Exploring the sequence variability of polymerization-involved residues in the production of levan- and inulin-type fructooligosaccharides with a levansucrase

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.

Strategies for modulating transglycosylation activity, substrate specificity, and product polymerization degree of engineered transglycosylases

Glycosides are widely used in many fields due to their favorable biological activity. The traditional plant extractions and chemical methods for glycosides production are limited by environmentally unfriendly, laborious protecting group strategies and low yields. Alternatively, enzymatic glycosylation has drawn special attention due to its mild reaction conditions, high catalytic efficiency, and specific stereo-/regioselectivity. Glycosyltransferases (GTs) and retaining glycoside hydrolases (rGHs) are two major enzymes for the formation of glycosidic linkages. Therein GTs generally use nucleotide phosphate activated donors. In contrast, GHs can use broader simple and affordable glycosyl donors, showing great potential in industrial applications. However, most rGHs mainly show hydrolysis activity and only a few rGHs, namely non-Leloir transglycosylases (TGs), innately present strong transglycosylation activities. To address this problem, various strategies have recently been developed to successfully tailor rGHs to alleviate their hydrolysis activity and obtain the engineered TGs. This review summarizes the current modification strategies in TGs engineering, with a special focus on transglycosylation activity enhancement, substrate specificity modulation, and product polymerization degree distribution, which provides a reference for exploiting the transglycosylation potentials of rGHs.

Characterization and alteration of product specificity of Beijerinckia indica subsp. indica β-fructosyltransferase

The trisaccharide 1-kestose, a major constituent of fructooligosaccharide, has strong prebiotic effects. We used high-performance liquid chromatography and 1H nuclear magnetic resonance spectroscopy to show that BiBftA, a β-fructosyltransferase belonging to glycoside hydrolase family 68, from Beijerinckia indica subsp. indica catalyzes transfructosylation of sucrose to produce mostly 1-kestose and levan polysaccharides. We substituted His395 and Phe473 in BiBftA with Arg and Tyr, respectively, and analyzed the reactions of the mutant enzymes with 180 g/L sucrose. The ratio of the molar concentrations of glucose and 1-kestose in the reaction mixture with wild-type BiBftA was 100:8.1, whereas that in the reaction mixture with the variant H395R/F473Y was 100:45.5, indicating that H395R/F473Y predominantly accumulated 1-kestose from sucrose. The X-ray crystal structure of H395R/F473Y suggests that its catalytic pocket is unfavorable for binding of sucrose while favorable for transfructosylation.

Plant glycosyltransferases for expanding bioactive glycoside diversity

Glycosylation is a successful strategy to alter the pharmacological properties of small molecules, and it has emerged as a unique approach to expand the chemical space of natural products that can be explored in drug discovery. Traditionally, most glycosylation events have been carried out chemically, often requiring many protection and deprotection steps to achieve a target molecule. Enzymatic glycosylation by glycosyltransferases could provide an alternative strategy for producing new glycosides. In particular, the glycosyltransferase family has greatly expanded in plants, representing a rich enzymatic resource to mine and expand the diversity of glycosides with novel bioactive properties. This article highlights previous and prospective uses for plant glycosyltransferases in generating bioactive glycosides and altering their pharmacological properties.

Rapid, real-time sucrase characterization: Showcasing the feasibility of a one-pot activity assay

Sucrases can modify numerous carbohydrates, and short-chain oligosaccharides produced by the unique transfructosylation activity of levansucrases are promising candidates for the growing sugar substitute market. These compounds could counteract the increasing number of diseases associated with the consumption of high-calorie sugars. Thus, there is great interest in the characterization of novel levansucrases. The commonly used method for sucrase activity determination is to quantify d-glucose released in the sucrose-splitting reaction. This is usually done in a discontinuous mode, i.e., several samples taken from the sucrase reaction are applied to a separately performed d-glucose determination (e.g., GOPOD assay). Employing the newly isolated levansucrase LevS_KK21 from Pseudomonas sp. KK21, the feasibility of a one-pot sucrase characterization was investigated by combining sucrase reaction and GOPOD-based d-glucose determination into a single, continuous assay (Real-time GOPOD). The enzyme was characterized with respect to kinetic parameters, ion dependency, pH value, and reaction temperature in a comparative approach employing Real-time GOPOD and HPLC. High data consistency for all investigated enzyme parameters demonstrated that current processes for sucrase characterization can be considerably accelerated by the continuous assay while maintaining data validity. However, the assay was not applicable at acidic pH, as decolorization of the quinoneimine dye formed during the GOPOD reaction was observed. Overall, the study presents valuable data on the potentials of real-time sucrase activity assessment for an accelerated discovery and characterization of interesting enzymes such as the hereby introduced levansucrase LevS_KK21. Progress in sucrase discovery will finally foster the development of health-promoting sucrose substitutes.

Directionally modulating the product chain length of an inulosucrase by semi-rational engineering for efficient production of 1-kestose

Microbial inulosucrase as a transfructosylation tool has been used to produce inulin and inulin-type fructooligosaccharides with various polymerization degrees. Tailor-made oligosaccharides could be generated by inulosucrase via chain length modulation. In this study, a semi-rational design based on the modeled structure of Lactobacillus reuteri 121 inulosucrase was carried out to screen and construct variants. The residues Arg541 and Arg544 were determined to be significant to the product chain elongation of L. reuteri 121 inulosucrase. The variant R544W altered the product specificity of inulosucrase and produced short-chain fructooligosaccharides with 1-kestose as the main component. Molecular dynamic simulations verified an increased binding free energy of variant R544W with 1-kestose than the wild-type enzyme with 1-kestose. After optimization, 1-kestose and total short-chain fructooligosaccharides production reached approximately 206 g/L and 307 g/L, respectively. This study suggests the great potential of variant R544W in the biotransformation from sucrose to functional sugar.

Comprehensive utilization of sucrose resources via chemical and biotechnological processes: A review

Sucrose, one of the most widespread disaccharides in nature, has been available in daily human life for many centuries. As an abundant and cheap sweetener, sucrose plays an essential role in our diet and the food industry. However, it has been determined that many diseases, such as obesity, diabetes, hyperlipidemia, etc., directly relate to the overconsumption of sucrose. It arouses many explorations for the conversion of sucrose to high-value chemicals. Production of valuable substances from sucrose by chemical methods has been studied since a half-century ago. Compared to chemical processes, biotechnological conversion approaches of sucrose are more environmentally friendly. Many enzymes can use sucrose as the substrate to generate functional sugars, especially those from GH68, GH70, GH13, and GH32 families. In this review, enzymatic catalysis and whole-cell fermentation of sucrose for the production of valuable chemicals were reviewed. The multienzyme cascade catalysis and metabolic engineering strategies were addressed.

Efficient substrate accessibility of cross-linked levanase aggregates using dialdehyde starch as a macromolecular cross-linker

Cross-linked enzyme aggregates (CLEAs) are influenced by mass diffusion limitations such as the degree of molecular cross-linking attained, which affects substrate accessibility. Thus, this study seeks to improve substrate accessibility using macromolecular cross-linkers in cross-linked levanase aggregates (CLLAs) formation for levan-type fructooligosaccharides (L-FOS) production. Dialdehyde starch-tapioca (DAST) was successfully developed and used to cross-link levanase to form CLLAs-D and with bovine serum albumin (BSA) to form CLLAs-DB which showed activity recoveries of 65.6% and 81.6%, respectively. After cross-linking, the pH (6-10) and thermal stability (30-40 °C) increased, and organic solvent tolerance resulted in the activation of CLLAs. Likewise, CLLAs-DB had higher substrate affinity and accessibility and a higher effectiveness factors than CLLAs-D. The total L-FOS yield of CLLAs-DB (78.9% (w/v)) was higher than that of CLLAs-D (62.4% (w/v)). Therefore, as a cross-linker, DAST may have application prospects as a promising and green biocatalyst for product formation.

Molecular insight into regioselectivity of transfructosylation catalyzed by GH68 levansucrase and β-fructofuranosidase

Glycoside hydrolase family 68 (GH68) enzymes catalyze β-fructosyltransfer from sucrose to another sucrose, the so-called transfructosylation. Although regioselectivity of transfructosylation is divergent in GH68 enzymes, there is insufficient information available on the structural factor(s) involved in the selectivity. Here, we found two GH68 enzymes, β-fructofuranosidase (FFZm) and levansucrase (LSZm), encoded tandemly in the genome of Zymomonas mobilis, displayed different selectivity: FFZm catalyzed the β-(2→1)-transfructosylation (1-TF), whereas LSZm did both of 1-TF and β-(2→6)-transfructosylation (6-TF). We identified His79^FFZm and Ala343^FFZm and their corresponding Asn84^LSZm and Ser345^LSZm respectively as the structural factors for those regioselectivities. LSZm with the respective substitution of FFZm-type His and Ala for its Asn84^LSZm and Ser345^LSZm (N84H/S345A-LSZm) lost 6-TF and enhanced 1-TF. Conversely, the LSZm-type replacement of His79^FFZm and Ala343^FFZm in FFZm (H79N/A343S-FFZm) almost lost 1-TF and acquired 6-TF. H79N/A343S-FFZm exhibited the selectivity like LSZm but did not produce the β-(2→6)-fructoside-linked levan and/or long levanooligosaccharides that LSZm did. We assumed Phe189^LSZm to be a responsible residue for the elongation of levan chain in LSZm and mutated the corresponding Leu187^FFZm in FFZm to Phe. An H79N/L187F/A343S-FFZm produced a higher quantity of long levanooligosaccharides than H79N/A343S-FFZm (or H79N-FFZm), although without levan formation, suggesting that LSZm has another structural factor for levan production. We also found that FFZm generated a sucrose analog, β-D-fructofuranosyl α-D-mannopyranoside, by β-fructosyltransfer to d-mannose and regarded His79^FFZm and Ala343^FFZm as key residues for this acceptor specificity. In summary, this study provides insight into the structural factors of regioselectivity and acceptor specificity in transfructosylation of GH68 enzymes.

Natural and engineered transglycosylases: Green tools for the enzyme-based synthesis of glycoproducts

An increasing number of transglycosylase-based processes provide access to oligosaccharides or glycoconjugates, some of them reaching performance levels compatible with industrial developments. Nevertheless, the full potential of transglycosylases has not been explored because of the challenges in transforming a glycoside hydrolase into an efficient transglycosylase. Advances in studying enzyme structure/function relationships, screening enzyme activity, and generating synthetic libraries guided by computational protein design or machine learning methods should considerably accelerate the development of these catalysts. The time has now come for researchers to uncover their possibilities and learn how to design and precisely refine their activity to respond more rapidly to the growing demand for well-defined glycosidic structures.

Enzyme promiscuity of carbohydrate active enzymes and their applications in biocatalysis

The application of biocatalysis for the synthesis of glycans and glycoconjugates is a well-established and successful strategy, both for small and large scale synthesis. Compared to chemical synthesis, is has the advantage of high selectivity, but biocatalysis had been largely limited to natural glycans both in terms of reactivity and substrates. This review describes recent advances in exploiting enzyme promiscuity to expand the range of substrates and reactions that carbohydrate active enzymes (CAZymes) can catalyse. The main focus is on formation and hydrolysis of glycosidic linkages, including sugar kinases, reactions that are central to glycobiotechnology. In addition, biocatalysts that generate sugar analogues and modify carbohydrates, such as oxidases, transaminases and acylases are reviewed. As carbohydrate active enzymes become more accessible and protein engineering strategies become faster, the application of biocatalysis in the generation of a wide range of glycoconjugates, beyond natural structures is expected to expand.

Implications of the mutation S164A on Bacillus subtilis levansucrase product specificity and insights into protein interactions acting upon levan synthesis

Mutation S164A largely affects the transfructosylation properties of Bacillus subtilis levansucrase (SacB). The variant uses acceptors such as glucose and short levans with an average molecular weight of 7.6 kDa more efficiently than SacB, leading to the enhanced synthesis of medium and high molecular weight polymer and a blasto-oligosaccharide series with a polymerization degree of 2-10. A 3-fold increase in blasto-oligosaccharides yield is provoked by the modified interplay between the variant and glucose. Despite its modified product specificity, protein-carbohydrate and protein-protein interactions are still a major factor affecting size and distribution of levan molecular weight. This study highlights the importance of critical factors such as protein concentration in the analysis of wild-type and mutagenized levansucrases. Docking experiments with the crystal structures of SacB and variant S164A - the latter obtained at a 2.6 Å resolution - identified unreported potential binding subsites for fructosyl moieties on the surface of both enzymes.

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.

The Structure of Sucrose-Soaked Levansucrase Crystals from Erwinia tasmaniensis reveals a Binding Pocket for Levanbiose

Given its potential role in the synthesis of novel prebiotics and applications in the pharmaceutical industry, a strong interest has developed in the enzyme levansucrase (LSC, EC 2.4.1.10). LSC catalyzes both the hydrolysis of sucrose (or sucroselike substrates) and the transfructosylation of a wide range of acceptors. LSC from the Gram-negative bacterium Erwinia tasmaniensis (EtLSC) is an interesting biocatalyst due to its high-yield production of fructooligosaccharides (FOSs). In order to learn more about the process of chain elongation, we obtained the crystal structure of EtLSC in complex with levanbiose (LBS). LBS is an FOS intermediate formed during the synthesis of longer-chain FOSs and levan. Analysis of the LBS binding pocket revealed that its structure was conserved in several related species. The binding pocket discovered in this crystal structure is an ideal target for future mutagenesis studies in order to understand its biological relevance and to engineer LSCs into tailored products.