Bacterial α-Glucan and Branching Sucrases from GH70 Family: Discovery, Structure-Function Relationship Studies and Engineering

Alternansucrase as a key enabling tool of biotransformation from molecular features to applications: A review

Alternansucrase (ASR), classified in GH70, produces unique α-glucans with alternating α-1,3 and α-1,6 glycosidic linkages in the backbone chain from renewable sucrose which is easily obtained from nature with low cost. ASR has synthesized many products with valuable functionalities that hold enormous commercial interest and promising applications. The influence of biocatalysis and fermentation parameters on the yields, and properties of products are critical for the propositions made to promote the enzyme application. Investigations on ASR have been compiled in the review to provide information on the enzyme, products and parameters. This review summarizes studies on the characteristics, conversion mechanism, products, and beneficial applications of ASR and exhibits structure-based technologies to improve enzyme activity, specificity, and thermostability for industrial applications. Finally, prospects for further development are also proposed for various ASR applications in food and other fields.

Regioselective thexyldimethylsilylation of (1➔3)-glucans - Does the linkage type matter

α- and β-(1➔3)-linked polysaccharides dissolved in N,N-dimethyl acetamide (DMA) were subjected to conversion with thexyldimethylchlorosilane (TDMS-Cl) in presence of pyridine as base. A degree of substitution of TDMS groups (DSSi) between 0.7 and 1.0 was achieved indicating that the β-(1➔3)-linked curdlan (DSSi 0.7) is less reactive than α-(1➔3)-linked glucans (DSSi ca. 1). The synthesis sequence of permethylation, desilylation, and acetylation afforded the corresponding acetyl-methyl derivatives, where unaffected OH groups were methylated and TDMS groups were replaced by acetyl moieties. NMR spectroscopic investigations revealed a highly selective silylation of the primary OH group at position 6 while leaving the secondary OH moieties unaffected. This pronounced selectivity was found to be distinctly higher compared to cellulose and starch. Conversion of (1➔4)-linked polysaccharides dissolved in DMA/LiCl with TDMS-Cl leads to products consisting of both 6-mono-O- and 2,6-di-O- silylated repeating units. Regioselective 6-mono-O-silylation requires the hazardous use of liquid ammonia.

Detailed structural characterization of five water-insoluble α-glucans produced by glucansucrases from Streptococcus spp

Water-insoluble α-glucans synthesized from sucrose by glucansucrases from Streptococcus spp. are essential in dental plaque and caries formation. Because limited information is available on the fine structure of these biopolymers, we analyzed the structures of unmodified glucans produced by five recombinant Streptococcus (S.) mutans DSM 20523 and S. salivarius DSM 20560 glucansucrases in detail. A combination of methylation analysis, endo-dextranase and endo-mutanase hydrolyses, and HPSEC-RI was used. Furthermore, crystal-like regions were analyzed by using XRD and 13C MAS NMR spectroscopy. Our results showed that the glucan structures were highly diverse: Two glucans with 1,3- and 1,6-linkages were characterized in detail besides an almost exclusively 1,3-linked and a linear 1,6-linked glucan. Furthermore, one glucan contained 1,3-, 1,4-, and 1,6-linkages and thus had an unusual, not yet described structure. It was demonstrated that the glucans had a varying structural architecture by using partial enzymatic hydrolyses. Furthermore, crystal-like regions formed by 1,3-glucopyranose units were observed for the two 1,3- and 1,6-linked glucans and the linear 1,3-linked glucan. 1,6-linked regions were mobile and not involved in the crystal-like areas. Altogether, our results broaden the knowledge of the structure of water-insoluble α-glucans from Streptococcus spp.

Unprecedented Diversity of the Glycoside Hydrolase Family 70: A Comprehensive Analysis of Sequence, Structure, and Function

The glycoside hydrolase family 70 (GH70) contains bacterial extracellular multidomain enzymes, synthesizing α-glucans from sucrose or starch-like substrates. A few dozen have been biochemically characterized, while crystal structures cover only the core domains and lack significant parts of auxiliary domains. Here we present a systematic overview of GH70 enzymes and their 3D structural organization and bacterial origin. A representative set of 234 permuted and 25 nonpermuted GH70 enzymes was generated, covering 12 bacterial families and 3 phyla and containing 185 predicted glucansucrases (GS), 15 branching sucrases (BrS), 8 "twin" GS-BrSs, and 51 α-glucanotransferases (α-GT). Analysis of AlphaFold models of all 259 entries showed that, apart from the core domains, the structural variation regarding auxiliary domains is far greater than anticipated, with nine different domain types. We analyzed the phylogenetic distribution and discuss the possible roles of auxiliary domains as well as possible correlations between enzyme specificity, auxiliary domain type, and bacterial origin.

The Discovery, Molecular Cloning, and Characterization of Dextransucrase LmDexA and Its Active Truncated Mutant from Leuconostoc mesenteroides NN710

Dextransucrases play a crucial role in the production of dextran from economical sucrose; therefore, there is a pressing demand to explore novel dextransucrases with better performance. This study characterized a dextransucrase enzyme, LmDexA, which was identified from the Leuconostoc mesenteroides NN710. This bacterium was isolated from the soil of growing dragon fruit in Guangxi province, China. We successfully constructed six different N-terminal truncated variants through sequential analysis. Additionally, a truncated variant, ΔN190LmDexA, was constructed by removing the 190 amino acids fragment from the N-terminal. This truncated variant was then successfully expressed heterologously in Escherichia coli and purified. The purified ΔN190LmDexA demonstrated optimal hydrolysis activity at a pH of 5.6 and a temperature of 30 °C. Its maximum specific activity was measured to be 126.13 U/mg, with a Km of 13.7 mM. Results demonstrated a significant improvement in the heterologous expression level and total enzyme activity of ΔN190LmDexA. ΔN190LmDexA exhibited both hydrolytic and transsaccharolytic enzymatic activities. When sucrose was used as the substrate, it primarily produced high-molecular-weight dextran (>400 kDa). However, upon the addition of maltose as a receptor, it resulted in the production of a significant amount of oligosaccharides. Our results can provide valuable information for enhancing the characteristics of recombinant dextransucrase and potentially converting sucrose into high-value-added dextran and oligosaccharides.

Bottom-Up Synthesized Glucan Materials: Opportunities from Applied Biocatalysis

Linear d-glucans are natural polysaccharides of simple chemical structure. They are comprised of d-glucosyl units linked by a single type of glycosidic bond. Noncovalent interactions within, and between, the d-glucan chains give rise to a broad variety of macromolecular nanostructures that can assemble into crystalline-organized materials of tunable morphology. Structure design and functionalization of d-glucans for diverse material applications largely relies on top-down processing and chemical derivatization of naturally derived starting materials. The top-down approach encounters critical limitations in efficiency, selectivity, and flexibility. Bottom-up approaches of d-glucan synthesis offer different, and often more precise, ways of polymer structure control and provide means of functional diversification widely inaccessible to top-down routes of polysaccharide material processing. Here the natural and engineered enzymes (glycosyltransferases, glycoside hydrolases and phosphorylases, glycosynthases) for d-glucan polymerization are described and the use of applied biocatalysis for the bottom-up assembly of specific d-glucan structures is shown. Advanced material applications of the resulting polymeric products are further shown and their important role in the development of sustainable macromolecular materials in a bio-based circular economy is discussed.

Glucans and applications in drug delivery

Glucan is a natural polysaccharide widely distributed in cereals and microorganisms that has various biological activities, including immunomodulatory, anti-infective, anti-inflammatory, and antitumor activities. In addition to wide applications in the broad fields of food, healthcare, and biomedicines, glucans hold promising potential as drug delivery carrier materials or ligands. Specifically, glucan microparticles or yeast cell wall particles are naturally enclosed vehicles with an interior cavity that can be exploited to carry and deliver drug payloads. The biological activities and targeting capacities of glucans depend largely on the recognition of glucan moieties by receptors such as dectin-1 and complement receptor 3, which are widely expressed on the cell membranes of mononuclear phagocytes, dendritic cells, neutrophils, and some lymphocytes. This review summarizes the chemical structures, sources, fundamental properties, extraction methods, and applications of these materials, with an emphasis on drug delivery. Glucans are utilized mainly as vaccine adjuvants, targeting ligands and as carrier materials for various drug entities. It is believed that glucans and glucan microparticles may be useful for the delivery of both small-molecule and macromolecular drugs, especially for potential treatment of immune-related diseases.

Improvement on wheat bread quality by in situ produced dextran-A comprehensive review from the viewpoint of starch and gluten

Deterioration of bread quality, characterized by the staling of bread crumb, the softening of bread crust and the loss of aroma, has caused a huge food waste and economic loss, which is a bottleneck restriction to the development of the breadmaking industry. Various bread improvers have been widely used to alleviate the issue. However, it is noteworthy that the sourdough technology has emerged as a pivotal factor in this regard. In sourdough, the metabolic breakdown of carbohydrates, proteins, and lipids leads to the production of exopolysaccharides, organic acids, aroma compounds, or prebiotics, which contributes to the preeminent ability of sourdough to enhance bread attributes. Moreover, sourdough exhibits a "green-label" feature, which satisfies the consumers' increasing demand for additive-free food products. In the past two decades, there has been a significant focus on sourdough with in situ produced dextran due to its exceptional performance. In this review, the behaviors of bread crucial compositions (i.e., starch and gluten) during dough mixing, proofing, baking and bread storing, as well as alterations induced by the acidic environment and the presence of dextran are systemically summarized. From the viewpoint of starch and gluten, results obtained confirm the synergistic amelioration on bread quality by the coadministration of acidity and dextran, and also highlight the central role of acidification. This review contributes to establishing a theoretical foundation for more effectively enhancing the quality of wheat breads through the application of in situ produced dextran.

Microbial dextran-hydrolyzing enzyme: Properties, structural features, and versatile applications

Dextran, an α-glucan mainly composed of (α1 → 6) linkages, has been widely applied in the food, cosmetic, and medicine industries. Dextranase can hydrolyze dextran to synthesize oligodextrans, which show prominent properties and promising applications in the food industry. Dextranases are widely distributed in bacteria, yeasts, and fungus, and classified into glycoside hydrolase (GH) 13, 15, 31, 49, and 66 families according to their sequence similarity, structural features, and reaction types. Dextranase, as a dextran-hydrolyzing enzyme, displays great application potential in the sugar-making, oral health care, medicine, and biotechnology industries. Here we mainly focused on presenting the enzymatic properties, structural features, and versatile (potential) applications of dextranase. To date, seven crystal structures of dextranases from GH 13, 15, 31, 49, and 66 families have been successfully solved. However, their molecular mechanisms for hydrolyzing dextran, especially on the size determinants of the hydrolysates, remain largely unknown. Additionally, the classification, microbial distribution, and immobilization technology of dextranase were also discussed in detail. This review discussed dextranase from different aspects with the ambition to present how they constitute the groundwork for promising future developments.

N1019D Mutant of Limosilactobacillus reuteri 121 4,6-α-Glucanotransferase GtfB Significantly Improved Catalytic Activity

Limosilactobacillus reuteri 121 4,6-α-glucanotransferase GtfB (Lr 121 GtfB), belonging to glycoside hydrolase family 70 (GH70), synthesizes linear isomalto/malto polysaccharides having (α1→6) linkages attached to the nonreducing ends of (α1→4) linked maltose oligosaccharide segments using starch or maltodextrin as a substrate. Since Lr 121 GtfB has low catalytic activity and efficiency, it leads to substrate regeneration and reduced substrate utilization. In this study, we superimposed the crystal structure of Lr 121 GtfB-ΔNΔV with that of L. reuteri NCC 2613 GtfB-ΔNΔV (Lr 2613 GtfB-ΔNΔV) to identify the acceptor binding subsites +1 to +3 and constructed five single-residue mutants and a random mutagenesis of N1019. Compared with the wild-type, N1019D Lr 121 GtfB-ΔN did not alter the product specificity, increased the catalytic activity and efficiency by 420 and 590%, respectively, and maintained >80% relative activity in the pH 3.5-6.5 interval. The findings will contribute to the industrial application of Lr 121 GtfB and provide new solutions for starch synthesis of higher value derivatives.

In Silico Analysis of the Dextransucrase Obtained From Leuconostoc mesenteroides Strain IBUN 91.2.98

The DSR-IBUN dextransucrase produced by Leuconostoc mesenteroides strain IBUN 91.2.98 has a short production time (4.5 hours), an enzymatic activity of 24.8 U/mL, and a specific activity of purified enzyme 2 times higher (331.6 U/mg) than that reported for similar enzymes. The aim of this study was to generate a structural model that, from an in silico approach, allows a better understanding, from the structural point of view, of the activity obtained by the enzyme of interest, which is key to continue with its study and industry application. For this, we translated the nucleotide sequence of the dsr_IBUN gene. With the primary structure of DSR-IBUN, the in silico prediction of physicochemical parameters, the possible subcellular localization, the presence of signal peptide, and the location of domains and functional and structural motifs of the protein were established. Subsequently, its secondary and tertiary structure were predicted and a homology model of the dextransucrase under study was constructed using Swiss-Model, performing careful template selection. The values obtained for the model, Global Model Quality Estimation (0.63), Quality Mean (-1.49), and root-mean-square deviation (0.09), allow us to affirm that the model for the enzyme dextransucrase DSR-IBUN is of adequate quality and can be used as a source of information for this protein.

Advances in Amylases-What's Going on?

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Probiogenomics of Leuconostoc Mesenteroides Strains F-21 and F-22 Isolated from Human Breast Milk Reveal Beneficial Properties

Bacteria of the Leuconostoc genus are Gram-positive bacteria that are commonly found in raw milk and persist in fermented dairy products and plant food. Studies have already explored the probiotic potential of L. mesenteroides, but not from a probiogenomic perspective, which aims to explore the molecular features responsible for their phenotypes. In the present work, probiogenomic approaches were applied in strains F-21 and F-22 of L. mesenteroides isolated from human milk to assess their biosafety at the molecular level and to correlate molecular features with their potential probiotic characteristics. The complete genome of strain F-22 is 1.99 Mb and presents one plasmid, while the draft genome of strain F-21 is 1.89 Mb and presents four plasmids. A high percentage of average nucleotide identity among other genomes of L. mesenteroides (≥ 96%) corroborated the previous taxonomic classification of these isolates. Genomic regions that influence the probiotic properties were identified and annotated. Both strains exhibited wide genome plasticity, cell adhesion ability, proteolytic activity, proinflammatory and immunomodulation capacity through interaction with TLR-NF-κB and TLR-MAPK pathway components, and no antimicrobial resistance, denoting their potential to be candidate probiotics. Further, the strains showed bacteriocin production potential and the presence of acid, thermal, osmotic, and bile salt resistance genes, indicating their ability to survive under gastrointestinal stress. Taken together, our results suggest that L. mesenteroides F-21 and F-22 are promising candidates for probiotics in the food and pharmaceutical industries.

Review on recent advances in the properties, production and applications of microbial dextranases

Dextranase is a type of hydrolase that is responsible for catalyzing the breakdown of high-molecular-weight dextran into low-molecular-weight polysaccharides. This process is called dextranolysis. A select group of bacteria and fungi, including yeasts and likely certain complex eukaryotes, produce dextranase enzymes as extracellular enzymes that are released into the environment. These enzymes join dextran's α-1,6 glycosidic bonds to make glucose, exodextranases, or isomalto-oligosaccharides (endodextranases). Dextranase is an enzyme that has a wide variety of applications, some of which include the sugar business, the production of human plasma replacements, the treatment of dental plaque and its protection, and the creation of human plasma replacements. Because of this, the quantity of studies carried out on worldwide has steadily increased over the course of the past couple of decades. The major focus of this study is on the most current advancements in the production, administration, and properties of microbial dextranases. This will be done throughout the entirety of the review.

Gluco-oligosaccharides as potential prebiotics: Synthesis, purification, structural characterization, and evaluation of prebiotic effect

Prebiotics have long been used to modulate the gut microbiota and improve host health. Most established prebiotics are nondigestible carbohydrates, especially short-chain oligosaccharides. Recently, gluco-oligosaccharides (GlcOS) with 2-10 glucose residues and one or more O-glycosidic linkage(s) have been found to exert prebiotic potentials (not fully established prebiotics) because of their selective fermentation by beneficial gut bacteria. However, the prebiotic effects (non-digestibility, selective fermentability, and potential health effects) of GlcOS are highly variable due to their complex structure originating from different synthesis processes. The relationship between GlcOS structure and their potential prebiotic effects has not been fully understood. To date, a comprehensive summary of the knowledge of GlcOS is still missing. Therefore, this review provides an overview of GlcOS as potential prebiotics, covering their synthesis, purification, structural characterization, and prebiotic effect evaluation. First, GlcOS with different structures are introduced. Then, the enzymatic and chemical processes for GlcOS synthesis are critically reviewed, including reaction mechanisms, substrates, catalysts, the structures of resultant GlcOS, and the synthetic performance (yield and selectivity). Industrial separation techniques for GlcOS purification and structural characterization methods are discussed in detail. Finally, in vitro and in vivo studies to evaluate the non-digestibility, selective fermentability, and associated health effects of different GlcOS are extensively reviewed with a special focus on the GlcOS structure-function relationship.

Mutations in the Different Residues between Dextransucrase Gtf-DSM and Reuteransucrase GtfO for the Investigation of Linkage Specificity Determinants

The dextransucrase Gtf-DSM has 99.3% sequence identity with the reuteransucrase GtfO, and only 11 out of 1045 residues are different between their N-terminally truncated recombinant forms. Gtf-DSM is capable of synthesizing a dextran with 1% (α1 → 2), 6% (α1 → 4), 24% (α1 → 3), and 69% (α1 → 6) linkages, while GtfO produces a reuteran with 21% (α1 → 6) and 79% (α1 → 4) linkages. In this work, using recombinant Gtf-DSM and GtfO as templates, parallel substitutions targeting these 11 distinguishing residues were performed to investigate their linkage specificity determinants. The combinatorial mutation (I937L/D977A/D1083V/Q1086K/K1087G) at the acceptor binding subsites +1 and +2 nearly converted the linkage specificity of Gtf-DSM to that of GtfO. Surprisingly, all of the individual or combinatorial mutations in four residues from domains IV and V of Gtf-DSM significantly altered the linkage specificity of Gtf-DSM. Additionally, all mutations in the 11 distinguishing residues of Gtf-DSM resulted in a dramatically reduced transferase/hydrolysis activity ratio, which was closer to that of GtfO. These mutation results suggested that the linkage specificity differences between Gtf-DSM and GtfO are determined by the distinct micro-physicochemical environments, formed by the concerted action of a series of residues not only from the acceptor binding subsites +1 and +2 but also from domains IV and V.

Whole Genome Sequence Analysis of a Novel Apilactobacillus Species from Giant Honeybee (Apis dorsata) Gut Reveals Occurrence of Genetic Elements Coding Prebiotic and Probiotic Traits

Apilactobacillus spp. are classified as obligate fructophilic lactic acid bacteria (FLAB) that inhabit fructose-rich niches such as honeybee gut. Lactic acid bacteria are an important component of the gut microbiome and play a crucial role in maintaining gut health. In this study, a new FLAB strain HBW1, capable of producing glucan-type exopolysaccharide, was isolated from giant honeybee (Apis dorsata) gut and subjected to whole genome sequencing (WHS) to determine its health-beneficial traits. The genome size of the isolate was 1.49 Mb with a GC content of 37.2%. For species level identity, 16S rDNA sequence similarity, genome to genome distance calculator (dDDH), and average nucleotide identity (ANI) values were calculated. Phylogenetic analysis showed that the isolate HBW1 belongs to the Apilactobacillus genus. The dDDH and ANI values in comparison with closely clustered Apilactobacillus kunkeei species were 52% and 93.10%, respectively. Based on these values, we concluded that HBW1 is a novel species of Apilactobacillus, and we propose the name Apilactobacillus waqarii HBW1 for it. Further, WHS data mining of HBW1 revealed that it harbors two glucosyltransferase genes for prebiotic glucan-type exopolysaccharide synthesis. Moreover, chaperon (clp) and methionine sulfoxide reductase (msrA, msrB, and msrC) genes as well as nutritional marker genes for folic acid (folD) and riboflavin biosynthesis (rib operon), important for conferring probiotic properties, were also detected. Occurrence of these genetic traits make HBW1 an excellent candidate for application to improve gut function.

Whole Genome Sequence Analysis of a Novel Apilactobacillus Species from Giant Honeybee (Apis dorsata) Gut Reveals Occurrence of Genetic Elements Coding Prebiotic and Probiotic Traits

Apilactobacillus spp. are classified as obligate fructophilic lactic acid bacteria (FLAB) that inhabit fructose-rich niches such as honeybee gut. Lactic acid bacteria are an important component of the gut microbiome and play a crucial role in maintaining gut health. In this study, a new FLAB strain HBW1, capable of producing glucan-type exopolysaccharide, was isolated from giant honeybee (Apis dorsata) gut and subjected to whole genome sequencing (WHS) to determine its health-beneficial traits. The genome size of the isolate was 1.49 Mb with a GC content of 37.2%. For species level identity, 16S rDNA sequence similarity, genome to genome distance calculator (dDDH), and average nucleotide identity (ANI) values were calculated. Phylogenetic analysis showed that the isolate HBW1 belongs to the Apilactobacillus genus. The dDDH and ANI values in comparison with closely clustered Apilactobacillus kunkeei species were 52% and 93.10%, respectively. Based on these values, we concluded that HBW1 is a novel species of Apilactobacillus, and we propose the name Apilactobacillus waqarii HBW1 for it. Further, WHS data mining of HBW1 revealed that it harbors two glucosyltransferase genes for prebiotic glucan-type exopolysaccharide synthesis. Moreover, chaperon (clp) and methionine sulfoxide reductase (msrA, msrB, and msrC) genes as well as nutritional marker genes for folic acid (folD) and riboflavin biosynthesis (rib operon), important for conferring probiotic properties, were also detected. Occurrence of these genetic traits make HBW1 an excellent candidate for application to improve gut function.

Construction of a Novel Chimeric Dextransucrase Fused to the Carbohydrate-Binding Module CBM2a

Lactic acid bacteria (LAB) have the potential to produce homoexopolysaccharides (HoPS). Their health benefits and physicochemical properties have been the subject of extensive research. The HoPS functional properties are determined by molecular weight, the type of glycosidic linkages, degrees of branching and chemical composition. The dextransucrases (DSases) produce a kind of HoPS (dextrans), which are among the first biopolymers produced at industrial scale with applications in medicine and biotechnology. The glycodiversification opens additional applications for DSases. Therefore, the design and characterization of new DSases is of prime importance. Previously, we described the isolation and characterization of a novel extracellular dextransucrase (DSR-F) encoding gene. In this study, from DSR-F, we design a novel chimeric dextransucrase DSR-F-∆SP-∆GBD-CBM2a, where DSR-F-∆SP-∆GBD (APY repeats and a CW repeat deleted) was fused to the carbohydrate-binding module (CBM2a) of the β-1-4 exoglucanase/xylanase Cex (Xyn10A) of Cellulomonas fimi ATCC 484. This dextransucrase variant is active and the specificity is not altered. The DSR-F-∆SP-∆GBD-CBM2a was purified by cellulose affinity chromatography for the first time. This research showed that hybrids and chimeric biocatalyst DSases with novel binding capacity to cellulose can be designed to purify and immobilize using renewable lignocellulosic materials as supports.