enzymes are not active on sucrose; instead, they use (α1 → 4) glucans (i.e. malto-oligosaccharides and starch) as substrates to synthesize novel α-glucans by introducing linear chains of (α1 → 6) linkages. All these GH70 enzymes are very interesting biocatalysts and hold strong potential for applications in the food, medicine and cosmetic industries. In this review, we summarize the microbiological distribution and the structure-function relationships of family GH70 enzymes, introduce the two newly identified GH70 subfamilies, and discuss evolutionary relationships between family GH70 and GH13 enzymes.

Effect of a single point mutation on the interaction of glucans with a glucansucrase from Leuconostoc mesenteroides NRRL B-1118

Our previous work showed that substitution of an amino acid that is coupled with the +2 subsite adjacent to the transition stabilizer of a glucansucrase, which produces a water-insoluble glucan, resulted in significant changes in the structures and yields of the water-insoluble glucans produced. We now describe how these changes affect the ability of the glucansucrase to bind to exogenous glucans, and how these glucans can influence the yield, product structures, and kinetics of the mutant glucansucrases. The activity of the wild-type enzyme, with threonine at position 654, is not significantly activated by added dextran, and the yield of water-insoluble glucan from sucrose is only slightly increased by dextran. Mutant T654Y is not affected at all by the addition of dextran. However, several mutant enzymes exhibit markedly lower yields of glucan relative to the wild type; these lower yields can be partially or completely overcome by the addition of water-soluble dextran. Although evidence indicates that the soluble dextran is incorporated into water-insoluble glucan, the increased yields cannot be accounted for solely by incorporation of the dextran into insoluble product. Furthermore, these DsrI mutants are significantly activated by exogenous glucans. The addition of dextran does not markedly change the KM for sucrose in the mutant enzymes, but does increase the Vmax of the reaction. These effects apparently depend on the presence of unbranched sequences of α1→6-linked D-glucose units in the glucan.

Glucansucrase Gtf180-ΔN of Lactobacillus reuteri 180: enzyme and reaction engineering for improved glycosylation of non-carbohydrate molecules

Glucansucrases have a broad acceptor substrate specificity and receive increased attention as biocatalysts for the glycosylation of small non-carbohydrate molecules using sucrose as donor substrate. However, the main glucansucrase-catalyzed reaction results in synthesis of α-glucan polysaccharides from sucrose, and this strongly impedes the efficient glycosylation of non-carbohydrate molecules and complicates downstream processing of glucosylated products. This paper reports that suppressing α-glucan synthesis by mutational engineering of the Gtf180-ΔN enzyme of Lactobacillus reuteri 180 results in the construction of more efficient glycosylation biocatalysts. Gtf180-ΔN mutants (L938F, L981A, and N1029M) with an impaired α-glucan synthesis displayed a substantial increase in monoglycosylation yields for several phenolic and alcoholic compounds. Kinetic analysis revealed that these mutants possess a higher affinity for the model acceptor substrate catechol but a lower affinity for its mono-α-d-glucoside product, explaining the improved monoglycosylation yields. Analysis of the available high resolution 3D crystal structure of the Gtf180-ΔN protein provided a clear understanding of how mutagenesis of residues L938, L981, and N1029 impaired α-glucan synthesis, thus yielding mutants with an improved glycosylation potential.

Synthesis of oligo- and polysaccharides by Lactobacillus reuteri 121 reuteransucrase at high concentrations of sucrose

GTFA, a glucansucrase enzyme of the probiotic bacterium Lactobacillus reuteri 121, is capable of synthesizing an α-glucan polysaccharide with (1 → 4) and (1 → 6) linkages from sucrose. With respect to its biosynthesis, the present study has shown that the ratio of oligosaccharide versus polysaccharide synthesized was directly proportional to the concentration of sucrose. It appears that the size distribution of products is kinetically controlled, but the linkage distribution in the polysaccharide material is not changed. At high sucrose concentrations the sucrose isomers leucrose and trehalulose were synthesized, using the accumulated fructose as acceptor, together with 4'- and 6'-α-D-glucosyl-leucrose and 6'-α-D-glucosyl-trehalulose. The finding of an additional branched hexasaccharide demonstrates that the enzyme is able to introduce branch-points already in relatively short oligosaccharides.

Residue Leu940 has a crucial role in the linkage and reaction specificity of the glucansucrase GTF180 of the probiotic bacterium Lactobacillus reuteri 180

Highly conserved glycoside hydrolase family 70 glucansucrases are able to catalyze the synthesis of α-glucans with different structure from sucrose. The structural determinants of glucansucrase specificity have remained unclear. Residue Leu(940) in domain B of GTF180, the glucansucrase of the probiotic bacterium Lactobacillus reuteri 180, was shown to vary in different glucansucrases and is close to the +1 glucosyl unit in the crystal structure of GTF180-ΔN in complex with maltose. Herein, we show that mutations in Leu(940) of wild-type GTF180-ΔN all caused an increased percentage of (α1→6) linkages and a decreased percentage of (α1→3) linkages in the products. α-Glucans with potential different physicochemical properties (containing 67-100% of (α1→6) linkages) were produced by GTF180 and its Leu(940) mutants. Mutant L940W was unable to form (α1→3) linkages and synthesized a smaller and linear glucan polysaccharide with only (α1→6) linkages. Docking studies revealed that the introduction of the large aromatic amino acid residue tryptophan at position 940 partially blocked the binding groove, preventing the isomalto-oligosaccharide acceptor to bind in an favorable orientation for the formation of (α1→3) linkages. Our data showed that the reaction specificity of GTF180 mutant was shifted either to increased polysaccharide synthesis (L940A, L940S, L940E, and L940F) or increased oligosaccharide synthesis (L940W). The L940W mutant is capable of producing a large amount of isomalto-oligosaccharides using released glucose from sucrose as acceptors. Thus, residue Leu(940) in domain B is crucial for linkage and reaction specificity of GTF180. This study provides clear and novel insights into the structure-function relationships of glucansucrase enzymes.

Glucansucrase acceptor reactions with d-mannose

The main acceptor product of glucansucrases with d-mannose has not previously been identified. We used glucansucrases that form water-insoluble α-d-glucans to produce increased yields of acceptor products from d-mannose, and identified the major product as 6-O-α-d-glucopyranosyl-d-mannose. Glucansucrases that synthesize insoluble α-d-glucans produced higher yields of the disaccharide compared to typical dextransucrases.