Rare sugar D-allulose: Potential role and therapeutic monitoring in maintaining obesity and type 2 diabetes mellitus

Engineering Escherichia coli for D-allulose biosynthesis from glycerol

D-allulose, a naturally occurring monosaccharide, is present in small quantities in nature. It is considered a valuable low-calorie sweetener due to its low absorption in the digestive tract and zero energy for growth. Most of the recent efforts to produce D-allulose have focused on in vitro enzyme catalysis. However, microbial fermentation is emerging as a promising alternative that offers the advantage of combining enzyme manufacturing and product synthesis within a single bioreactor. Here, a novel approach was proposed for the efficient biosynthesis of D-allulose from glycerol using metabolically engineered Escherichia coli. FbaA, Fbp, AlsE, and A6PP were used to construct the D-allulose synthesis pathway. Subsequently, PfkA, PfkB, and Pgi were disrupted to block the entry of the intermediate fructose-6-phosphate (F6P) into the Embden-Meyerhof-Parnas (EMP) and pentose phosphate (PP) pathways. Additionally, GalE and FryA were inactivated to reduce D-allulose consumption by the cells. Finally, a fed-batch fermentation process was implemented to optimize the performance of the cell factory. As a result, the titer of D-allulose reached 7.02 g/L with a maximum yield of 0.287 g/g.

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.

Semi-rational engineering of D-allulose 3-epimerase for simultaneously improving the catalytic activity and thermostability based on D-allulose biosensor

BACKGROUND

D-Allulose is one of the most well-known rare sugars widely used in food, cosmetics, and pharmaceutical industries. The most popular method for D-allulose production is the conversion from D-fructose catalyzed by D-allulose 3-epimerase (DAEase). To address the general problem of low catalytic efficiency and poor thermostability of wild-type DAEase, D-allulose biosensor was adopted in this study to develop a convenient and efficient method for high-throughput screening of DAEase variants.

RESULTS

The catalytic activity and thermostability of DAEase from Caballeronia insecticola were simultaneously improved by semi-rational molecular modification. Compared with the wild-type enzyme, DAEaseS37N/F157Y variant exhibited 14.7% improvement in the catalytic activity and the half-time value (t1/2) at 65°C increased from 1.60 to 27.56 h by 17.23-fold. To our delight, the conversion rate of D-allulose was 33.6% from 500-g L-1 D-fructose in 1 h by Bacillus subtilis WB800 whole cells expressing this DAEase variant. Furthermore, the practicability of cell immobilization was evaluated and more than 80% relative activity of the immobilized cells was maintained from the second to seventh cycle.

CONCLUSION

All these results indicated that the DAEaseS37N/F157Y variant would be a potential candidate for the industrial production of D-allulose.

The Effect of Allulose on the Attenuation of Glucose Release from Rice in a Static In Vitro Digestion Model

Allulose is a rare sugar that provides <10% of the energy but 70% of the sweetness of sucrose. Allulose has been shown to attenuate glycemic responses to carbohydrate-containing foods in vivo. This study aimed to determine the optimal allulose dose for minimizing in vitro glucose release from rice compared to a rice control and fructose. A triphasic static in vitro digestion method was used to evaluate the in vitro digestion of a rice control compared to the co-digestion of rice with allulose (10 g, 20 g, and 40 g) and fructose (40 g). In vitro glucose release was affected by treatment (p < 0.001), time (p < 0.001), and treatment-by-time interaction (p = 0.002). Allulose (40 g) resulted in a reduction in in vitro glucose release from rice alone and rice digested with allulose (10 g), allulose (20 g), and fructose. The incremental area under the curve (iAUC) for in vitro glucose release was lower after allulose (40 g) (p = 0.005) compared to rice control and allulose (10 g) but did not differ from allulose (20 g) or fructose. This study demonstrates that allulose reduces glucose release from carbohydrates, particularly at higher doses, underscoring its potential as a food ingredient with functional benefits.

Impact of Rare Sugar D-Allulose on Hardening of Starch Gels during Refrigerated Storage

The rare sugar D-allulose (Alu), with ca. 10% calories of sucrose (Suc), is a promising alternative sugar that can be used to improve the quality of starch gels in storage. The effects of Alu (compared to Suc) on the hardening and microstructural and molecular order of amylopectin-rich (glutinous rice (GR) and corn amylopectin (CAP)) and amylose-rich (corn (C)) starch gels were investigated. Alu and Suc both suppressed hardening in C gels, while Alu but not Suc was effective in GR and CAP gels. SEM results showed that Alu-containing GR and CAP maintained a relatively large pore size compared to Suc-containing gels. The deconvolution of FTIR spectra revealed that Alu-containing GR and CAP gels had lower ratios of intermolecular hydrogen bonds and higher ratios of loose hydrogen bonds than Suc-containing gels. For amylose-rich C gels, on the other hand, such tendencies were not observed. The influence of Alu on amylopectin-rich gels could be because Alu reduced the ratio of intermolecular hydrogen bonds, which might be involved in amylopectin recrystallization, and increased that of loose hydrogen bonds. The results suggest that Alu is more effective than Suc in inhibiting the hardening of amylopectin-rich starch gels during refrigerated storage.

The Metabolic and Endocrine Effects of a 12-Week Allulose-Rich Diet

The global rise in type 2 diabetes (T2D) and obesity necessitates innovative dietary interventions. This study investigates the effects of allulose, a rare sugar shown to reduce blood glucose, in a rat model of diet-induced obesity and T2D. Over 12 weeks, we hypothesized that allulose supplementation would improve body weight, insulin sensitivity, and glycemic control. Our results showed that allulose mitigated the adverse effects of high-fat, high-sugar diets, including reduced body weight gain and improved insulin resistance. The allulose group exhibited lower food consumption and increased levels of glucagon-like peptide-1 (GLP-1), enhancing glucose regulation and appetite control. Additionally, allulose prevented liver triglyceride accumulation and promoted mitochondrial uncoupling in adipose tissue. These findings suggest that allulose supplementation can improve metabolic health markers, making it a promising dietary component for managing obesity and T2D. Further research is needed to explore the long-term benefits and mechanisms of allulose in metabolic disease prevention and management. This study supports the potential of allulose as a safe and effective intervention for improving metabolic health in the context of dietary excess.

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.

Effects of d-allose on anti-brain edema effects and reduction of tumor necrosis factor-alpha and interleukin-6 in the water intoxication model

Rare sugars, which exist only in very small quantities in nature, have recently attracted attention for their various biological functions in medicine. Among them, d-allose is known to have cytoprotective effects by antioxidant effects. In this study, we investigated whether the antioxidant effects of d-allose reduce brain edema in a water intoxication model of cytotoxic brain edema. Methods: Mice were injected intraperitoneally with distilled water (10 % of body weight) to create a model of brain edema. d-allose was administered orally at 400 mg/kg 30 min before the model was created. Two hours later, the degree of brain edema was measured by the dry-weight method to determine whether d-allose reduced brain edema. As an index of antioxidant effects, we measured changes over time in inflammatory cytokines (tumor necrosis factor-alpha, interleukin-6) induced by the water intoxication model, and whether d-allose reduced inflammatory cytokines 4 h after model creation. Results: Administration of d-allose significantly suppressed brain edema formation of the water-intoxication model. And it significantly reduced inflammatory cytokines (tumor necrosis factor-alpha, interleukin-6). These results suggest that the antioxidant effect of d-allose exerts an anti-inflammatory effect and reduces brain edema.

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.

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.

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.

Stereocontrolled synthesis of α-d-allulofuranosides using α-selective d-fructofuranosidation reaction

Stereocontrolled synthesis of rare sugar derivatives, namely α-d-allulofuranosides, was achieved using d-fructose, one of the most abundant carbohydrates in nature. The following are the key steps of the α-d-allulofuranosides' synthesis. (1) An α-selective glycosidation reaction of 1,3,4,6-tetra-O-benzoylated d-fructofuranosyl donor to obtain α-d-fructofuranosides with 98 %-75 % isolated yields. (2) A regioselective 1,4,6-tri-O-pivaloylation reaction of the tetraol of α-d-fructofuranosides with the C3-hydroxy group remaining intact. (3) The oxidation of the C3-hydroxy group followed by the stereoselective reduction of the C3-carbonyl group. Primary and secondary alcohols and sugars can be used as glycosyl acceptors and aglycones for the following pivaloylation and stereoinversion reactions to obtain α-d-allulofuranosides.

Effect of different sweeteners on the thermal, rheological, and water mobility properties of soft wheat flour and their application to cookies as an alternative to sugar

The effects of different sweeteners on the physicochemical properties of soft wheat flour were investigated mainly in terms of thermal, rheological, and water mobility features, and their feasibilities as an alternative to sugar were evaluated in the cookie system. Kestose significantly reduced the solvent retention capacity of wheat flour, followed by sucrose, fructose, and allulose. Thermal analysis showed that the sucrose and kestose distinctly led to an increase in the gelatinization temperature of wheat flour, which was explained by lower T2 relaxation times. In addition, the pasting viscosities and thermo-mechanical properties of wheat flour containing kestose became lower compared to allulose, and these differences were morphologically confirmed by the real-time microscopic measurements during heating. Furthermore, when the sweeteners were incorporated into the cookie formulations, kestose played a positive role as a sugar replacer in the cookie system by presenting a comparable spread factor, texture, and color to cookies with sucrose.

Applications of Bacillus subtilis Protein Display for Medicine, Catalysis, Environmental Remediation, and Protein Engineering

Bacillus subtilis spores offer several advantages that make them attractive for protein display. For example, protein folding issues associated with unfolded polypeptide chains crossing membranes are circumvented. In addition, they can withstand physical and chemical extremes such as heat, desiccation, radiation, ultraviolet light, and oxidizing agents. As a result, the sequence of the displayed protein can be easily obtained even under harsh screening conditions. Next, immobilized proteins have many economic and technological advantages. They can be easily separated from the reaction and the protein stability is increased in harsh environments. In traditional immobilization methods, proteins are expressed and purified and then they are attached to a matrix. In contrast, immobilization occurs naturally during the sporulation process. They can be easily separated from the reaction and the protein stability is increased in harsh environments. Spores are also amenable to high-throughput screening for protein engineering and optimization. Furthermore, they can be used in a wide array of biotechnological and industrial applications such as vaccines, bioabsorbants to remove toxic chemicals, whole-cell catalysts, bioremediation, and biosensors. Lastly, spores are easily produced in large quantities, have a good safety record, and can be used as additives in foods and drugs.

Isomerization and epimerization of fructose in phosphate buffer under subcritical water conditions

Isomerization and epimerization of fructose to glucose, mannose, allulose, and allose were executed using a subcritical phosphate buffer solution to effectively produce useful monosaccharides. The conversion of the substrate and the yield of products were dependent on the reaction temperature, initial pH, initial substrate concentration, and buffer concentration. A high yield of mannose was achieved under the optimal reaction conditions we identified. We subsequently performed the kinetic analysis based on the proposed reaction network, and evaluated the effects of temperature and pH on the reactions. We then estimated the apparent activation energy values for each reaction.

A comprehensive review of recent advances in the characterization of L-rhamnose isomerase for the biocatalytic production of D-allose from D-allulose

D-Allose and D-allulose are two important rare natural monosaccharides found in meager amounts. They are considered to be the ideal substitutes for table sugar (sucrose) for, their significantly lower calorie content with around 80 % and 70 % of the sweetness of sucrose, respectively. Additionally, both monosaccharides have gained much attention due to their remarkable physiological properties and excellent health benefits. Nevertheless, D-allose and D-allulose are rare in nature and difficult to produce by chemical methods. Consequently, scientists are exploring bioconversion methods to convert D-allulose into D-allose, with a key enzyme, L-rhamnose isomerase (L-RhIse), playing a remarkable role in this process. This review provides an in-depth analysis of the extractions, physiological functions and applications of D-allose from D-allulose. Specifically, it provides a detailed description of all documented L-RhIse, encompassing their biochemical properties including, pH, temperature, stabilities, half-lives, metal ion dependence, molecular weight, kinetic parameters, specific activities and specificities of the substrates, conversion ratio, crystal structure, catalytic mechanism as well as their wide-ranging applications across diverse fields. So far, L-RhIses have been discovered and characterized experimentally by numerous mesophilic and thermophilic bacteria. Furthermore, the crystal forms of L-RhIses from E. coli and Stutzerimonas/Pseudomonas stutzeri have been previously cracked, together with their catalytic mechanism. However, there is room for further exploration, particularly the molecular modification of L-RhIse for enhancing its catalytic performance and thermostability through the directed evolution or site-directed mutagenesis.

Intestinal absorption of D-fructose isomers, D-allulose, D-sorbose and D-tagatose, via glucose transporter type 5 (GLUT5) but not sodium-dependent glucose cotransporter 1 (SGLT1) in rats

D-allulose, D-sorbose and D-tagatose are D-fructose isomers that are called rare sugars. These rare sugars have been studied intensively in terms of biological production and food application as well as physiological effects. There are limited papers with regard to the transporters mediating the intestinal absorption of these rare sugars. We examined whether these rare sugars are absorbed via sodium-dependent glucose cotransporter 1 (SGLT1) as well as via GLUT type 5 (GLUT5) using rats. High-fructose diet fed rats, which express more intestinal GLUT5, exhibited significantly higher peripheral concentrations, Cmax and AUC0–180 min when D-allulose, D-sorbose and D-tagatose were orally administrated. KGA-2727, a selective SGLT1 inhibitor, did not affect the peripheral and portal vein concentrations and pharmacokinetic parameters of these rare sugars. The results suggest that D-allulose, D-sorbose and D-tagatose are likely transported via GLUT5 but not SGLT1 in rat small intestine.

Calorie restriction mimetic drugs could favorably influence gut microbiota leading to lifespan extension

Calorie restriction (CR) can prolong human lifespan, but enforcing long-term CR is difficult. Thus, a drug that reproduces the effects of CR without CR is required. More than 10 drugs have been listed as CR mimetics (CRM), and some of which are conventionally categorized as upstream-type CRMs showing glycolytic inhibition, whereas the others are categorized as downstream-type CRMs that regulate or genetically modulate intracellular signaling proteins. Intriguingly, recent reports have revealed the beneficial effects of CRMs on the body such as improving the host body condition via intestinal bacteria and their metabolites. This beneficial effect of gut microbiota may lead to lifespan extension. Thus, CRMs may have a dual effect on longevity. However, no reports have collectively discussed them as CRMs; hence, our knowledge about CRM and its physiological effects on the host remains fragmentary. This study is the first to present and collectively discuss the accumulative evidence of CRMs improving the gut environments for healthy lifespan extension, after enumerating the latest scientific findings related to the gut microbiome and CR. The conclusion drawn from this discussion is that CRM may partially extend the lifespan through its effect on the gut microbiota. CRMs increase beneficial bacteria abundance by decreasing harmful bacteria rather than increasing the diversity of the microbiome. Thus, the effect of CRMs on the gut could be different from that of conventional prebiotics and seemed similar to that of next-generation prebiotics.

An artificial multienzyme cascade for the whole-cell synthesis of rare ketoses from glycerol

PURPOSE

In our previous study, we constructed a one-pot multi-enzyme system for rare ketoses synthesis based on L-rhamnulose-1-phosphate aldolase (RhaD) from accessible glycerol in vitro. To eliminate tedious purification of enzymes, a facile Escherichia coli whole-cell cascade platform was established in this study.

METHODS

To enhance the conversion rate, the reaction conditions, substrate concentrations and expressions of related enzymes were extensively optimized.

RESULTS

The biosynthetic route for the cascade synthesis of rare ketoses in whole cells was successfully constructed and three rare ketoses including D-allulose, D-sorbose and L-fructose were produced using glycerol and D/L-glyceraldehyde (GA). Under optimized conditions, the conversion rates of rare ketoses were 85.0% and 93.0% using D-GA and L-GA as the receptor, respectively. Furthermore, alditol oxidase (AldO) was introduced to the whole-cell system to generate D-GA from glycerol, and the total production yield of D-sorbose and D-allulose was 8.2 g l-1 only from the sole carbon source glycerol.

CONCLUSION

This study demonstrates a feasible and cost-efficient method for rare sugars synthesis and can also be applied to the green synthesis of other value-added chemicals from glycerol.

Short-term effects of allulose consumption on glucose homeostasis, metabolic parameters, incretin levels, and inflammatory markers in patients with type 2 diabetes: a double-blind, randomized, controlled crossover clinical trial

PURPOSE

Allulose is a rare monosaccharide with almost zero calories. There is no study of short-term allulose consumption in patients with type 2 diabetes (T2D). Thus, we aimed to study the effect of allulose consumption for 12 weeks on glucose homeostasis, lipid profile, body composition, incretin levels, and inflammatory markers in patients with T2D.

METHODS

A double-blind, randomized, controlled crossover study was conducted on sixteen patients with T2D. Patients were randomly assigned to allulose 7 g twice daily or aspartame 0.03 g twice daily for 12 weeks. After a 2-week washout, patients were crossed over to the other sweetener for an additional 12 weeks. Oral glucose tolerance tests, laboratory measurements, and dual-energy X-ray absorptiometry were conducted before and after each phase.

RESULTS

This study revealed that short-term allulose consumption exerted no significant effect on glucose homeostasis, incretin levels, or body composition but significantly increased MCP-1 levels (259 ± 101 pg/ml at baseline vs. 297 ± 108 pg/mL after 12 weeks of allulose, p = 0.002). High-density lipoprotein cholesterol (HDL-C) significantly decreased from 51 ± 13 mg/dl at baseline to 41 ± 12 mg/dL after 12 weeks of allulose, p < 0.001.

CONCLUSION

Twelve weeks of allulose consumption had a neutral effect on glucose homeostasis, body composition, and incretin levels. Additionally, it decreased HDL-C levels and increased MCP-1 levels.

TRIAL REGISTRATION

This trial was retrospectively registered on the Thai Clinical Trials Registry (TCTR20220516006) on December 5, 2022.

D-Allulose Ameliorates Dysregulated Macrophage Function and Mitochondrial NADH Homeostasis, Mitigating Obesity-Induced Insulin Resistance

D-allulose, a rare sugar, has been proposed to have potential benefits in addressing metabolic disorders such as obesity and type 2 diabetes (T2D). However, the precise mechanisms underlying these effects remain poorly understood. We aimed to elucidate the mechanisms by which D-allulose influences obesity-induced insulin resistance. We conducted gene set enrichment analysis on the liver and white adipose tissue of mice exposed to a high-fat diet (HFD) along with the white adipose tissue of individuals with obesity. Our study revealed that D-allulose effectively suppressed IFN-γ, restored chemokine signaling, and enhanced macrophage function in the livers of HFD-fed mice. This implies that D-allulose curtails liver inflammation, alleviating insulin resistance and subsequently impacting adipose tissue. Furthermore, D-allulose supplementation improved mitochondrial NADH homeostasis and translation in both the liver and white adipose tissue of HFD-fed mice. Notably, we observed decreased NADH homeostasis and mitochondrial translation in the omental tissue of insulin-resistant obese subjects compared to their insulin-sensitive counterparts. Taken together, these results suggest that supplementation with allulose improves obesity-induced insulin resistance by mitigating the disruptions in macrophage and mitochondrial function. Furthermore, our data reinforce the crucial role that mitochondrial energy expenditure plays in the development of insulin resistance triggered by obesity.

The Engineering, Expression, and Immobilization of Epimerases for D-allulose Production

The rare sugar D-allulose is a potential replacement for sucrose with a wide range of health benefits. Conventional production involves the employment of the Izumoring strategy, which utilises D-allulose 3-epimerase (DAEase) or D-psicose 3-epimerase (DPEase) to convert D-fructose into D-allulose. Additionally, the process can also utilise D-tagatose 3-epimerase (DTEase). However, the process is not efficient due to the poor thermotolerance of the enzymes and low conversion rates between the sugars. This review describes three newly identified DAEases that possess desirable properties for the industrial-scale manufacturing of D-allulose. Other methods used to enhance process efficiency include the engineering of DAEases for improved thermotolerance or acid resistance, the utilization of Bacillus subtilis for the biosynthesis of D-allulose, and the immobilization of DAEases to enhance its activity, half-life, and stability. All these research advancements improve the yield of D-allulose, hence closing the gap between the small-scale production and industrial-scale manufacturing of D-allulose.

Effect of diabetes-specific oral nutritional supplements with allulose on weight and glycemic profiles in overweight or obese type 2 diabetic patients

BACKGROUND/OBJECTIVES

Diabetes-specific oral nutritional supplements (ONS) have anti-hyperglycemic effects, while D-allulose exerts anti-diabetic and anti-obesity effects. In this study, we investigated the efficacy and safety of diabetes-specific ONS, including allulose, on glycemic and weight changes in overweight or obese patients with type 2 diabetes mellitus (T2DM).

SUBJECTS/METHODS

A single-arm, historical-control pilot clinical trial was conducted on 26 overweight or obese patients with T2DM (age range: 30-70 yrs). The participants were administered 2 packs of diabetes-specific ONS, including allulose (200 kcal/200 mL), every morning for 8 weeks. The glycemic profiles, obesity-related parameters, and lipid profiles were assessed to evaluate the efficacy of ONS.

RESULTS

After 8 weeks, fasting blood glucose (FBG) level significantly decreased from 139.00 ± 29.66 mg/dL to 126.08 ± 32.00 mg/dL (P = 0.007) and glycosylated hemoglobin (HbA1c) improved (7.23 ± 0.82% vs. 7.03 ± 0.69%, P = 0.041). Moreover, the fasting insulin (δ: -1.81 ± 3.61 μU/mL, P = 0.017) and homeostasis model assessment for insulin resistance (HOMA-IR) (δ: -0.87 ± 1.57, P = 0.009) levels decreased at 8 weeks, and body weight significantly decreased from 67.20 ± 8.29 kg to 66.43 ± 8.12 kg (P = 0.008). Body mass index (BMI) also decreased in accordance with this (from 25.59 ± 1.82 kg/m2 to 25.30 ± 1.86 kg/m2, P = 0.009), as did waist circumference (δ: -1.31 ± 2.04 cm, P = 0.003).

CONCLUSIONS

The consumption of diabetes-specific ONS with allulose in overweight or obese patients with T2DM improved glycemic profiles, such as FBG, HbA1c, and HOMA-IR, and reduced body weight and BMI.

A Novel D-Psicose 3-Epimerase from Halophilic, Anaerobic Iocasia fonsfrigidae and Its Application in Coconut Water

D-Psicose is a rare, low-calorie sugar that is found in limited quantities in national products. Recently, D-psicose has gained considerable attention due to its potential applications in the food, nutraceutical, and pharmaceutical industries. In this study, a novel D-psicose 3-epimerase (a group of ketose 3-epimerase) from an extremely halophilic, anaerobic bacterium, Iocasia fonsfrigidae strain SP3-1 (IfDPEase), was cloned, expressed in Escherichia coli, and characterized. Unlike other ketose 3-epimerase members, IfDPEase shows reversible epimerization only for D-fructose and D-psicose at the C-3 position but not for D-tagatose, most likely because the Gly218 and Cys6 at the substrate-binding subsites of IfDPEase, which are involved in interactions at the O-1 and O-6 positions of D-fructose, respectively, differ from those of other 3-epimerases. Under optimum conditions (5 µM IfDPEase, 1 mM Mn2+, 50 °C, and pH 7.5), 36.1% of D-psicose was obtained from 10 mg/mL D-fructose. The IfDPEase is highly active against D-fructose under NaCl concentrations of up to 500 mM, possibly due to the excessive negative charges of acidic amino acid residues (aspartic and glutamic acids), which are localized on the surface of the halophilic enzyme. These negative charges may protect the enzyme from Na+ ions from the environment and result in the lowest pI value compared to those of other 3-epimerase members. Moreover, without adjusting any ingredients, IfDPEase could improve coconut water quality by converting D-fructose into D-psicose with a yield of 26.8%. Therefore, IfDPEase is an attractive alternative to enhancing the quality of fructose-containing foods.

Rare Sugar Metabolism and Impact on Insulin Sensitivity along the Gut-Liver-Muscle Axis In Vitro

Rare sugars have recently attracted attention as potential sugar replacers. Understanding the biochemical and biological behavior of these sugars is of importance in (novel) food formulations and prevention of type 2 diabetes. In this study, we investigated whether rare sugars may positively affect intestinal and liver metabolism, as well as muscle insulin sensitivity, compared to conventional sugars. Rare disaccharide digestibility, hepatic metabolism of monosaccharides (respirometry) and the effects of sugars on skeletal muscle insulin sensitivity (impaired glucose uptake) were investigated in, respectively, Caco-2, HepG2 and L6 cells or a triple coculture model with these cells. Glucose and fructose, but not l-arabinose, acutely increased extracellular acidification rate (ECAR) responses in HepG2 cells and impaired glucose uptake in L6 cells following a 24 h exposure at 28 mM. Cellular bioenergetics and digestion experiments with Caco-2 cells indicate that especially trehalose (α1-1α), D-Glc-α1,2-D-Gal, D-Glc-α1,2-D-Rib and D-Glc-α1,3-L-Ara experience delayed digestion and reduced cellular impact compared to maltose (α1-4), without differences on insulin-stimulated glucose uptake in a short-term setup with a Caco-2/HepG2/L6 triple coculture. These results suggest a potential for l-arabinose and specific rare disaccharides to improve metabolic health; however, additional in vivo research with longer sugar exposures should confirm their beneficial impact on insulin sensitivity in humans.

Rare sugar L-sorbose exerts antitumor activity by impairing glucose metabolism

Rare sugars are monosaccharides with low natural abundance. They are structural isomers of dietary sugars, but hardly be metabolized. Here, we report that rare sugar L-sorbose induces apoptosis in various cancer cells. As a C-3 epimer of D-fructose, L-sorbose is internalized via the transporter GLUT5 and phosphorylated by ketohexokinase (KHK) to produce L-sorbose-1-phosphate (S-1-P). Cellular S-1-P inactivates the glycolytic enzyme hexokinase resulting in attenuated glycolysis. Consequently, mitochondrial function is impaired and reactive oxygen species are produced. Moreover, L-sorbose downregulates the transcription of KHK-A, a splicing variant of KHK. Since KHK-A is a positive inducer of antioxidation genes, the antioxidant defense mechanism in cancer cells can be attenuated by L-sorbose-treatment. Thus, L-sorbose performs multiple anticancer activities to induce cell apoptosis. In mouse xenograft models, L-sorbose enhances the effect of tumor chemotherapy in combination with other anticancer drugs. These results demonstrate L-sorbose as an attractive therapeutic reagent for cancer treatment.

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.

Efficient enzymatic synthesis of d-allulose using a novel d-allulose-3-epimerase from Caballeronia insecticola

BACKGROUND

Rare sugars have become promising 'sugar alternatives' because of their low calories and unique physiological functions. Among the family of rare sugars, d-allulose is one of the sugars attracting interest. Ketose 3-epimerases (KEase), including d-tagatose 3-epimerase (DTEase) and d-allulose 3-epimerase (DAEase), are mainly used for d-allulose production.

RESULTS

In this study, a putative xylose isomerase from Caballeronia insecticola was characterized and identified as a novel DAEase. Caballeronia insecticola DAEase displayed prominent enzymatic properties, and 150 g L^-1 d-allulose was produced from 500 g L^-1 d-fructose in 45 min with a conversion rate of 30% and high productivity of 200 g L^-1  h^-1 . Furthermore, DAEase was employed in a phosphorylation-dephosphorylation cascade reaction, which significantly increased the conversion rate of d-allulose. Under optimized conditions, the conversion rate of d-allulose was approximately 100% when the concentration of d-fructose was 50 mmol L^-1 .

CONCLUSION

This research described a very beneficial and facile approach for d-allulose production based on C. insecticola DAEase. © 2022 Society of Chemical Industry.

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.

Effects of Dietary Allitol on Body Fat Accumulation in Rats

Allitol is a rare sugar alcohol obtained by reducing d-allulose (d-psicose). However, information on the effects of long-term dietary allitol intake is limited. This study aimed to investigate the effect of allitol supplementation, as a sugar substitute, on body fat accumulation in rats compared with sucrose, rare sugar d-allulose, or erythritol. Thirty-two male Wistar rats (3 wk old) were fed experimental diets including 5% sucrose, allitol, erythritol, or d-allulose for 8 wk ad libitum. Weight gain, food intake, and food efficiency did not differ among the groups. The total body fat mass and percentage, and intra-abdominal adipose tissue weights were significantly lower in rats fed with the allitol diet than in those fed with the sucrose diet. These body fat indicators tended to be lower in rats fed with the erythritol and d-allulose diets than in those fed with the sucrose diet, but there was no significant difference. The serum glucose-lowering effect obtained in rats fed with the d-allulose diet did not appear in rats fed with the allitol diet. These results suggest that the anti-obesity effect of allitol may be equal to or greater than that of d-allulose.

Dietary D-Allulose Reduces Body Fat Accumulation in Rats with and without Medium-Chain Triacylglycerol Supplementation

d-Allulose (d-psicose) is a rare sugar, that contains no calories and exhibits 70% relative sweetness when compared with sucrose. Recently, several studies have demonstrated the anti-obesity effect of d-allulose, mediated by suppressing lipogenesis and increasing energy expenditure. Medium-chain triacylglycerols (MCTs) are lipids formed by 3 medium-chain fatty acids (MCFAs) with 6-12 carbon atoms attached to glycerol. MCTs have been expensively studied to reduce body fat accumulation in rats and humans. The anti-obesity effect of MCTs was not confirmed depending on the nutritional conditions because MCT might promote lipogenesis. In the present study, we examined the effects of simultaneous intake of diets containing low (5%) or high (13%) MCTs, with or without 5% d-allulose, on body fat accumulation in rats (Experiment 1). Furthermore, we assessed the interaction between 5% MCT and 5% d-allulose in the diet (Experiment 2). In Experiment 1, intra-abdominal adipose tissue weight was significantly greater in the high MCT diet groups than in the commercial diet (control) group. d-Allulose significantly decreased weights of intra-abdominal adipose tissue, carcass fat, and total body fat, however, these weights increased as the amount of MCT added increased. In Experiment 2, d-allulose significantly decreased almost all body fat indicators, and these values were not influenced by the presence or absence of MCT addition. The anti-obesity effect of d-allulose was observed with or without dietary MCT, and no synergistic effect was detected between d-allulose and MCT. These results suggest that d-allulose is a beneficial food ingredient in diets aimed at reducing body fat accumulation. However, further research is required on the synergistic effects between d-allulose and MCTs.

Altering the substrate specificity of recombinant l-rhamnose isomerase from Thermoanaerobacterium saccharolyticum NTOU1 to favor d-allose production

l-Rhamnose isomerase (l-RhI) catalyzes rare sugar isomerization between aldoses and ketoses. In an attempt to alter the substrate specificity of Thermoanaerobacterium saccharolyticus NTOU1 l-RhI (TsRhI), residue Ile102 was changed to other polar or charged amino acid residues by site-directed mutagenesis. The results of activity-screening using different substrates indicate that I102N, I102Q, and I102R TsRhIs can increase the preference against d-allose in comparison with the wild-type enzyme. The catalytic efficiencies of the purified I102N, I102Q, and I102R TsRhIs against d-allose are 148 %, 277 %, and 191 %, respectively, of that of wild-type enzyme, while those against l-rhamnose are 100 %, 167 % and 87 %, respectively. Mutant I102N, I102Q, and I102R TsRhIs were noted to have the altered substrate specificity, and I102Q TsRhI has the highest catalytic efficiency against d-allose presumably through the formation of an additional hydrogen bond with d-allose. The purified wild-type and mutant TsRhIs were further used to produce d-allose from 100 g/L d-fructose in the presence of d-allulose 3-epimerase, and the yields can reach as high as 22 % d-allulose and 12 % d-allose upon equilibrium. I102Q TsRhI takes only around half of the time to reach the same 12 % d-allose yield, suggesting that this mutant enzyme has a potential to be applied in d-allose production.

Does Allulose Appeal to Consumers? Results from a Discrete Choice Experiment in Germany

Reducing the sugar content in food is an important goal in many countries in order to counteract obesity and unhealthy eating. Currently, many consumers eat a number of foods with too much sugar content. However, mankind has an innate preference for sweet foods, and thus one strategy is to have food products which taste sweet but consist of a reduced calorie and sugar content. Allulose is a rare monosaccharide and is considered a safe ingredient in foods, for example in the US, Japan, Singapore, and Mexico, while in Europe, it is in the approval process as a novel food. Thus, it is relevant to find out how consumers perceive the different attributes of allulose in comparison to other sweeteners. Therefore, an online survey consisting of a choice experiment was conducted in Germany to find out consumer preferences of sweeteners. The survey data were analyzed using a mixed logit model. The results reveal that taste is the most important attribute for sweeteners, which explains about 40% of the choice. In the attribute level, a typical sugar taste is preferred. As allulose has a typical sugar taste, the likelihood that it appeals to consumers is high. The second most important attribute is the base product.

Comparative Effects of Allulose, Fructose, and Glucose on the Small Intestine

Despite numerous studies on the health benefits of the rare sugar allulose, its effects on intestinal mucosal morphology and function are unclear. We therefore first determined its acute effects on the small intestinal transcriptome using DNA microarray analysis following intestinal allulose, fructose and glucose perfusion in rats. Expression levels of about 8-fold more genes were altered by allulose compared to fructose and glucose perfusion, suggesting a much greater impact on the intestinal transcriptome. Subsequent pathway analysis indicated that nutrient transport, metabolism, and digestive system development were markedly upregulated, suggesting allulose may acutely stimulate these functions. We then evaluated whether allulose can restore rat small intestinal structure and function when ingested orally following total parenteral nutrition (TPN). We also monitored allulose effects on blood levels of glucagon-like peptides (GLP) 1 and 2 in TPN rats and normal mice. Expression levels of fatty acid binding and gut barrier proteins were reduced by TPN but rescued by allulose ingestion, and paralleled GLP-2 secretion potentially acting as the mechanism mediating the rescue effect. Thus, allulose can potentially enhance disrupted gut mucosal barriers as it can more extensively modulate the intestinal transcriptome relative to glucose and fructose considered risk factors of metabolic disease.

d-Allulose Inhibits Ghrelin-Responsive, Glucose-Sensitive and Neuropeptide Y Neurons in the Arcuate Nucleus and Central Injection Suppresses Appetite-Associated Food Intake in Mice

d-allulose, a rare sugar, has sweetness with few calories. d-allulose regulates feeding and glycemia, and ameliorates hyperphagia, obesity and diabetes. All these functions involve the central nervous system. However, central mechanisms underlying these effects of d-allulose remain unknown. We recently reported that d-allulose activates the anorexigenic neurons in the hypothalamic arcuate nucleus (ARC), the neurons that respond to glucagon-like peptide-1 and that express proopiomelanocortin. However, its action on the orexigenic neurons remains unknown. This study investigated the effects of d-allulose on the ARC neurons implicated in hunger, by measuring cytosolic Ca²⁺ concentration ([Ca²⁺]_i) in single neurons. d-allulose depressed the increases in [Ca²⁺]_i induced by ghrelin and by low glucose in ARC neurons and inhibited spontaneous oscillatory [Ca²⁺]_i increases in neuropeptide Y (NPY) neurons. d-allulose inhibited 10 of 35 (28%) ghrelin-responsive, 18 of 60 (30%) glucose-sensitive and 3 of 8 (37.5%) NPY neurons in ARC. Intracerebroventricular injection of d-allulose inhibited food intake at 20:00 and 22:00, the early dark phase when hunger is promoted. These results indicate that d-allulose suppresses hunger-associated feeding and inhibits hunger-promoting neurons in ARC. These central actions of d-allulose represent the potential of d-allulose to inhibit the hyperphagia with excessive appetite, thereby counteracting obesity and diabetes.

D-Allulose cooperates with glucagon-like peptide-1 and activates proopiomelanocortin neurons in the arcuate nucleus and central injection inhibits feeding in mice

A rare sugar D-Allulose has sweetness without calorie. Previous studies have shown that D-Allulose improves glucose and energy metabolism and ameliorates obesity. However, underlying mechanisms remain elusive. This study explored the effect of central injection of D-Allulose on feeding behavior in mice. We also examined direct effects of D-Allulose on the neurons in the hypothalamic arcuate nucleus (ARC) that regulate feeding, including the anorexigenic glucagon-like peptide-1 (GLP-1)-responsive neurons and proopiomelanocortin (POMC) neurons. Single neurons were isolated from ARC and cytosolic Ca²⁺ concentration ([Ca²⁺]_i) was measured by fura-2 microfluorometry. Administration of D-Allulose at 5.6, 16.7 and 56 mM concentration-dependently increased [Ca²⁺]_i in ARC neurons. The [Ca²⁺]_i increases took place similarly when the osmolarity of superfusion solution was kept constant. The majority (40%) of the D-Allulose-responsive neurons also responded to GLP-1 with [Ca²⁺]_i increases. D-Allulose increased [Ca²⁺]_i in 33% of POMC neurons in ARC. D-Allulose potentiated the GLP-1 action to increase [Ca²⁺]_i in ARC neurons including POMC neurons. Intracerebroventricular injection of D-Allulose significantly decreased food intake at 1 and 2 h after injection. These results demonstrate that D-Allulose cooperates with glucagon-like peptide-1 and activates the ARC neurons including POMC neurons. Furthermore, central injection of D-Allulose inhibits feeding. These central actions of D-Allulose may underlie the ability of D-Allulose to counteract obesity and diabetes.

A pilot study on the effect of D-allulose on postprandial glucose levels in patients with type 2 diabetes mellitus during Ramadan fasting

BACKGROUND

During Ramadan fasting, postprandial hyperglycemia is commonly observed after iftar (break of fast at sunset) meal. D-allulose is a rare sugar and is reported to have several health benefits, including the suppression of increase in postprandial glucose levels. This study investigates whether D-allulose (a C-3 epimer of D-fructose) improves the postprandial glucose in patients with type 2 diabetes mellitus (T2DM) during Ramadan.

METHODS

This was a pilot, prospective single-arm study design that was conducted for 10 consecutive days; 5 days of control and 5 days of consumption. The primary outcome was postprandial peak glucose levels. During the consumption period, 8.5 g of D-allulose was consumed by the participants before iftar meal. Postprandial glucose was measured using a continuous glucose monitoring system.

RESULTS

A total of 12 participants completed the study. Significant lower (p < 0.01) postprandial glucose values and the glucose incremental area under the curve (iAUC) were observed from 0 to 180 min during the consumption period compared to the control period. The consumption period demonstrated significantly higher percentages of time in which glucose values were found in the target range (p = 0.0032), and when the glucose levels above the target range were reduced (p = 0.0015).

CONCLUSIONS

The supplementation with D-allulose has the potential to improve postprandial hyperglycemia in patients with T2DM after iftar during Ramadan. Further studies are needed to confirm these findings. Trial registration ClinicalTrials.gov NCT05071950. Retrospectively registered, 8 October 2021.

Expression and characterization of thermostable D-allulose 3-epimerase from Arthrobacter psychrolactophilus (Ap DAEase) with potential catalytic activity for bioconversion of D-allulose from d-fructose

A novel D-allulose 3-epimerase (DAEase) from Arthrobacter psychrolactophilus (Ap DAEase) was first characterized in this study. The enzyme catalyzes the epimerization of d-fructose into a functional rare sugar, D-allulose. Ap DAEase was the first record of DAEase identified as a homotrimer with the molecular weight of its subunit at approximately 34 kDa. It had an optimum activity at pH 8.5 and 70 °C in the presence of 1 mM Mg²⁺. Ap DAEase was found to be an excellent thermostable enzyme. The half-life value at 70 °C was 128.4 min. The k_cat and catalytic efficiency of the enzyme toward d-fructose were 2920.00 s^-1 and 3.953 mM^-1 s^-1, respectively. To the best of our knowledge, Ap DAEase possesses the highest k_cat among the previously reported DAEases. The conversion ratio of 500 and 100 mg L^-1d-fructose to D-allulose was approximately 27 % in 15 and 90 min, respectively. These research findings suggest that Ap DAEase is a promising candidate for the industrial production of D-allulose.

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.

Combined effects of exercise training and D-allulose intake on endurance capacity in mice

This study investigated the combined effects of exercise training and D-allulose intake on endurance capacity in mice. Male C57BL/6J mice were fed either a control diet (Con) or a 3% D-allulose diet (Allu) and further divided into the sedentary (Sed) or exercise training (Ex) groups (Con-Sed, Con-Ex, Allu-Sed, Allu-Ex, respectively; n = 6-7/group). The mice in the Ex groups were trained on a motor-driven treadmill 5 days/week for 4 weeks (15-18 m/min, 60 min). After the exercise training period, all mice underwent an exhaustive running test to assess their endurance capacity. At 48 h after the running test, the mice in the Ex groups were subjected to run at 18 m/min for 60 min again. Then the gastrocnemius muscle and liver were sampled immediately after the exercise bout. The running time until exhaustion tended to be higher in the Allu-Ex than in the Con-Ex group (p = 0.08). The muscle glycogen content was significantly lower in the Con-Ex than in the Con-Sed group and was significantly higher in the Allu-Ex than in the Con-Ex group (p < 0.05). Moreover, exercise training increased the phosphorylation levels of adenosine monophosphate-activated protein kinase (AMPK) in the muscle and liver. The phosphorylation levels of acetyl coenzyme A carboxylase (ACC), a downstream of AMPK, in the muscle and liver were significantly higher in the Allu-Ex than in the Con-Sed group (p < 0.05), suggesting that the combination of exercise training and D-allulose might have activated the AMPK-ACC signaling pathway, which is associated with fatty acid oxidation in the muscle and liver. Taken together, our data suggested the combination of exercise training and D-allulose intake as an effective strategy to upregulate endurance capacity in mice. This may be associated with sparing glycogen content and enhancing activation of AMPK-ACC signaling in the skeletal muscle.

Ratiometric fluorescence sensing of d-allulose using an inclusion complex of γ-cyclodextrin with a benzoxaborole-based probe

Because d-allulose has been attracting attention as a zero-calorie sugar, the selective sensing of d-allulose is desired to investigate its health benefits. We report herein a novel fluorescence chemosensor that is based on an inclusion complex of γ-cyclodextrin (γ-CyD) with a benzoxaborole-based probe. Two inclusion complexes, 1/γCyD and 2/γCyD, were prepared by mixing γ-CyD with their corresponding probes in a water-rich solvent, where γ-CyD encapsulates two molecules of the probes inside its cavity to form a pyrene dimer. Both 1/γCyD and 2/γCyD exhibit monomeric and dimeric fluorescence from the pyrene moieties. By the reaction of 1/γCyD with saccharides, the intensities of monomeric and dimeric fluorescence remained unchanged and decreased, respectively. We have demonstrated that 1/γCyD has much higher affinity for d-allulose than for the other saccharides (d-fructose, d-glucose, and d-galactose). The conditional equilibrium constants for the reaction systems were determined to be 498 ± 35 M^-1 for d-fructose, 48.4 ± 25.3 M^-1 for d-glucose, 15.0 ± 3.3 M^-1 for d-galactose, and (8.05 ± 0.59) × 10³ M^-1 for d-allulose. These features of 1/γCyD enable ratiometric fluorescence sensing with high sensitivity and selectivity for d-allulose. The limits of detection and quantification of 1/γCyD for d-allulose at pH 8.0 were determined to be 6.9 and 21 μM, respectively. Induced circular dichroism spectral study has shown that the reaction of 1/γCyD with d-allulose causes the monomerisation of the dimer of probe 1 that is encapsulated by γ-CyD, which leads to the diminishment of the dimeric fluorescence.

Computer-Aided Targeted Mutagenesis of Thermoclostridium caenicola d-Allulose 3-Epimerase for Improved Thermostability

d-Allulose 3-epimerase (DAEase) is a key enzyme in d-allulose bioproduction. DAEase from Thermoclostridium caenicola suffers from poor thermostability, hampering its large-scale applications in industry. In this study, mutants A70P, G107P, F155Y, and D162T with increased melting point temperature (T_m) were obtained by targeted mutagenesis based on the calculation of the free energy of folding. The optimal single-point mutant G107P showed 11.08 h, 5, and 5.70 °C increases in the values of half-life (t_1/2) at 60 °C, the optimum temperature (T_opt), and T_m, respectively. Beneficial mutations were combined by ordered recombination mutagenesis, and the combinational mutant Var3 (G107P/F155Y/D162T/A70P) was generated with ΔT_opt of 10 °C and ΔT_m of 12.25 °C. Its t_1/2 value at 65 °C was more than 140 times higher than that of the wild-type enzyme. Molecular dynamics simulations and homology modeling analysis indicated that the enhanced overall rigidity, increased hydrogen bonds between subunits, and redistributed surface electrostatic charges might be responsible for the improved thermostability of the mutant Var3.

d-Allulose Improves Endurance and Recovery from Exhaustion in Male C57BL/6J Mice

d-Allulose, a rare sugar, improves glucose metabolism and has been proposed as a candidate calorie restriction mimetic. This study aimed to investigate the effects of d-allulose on aerobic performance and recovery from exhaustion and compared them with the effects of exercise training. Male C57BL/6J mice were subjected to exercise and allowed to run freely on a wheel. Aerobic performance was evaluated using a treadmill. Glucose metabolism was analyzed by an intraperitoneal glucose tolerance test (ipGTT). Skeletal muscle intracellular signaling was analyzed by Western blotting. Four weeks of daily oral administration of 3% d-allulose increased running distance and shortened recovery time as assessed by an endurance test. d-Allulose administration also increased the maximal aerobic speed (MAS), which was observed following treatment for >3 or 7 days. The improved performance was associated with lower blood lactate levels and increased liver glycogen levels. Although d-allulose did not change the overall glucose levels as determined by ipGTT, it decreased plasma insulin levels, indicating enhanced insulin sensitivity. Finally, d-allulose enhanced the phosphorylation of AMP-activated protein kinase and acetyl-CoA carboxylase and the expression of peroxisome proliferator-activated receptor γ coactivator 1α. Our results indicate that d-allulose administration enhances endurance ability, reduces fatigue, and improves insulin sensitivity similarly to exercise training. d-Allulose administration may be a potential treatment option to alleviate obesity and enhance aerobic exercise performance.