D-allulose, a versatile rare sugar: recent biotechnological advances and challenges

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.

Eco-friendly production, separation and purification of D-tagatose and D-allulose from whey powder via one-pot whole-cells biotransformation, yeast fermentation and chromatography

Whey powder (WP), a dairy by-product with high biochemical oxygen demand (BOD) and chemical oxygen demand (COD), presents challenges due to its high production, low-value utilization, and environmental pollution. Based on the idea of turning waste into treasure, high-value use of WP was studied. Firstly, an engineered Bacillus subtilis co-expressing β-galactosidase (β-Gal) and L-arabinose isomerase (LAI) was constructed, which ultimately yielded 77.5 g/L D-tagatose from 500 g/L lactose. Subsequently, an engineered Escherichia coli co-expressing glucose isomerase (GI) and D-allulose 3-epimerase (DAE) was used together with above recombinant B. subtilis in a one-pot whole-cell biotransformation, and 29.11 g/L D-tagatose and 11.45 g/L D-allulose were derived from 200 g/L WP (equating to 140 g/L lactose) with yield of 0.29 g rare sugars/g lactose. In addition, the d-glucose, d-fructose and D-galactose in the reaction solution were removed by Saccharomyces cerevisiae S288C fermentation, and finally chromatography was used in separation of D-tagatose and D-allulose to obtain the purified products with 97.5 % and 95.0 % purities, respectively. This study showcases the eco-friendly production of D-tagatose and D-allulose from WP, with their separation and purification via yeast fermentation and chromatography successfully carried out for the first time.

Effects of oligosaccharides, sodium carboxymethyl cellulose and d-allulose as a compound improver on the quality of sugar-reduced bread

BACKGROUND

Growing public concern over the health risks of high-sugar diets has led to a consensus on the necessity of sugar reduction. This research evaluated the effects of oligosaccharides (OS), sodium carboxymethyl cellulose (CMCNa) and d-allulose as a compound improver on the quality of sugar-reduced bread, aiming to assess the feasibility of substantial sucrose reduction at the same time as maintaining desirable bread characteristics.

RESULTS

Compared to bread with a 90% sugar reduction, the improved formulation increased specific volume by 50.14% and reduced hardness by 66.69%. It exhibited a homogeneous structure and crust color closely resembling that of full-sugar bread. Low-field NMR analysis revealed better moisture control, delaying water loss and starch retrogradation, with relative crystallinity and retrogradation enthalpy decreased by 45.31% and 59.96%, respectively. Additionally, the combination of XOS and d-allulose boosted volatile compound production, increasing the abundance of aldehydes, esters and heterocyclic compounds, enriching the flavor with fruity and baked aromas.

CONCLUSION

The improvers enhanced the texture, appearance, flavor and storage stability of sugar-reduced bread, yielding qualities that are comparable to or even surpass conventional bread. These findings provide a new insight for the development of quality improvers designed for sugar-reduced bakery products. © 2024 Society of Chemical Industry.

Unleashing the innate ability of Escherichia coli to produce D-Allose

D-allose is a rare monosaccharide, found naturally in low abundances. Due to its low-calorie profile and similar taste to sucrose, D-allose has the potential to become an ideal sugar substitute. D-allose also displays unique properties and health benefits that can be applied to various fields, including food and medicine. D-allose can be produced using two enzymatic steps in vitro: the epimerization of D-fructose, then the isomerization of the resulting D-psicose. This method suffers from poor yield due to the reversible nature of both reactions. We found that Escherichia coli possesses all of the required enzymes to convert D-glucose to D-allose with a thermodynamically favorable pathway, through a series of phosphorylation-epimerization-isomerization-dephosphorylation steps. To increase carbon flux toward D-allose production, the pathway genes were additionally expressed, and the competing pathways were removed. The engineered strains achieved production of D-allose, at a titer of 56.4 g L-1, a productivity of 0.65 g L-1 hr-1, and a yield of 41.4% under test tube conditions.

D-allulose 3-epimerase for low-calorie D-allulose synthesis: microbial production, characterization, and applications

D-allulose, an epimer of D-fructose at C-3 position, is a low-calorie rare sugar with favorable physiochemical properties and special physiological functions, which displays promising perspectives in the food and pharmaceutical industries. Currently, D-allulose is extremely sparse in nature and is predominantly biosynthesized through the isomerization of D-fructose by D-allulose 3-epimerase (DAEase). In recent years, D-allulose 3-epimerase as the key biocatalyst for D-allulose production has received increasing interest. The current review begins by providing a summary of D-allulose regarding its characteristics and applications, as well as different synthesis pathways dominated by biotransformation. Then, the research advances of D-allulose 3-epimerase are systematically reviewed, focusing on heterologous expression and biochemical characterization, crystal structure and molecular modification, and application in D-allulose production. Concerning the constraint of low yield of DAEase for industrial application, this review addresses the various attempts made to promote the production of DAEase in different expression systems. Also, various strategies have been adopted to improve its thermotolerance and catalytic activity, which is mainly based on the structure-function relationship of DAEase. The application of DAEase in D-allulose biosynthesis from D-fructose or low-cost feedstocks through single- or multi-enzymatic cascade reaction has been discussed. Finally, the prospects for related research of D-allulose 3-epimerase are also proposed, facilitating the industrialization of DAEase and more efficient and economical bioproduction of D-allulose.

Dietary Guidance, Sensory, Health and Safety Considerations When Choosing Low and No-Calorie Sweeteners

The growing global focus on the adverse health conditions associated with excessive sugar consumption has prompted health and policy organizations as well as the public to take a more mindful approach to health and wellness. In response, food and beverage companies have proactively innovated and reformulated their product portfolios to incorporate low and no-calorie sweeteners (LNCSs) as viable alternatives to sugar. LNCSs offer an effective and safe approach to delivering sweetness to foods and beverages and reducing calories and sugar intake while contributing to the enjoyment of eating. The objective of this paper is to enhance the understanding of LNCSs segmentation and definitions, dietary consumption and reduction guidance, front-of-package labeling, taste and sensory perception and physiology, metabolic efficacy and impact, as well as the overall safety of LNCSs and sugar.

Synthesis of value-added uridine 5'-diphosphate-glucose from sucrose applying an engineered sucrose synthase counteracts the activity-stability trade-off

The one-step deconstruction of sucrose into uridine 5'-diphosphate-glucose (UDP-Glc), an important sugar donor for transglycosylation, employing sucrose synthase (Susy) is emerging as a valuable sucrose utilization process. The insufficient activity and stability of Susy limit the productivity of UDP-Glc from sucrose. Here, an engineered Susy (SusyM6) that counteracted the activity-stability trade-off was developed with the half-life time and activity being 43-fold and 1.4-fold of wild-type, respectively. Tighter hydrophobic patches and stabilization of the SSN2 domain contributed to greater activity and stability. The use of SusyM6 in UDP-Glc production resulted in a satisfactory space-time yield of 73 g/L/h within 1 h. The cascade of different biocatalysts with SusyM6, focusing on utilizing two products of sucrose decomposition, fructose and UDP-Glc, expanded sucrose utilization, efficiently promoting the UDP-Glc productivity and giving a cost-effective method for UDP-galactose (UDP-Gal) synthesis. This study demonstrated promising green pathways for producing multiple value-added products from sucrose using Susy.

Advances in the bioproduction of d-allulose: A comprehensive review of current status and future prospects

As living standards rise, the overconsumption of sugary and calorific foods has led to a rise in obesity, diabetes, and other diseases. In response to the increasing demand for healthier diets, the food industry is actively seeking sugar alternatives. Among these alternatives, d-allulose as a functional sweetener has garnered significant attention for its low-calorie content, low glycemic index, and health benefits. This review summarizes recent advancements in d-allulose research, including its physiological functions, potential applications, and bioproduction methods. This review consolidates the known physiological functions of d-allulose and assesses its potential applications in the food and medical industries. Furthermore, the review explores recent progress in biotechnological production technologies, such as enzymatic conversion and microbial fermentation, which are key to producing d-allulose. d-Allulose is a standout natural sweetener with low calories and a low glycemic index, providing health benefits like lowering blood sugar and lipids, antioxidants, preventing obesity, and regulating metabolism. In the food industry, d-allulose is suitable for use in a variety of products, including baked goods, beverages, confectionery, and yogurt. The primary methods for its production are enzymatic conversion and microbial fermentation, both of which offer scalable and sustainable approaches. Recent research has advanced the production of d-allulose using low-cost raw materials, including agricultural and forestry waste, and even CO2, highlighting a move towards more sustainable production methods. With its diverse physiological functions and broad application prospects, d-allulose holds significant potential for growth in both the food and healthcare sectors.

Photocatalytic production and biological activity of D-arabino-1,4-lactone from D-fructose

Lactones play crucial roles in various fields, such as pharmaceuticals, food, and materials science, due to their unique structures and diverse biological activities. However, certain lactones are difficult to obtain in large quantities from natural sources, necessitating their synthesis to study their properties and potential. In this study, we investigated the photocatalytic conversion of D-fructose, a biomass-derived and naturally abundant sugar, using a TiO2 photocatalyst under light irradiation in ambient conditions. The resulting products were identified using HPLC, LCMS, MALDI TOF MS, and 1H NMR. The results confirmed the successful production of D-arabino-1,4-lactone as a key product, along with the formation of other valuable compounds, including rare sugars such as erythrose and glyceraldehyde. Analysis of the reaction mechanism revealed that D-arabino-1,4-lactone can be directly produced by the α scission (C1-C2 position cleavage) of D-fructose. Furthermore, erythrose and glyceraldehyde, as rare sugars, can be produced from the decomposition of D-arabino-1,4-lactone, which means that D-arabino-1,4-lactone can be used as a source of rare sugars. Furthermore, to investigate the biological activity of D-arabino-1,4-lactone, it was administered to Bifidobacterium. The results showed that Bifidobacterium proliferated and produced more lactic acid than when cultured in a medium without D-arabino-1,4-lactone, suggesting that Bifidobacterium can utilize D-arabino-1,4-lactone.

Tailored magnetic silica-immobilized D-allulose 3-epimerase with enhanced stability and recyclability for efficient D-allulose production

D-allulose, a low-calorie functional sweetener, is produced by the enzymatic conversion of d-fructose via D-allulose 3-epimerase (DAE) and holds significant market potential, particularly for individuals with obesity and diabetes. However, the limited reusability and stability of DAE have restricted its industrial application. In this study, we developed functional superparamagnetic supports by integrating diatomite, a biomineralized silica-based material, with cobalt ferrite nanoparticles through a green chemical co-precipitation method. The covalent attachment of DAE enzymes to these magnetic supports resulted in enzyme-metal hybrid catalysts (DAE@mDE-NH2) that exhibited enhanced stability and facilitated recovery and reuse via magnetic separation. These catalysts showed superior stability in acidic conditions and high temperatures, with a 24-fold increase in half-life at 60 °C compared to free DAE. They also exhibited remarkable durability, retaining 95.36 % of their activity after six months of storage at 4 °C and 70.08 % activity after 12 consecutive cycles. Utilizing this robust and recyclable biocatalyst, 147.7 g/L of D-allulose was obtained from 500 g/L of d-fructose. This study presents a sustainable strategy for advancing the production of high-value functional sweeteners like D-allulose while providing new insights into enzyme immobilization for biocatalytic processes.

Multienzyme Cascade Synthesis of Rare Sugars From Glycerol in Bacillus subtilis

BACKGROUND

Rare sugars are valuable and unique monosaccharides extensively utilized in the food, cosmetics, and pharmaceutical industries. Considering the high purification costs and the complex processes of enzymatic synthesis, whole-cell conversion has emerged as a significantly important alternative. The Escherichia coli strain was initially used in whole-cell synthesis of rare sugars. However, its pathogenic nature poses limitations to its widespread applications. Consequently, there is an urgent need to explore biologically safe strains for the efficient production of rare sugars.

RESULTS

In this study, the generally regarded as safe (GRAS) strain Bacillus subtilis was employed as the chassis cells to produce rare sugars via whole-cell conversion. Three genes encoding alditol oxidase (AldO), L-rhamnulose-1-phosphate aldolase (RhaD), and fructose-1-phosphatase (YqaB) involved in rare sugars biosynthesis were heterogeneously expressed in B. subtilis to convert the only substrate glycerol into rare sugars. To enhance the expression levels of the relevant enzymes in B. subtilis, different promoters for aldO, rhaD, and yqaB were investigated and optimized in this system. Under the optimized reaction conditions, the maximum total production titer was 16.96 g/L of D-allulose and D-sorbose with a conversion yield of 33.9% from glycerol. Furthermore, the engineered strain produced 26.68 g/L of D-allulose and D-sorbose through fed-batch for the whole-cell conversion, representing the highest titer from glycerol reported to date.

CONCLUSION

This study demonstrated an efficient and cost-effective method for the synthesis of rare sugars, providing a food-grade platform with the potential to meet the growing demand for rare sugars in industries.

Advances in the biosynthesis of D-allulose

D-allulose is a rare monosaccharide and a C-3 epimer of D-fructose. It has physiological functions, such as antihyperglycemic, obesity-preventing, neuroprotective, and reactive oxygen species (ROS) scavenging effects, making it an ideal sugar substitute. The synthesis methods for D-allulose include chemical synthesis and biosynthesis. Chemical synthesis requires strict reaction conditions and tends to produce byproducts. Biosynthesis is mainly an enzymatic process. Enzymatic catalysis for the conversion of starch or glycerol to D-allulose is performed mainly by enzymes such as isoamylase (IA), glucose isomerase (GI), D-allulose 3-epimerase (DPE), D-allulose-6-phosphate 3-epimerase (A6PE), D-allulose 6-phosphate phosphatase (A6PP), ribitol 2-dehydrogenase (RDH), glycerophosphate kinase (GK), glycerophosphate oxidase (GPO), and dihydroxyacetone phosphate (DHAP)-dependent aldolase. Biosynthesis is a more energy-efficient process, producing fewer harmful by-products and pollutants, and significantly reducing negative environmental impacts. Furthermore, the specific catalytic activity of enzymes facilitates the production of compounds of higher purity, thereby facilitating the isolation and purification of the products. It has thus become the main method for producing D-allulose. This article reviews the progress in research on the biosynthetic production of D-allulose, focusing on the enzymes involved and their enzymatic properties, and discusses the production prospects for D-allulose.

Low-calorie d-allulose as a sucrose alternative modulates the physicochemical properties and volatile profile of sponge cake

d-Allulose, a C-3 epimer of d-fructose, is a rare sugar with ∼70% of the sweetness of sucrose but a caloric content of only 0.4 kcal/g. Due to its low-calorie nature, d-allulose has garnered increasing interest in the food industry. This study was the first attempt to explore the effect of d-allulose as a sucrose replacer on the properties of sponge cake, a widely consumed high-sugar product. Substituting sucrose with d-allulose generated negligible impact on the batter system, while pronounced differences in physicochemical properties of cakes were detected, including specific volume, texture, microstructure, color, and antioxidant activity. In addition, sponge cake containing d-allulose displayed a distinctive aroma volatile profile, with more furans and pyrazines generation. Furthermore, correlations of physicochemical properties across all formulations were depicted, and the potential mechanism behind the property alterations modulated by d-allulose was revealed from the perspectives of starch gelatinization and browning reactions. Overall, this study provides insights into the application potential of d-allulose as a sucrose substitute in bakery product. PRACTICAL APPLICATION: This study elucidates the effect of d-allulose as a low-calorie sugar substitute on sponge cakes. This finding is valuable for the food industry, providing insights into a healthier alternative to traditional sugar in baked goods.

Comprehensive Analysis of Allulose Production: A Review and Update

Advancements in D-allulose production have seen significant strides in recent years, focusing on enzymatic conversion methods. Key developments include traditional immobilization techniques, the discovery of novel enzymes, directed evolution studies, and biosynthesis through metabolic pathway modification. Enzymatic conversion, particularly utilizing D-allulose 3-epimerase, remains fundamental for industrial-scale production. Innovative immobilization strategies, such as functionalized nano-beads and magnetic MOF nanoparticles, have significantly enhanced enzyme stability and reusability. Directed evolution has led to improved enzyme thermostability and catalytic efficiency, while synthetic biology methods, including phosphorylation-driven and thermodynamics-driven pathways, have optimized production processes. High-throughput screening methods have been crucial in identifying and refining enzyme variants for industrial applications. Collectively, these advancements not only enhance production efficiency and cost-effectiveness but also adhere to sustainable and economically viable manufacturing practices. The past five years have witnessed critical developments with significant potential impact on the commercial viability and global demand for allulose.

A comprehensive review on natural sweeteners: impact on sensory properties, food structure, and new frontiers for their application

In recent years, the worldwide increase in lifestyle diseases and metabolic disorders has been ascribed to the excessive consumption of sucrose and added sugars. For this reason, many approaches have been developed in order to replace sucrose in food and beverage formulations with alternative sweetening compounds. The raising awareness concerning the synthetic sweeteners due to their negative impact on health, triggered the need to search for alternative substances. Natural sweeteners may be classified in: (i) non-nutritive (e.g., neohesperidine dihydrochalcone, thaumatin, glycyrrhizin mogroside and stevia) and (ii) bulk sweeteners, including both polyols (e.g., maltitol, mannitol, erythritol) and rare sugars (e.g., tagatose and allulose). In this review we discuss the most popular natural sweeteners and their application in the main food sectors (e.g., bakery, dairy, confectionary and beverage), providing a full understanding of their impact on the textural and sensory properties in comparison to sucrose. Furthermore, we analyze the use of natural sweeteners in blends, which in addition to enabling an effective replacement of sugar, in order to complement the merits and limits of individual compounds. Finally, microencapsulation technology is presented as an alternative strategy to solving some issues such as aftertaste, bitterness, unpleasant flavors, but also to enhance their stability and ease of use.

Directed Evolution of Escherichia coli Nissle 1917 to Utilize Allulose as Sole Carbon Source

Sugar substitutes are popular due to their akin taste and low calories. However, excessive use of aspartame and erythritol can have varying effects. While D-allulose is presently deemed a secure alternative to sugar, its excessive consumption is not devoid of cellular stress implications. In this study, the evolution of Escherichia coli Nissle 1917 (EcN) is directed to utilize allulose as sole carbon source through a combination of adaptive laboratory evolution (ALE) and fluorescence-activated droplet sorting (FADS) techniques. Employing whole genome sequencing (WGS) and clustered regularly interspaced short palindromic repeats interference (CRISPRi) in conjunction with compensatory expression displayed those genetic mutations in sugar and amino acid metabolic pathways, including glnP, glpF, gmpA, nagE, pgmB, ybaN, etc., increased allulose assimilation. Enzyme-substrate dynamics simulations and deep learning predict enhanced substrate specificity and catalytic efficiency in nagE A247E and pgmB G12R mutants. The findings evince that these mutations hold considerable promise in enhancing allulose uptake and facilitating its conversion into glycolysis, thus signifying the emergence of a novel metabolic pathway for allulose utilization. These revelations bear immense potential for the sustainable utilization of D-allulose in promoting health and well-being.

Rational design improves both thermostability and activity of a new D-tagatose 3-epimerase from Kroppenstedtia eburnean to produce D-allulose

D-allulose is a naturally occurring rare sugar and beneficial to human health. However, the efficient biosynthesis of D-allulose remains a challenge. Here, we mined a new D-tagatose 3-epimerase from Kroppenstedtia eburnean (KeDt3e) with high catalytic efficiency. Initially, crucial factors contributing to the low conversion of KeDt3e were identified through crystal structure analysis, density functional theory calculations (DFT), and molecular dynamics (MD) simulations. Subsequently, based on the mechanism, combining restructuring the flexible region, proline substitution based onconsensus sequence analysis, introducing disulfide bonds, and grafting properties, and reshaping the active center, the optimal mutant M5 of KeDt3e was obtained with enhanced thermostability and activity. The optimal mutant M5 exhibited an enzyme activity of 130.8 U/mg, representing a 1.2-fold increase; Tm value increased from 52.7 °C to 71.2 °C; and half-life at 55 °C extended to 273.7 min, representing a 58.2-fold improvement, and the detailed mechanism of performance improvement was analyzed. Finally, by screening the ribosome-binding site (RBS) of the optimal mutant M5 recombinant bacterium (G01), the engineered strain G08 with higher expression levels was obtained. The engineered strain G08 catalyzed 500 g/L D-fructose to produce 172.4 g/L D-allulose, with a conversion of 34.4% in 0.5 h and productivity of 344.8 g/L/h on a 1 L scale. This study presents a promising approach for industrial-scale production of D-allulose.

Designable immobilization of D-allulose 3-epimerase on bimetallic organic frameworks based on metal ion compatibility for enhanced D-allulose production

D-allulose, a low-calorie rare sugar catalyzed by D-allulose 3-epimerase (DAE), is highly sought after for its potential health benefits. However, poor reusability and stability of DAE limited its popularization in industrial applications. Although metal-organic frameworks (MOFs) offer a promising enzyme platform for enzyme immobilization, developing customized strategies for MOF immobilization of enzymes remains challenging. In this study, we introduce a designable strategy involving the construction of bimetal-organic frameworks (ZnCo-MOF) based on metal ions compatibility. The DAE@MOFs materials were prepared and characterized, and the immobilization of DAE and the enzymatic characteristics of the MOF-immobilized DAE were subsequently evaluated. Remarkably, DAE@ZnCo-MOF exhibited superior recyclability which could maintain 95 % relative activity after 8 consecutive cycles. The storage stability is significantly improved compared to the free form, with a relative activity of 116 % remaining after 30 days. Molecular docking was also employed to investigate the interaction between DAE and the components of MOFs synthesis. The results demonstrate that the DAE@ZnCo-MOF exhibited enhanced catalytic efficiency and increased stability. This study introduces a viable and adaptable MOF-based immobilization strategy for enzymes, which holds the potential to expand the implementation of enzyme biocatalysts in a multitude of disciplines.

Impact of artificial sweeteners and rare sugars on the gut microbiome

Alternative sugars are often used as sugar substitutes because of their low calories and glycemic index. Recently, consumption of these sweeteners in diet foods and beverages has increased dramatically, raising concerns about their health effects. This review examines the types and characteristics of artificial sweeteners and rare sugars and analyzes their impact on the gut microbiome. In the section on artificial sweeteners, we have described the chemical structures of different sweeteners, their digestion and absorption processes, and their effects on the gut microbiota. We have also discussed the biochemical properties and production methods of rare sugars and their positive and negative effects on gut microbial communities. Finally, we have described how artificial sweeteners and rare sugars alter the gut microbiome and how these changes affect the gut environment. Our observations aim to improve our understanding regarding the potential health implications of the consumption of artificial sweeteners and low-calorie sugars.

The Formation of D-Allulose 3-Epimerase Hybrid Nanoflowers and Co-Immobilization on Resins for Improved Enzyme Activity, Stability, and Processability

As a low-calorie sugar, D-allulose is produced from D-fructose catalyzed by D-allulose 3-epimerase (DAE). Here, to improve the catalytic activity, stability, and processability of DAE, we reported a novel method by forming organic-inorganic hybrid nanoflowers (NF-DAEs) and co-immobilizing them on resins to form composites (Re-NF-DAEs). NF-DAEs were prepared by combining DAE with metal ions (Co2+, Cu2+, Zn2+, Ca2+, Ni2+, Fe2+, and Fe3+) in PBS buffer, and were analyzed by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy, and X-ray diffraction. All of the NF-DAEs showed higher catalytic activities than free DAE, and the NF-DAE with Ni2+ (NF-DAE-Ni) reached the highest relative activity of 218%. The NF-DAEs improved the thermal stability of DAE, and the longest half-life reached 228 min for NF-DAE-Co compared with 105 min for the free DAE at 55 °C. To further improve the recycling performance of the NF-DAEs in practical applications, we combined resins and NF-DAEs to form Re-NF-DAEs. Resins and NF-DAEs co-effected the performance of the composites, and ReA (LXTE-606 neutral hydrophobic epoxy-based polypropylene macroreticular resins)-based composites (ReA-NF-DAEs) exhibited outstanding relative activities, thermal stabilities, storage stabilities, and processabilities. The ReA-NF-DAEs were able to be reused to catalyze the conversion from D-fructose to D-allulose, and kept more than 60% of their activities after eight cycles.

Enhancing the stability of a novel D-allulose 3-epimerase from Ruminococcus sp. CAG55 by interface interaction engineering and terminally attached a self-assembling peptide

D-allulose, a highly desirable sugar substitute, is primarily produced using the D-allulose 3-epimerase (DAE). However, the availability of usable DAE enzymes is limited. In this study, we discovered and engineered a novel DAE Rum55, derived from a human gut bacterium Ruminococcus sp. CAG55. The activity of Rum55 was strictly dependent on the presence of Co2+, and it exhibited an equilibrium conversion rate of 30.6 % and a half-life of 4.5 h at 50 °C. To enhance its performance, we engineered the interface interaction of Rum55 to stabilize its tetramer structure, and the best variant E268R was then attached with a self-assembling peptide to form active enzyme aggregates as carrier-free immobilization. The half-life of the best variant E268R-EKL16 at 50 °C was dramatically increased 30-fold to 135.3 h, and it maintained 90 % of its activity after 13 consecutive reaction cycles. Additionally, we identified that metal ions played a key role in stabilizing the tetramer structure of Rum55, and the dependence on metal ions for E268R-EKL16 was significantly reduced. This study provides a useful route for improving the thermostability of DAEs, opening up new possibilities for the industrial production of D-allulose.

D-allose, a typical rare sugar: properties, applications, and biosynthetic advances and challenges

D-allose, a C-3 epimer of D-glucose and an aldose-ketose isomer of D-allulose, exhibits 80% of sucrose's sweetness while being remarkably low in calories and nontoxic, making it an appealing sucrose substitute. Its diverse physiological functions, particularly potent anticancer and antitumor effects, render it a promising candidate for clinical treatment, garnering sustained attention. However, its limited availability in natural sources and the challenges associated with chemical synthesis necessitate exploring biosynthetic strategies to enhance production. This overview encapsulates recent advancements in D-allose's physicochemical properties, physiological functions, applications, and biosynthesis. It also briefly discusses the crucial role of understanding aldoketose isomerase structure and optimizing its performance in D-allose synthesis. Furthermore, it delves into the challenges and future perspectives in D-allose bioproduction. Early efforts focused on identifying and characterizing enzymes responsible for D-allose production, followed by detailed crystal structure analysis to improve performance through molecular modification. Strategies such as enzyme immobilization and implementing multi-enzyme cascade reactions, utilizing more cost-effective feedstocks, were explored. Despite progress, challenges remain, including the lack of efficient high-throughput screening methods for enzyme modification, the need for food-grade expression systems, the establishment of ordered substrate channels in multi-enzyme cascade reactions, and the development of downstream separation and purification processes.

Enhancement of the d-Allulose 3-Epimerase Expression in Bacillus subtilis through Both Transcriptional and Translational Regulations

d-Allulose, a functional bulk sweetener, has recently attracted increasing attention because of its low-caloric-ness properties and diverse health effects. d-Allulose is industrially produced by the enzymatic epimerization of d-fructose, which is catalyzed by ketose 3-epimerase (KEase). In this study, the food-grade expression of KEase was studied using Bacillus subtills as the host. Clostridium sp. d-allulose 3-epimerase (Clsp-DAEase) was screened from nine d-allulose-producing KEases, showing better potential for expression in B. subtills WB600. Promoter-based transcriptional regulation and N-terminal coding sequence (NCS)-based translational regulation were studied to enhance the DAEase expression level. In addition, the synergistic effect of promoter and NCS on the Clsp-DAEase expression was studied. Finally, the strain with the combination of a PHapII promoter and gln A-Up NCS was selected as the best Clsp-DAEase-producing strain. It efficiently produced Clsp-DAEase with a total activity of 333.2 and 1860.6 U/mL by shake-flask and fed-batch cultivations, respectively.

Growth-Coupled Evolutionary Pressure Improving Epimerases for D-Allulose Biosynthesis Using a Biosensor-Assisted In Vivo Selection Platform

Fast screening strategies that enable high-throughput evaluation and identification of desired variants from diversified enzyme libraries are crucial to tailoring biocatalysts for the synthesis of D-allulose, which is currently limited by the poor catalytic performance of ketose 3-epimerases (KEases). Here, the study designs a minimally equipment-dependent, high-throughput, and growth-coupled in vivo screening platform founded on a redesigned D-allulose-dependent biosensor system. The genetic elements modulating regulator PsiR expression levels undergo systematic optimization to improve the growth-responsive dynamic range of the biosensor, which presents ≈30-fold facilitated growth optical density with a high signal-to-noise ratio (1.52 to 0.05) toward D-allulose concentrations from 0 to 100 mm. Structural analysis and evolutionary conservation analysis of Agrobacterium sp. SUL3 D-allulose 3-epimerase (ADAE) reveal a highly conserved catalytic active site and variable hydrophobic pocket, which together regulate substrate recognition. Structure-guided rational design and directed evolution are implemented using the growth-coupled in vivo screening platform to reprogram ADAE, in which a mutant M42 (P38N/V102A/Y201L/S207N/I251R) is identified with a 6.28-fold enhancement of catalytic activity and significantly improved thermostability with a 2.5-fold increase of the half-life at 60 °C. The research demonstrates that biosensor-assisted growth-coupled evolutionary pressure combined with structure-guided rational design provides a universal route for engineering KEases.

Enhancing enzyme immobilization: Fabrication of biosilica-based organic-inorganic composite carriers for efficient covalent binding of D-allulose 3-epimerase

D-allulose, an ideal low-calorie sweetener, is primarily produced through the isomerization of d-fructose using D-allulose 3-epimerase (DAE; EC 5.1.3.30). Addressing the gap in available immobilized DAE enzymes for scalable commercial D-allulose production, three core-shell structured organic-inorganic composite silica-based carriers were designed for efficient covalent immobilization of DAE. Natural inorganic diatomite was used as the core, while 3-aminopropyltriethoxysilane (APTES), polyethyleneimine (PEI), and chitosan organic layers were coated as the shells, respectively. These tailored carriers successfully formed robust covalent bonds with DAE enzyme conjugates, cross-linked via glutaraldehyde, and demonstrated enzyme activities of 372 U/g, 1198 U/g, and 381 U/g, respectively. These immobilized enzymes exhibited an expanded pH tolerance and improved thermal stability compared to free DAE. Particularly, the modified diatomite with PEI exhibited a higher density of binding sites than the other carriers and the PEI-coated immobilized DAE enzyme retained 70.4 % of its relative enzyme activity after ten cycles of reuse. This study provides a promising method for DAE immobilization, underscoring the potential of using biosilica-based organic-inorganic composite carriers for the development of robust enzyme systems, thereby advancing the production of value-added food ingredients like D-allulose.

Allulose mitigates chronic enteritis by reducing mitochondria dysfunction via regulating cathepsin B production

Metabolic changes have been linked to the development of inflammatory bowel disease (IBD), which includes colitis. Allulose, an endogenous bioactive monosaccharide, is vital to the synthesis of numerous compounds and metabolic processes within living organisms. Nevertheless, the precise biochemical mechanism by which allulose inhibits colitis remains unknown. Allulose is an essential and intrinsic protector of the intestinal mucosal barrier, as it maintains the integrity of tight junctions in the intestines, according to the current research. It is also important to know that there is a link between the severity of inflammatory bowel disease (IBD) and colorectal cancer (CRC), chemically-induced colitis in rodents, and lower levels of allulose in the blood. Mice with colitis, either caused by dextran sodium sulphate (DSS) or naturally occurring colitis in IL-10-/- mice, had less damage to their intestinal mucosa after being given allulose. Giving allulose to a colitis model starts a chain of reactions because it stops cathepsin B from ejecting and helps lysosomes stick together. This system effectively stops the activity of myosin light chain kinase (MLCK) when intestinal epithelial damage happens. This stops the breakdown of tight junction integrity and the start of mitochondrial dysfunction. To summarise, the study's findings have presented data that supports the advantageous impact of allulose in reducing the advancement of colitis. Its ability to stop the disruption of the intestinal barrier enables this. Therefore, allulose has potential as a medicinal supplement for treating colitis.

Biochemical characterization, structure-guided mutagenesis, and application of a recombinant D-allulose 3-epimerase from Christensenellaceae bacterium for the biocatalytic production of D-allulose

D-Allulose has become a promising alternative sweetener due to its unique properties of low caloric content, moderate sweetness, and physiological effects. D-Allulose 3-epimerase (DAEase) is a promising enzyme for D-Allulose production. However, the low catalytic efficiency limited its large-scale industrial applications. To obtain a more effective biocatalyst, a putative DAEase from Christensenellaceae bacterium (CbDAE) was identified and characterized. The recombinant CbDAE exhibited optimum activity at pH 7.5°C and 55°C, retaining more than 60% relative activity from 40°C to 70°C, and the catalytic activity could be significantly increased by Co2+ supplementation. These enzymatic properties of purified CbDAE were compared with other DAEases. CbDAE was also found to possess desirable thermal stability at 55°C with a half-life of 12.4 h. CbDAE performed the highest relative activity towards D-allulose and strong affinity for D-fructose but relatively low catalytic efficiency towards D-fructose. Based on the structure-guided design, the best double-mutation variant G36N/W112E was obtained which reached up to 4.21-fold enhancement of catalytic activity compared with wild-type (WT) CbDAE. The catalytic production of G36N/W112E with 500 g/L D-fructose was at a medium to a higher level among the DAEases in 3.5 h, reducing 40% catalytic reaction time compared to the WT CbDAE. In addition, the G36N/W112E variant was also applied in honey and apple juice for D-allulose conversion. Our research offers an extra biocatalyst for D-allulose production, and the comprehensive report of this enzyme makes it potentially interesting for industrial applications and will aid the development of industrial biocatalysts for D-allulose.

Bioproduction of Rare d-Allulose from d-Glucose via Borate-Assisted Isomerization

d-Allulose is a low-calorie functional rare sugar with excellent processing suitability and unique physiological efficacy. d-Allulose is primarily produced from d-fructose through enzymatic epimerization, facing the constraints of a low conversion yield and high production cost. In this study, a double-enzyme cascade system with tetraborate-assisted isomerization was constructed for the efficient production of d-allulose from inexpensive d-glucose. With the introduction of sodium tetraborate (STB), capable of forming complexes with diol-bearing sugars, the conversion yield of d-allulose from d-glucose substantially escalated from the initial 17.37% to 44.97%. Furthermore, d-allulose was found to exhibit the most pronounced binding affinity for STB with an association constant of 1980.51 M-1, notably surpassing that of d-fructose (183.31 M-1) and d-glucose (35.37 M-1). Additionally, the structural analysis of the sugar-STB complexes demonstrated that d-allulose reacted with STB via the cis 2,3-hydroxyl groups in the α-furanose form. Finally, the mechanism underlying STB-assisted isomerization was proposed, emphasizing the preferential formation of an allulose-STB complex that effectively shifts the isomerization equilibrium to the allulose side, thereby resulting in high yield of d-allulose. Such an STB-facilitated isomerization system would also provide a guidance for the cost-effective synthesis of other rare sugars.

Pectin conformation influences the bioaccessibility of cherry laurel polyphenols and gut microbiota distribution following in vitro gastrointestinal digestion and fermentation

Interactions between dietary fiber and phenolic compounds in foods can influence their gastrointestinal fate. This study aimed to examine the effect of four types of pectin on the polyphenols of cherry laurel puree and human gut microbiota during a simulated in vitro gastrointestinal digestion and large intestine fermentation. Results revealed that the combined addition of different pectins and pectinase to cherry laurel puree significantly affected the content and bioaccessibility of phenolics. The addition of pectins and pectinase distinctively impacted the phenolic subclasses in both raw and post-digested/fermented cherry laurel puree, suggesting differential interactions due to structural features. Both pectins and pectinase modulated the composition of fecal microbiota after in vitro fermentation, increasing bacterial diversity following pectinase treatment. The combined addition of pectins followed by pectinase had differential impacts on polyphenol bioaccessibility and gut microbiome diversity, hence having a potential outcome in terms of human health.

Sequence- and Structure-Based Mining of Thermostable D-Allulose 3-Epimerase and Computer-Guided Protein Engineering To Improve Enzyme Activity

D-Allulose, a functional sweetener, can be synthesized from fructose using D-allulose 3-epimerase (DAEase). Nevertheless, a majority of the reported DAEases have inadequate stability under harsh industrial reaction conditions, which greatly limits their practical applications. In this study, big data mining combined with a computer-guided free energy calculation strategy was employed to discover a novel DAEase with excellent thermostability. Consensus sequence analysis of flexible regions and comparison of binding energies after substrate docking were performed using phylogeny-guided big data analyses. TtDAE from Thermogutta terrifontis was the most thermostable among 358 candidate enzymes, with a half-life of 32 h at 70 °C. Subsequently, structure-guided virtual screening and a customized strategy based on a combinatorial active-site saturation test/iterative saturation mutagenesis were utilized to engineer TtDAE. Finally, the catalytic activity of the M4 variant (P105A/L14C/T63G/I65A) was increased by 5.12-fold. Steered molecular dynamics simulations indicated that M4 had an enlarged substrate-binding pocket, which enhanced the fit between the enzyme and the substrate. The approach presented here, combining DAEases mining with further rational modification, provides guidance for obtaining promising catalysts for industrial-scale production.

Isomerase and epimerase: overview and practical application in production of functional sugars

The biosynthesis of functional sugars has gained significant attention due to their potential health benefits and increasing demand in the food industry. Enzymatic synthesis has emerged as a promising approach, offering high catalytic efficiency, chemoselectivity, and stereoselectivity. However, challenges such as poor thermostability, low catalytic efficiency, and food safety concerns have limited the commercial production of functional sugars. Protein engineering, including directed evolution and rational design, has shown promise in overcoming these barriers and improving biocatalysts for large-scale production. Furthermore, enzyme immobilization has proven effective in reducing costs and facilitating the production of functional sugars. To ensure food safety, the use of food-grade expression systems has been explored. However, downstream technologies, including separation, purification, and crystallization, still pose challenges in terms of efficiency and cost-effectiveness. Addressing these challenges is crucial to optimize the overall production process. Despite the obstacles, the future outlook for functional sugars is promising, driven by increasing awareness of their health benefits and continuous technological advancements. With further research and technological breakthroughs, industrial-scale production of functional sugars through biosynthesis will become a reality, leading to their widespread incorporation in various industries and products.

Oat β-glucan supplementation pre- and during pregnancy alleviates fetal intestinal immunity development damaged by gestational diabetes in rats

Oat β-glucan (OG) has been shown to improve intestinal microecology in gestational diabetes mellitus (GDM), but the effect on fetal intestine health is unknown. Herein, we aimed to investigate the effects of OG supplementation during gestation in GDM dams on fetal intestinal immune development. OG was supplemented one week before mating until the end of the experiment. GDM rats were made with a high-fat diet (HFD) with a minimal streptozotocin (STZ) dose. The fetal intestines were sampled at gestation day (GD) 19.5, and the intestinal morphology, chemical barrier molecules, intraepithelial immune cell makers, and levels of inflammatory cytokines were investigated. The results showed that OG supplementation alleviated the decrease of the depth of fetal intestinal villi and crypts, the number of goblet cells (GCs), protein expression of mucin-1 (Muc1) and Muc2, the mRNA levels of Gpr41, Gpr43, and T cell markers, and increased the number of paneth cells (PCs), the mRNA levels of defensin-6 (defa6), and macrophage (Mø) marker and the expression of cytokines induced by GDM. In addition, OG supplementation alleviated the function of immune cell self-proliferation, chemotaxis and assembly capabilities, protein, fat, folic acid, and zinc absorption damaged by GDM. As indicated by these findings, OG supplementation before and during pregnancy improved the fetal intestinal chemical barriers, immune cells, cytokines, and the metabolism of nutrients to protect the fetal intestinal immunity.

Substantial Improvement of an Epimerase for the Synthesis of D-Allulose by Biosensor-Based High-Throughput Microdroplet Screening

Biosynthesis of D-allulose has been achieved using ketose 3-epimerases (KEases), but its application is limited by poor catalytic performance. In this study, we redesigned a genetically encoded biosensor based on a D-allulose-responsive transcriptional regulator for real-time monitoring of D-allulose. An ultrahigh-throughput droplet-based microfluidic screening platform was further constructed by coupling with this D-allulose-detecting biosensor for the directed evolution of the KEases. Structural analysis of Sinorhizobium fredii D-allulose 3-epimerase (SfDAE) revealed that a highly flexible helix/loop region exposes or occludes the catalytic center as an essential lid conformation regulating substrate recognition. We reprogrammed SfDAE using structure-guided rational design and directed evolution, in which a mutant M3-2 was identified with 17-fold enhanced catalytic efficiency. Our research offers a paradigm for the design and optimization of a biosensor-based microdroplet screening platform.

Low-calorie bulk sweeteners: Recent advances in physical benefits, applications, and bioproduction

At present, with the continuous improvement of living standards, people are paying increasing attention to dietary nutrition and health. Low sugar and low energy consumption have become important dietary trends. In terms of sugar control, more and more countries have implemented sugar taxes in recent years. Hence, as the substitute for sugar, low-calorie sweeteners have been widely used in beverage, bakery, and confectionary industries. In general, low-calorie sweeteners consist of high-intensity and low-calorie bulk sweeteners (some rare sugars and sugar alcohols). In this review, recent advances and challenges in low-calorie bulk sweeteners are explored. Bioproduction of low-calorie bulk sweeteners has become the focus of many researches, because it has the potential to replace the current industrial scale production through chemical synthesis. A comprehensive summary of the physicochemical properties, physiological functions, applications, bioproduction, and regulation of typical low-calorie bulk sweeteners, such as D-allulose, D-tagatose, D-mannitol, sorbitol, and erythritol, is provided.

Engineering of Acid-Resistant d-Allulose 3-Epimerase for Functional Juice Production

d-Allulose, a rare sugar and functional sweetener, can be biosynthesized by d-allulose 3-isomerase (DAE). However, most of the reported DAEs exhibit poor resistance under acidic conditions, which severely limited their application. Here, surface charge engineering and random mutagenesis were used to construct a mutant library of CcDAE from Clostridium cellulolyticum H10, combined with high-throughput screening to identify mutants with high activity and resistance under acidic conditions. The mutant M3 (I114R/K123E/H209R) exhibited high activity (3.36-fold of wild-type) and acid resistance (10.6-fold of wild-type) at pH 4.5. The structure-function relationship was further analyzed by molecular dynamics (MD) simulations, which indicated that M3 had a higher number of hydrogen bonds and negative surface charges than the wild type. A multienzyme cascade system including M3 was used to convert high-calorie sugars in acidic juices, and functional juices containing 7.8-15.4 g/L d-allulose were obtained. Our study broadens the manufacture of functional foods containing d-allulose.

Efficient Utilization of Fruit Peels for the Bioproduction of D-Allulose and D-Mannitol

Currently, the demand for low-calorie sweeteners has grown dramatically because consumers are more mindful of their health than they used to be. Therefore, bioproduction of low-calorie sweeteners from low-cost raw materials becomes a hot spot. In this study, a two-stage strategy was established to efficiently utilize D-fructose from fruit and vegetable wastes. Firstly, ketose 3-epimerase was used to produce D-allulose from D-fructose of pear peels. Secondly, the residual D-fructose was converted to D-mannitol by the engineered strain co-expression of D-mannitol 2-dehydrogenase and formate dehydrogenase. Approximately 29.4% D-fructose of pear peels was converted to D-allulose. Subsequently, under optimal conditions (35 °C, pH 6.5, 1 mM Mn²⁺, 2 g/L dry cells), almost all the residual D-fructose was transformed into D-mannitol with a 93.5% conversion rate. Eventually, from 1 kg fresh pear peel, it could produce 10.8 g of D-allulose and 24.6 g of D-mannitol. This bioprocess strategy provides a vital method to biosynthesize high-value functional sugars from low-cost biomass.

Boosting the Heterologous Expression of d-Allulose 3-Epimerase in Bacillus subtilis through Protein Engineering and Catabolite-Responsive Element Box Engineering

As a natural sweetener with low calories and various physiological activities, d-allulose has drawn worldwide attention. Currently, d-allulose 3-epimerase (DAEase) is mainly used to catalyze the epimerization of d-fructose to d-allulose. Therefore, it is quite necessary to enhance the food-grade expression of DAEase to meet the surging market demand for d-allulose. In this study, initially, the promising variant H207L/D281G/C289R of Clostridium cellulolyticum H10 DAEase (CcDAEase) was generated by protein engineering, the specific activity and the T_1/2 of which were 2.24-fold and 13.45-fold those of the CcDAEase wild type at 60 °C, respectively. After that, PamyE was determined as the optimal promoter for the recombinant expression of CcDAEase in Bacillus subtilis, and catabolite-responsive element (CRE) box engineering was further performed to eliminate the carbon catabolite repression (CCR) effect. Lastly, high-density fermentation was carried out and the final activity peaked at 4971.5 U mL^-1, which is the highest expression level and could effectively promote the industrial production of DAEase. This research provides a theoretical basis and technical support for the molecular modification of DAEase and its efficient fermentation preparation.

Crystal structure of a novel homodimeric D-allulose 3-epimerase from a Clostridia bacterium

D-Allulose, a low-calorie rare sugar with various physiological functions, is mainly produced through the isomerization of D-fructose by ketose 3-epimerases (KEases), which exhibit various substrate specificities. A novel KEase from a Clostridia bacterium (CDAE) was identified to be a D-allulose 3-epimerase and was further characterized as thermostable and metal-dependent. In order to explore its structure–function relationship, the crystal structure of CDAE was determined using X-ray diffraction at 2.10 Å resolution, revealing a homodimeric D-allulose 3-epimerase structure with extensive interactions formed at the dimeric interface that contribute to structure stability. Structural analysis identified the structural features of CDAE, which displays a common (β/α)8-TIM barrel and an ordered Mn2+-binding architecture at the active center, which may explain the positive effects of Mn2+ on the activity and stability of CDAE. Furthermore, comparison of CDAE and other KEase structures revealed several structural differences, highlighting the remarkable differences in enzyme–substrate binding at the O4, O5 and O6 sites of the bound substrate, which are mainly induced by distinct hydrophobic pockets in the active center. The shape and hydrophobicity of this pocket appear to produce the differences in specificity and affinity for substrates among KEase family enzymes. Exploration of the crystal structure of CDAE provides a better understanding of its structure–function relationship, which might provide a basis for molecular modification of CDAE and further provides a reference for other KEases.

Research progress on extraction technology and biomedical function of natural sugar substitutes

Improved human material living standards have resulted in a continuous increase in the rate of obesity caused by excessive sugar intake. Consequently, the number of diabetic patients has skyrocketed, not only resulting in a global health problem but also causing huge medical pressure on the government. Limiting sugar intake is a serious problem in many countries worldwide. To this end, the market for sugar substitute products, such as artificial sweeteners and natural sugar substitutes (NSS), has begun to rapidly grow. In contrast to controversial artificial sweeteners, NSS, which are linked to health concepts, have received particular attention. This review focuses on the extraction technology and biomedical function of NSS, with a view of generating insights to improve extraction for its large-scale application. Further, we highlight research progress in the use of NSS as food for special medical purpose (FSMP) for patients.

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.

Recent Advances Regarding the Physiological Functions and Biosynthesis of D-Allulose

D-Allulose, a generally regarded as safe (GRAS) sugar, is rare in nature. It is among the most promising sweeteners for future use due to its low caloric content, sucrose-like taste, and unique functions. D-Allulose has many physiological effects, such as antiobesity, antihyperglycemia, antidiabetes, anti-inflammatory, antioxidant, and neuroprotective effects. Therefore, D-allulose has important application value in the food, pharmaceutical, and healthcare industries. However, the high cost of D-allulose production limits its large-scale application. Currently, biotransformation is very attractive for D-allulose synthesis, with the two main methods of biosynthesis being the Izumoring strategy and the DHAP-dependent aldolase strategy. This article reviews recent advances regarding the physiological functions and biosynthesis of D-allulose. In addition, future perspectives on the production of D-allulose are presented.