초록
<P><SMALL>D</SMALL>-Allulose is a low-calorie functional rare sugar with excellent processing suitability and unique physiological efficacy. <SMALL>D</SMALL>-Allulose is primarily produced from <SMALL>D</SMALL>-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 <SMALL>D</SMALL>-allulose from inexpensive <SMALL>D</SMALL>-glucose. With the introduction of sodium tetraborate (STB), capable of forming complexes with diol-bearing sugars, the conversion yield of <SMALL>D</SMALL>-allulose from <SMALL>D</SMALL>-glucose substantially escalated from the initial 17.37% to 44.97%. Furthermore, <SMALL>D</SMALL>-allulose was found to exhibit the most pronounced binding affinity for STB with an association constant of 1980.51 M<SUP>-1</SUP>, notably surpassing that of <SMALL>D</SMALL>-fructose (183.31 M<SUP>-1</SUP>) and <SMALL>D</SMALL>-glucose (35.37 M<SUP>-1</SUP>). Additionally, the structural analysis of the sugar-STB complexes demonstrated that <SMALL>D</SMALL>-allulose reacted with STB via the <I>cis</I> 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 <SMALL>D</SMALL>-allulose. Such an STB-facilitated isomerization system would also provide a guidance for the cost-effective synthesis of other rare sugars.</P><BR>[FIG OMISSION]</BR>