Food properties of egg white protein modified by rare ketohexoses through Maillard reaction

Effects of saccharide type and extended heating on the Maillard reaction and physicochemical properties of high-solid gelatin gels

This research delves into the Maillard reaction (MR) in high-solid gelatin-saccharide mixtures consisting of 8% and 72% of allulose, fructose, or fructo-oligosaccharides, which were subjected to varied duration (0-60min) of thermal processing prior to gelation. Physicochemical properties of the gels, including color, chemical composition, protein crosslinking, mechanical strength, in-vitro digestibility and antioxidant activities, were characterized. At pH ∼5.5 and intermediate water activities (0.6-0.7), fast browning was observed through sugar degradation and sugar-amine interactions, which were intensified by prolonged heating. The MR reactivity of saccharides followed: AL > FRU > FOS. Characteristic products (MRPs, e.g., α-dicarbonyls, 5-hydroxymethylfurfural, and advanced glycation end products) were identified, with the spectra of MRPs varying significantly between monosaccharides and oligosaccharides. The MR-induced protein glycation and crosslinking exhibited certain negative impacts on the gel strength and in-vitro protein digestibility. Furthermore, all gelatin-saccharide mixtures exhibited augmented antioxidant properties, with the gelatin-AL mixtures displaying the highest free radical scavenging rates.

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.

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.

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.

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.

Effects of moisture content on the enolization products formation in glucose-proline Maillard reaction models

BACKGROUND

2,3-Dihydro-3,5-dihydroxy-6-methyl-4(H)-pyran-4-one (DDMP) and 5-hydroxymethylfurfural (HMF) are two main enolization products in the Maillard reaction and found in some foodstuffs. For many years, whether they are functional or noxious to human health has been a matter of debate. Thus, insight into their formation pathways is important to manage Maillard reaction products. In this study, DDMP and HMF were quantified and compared with regard to their formation and degradation in the d-glucose and l-proline Maillard reaction models using different moisture contents (0, 0.1, 0.5, 1.0, and 4.0 mL) at 150 °C for various heating times.

RESULTS

DDMP was predominantly generated in dry or low water-content heating models along with n increased 1-deoxyglucosone (1-DG) generation via 2,3-enolization. However, increasing moisture content resulted in a decay of reaction intensity, 1-DG, and DDMP due to a change in the reaction mechanism from 2,3-enolization to 1,2-enolization, which facilitated 3-deoxyglucosone (3-DG) and HMF formation.

CONCLUSION

Increased moisture content in glucose-proline models reduced reaction intensity and also inhibited DDMP and facilitated HMF formation by promoting the pathway change from 2,3-enolization to 1,2-enolization to generate more 3-DG. A water content of 1.0 mL was identified as a critical value, from which the 1,2-enolization became a primary pathway occurring in the Maillard reaction. © 2022 Society of Chemical Industry.

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

D-Allulose is the C-3 epimer of D-fructose, and widely regarded as a promising substitute for sucrose. It's an excellent low-calorie sweetener, with 70% sweetness of sucrose, 0.4 kcal/g dietary energy, and special physiological functions. It has been approved as GRAS by the U.S. Food and Drug Administration, and is allowed to be excluded from total and added sugar counts on the food labels. Therefore, D-allulose gradually attracts more public attention. Owing to scarcity in nature, the bioproduction of D-allulose by using ketose 3-epimerase (KEase) has become the research hotspot. Herein, we give a summary of the physicochemical properties, physiological function, applications, and the chemical and biochemical synthesis methods of D-allulose. In addition, the recent progress in the D-allulose bioproduction using KEases, and the possible solutions for existing challenges in the D-allulose industrial production are comprehensively discussed, focusing on the molecular modification, immobilization, food-grade expression, utilizing low-cost biomass as feedstock, overcoming thermodynamic limitation, as well as the downstream separation and purification. Finally, Prospects for further development are also proposed.

Biochemical identification of a hyperthermostable l-ribulose 3-epimerase from Labedella endophytica and its application for d-allulose bioconversion

Currently, low sugar and low energy have become an important trend in the food industries. Therefore, the bioconversion of the functional low-calorie rare sugars attracts more and more attention. l-Ribulose 3-epimerase (LREase) belongs to the ketose 3-epimerase (KEase) family, which could not only efficiently catalyze the reversible C-3 epimerization between l-ribulose and l-xylulose but also between d-fructose and d-allulose. In this paper, a hyperthermostable LREase from Labedella endophytica was identified and characterized. It exhibited maximum catalytic activity at pH 6.0 and 80 °C with 1 mM Ni²⁺. In the presence of Co²⁺, the t_1/2 values at 60, 65, and 70 °C were 37.7, 9.0, and 4.6 h, respectively, and T_m value was 80.9 °C. From 500 g/L d-fructose, it could produce 154.2 g/L d-allulose with a conversion rate of 30.8% in 10 h. In view of its strong thermostability and high catalytic efficiency, L. endophytica LREase might be a good potential alternative for d-allulose industrial production.

Characterization of Pectin-Based Gels: A 1H Nuclear Magnetic Resonance Relaxometry Study

Rare sugars are monosaccharides and their derivatives that are not commonly found in nature. d-Allulose is a rare sugar that is C-3 epimer of fructose and presents an alternative to sucrose with potential health benefits. In this study, different amounts of sucrose, d-allulose, and soy protein isolate (SPI) were used to prepare a set of pectin gels. The effect of these ingredients on the gels was studied at both a molecular level, by ¹H nuclear magnetic resonance (NMR) relaxometry, and a macroscopic level, through the assessment of viscoelastic properties as well as hardness and moisture content measurements. The NMR dispersion profiles were analyzed considering relaxation mechanisms associated with rotational and translational diffusion motions of mono- and disaccharides as well as bound water molecules. Significant variations of the local diffusion coefficient for the studied formulations were evidenced by the model fitting analysis. The viscosity trends observed within each group of samples having the same amount of SPI were mostly in agreement with the diffusion coefficients obtained from the NMR relaxometry. The observed discrepancies could be explained considering hardness and moisture content results, which put into evidence the fact that decreasing the moisture (mainly free water) affects the macroscopic properties of the systems, such as hardness and viscosity, but not the local diffusion processes probed by NMR relaxometry. These findings show the importance of combining both micro- and macroscopic information to analyze the different properties of food products.

Research Advances of d-allulose: An Overview of Physiological Functions, Enzymatic Biotransformation Technologies, and Production Processes

d-allulose has a significant application value as a sugar substitute, not only as a food ingredient and dietary supplement, but also with various physiological functions, such as improving insulin resistance, anti-obesity, and regulating glucolipid metabolism. Over the decades, the physiological functions of d-allulose and the corresponding mechanisms have been studied deeply, and this product has been applied to various foods to enhance food quality and prolong shelf life. In recent years, biotransformation technologies for the production of d-allulose using enzymatic approaches have gained more attention. However, there are few comprehensive reviews on this topic. This review focuses on the recent research advances of d-allulose, including (1) the physiological functions of d-allulose; (2) the major enzyme families used for the biotransformation of d-allulose and their microbial origins; (3) phylogenetic and structural characterization of d-allulose 3-epimerases, and the directed evolution methods for the enzymes; (4) heterologous expression of d-allulose ketose 3-epimerases and biotransformation techniques for d-allulose; and (5) production processes for biotransformation of d-allulose based on the characterized enzymes. Furthermore, the future trends on biosynthesis and applications of d-allulose in food and health industries are discussed and evaluated in this review.

A review on l-ribose isomerases for the biocatalytic production of l-ribose and l-ribulose

Presently, because of the extraordinary roles and potential applications, rare sugars turn into a focus point for countless researchers in the field of carbohydrates. l-ribose and l-ribulose are rare sugars and isomers of each other. This aldo and ketopentose are expensive but can be utilized as an antecedent for the manufacturing of various rare sugars and l-nucleoside analogue. The bioconversion approach turns into an excellent alternative method to l-ribulose and l-ribose production, as compared to the complex and lengthy chemical methods. The basic purpose of this research was to describe the importance of rare sugars in various fields and their easy production by using enzymatic methods. l-Ribose isomerase (L-RI) is an enzyme discovered by Tsuyoshi Shimonishi and Ken Izumori in 1996 from Acinetobacter sp. strain DL-28. L-RI structure was cupin-type-β-barrel shaped with a catalytic site between two β-sheets surrounded by metal ions. The crystal structures of the L-RI showed that it contains a homotetramer structure. Current review have concentrated on the sources, characteristics, applications, conclusions and future prospects including the potentials of l-ribose isomerase for the commercial production of l-ribose and l-ribulose. The MmL-RIse and CrL-RIse have the potential to produce the l-ribulose up to 32% and 31%, respectively. The CrL-RIse is highly stable as compared to other L-RIs. The results explained that the L-RIs have great potential in the production of rare sugars especially, l-ribose and l-ribulose, while the immobilization technique can enhance its functionality and properties. The present study precises the applications of L-RIs acquired from various sources for l-ribose and l-ribulose production.

Improving the enzyme property of D-allulose 3-epimerase from a thermophilic organism of Halanaerobium congolense through rational design

The rare sugar d-allulose is an attractive sucrose substitute due to its sweetness and ultra-low caloric value. It can be produced from D-fructose using d-allulose 3-epimerase (DAE) as the biocatalyst. However, most of the reported DAEs show low catalytic efficiency and poor thermostability, which limited their further use in food industrial. Here, a putative d-allulose 3-epimerase from a thermophilic organism of Halanaerobium congolense (HcDAE) was characterized, showing optimal activity at pH 8.0 and 70 °C in the presence of Mg²⁺. Saturation mutagenesis of Y7, C66, and I108, the putative residues responsible for substrate recognition at the O-4, -5, and -6 atoms of D-fructose was performed, and it yielded the triple mutant Y7H/C66L/I108A with improved activity toward D-fructose (345 % of wild-type enzyme). The combined mutant Y7H/C66L/I108A/R156C/K260C exhibited a half-half (t_1/2) of 5.2 h at 70 °C and an increase of the T_m value by 6.5 °C due to the introduction of disulfide bridges between intersubunit with increased interface interactions. The results indicate that mutants could be used as industrial biocatalysts for d-allulose production.

A Novel d-Allulose 3-Epimerase Gene from the Metagenome of a Thermal Aquatic Habitat and d-Allulose Production by Bacillus subtilis Whole-Cell Catalysis

A novel d-allulose 3-epimerase gene (daeM) has been identified from the metagenomic resource of a hot-water reservoir. The enzyme epimerizes d-fructose into d-allulose, a functional sugar of rare abundance in nature. The metagenomic DNA fragment was cloned and expressed in Escherichia coli The purified recombinant protein (DaeM) was found to be metal dependent (Co2+ or Mn2+). It displayed the maximal levels of catalytic activity in a pH range of 6 to 11 and a temperature range of 75°C to 80°C. The enzyme exhibited remarkably high thermal stability at 60°C and 70°C, with half-life values of 9,900 and 3,240 min, respectively. To the best of our knowledge, this is the highest thermal stability demonstrated by a d-allulose 3-epimerase that has been characterized to date. The enzymatic treatment of 700 mg·ml-1 d-fructose yielded about 217 mg·ml-1 d-allulose, under optimal condition. The catalytic product was purified, and its nuclear magnetic resonance (NMR) spectra were found to be indistinguishable from those of standard d-allulose. For biomolecule production, the whole-cell catalysis procedure avoids the tedious process of extraction and purification of enzyme and also offers better biocatalyst stability. Further, it is desirable to employ safe-grade microorganisms for the biosynthesis of a product. The daeM gene was expressed intracellularly in Bacillus subtilis A whole-cell catalysis reaction performed with a reaction volume of 1 liter at 60°C yielded approximately 196 g·liter-1 d-allulose from 700 g·liter-1 d-fructose. Further, the whole recombinant cells were able to biosynthesize d-allulose in apple juice, mixed fruit juice, and honey.IMPORTANCE d-Allulose is a noncaloric sugar substitute with antidiabetes and antiobesity potential. With several characteristics of physiological significance, d-allulose has wide-ranging applications in the food and pharmacology industries. The development of a thermostable biocatalyst is an objective of mainstream research aimed at achieving industrial acceptability of the enzyme. Aquatic habitats of extreme temperatures are considered a potential metagenomic resource of heat-tolerant biocatalysts of industrial importance. The present study explored the thermal-spring metagenome of the Tattapani geothermal region, Chhattisgarh, India, discovering a novel d-allulose 3-epimerase gene, daeM, encoding an enzyme of high-level heat stability. The daeM gene was expressed in the microbial cells of a nonpathogenic and safe-grade species, B. subtilis, which was found to be capable of performing d-fructose to d-allulose interconversion via a whole-cell catalysis reaction. The results indicate that DaeM is a potential biocatalyst for commercial production of the rare sugar d-allulose. The study established that extreme environmental niches represent a genomic resource of functional sugar-related biocatalysts.

Role of 'D-allulose' in a starch based composite gel matrix

Type of sugar and gelling agents used in confectionery formulations have vital importance since they directly influence physicochemical properties during storage. In this study, the effect of a non-caloric rare sugar, D-allulose (formerly called D-psicose) on the starch based confectionery gels were investigated in the presence and absence of soy protein isolate (SPI) using different experimental techniques for 28 days. For characterization of the formulized gel systems, common techniques were used (SEM, DSC, XRD, moisture content, water activity, hardness and color). Time Domain Nuclear Magnetic Resonance (TD-NMR) technique was also employed to explain dynamics in the systems. Sugar type was found to be a very significant factor affecting gel characteristics and retrogradation. Results showed that D-allulose containing formulations were less prone to retrogradation and showed smaller changes upon storage by supporting presence of better gel network. According to X-ray results, sucrose containing formulations were more susceptible to crystallization. T₂ relaxation spectra obtained from NMR experiments showed that number of distinct peaks reduced with the addition of SPI while relaxation times of peaks changed when different type of sugar.

Modification of Glutenin and Associated Changes in Digestibility Due to Methylglyoxal during Heat Processing

Glutenin is the main protein of flour and is a very important source of protein nutrition for humans. Methylglyoxal (MGO) is an important product of the Maillard reaction that occurs during the hot-processing of flour products, and it reacts with glutenin to facilitate changes in glutenin properties. Here, the effects of MGO on glutenin digestion during the heating process were investigated using a simulated MGO-glutenin system. MGO significantly reduced the digestibility of glutenin. The structure of MGO-glutenin and physicochemical properties were studied to understand the mechanism of the decrease of digestibility. These data suggest that changes in digestibility were caused by decreases in surface hydrophobicity and increases in disulfide bonds. MGO induces strong aggregation of glutenin after heating that led to the masking of cleavage sites for proteases. Moreover, carbonyl oxidation induced by MGO leads to intermolecular cross-linking of glutenin that increasingly masks or even destroys cleavage sites, further decreasing digestibility.

Insights into the Regulation Effects of Certain Phenolic Acids on 2,3-Dihydro-3,5-dihydroxy-6-methyl-4(H)-pyran-4-one Formation in a Microaqueous Glucose-Proline System

The control of 2,3-dihydro-3,5-dihydroxy-6-methyl-4(H)-pyran-4-one (DDMP) formation in the Maillard reaction is important to improve the thermally treated food quality as a result of its intense bitterness and potential toxicity. In this work, phenolic acids, such as gallic, protocatechuic, caffeic, and ferulic acids, were applied to modulate DDMP formation in a microaqueous glucose-proline model. The formation of DDMP was inhibited at low concentrations (from 0.1 to 5.0 mM) while enhanced at 10.0 mM gallic, protocatechuic, and caffeic acids. Ferulic acid always inhibited DDMP formation as a result of the absence of catechol groups on its benzene ring. The result indicated that the control of DDMP formation depended upon the concentration and chemical structures of phenolic acids, such as the number of hydroxyl groups. Further studies indicated that the hydroxyl distribution of phenolic acids regulated the peroxide formation in the model reaction system and further changed the development of the oxidation reaction, which affected the degradation of glucose via caramel or Maillard reaction, Amadori rearrangement product oxidation, and 1-deoxyglucosone degradation to form the intermediates.

Dietary glycerol monolaurate supplementation for the modification of functional properties of egg white protein

BACKGROUND

Understanding the interactions between feed additives and the functional properties of egg white protein (EWP) may offer novel insights into the effects of feed additives on laying hens and may provide an alternative for modification of the functional properties of EWP by using laying hens as bioreactors. Glycerol monolaurate (GML) is widely used in the food industry as an effective antibacterial emulsifier. In this work, the effects of three doses of dietary GML supplementation (150, 300, and 450 mg kg^-1 hen) on the functional properties of EWP were investigated.

RESULTS

The hardness of EWP gels was significantly improved by 300 and 450 mg kg^-1 GML supplementation. Foaming capacity (FC) and foaming stability (FS) were increased after GML treatment; 450 mg kg^-1 GML supplementation showed the most significant improvements, with 44.82% in FC and 23.39% in FS. Stabilization of EWP-oil emulsions was also improved, supported by a slowed creaming process and the formation of smaller oil droplets. The heat denaturation temperature and rheological properties were also modified by dietary GML supplementation, implying improved thermal stability.

CONCLUSION

Our study demonstrated that GML supplementation has the potential to modify the functional properties of EWP, broadening the application of GML and providing a new perspective for evaluation of the efficacy of feed additives. © 2019 Society of Chemical Industry.

The development of low-calorie sugar and functional jujube food using biological transformation and fermentation coupling technology

Jujube juice has been used as ingredient in a range of foods and dietary supplements. In this study, an enzyme transformation and fermentation coupling technology was applied to increase the nutritional value of concentrated/extracted Jinsi jujube juice. Two enzymes, D-glucose isomerase (GI) and D-allulose 3-epimerase (DAE), were employed to convert the glucose and fructose to a low-calorie sweeter D-allulose with a concentration of 110 g/L in jujube juice. Furthermore, the mixed cultures of Pediococcus pentosaceus PC-5 and Lactobacillus plantarum M were employed to increase the content of nutrition components related to bioactivities and flavor volatiles in jujube juice. Accordingly, this fermentation accumulated 100 mg/L gamma-aminobutyric acid (GABA), which has neurotransmission, hypotension, diuretic, and tranquilizer effects, and increased the content of branched-chain amino acids (BCAAs) and many free amino acids (Asp, Glu, Gly, and Ala) at different level. The fermentation not only maintained the concentration of native functional components such as cyclic adenosine monophosphate (cAMP) and minerals, but also increased the content of iron (Fe2+) and zinc (Zn2+), which have blood and eyesight tonic function. The value-added jujube juice might serve as a low-calorie and probiotic functional beverage and show high application potential in food industry.

Biochemical characterization and biocatalytic application of a novel d-tagatose 3-epimerase from Sinorhizobium sp

Sinorhizobium sp. d-tagatose 3-epimerase (sDTE) catalyzes the conversion of d-tagatose to d-sorbose. It also recognizes d-fructose as a substrate for d-allulose production. The optimal temperature and pH of the purified sDTE was 50 °C and 8.0, respectively. Based on the sDTE homologous model, Glu154, Asp187, Gln213, and Glu248, form a hydrogen bond network with the active-site Mn²⁺ and constitute the catalytic tetrad. The amino acid residues around O-1, -2, and -3 atoms of the substrates (d-tagatose/d-fructose) are strictly conserved and thus likely regulate the catalytic reaction. However, the residues at O-4, -5, and -6, being responsible for the substrate-binding, are different. In particular, Arg65 and Met9 were found to form a unique interaction with O-4 of d-fructose and d-tagatose. The whole cells with recombinant sDTE showed a higher bioconversion rate of 42.5% in a fed-batch bioconversion using d-fructose as a substrate, corresponding to a production of 476 g L⁻¹d-allulose. These results suggest that sDTE is a potential industrial biocatalyst for the production of d-allulose in fed-batch mode.

Sinorhizobium sp. d-tagatose 3-epimerase (sDTE) catalyzes the conversion of d-tagatose to d-sorbose.

Thermostability Improvement of the d-Allulose 3-Epimerase from Dorea sp. CAG317 by Site-Directed Mutagenesis at the Interface Regions

d-Allulose is a low-calorie sweetener and has broad applications in the food, cosmetics, and pharmaceutical industries. Recently, most studies focus on d-allulose production from d-fructose by d-allulose 3-epimerase (DAEase). However, the major blocker of industrial production of d-allulose is the poor thermostability. In this study, site-directed mutagenesis at the interface regions of Dorea sp. DAEase was carried out, and the F154Y/E191D/I193F mutation was obtained. The mutant protein displayed much higher thermostability, with a t_1/2 value of 20.47 h (50 °C) and a T_m value of 74.18 °C. Compared with the wild-type DAEase, the t_1/2 value at 50 °C increased by 5.4-fold, and the T_m value increased by 17.54 °C. In the d-allulose production from 500 g/L d-fructose, 148.2 g/L d-allulose could be obtained by F154Y/E191D/I193F mutant protein. The results suggest that site-directed mutagenesis at the interface regions is an efficient approach for improving the thermostability of DAEase.

Metabolic engineering pathways for rare sugars biosynthesis, physiological functionalities, and applications-a review

Biomolecules like rare sugars and their derivatives are referred to as monosaccharides particularly uncommon in nature. Remarkably, many of them have various known physiological functions and biotechnological applications in cosmetics, nutrition, and pharmaceutical industries. Also, they can be exploited as starting materials for synthesizing fascinating natural bioproducts with significant biological activities. Regrettably, most of the rare sugars are quite expensive, and their synthetic chemical routes are both limited and economically unfeasible due to expensive raw materials. On the other hand, their production by enzymatic means often suffers from low space-time yields and high catalyst costs due to hasty enzyme denaturation/degradation. In this context, biosynthesis of rare sugars with industrial importance is receiving renowned scientific attention, across the globe. Moreover, the utilization of renewable resources as energy sources via microbial fermentation or microbial metabolic engineering has appeared a new tool. This article presents a comprehensive review of physiological functions and biotechnological applications of rare ketohexoses and aldohexoses, including D-psicose, D-tagatose, L-tagatose, D-sorbose, L-fructose, D-allose, L-glucose, D-gulose, L-talose, L-galactose, and L-fucose. Novel in-vivo recombination pathways based on aldolase and phosphatase for the biosynthesis of rare sugars, particularly D-psicose and D-sorbose using robust microbial strains are also deliberated.

Development of a thermo-stable and recyclable magnetic nanobiocatalyst for bioprocessing of fruit processing residues and D-allulose synthesis

The aim of the study was to covalently immobilize Smt3-D-psicose 3-epimerase onto functionalized iron oxide magnetic nanoparticles. After immobilization, K_m of the immobilized enzyme increased, however, V_max was nearly the same as that of its free form, indicating that immobilization has no detrimental effects on its catalytic output. The covalent immobilization caused a reduction in the deactivation rate constant (k_d) values leading to 4-5 fold enhancement in its half-life at 50-65°C, indicating significant thermal stability of the iron-enzyme nanobioconjugate. The immobilized enzyme showed excellent storage stability by losing only 20% activity even after 60days of storage at 4°C. The immobilized enzyme retained up to 90% of its initial activity even after 10 consecutive cycles of catalyzing D-fructose epimerization reactions. Thus, after immobilization the enzyme exhibited remarkable improvements in thermal tolerance, storage stability and recycling efficiency, useful for development of industrially exploitable process for D-allulose production.

Production of d-allulose from d-glucose by Escherichia coli transformant cells co-expressing d-glucose isomerase and d-psicose 3-epimerase genes

BACKGROUND

d-Allulose is a novel and low-calorie rare monosaccharide that is a C-3 epimer of d-fructose. Because of its excellent physiological properties and commercial potential, d-allulose has attracted researchers' interests. Based on the Izumoring strategy, d-allulose is converted from d-fructose by d-psicose 3-epimerase (DPEase), while d-fructose is converted from d-glucose by d-glucose isomerase (GIase). In this study, we created a cellular system capable of converting d-glucose to d-allulose in a one-step process that co-expressed the GIase from Acidothermus cellulolyticus and the DPEase from Dorea sp. CAG.

RESULTS

The co-expression plasmid pETDuet-Dosp-DPE/Acce-GI was generated and transformed into Escherichia coli BL21(DE3) cells. The recombinant co-expression cells exhibited maximum catalytic activity at pH 6.5 and 75 °C. These cells were thermostable at less than 60 °C. The addition of Co²⁺ significantly increased the catalytic activity by 10.8-fold. When the reaction equilibrium was reached, the ratio of d-glucose, d-fructose and d-allulose was approximately 6.5:7:3, respectively.

CONCLUSION

A recombinant co-expression strain that catalysed the bioconversion of d-allulose from d-glucose in a one-step process was created and characterised. When adding 500 g L^-1 d-glucose as a substrate, 204.3 g L^-1 d-fructose and 89.1 g L^-1 d-allulose were produced. © 2016 Society of Chemical Industry.