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.