Enhanced biosynthesis of d-tagatose from maltodextrin through modular pathway engineering of recombinant Escherichia coli

Characterization of a tagatose 4-epimerase and optimizing its expression for efficient D-tagatose bioconversion from fructose

D-Tagatose is a promising functional sweetener due to its safety profile and versatile physiological effects. Recently, the bioconversion of D-tagatose from cost-effective natural resources has gained increasing attention. In this study, the hypothetical protein DRJ43_04605 from Thermoprotei archaeon, abbreviated as DRJ43-T4Ease, was identified as a tagatose 4-epimerase capable of efficiently converting d-fructose into D-tagatose. DRJ43-T4Ease exhibited optimal specific activity of 0.79 U/mg at pH 7.5 and 90 °C in the presence of 1 mM Ni2+. The denaturation temperature (Tm) of the enzyme was determined to be 105.9 °C, with a half-life of 7.19, 6.56, and 4.84 h at 85, 90, and 95 °C, respectively. The kinetic parameters for d-fructose conversion, including Km, kcat, and kcat/Km, were calculated to be 51.86 mM, 127.22 min-1, and 2.45 mM-1 min-1, respectively. Furthermore, the expression of DRJ43-T4Ease in E. coli was optimized by engineering the translation initiation sequence. The engineered strain EcDRJ43-T4Ease-M1 achieved a volume activity of 98.4 U/L through shake-flask fermentation and a conversion efficiency of 17.2 % using 100 g/L d-fructose as the substrate. This study demonstrates the potential for D-tagatose production from d-fructose via whole-cell bioconversion, providing a promising approach for the industrial-scale production of this valuable sweetener.

Protein Engineering of Tagatose 4-Epimerase for D-Tagatose Production

D-Tagatose, a promising sugar substitute with various functional properties and commercial applications, can be enzymatically converted from D-fructose by tagatose 4-epimerase. The development of an efficient tagatose 4-epimerase that catalyzes the conversion of D-fructose into D-tagatose is essential to make the production technology of D-tagatose applicable. In this study, tagatose 4-epimerase from Thermotogae (TsT4Ease) was engineered through semi-rational design and directed evolution, resulting in a 2.8-fold improvement in catalytic activity compared to the wild type (WT). The production of D-tagatose reached 42 g/L in 2 h. Crystal structure analysis determined the structural features with a common (α/β)8-TIM barrel and a Zn2+-binding architecture at the active center. Subsequent molecular dynamics (MD) simulations revealed that the substitutions improved substrate binding energy and stabilized the active pocket. This study offers new insights into the structure-function relationship of TsT4Ease and provides a candidate tagatose 4-epimerase.

Biochemical characterization and biocatalytic application of a hyperthermostable tagatose 4-epimerase from Infirmifilum uzonense

D-Tagatose is a representative rare sugar with the physiochemical properties of low energy and high sweetness, as well as excellent physiological functions such as blood sugar regulation, enhancement of intestinal flora, and prevention of dental caries. At present, D-tagatose production involves lactose hydrolysis and D-galactose isomerization processes, resulting in high production costs that hinder its industrial advancement. Tagatose 4-epimerase (T4Ease) has the capability to directly convert d-fructose into D-tagatose through C-4 epimerization, providing a new approach for D-tagatose production. In this study, a hyperthermostable T4Ease from Infirmifilum uzonense (Inuz-TE4ase) was identified from the Foldseek clustered AlphaFold database and its biochemical properties were characterized in detail. Under the optimal reaction conditions of 90 °C and pH 8.5 (Tris-HCl) with the addition of 1 mM Ni2+, the maximum catalytic activity towards d-fructose was determined to be 0.680 U/mg. Inuz-TE4ase exhibited exceptional thermostability, with half-life (t1/2) values of 19.3 h at 85 °C and 8.9 h at 90 °C, respectively. Inuz-TE4ase was strictly metal-dependent, and its stability could be enhanced by Ni2+ with an increase in the melting temperature (Tm) value from 101.1 °C to 105.7 °C. When 100 g/L d-fructose was used as the substrate, Inuz-TE4ase could catalyze the production of 21.67 g/L D-tagatose, indicating its significant potential for D-tagatose bioproduction.

Biosynthesis of a healthy natural sugar D-tagatose: advances and opportunities

D-tagatose is a natural low-calorie rare sugar with nearly the same sweet taste as sucrose. It has nutritional and functional properties of great interest for health, such as anti-diabetes, anti-caries, anti-atherosclerosis, anti-hyperlipidemia, anti-aging, improvement of intestinal microflora, etc. The production of D-tagatose from D-galactose catalyzed by an alkali suffers from limited supplies of costly feedstock (i.e., lactose) and high manufacturing costs due to harsh reaction conditions, costly separation, as well as severe degradation and pollution. In this review, we briefly present the properties of D-tagatose and its physiological effects, review the recent advances in the biosynthesis of D-tagatose from inexpensive and abundant glucans (e.g., starch) and their derivatives (e.g., D-glucose and D-fructose) and from lactose, including both academic literature and industrial patents, as well as discuss its future challenges and opportunities. The biosynthesis of D-tagatose can be catalyzed by four types of biocatalysts: enzymes, whole-cells, microbial fermentation, and in vitro multi-enzyme molecular machines. The biomanufacturing of starchy D-tagatose catalyzed by multi-enzyme molecular machines could be the most promising approach because it not only makes D-tagatose from ample starch but also surpasses the equilibria of monosaccharide isomerization reactions (e.g., D-fructose-to-D-tagatose, D-galactose-to-D-tagatose). D-tagatose as a filler for a variety of food and drinks or a key component mixed with other sweeteners would become a predominant starch-derived sweetener and partially replace high-fructose corn sirup in the future.

The use of isomerases and epimerases for the production of the functional sugars mannose, allulose and tagatose from Fructose

Fructose, a common monosaccharide in nature extensively utilized in the food industry, poses a risk of elevated chronic disease incidence with excessive consumption. The global movement for a healthier living has sparked a quest for sugar reduction in foodstuff. The growing concern regarding the adverse impact of excessive sugar consumption on public health has led to significant interest in investigating healthier sugar alternatives. Research efforts have refocused on converting fructose into high-value, reduced-calorie functional sugars. Fructose can undergo direct conversion into three such sugars-mannose, allulose, and tagatose-via a streamlined bioconversion process. Allulose and tagatose, epimers of fructose, are derivable directly from fructose through C-3 and C-4 epimerization processes, whereas mannose, the aldose isomer of fructose, can be synthesized via isomerization pathways. This article aims to present recent advancements in the physiological functions, production methods, and applications of functional sugars derived from fructose. Particularly, it focuses on the bioproduction of mannose, allulose, and tagatose from fructose, encompassing discussions on the recent progress in the related isomerases and epimerases, such as mannose isomerase/lyxose isomerase, ketose 3-epimerase, and tagatose 4-epimerase. This review will provide a fresh perspective on the high-value biological utilization of fructose resources.

Green biotechnological synthesis of rare sugars/alcohols: D-allulose, allitol, D-tagatose, L-xylulose, L-ribose

Rare sugars are paid more attention because of which have the characteristics of low calorie, low absorption and excellent physiological functions. Biotechnological synthesis of rare sugars has the advantages of being green, clean, simple and economic compared to chemical synthesis. Abundant enzymes for rare sugars biosynthesis are introduced and multienzyme cascade catalytic system (MECCS) used in biosynthesis of rare sugars is highlighted in this paper. Different biosynthesis pathways of five important rare sugars (D-allulose, allitol, D-tagatose, L-xylulose, l-ribose), mainly containing isomerization/epimerization reaction (existing thermodynamic equilibrium limitation), reduction-oxidation reaction (needing expensive cofactors) and phosphorylation-dephosphorylation reaction pathways (inherent constraint of thermodynamic equilibrium and requirement high-cost cofactors) etc., are reviewed. Furthermore, techniques of cofactor regeneration and enzyme/cell immobilization are provided. Finally, unique insights and expectations for future development in biosynthesis of rare sugars are given. This review provides a comprehensive analysis of the latest biotechnological advancements in the biosynthesis of rare sugars/alcohols, highlighting innovative multienzyme cascade catalytic systems and cofactor regeneration strategies.

Activating the d-Tagatose Production Capacity of Escherichia coli with Structural Insights into C4 Epimerase Specificity

d-Tagatose, a rare low-calorie sweetener, is ideal for beverages due to its high solubility and low viscosity. Current enzymatic production methods from d-galactose or d-galactitol are limited by reaction reversibility, affecting the yield and purity. This study demonstrates that Escherichia coli harbors a thermodynamically favorable pathway for producing d-tagatose from d-glucose via phosphorylation-epimerization-dephosphorylation steps. GatZ and KbaZ, annotated as aldolase chaperones, exhibit C4 epimerization activity, converting d-fructose-6-phosphate to d-tagatose-6-phosphate. Structural analysis reveals active site differences between these enzymes and class II aldolases, indicating functional divergence. By exploiting the strains' inability to metabolize d-tagatose, carbon starvation was applied to remove sugar byproducts. The engineered strains converted 45 g L-1 d-glucose to d-tagatose, achieving a titer of 7.3 g L-1 and a productivity of 0.1 g L-1 h-1 under test tube conditions. This approach highlights E. coli as a promising host for efficient d-tagatose production.

Construction of a multienzyme cascade reaction system and its application in D-tagatose biosynthesis

D-tagatose, a low-calorie rare sugar, has significant potential in food, medicine, cosmetics, and other industries owing to its high application value and market potential. In this study, Escherichia coli BL21 was used as the starting strain to express the β-galactosidase (β-Gal) gene-BgaB-derived from Bacillus stearothermophilus and the L-arabinose isomerase (L-AI) gene-araA-derived from Thermus sp., yielding the genetically engineered strains E. coli BL21-pET28a-BgaB and E. coli BL21-pET28a-araA. These strains synthesized D-tagatose using β-Gal and L-AI with a conversion rate of 23.73%. Based on this, we constructed a multienzyme cascade pathway comprising β-Gal, L-AI, glucose isomerase (GI), fructose kinase (FK), D-tagatose-bisphosphate aldolase (GatZ), polyphosphate kinase (PPK), and phosphatase (PGP), further enhancing D-tagatose biosynthesis. This multienzyme approach improved the conversion of the intermediate product D-glucose to D-tagatose by 3.84% compared with the dual-enzyme system. Thus, our study provides a theoretical basis and technical support for the industrial production of D-tagatose.

Discovery of a Thermostable Tagatose 4-Epimerase Powered by Structure- and Sequence-Based Protein Clustering

d-Tagatose is a highly promising functional sweetener known for its various physiological functions. In this study, a novel tagatose 4-epimerase from Thermoprotei archaeon (Thar-T4Ease), with the ability to convert d-fructose to d-tagatose, was discovered through a combination of structure similarity search and sequence-based protein clustering. The recombinant Thar-T4Ease exhibited optimal activity at pH 8.5 and 85 °C, in the presence of 1 mM Ni2+. Its kcat and kcat/Km values toward d-fructose were measured to be 248.5 min-1 and 2.117 mM-1·min-1, respectively. Notably, Thar-T4Ease exhibited remarkable thermostability, with a t1/2 value of 198 h at 80 °C. Moreover, it achieved a conversion ratio of 18.9% using 100 g/L d-fructose as the substrate. Finally, based on sequence and structure analysis, crucial residues for the catalytic activity of Thar-T4Ease were identified by molecular docking and site-directed mutagenesis. This research expands the repertoire of enzymes with C4-epimerization activity and opens up new possibilities for the cost-effective production of d-tagatose from d-fructose.

Permeabilized whole cells containing co-expressed cyclomaltodextrinase and maltooligosyltrehalose synthase facilitate the synthesis of nonreducing maltoheptaose (N-G7) from β-cyclodextrin

BACKGROUND

Maltodextrin is an important bulk ingredient in food and other industries; however, drawbacks such as uneven polymerization and high reducibility limit its utilization. Nonreducing maltoheptaose (N-G7) is a good substitute for maltodextrin owing to its single degree of polymerization and its nonreducing properties. In this study, in vitro cell factory biotransformation of β-cyclodextrin (β-CD) to N-G7 is demonstrated using coexpressed cyclomaltodextrinase (CDase, EC 3.2.1.54) and maltooligosyltrehalose synthase (MTSase, EC 5.4.99.15). However, the cell membrane prevents β-CD from entering the cell owing to its large diameter.

RESULTS

The amylase-deficient permeabilized host ΔycjM-ΔmalS-ΔlpxM is utilized for the coexpression of recombinant CDase and MTSase. Deletion of lpxM effectively allows the entry of β-cyclodextrin into the cell, despite its large diameter, without requiring any relevant cell membrane permeability-promoting reagent. This results in a 28.44% increase in the efficiency of β-CD entry into the cell, thus enabling intracellular N-G7 synthesis without the extracellular secretion of recombinant CDase and MTSase. After reacting for 5.5 h, the highest purity of N-G7 (65.50%) is obtained. However, hydrolysis decreases the purity of N-G7 to 49.30%, thus resulting in a conversion rate of 40.16% for N-G7 when the reaction lasts 6 h. Precise control of reaction time is crucial for obtaining high-purity N-G7.

CONCLUSION

Whole-cell catalysis avoids cell fragmentation and facilitates the creation of an eco-friendly, energy-efficient biotransformation system; thus, it is a promising approach for N-G7 synthesis. © 2023 Society of Chemical Industry.

Recent Advances in the Biosynthesis of Natural Sugar Substitutes in Yeast

Natural sugar substitutes are safe, stable, and nearly calorie-free. Thus, they are gradually replacing the traditional high-calorie and artificial sweeteners in the food industry. Currently, the majority of natural sugar substitutes are extracted from plants, which often requires high levels of energy and causes environmental pollution. Recently, biosynthesis via engineered microbial cell factories has emerged as a green alternative for producing natural sugar substitutes. In this review, recent advances in the biosynthesis of natural sugar substitutes in yeasts are summarized. The metabolic engineering approaches reported for the biosynthesis of oligosaccharides, sugar alcohols, glycosides, and rare monosaccharides in various yeast strains are described. Meanwhile, some unresolved challenges in the bioproduction of natural sugar substitutes in yeast are discussed to offer guidance for future engineering.

Biocatalytic Foams from Microdroplet-Formulated Self-Assembling Enzymes

Industrial biocatalysis plays an important role in the development of a sustainable economy, as enzymes can be used to synthesize an enormous range of complex molecules under environmentally friendly conditions. To further develop the field, intensive research is being conducted on process technologies for continuous flow biocatalysis in order to immobilize large quantities of enzyme biocatalysts in microstructured flow reactors under conditions that are as gentle as possible in order to realize efficient material conversions. Here, monodisperse foams consisting almost entirely of enzymes covalently linked via SpyCatcher/SpyTag conjugation are reported. The biocatalytic foams are readily available from recombinant enzymes via microfluidic air-in-water droplet formation, can be directly integrated into microreactors, and can be used for biocatalytic conversions after drying. Reactors prepared by this method show surprisingly high stability and biocatalytic activity. The physicochemical characterization of the new materials is described and exemplary applications in biocatalysis are shown using two-enzyme cascades for the stereoselective synthesis of chiral alcohols and the rare sugar tagatose.

Metabolic Engineering of Escherichia coli for High-Level Production of Lacto-N-neotetraose and Lacto-N-tetraose

Lacto-N-neotetraose (LNnT) and lacto-N-tetraose (LNT) are important oligosaccharides found in breast milk and are commonly used as nutritional supplements in infant formula. We used metabolic engineering techniques to optimize the modified Escherichia coli BL21 star (DE3) strain for efficient synthesis of LNnT and LNT using β-1,4-galactosyltransferase (HpgalT) from Helicobacter pylori and β-1,3-galactosyltransferase (SewbdO) from Salmonella enterica subsp. salamae serovar, respectively. Further, we optimized the expression of three key genes, lgtA, galE, and HpgalT (SewbdO), to synthesize LNnT or LNT and deleted several genes (ugd, ushA, agp, wcaJ, otsA, and wcaC) to block competition in the UDP-galactose synthesis pathway. The optimized strain produced LNnT or LNT with a titer of 22.07 or 48.41 g/L, respectively, in a supplemented batch culture, producing 0.41 or 0.73 g/L/h, respectively. The strategies used in this study contribute to the development of cell factories for high-level LNnT and LNT and their derivatives.

Analysis of Akkermansia muciniphila in Mulberry Galacto-Oligosaccharide Medium via Comparative Transcriptomics

Akkermansia muciniphila is a common member of the human gut microbiota and belongs to the phylum Verrucomicrobia. Decreased levels of A. muciniphila are associated with many diseases, so it is thought to be a beneficial resident of the intestinal mucosal layer. In this study, we found that different prebiotics promoted the proliferation of A. muciniphila, and mulberry galacto-oligosaccharide (MGO) had the greatest effect. We cultured A. muciniphila in a brian heart infusion (BHI) medium containing 5% galactooligosaccharides (GOS), mulberry polysaccharide solution (MPS), and MGO, and transcriptomic analyses were performed. The results revealed that, after 6 days of cultivation, the numbers of upregulated functional genes (based on Gene Ontology) were approximately 0.7 and 19% higher with MPS and MGO, respectively, than with GOS. Analysis using the Kyoto Encyclopedia of Genes and Genomes showed that, when A. muciniphila was cultured with MGO, genes that were upregulated were enriched in the carbohydrate metabolism, the metabolism of cofactors and vitamins, the energy metabolism, the amino acid metabolism, and the lipid metabolism. Upregulated genes included galM and pfkA in the galactose metabolism, and pgi, pfk, fbaA, tpiA, gapA, pgk, gpml, eno, pyk, and lpd in the glycolysis/gluconeogenesis pathway. Real-time quantitative PCR results were consistent with the RNA-Seq data. This work provides valuable knowledge which can be available for the functional application of A. muciniphila and MGO.

Isolation, identification, and biochemical characterization of a novel bifunctional phosphomannomutase/phosphoglucomutase from the metagenome of the brown alga Laminaria digitata

Macroalgae host diverse epiphytic bacterial communities with potential symbiotic roles including important roles influencing morphogenesis and growth of the host, nutrient exchange, and protection of the host from pathogens. Macroalgal cell wall structures, exudates, and intra-cellular environments possess numerous complex and valuable carbohydrates such as cellulose, hemi-cellulose, mannans, alginates, fucoidans, and laminarin. Bacterial colonizers of macroalgae are important carbon cyclers, acquiring nutrition from living macroalgae and also from decaying macroalgae. Seaweed epiphytic communities are a rich source of diverse carbohydrate-active enzymes which may have useful applications in industrial bioprocessing. With this in mind, we constructed a large insert fosmid clone library from the metagenome of Laminaria digitata (Ochrophyta) in which decay was induced. Subsequent sequencing of a fosmid clone insert revealed the presence of a gene encoding a bifunctional phosphomannomutase/phosphoglucomutase (PMM/PGM) enzyme 10L6AlgC, closely related to a protein from the halophilic marine bacterium, Cobetia sp. 10L6AlgC was subsequently heterologously expressed in Escherichia coli and biochemically characterized. The enzyme was found to possess both PMM and PGM activity, which had temperature and pH optima of 45°C and 8.0, respectively; for both activities. The PMM activity had a K m of 2.229 mM and V max of 29.35 mM min-1 mg-1, while the PGM activity had a K m of 0.5314 mM and a V max of 644.7 mM min-1 mg-1. Overall characterization of the enzyme including the above parameters as well as the influence of various divalent cations on these activities revealed that 10L6AlgC has a unique biochemical profile when compared to previously characterized PMM/PGM bifunctional enzymes. Thus 10L6AlgC may find utility in enzyme-based production of biochemicals with different potential industrial applications, in which other bacterial PMM/PGMs have previously been used such as in the production of low-calorie sweeteners in the food industry.