High-level extracellular production of D-Psicose-3-epimerase with recombinant Escherichia coli by a two-stage glycerol feeding approach

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.

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.

Bioproduction of D-allulose: Properties, applications, purification, and future perspectives

D-allulose is the C-3 epimer of D-fructose, which rarely exists in nature, and can be biosynthesized from D-fructose by the catalysis of D-psicose 3-epimerase. D-allulose is safe for human consumption and was recently approved by the United States Food and Drug Administration for food applications. It is not only able be used in food and dietary supplements as a low-calorie sweetener, but also modulates a variety of physiological functions. D-allulose has gained increasing attention owing to its excellent properties. This article presents a review of recent progress on the properties, applications, and bioproduction progress of D-allulose.

Review on D-Allulose: In vivo Metabolism, Catalytic Mechanism, Engineering Strain Construction, Bio-Production Technology

Rare sugar D-allulose as a substitute sweetener is produced through the isomerization of D-fructose by D-tagatose 3-epimerases (DTEases) or D-allulose 3-epimerases (DAEases). D-Allulose is a kind of low energy monosaccharide sugar naturally existing in some fruits in very small quantities. D-Allulose not only possesses high value as a food ingredient and dietary supplement, but also exhibits a variety of physiological functions serving as improving insulin resistance, antioxidant enhancement, and hypoglycemic controls, and so forth. Thus, D-allulose has an important development value as an alternative to high-energy sugars. This review provided a systematic analysis of D-allulose characters, application, enzymatic characteristics and molecular modification, engineered strain construction, and processing technologies. The existing problems and its proposed solutions for D-allulose production are also discussed. More importantly, a green and recycling process technology for D-allulose production is proposed for low waste formation, low energy consumption, and high sugar yield.

Cell regeneration and cyclic catalysis of engineered Kluyveromyces marxianus of a D-psicose-3-epimerase gene from Agrobacterium tumefaciens for D-allulose production

D-Allulose as a low-energy and special bioactive monosaccharide sugar is essential for human health. In this study, the D-psicose-3-epimerase gene (DPEase) of Agrobacterium tumefaciens was transferred into thermotolerant Kluyveromyces marxianus to decrease the production cost of D-allulose and reduce the number of manufacturing procedures. The cell regeneration of K. marxianus and cyclic catalysis via whole-cell reaction were investigated to achieve the sustainable application of K. marxianus and the consumption of residual D-fructose. Results showed that DPEase, encoding a 33 kDa protein, could be effectively expressed in thermotolerant K. marxianus. The engineered K. marxianus produced 190 g L^-1 D-allulose with 750 g L^-1 D-fructose as a substrate at 55 °C within 12 h. Approximately 100 g of residual D-fructose was converted into 34 g of ethanol, and 15 g of the engineered K. marxianus cells was regenerated after fermentation at 37 °C for 21 h. The purity of D-allulose of more than 90% could be obtained without isolating it from D-allulose and D-fructose mixture through residual D-fructose consumption. This study provided a valuable pathway to regenerate engineered K. marxianus cells and achieve cyclic catalysis for D-allulose production.

Production of d-psicose from d-fructose by whole recombinant cells with high-level expression of d-psicose 3-epimerase from Agrobacterium tumefaciens

The specific activity of recombinant Escherichia coli cells expressing the double-site variant (I33L-S213C) d-psicose 3-epimerase (DPEase) from Agrobacterium tumefaciens was highest at 24 h of cultivation time in Terrific Broth (TB) medium among the media tested. The contents of crude protein and DPEase in recombinant cells at 24 h were 37.0 and 8.6% (w/w), respectively, indicating that the enzyme was highly expressed. The reaction conditions for the production of d-psicose from d-fructose by whole recombinant cells with the highest specific activity were optimal at 60°C, pH 8.5, 4 g/l cells, and 700 g/l d-fructose. Under these conditions, whole recombinant cells produced 230 g/l d-psicose after 40 min, with a conversion yield of 33% (w/w), a volumetric productivity of 345 g/l/h, and a specific productivity of 86.2 g/g/h. These are the highest conversion yield and volumetric and specific productivities of d-psicose from d-fructose by cells reported thus far.