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

Abstract

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.

Keywords: d-allulose; directed evolution; enzymatic biotransformation; ketose 3-epimerase; physiological functions; production processes; sweetener.

Figure 1

Multiple physiological functions of d-allulose.

Figure 2

Overview of enzymatic biotransformation processes of d-allulose based on d-tagatose 3-epimerase (DTEase)/ d-allulose 3-epimerase (DAEase).

Figure 3

The epimerization reactions of substrates d-fructose and d-tagatose catalyzed by the DAEase/DTEase family.

Figure 4

Phylogenetic tree of d-allulose-3 epimerases generated based on the amino acid sequences, using the MEGA X program with the neighbor-joining method.

Figure 5

(A) Multiple sequence alignment of d-tagatose-3 epimerases and d-allulose-3 epimerases. The red-boxed characters are the strictly identical amino acid residues. (B) Superimposition of the crystal structure of PDB:4PFH. The purple ball is the Mn²⁺ ion. The green dots are the ligand residues (in Chain B) with interactions with the Mn²⁺ ion and the substrate. The substrate d-fructose (FUD) and the product d-psicose (PSJ) are shown with arrows. The figure was visualized with an NGL engine powered by the MMTF program.