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

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

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

Transforming monosaccharides: Recent advances in rare sugar production and future exploration

Rare sugars are defined as monosaccharides and their derivatives that do not exist in nature at all or that exist in extremely limited amounts despite being theoretically possible. At present, no comprehensive dogma has been presented regarding how and why these rare sugars have deviated from the naturally selected monosaccharides. In this minireview, we adopt a hypothesis on the origin and evolution of elementary hexoses, previously presented by one of the authors (Hirabayashi, Q Rev Biol, 1996, 71:365-380). In this scenario, monosaccharides, which constitute various kinds of glycans in nature, are assumed to have been generated by formose reactions on the prebiotic Earth (chemical evolution era). Among them, the most stable hexoses, i.e., fructose, glucose, and mannose remained accumulated. After the birth of life, the "chemical origin" saccharides thus survived were transformed into a variety of "bricolage products", which include galactose and other recognition saccharides like fucose and sialic acid through the invention of diverse metabolic pathways (biological evolution era). The remaining monosaccharides that have deviated from this scenario are considered rare sugars. If we can produce rare sugars as we wish, it is expected that various more useful biomaterials will be created by using them as raw materials. Thanks to the pioneering research of the Izumori group in the 1990's, and to a few other investigations by other groups, almost all monosaccharides including l-sugars can now be produced by combining both chemical and enzymatic approaches. After briefly giving an overview of the origin of elementary hexoses and the current state of the rare sugar production, we will look ahead to the next generation of monosaccharide research which also targets glycosides including disaccharides.

Enhancing Stability and Catalytic Activity of d-Allulose 3-Epimerase through Multistrategy Computational Design and Cross-Regional Advantageous Mutations

d-Allulose 3-epimerase (DAEase) derived from Clostridium bolteae has excellent properties in the catalytic production of d-allulose, a rare sugar with unique biological functions. However, the industrial application of C. bolteae DAEase (Cb-DAEase) for d-allulose production is hindered by its low enzyme activity, poor long-term thermostability, and pH tolerance. In this study, we identified potential noncatalytic residues in Cb-DAEase using methods such as proline substitution, surface charge engineering, and surface residue prediction. The effects of these residues were experimentally validated, followed by structural analysis, which led to the generation of multisite mutants through cross-regional structural combinations. The obtained mutant Cb-R2P-E6P-D137C showed 155.6% of the enzyme activity of the wild type, and the Kcat/Km increased by 1.3-fold, an elevated half-life of 15.7 min, and an elevated Tm value of 1.1 °C. The mutant Cb-R2P-E6P-A83D-D137C had 139.7% of the enzyme activity of the wild type, the Kcat/Km increased by 1.2-fold, with an elevated half-life of 12.3 min, an elevated Tm value of 0.8 °C, and maintained 68% of the enzyme activity at pH 5.0. The findings outlined a feasible approach for the rational design of multiple preset functions of target enzymes simultaneously.

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

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