PDA/SiO₂ microspheres immobilize dual enzymes to increase temperature tolerance and catalyze the production of D-ribulose and D-xylulose from D-xylose

D-xylose, a biomass derivative, is used as a substrate to generate D-xylulose and D-ribulose with high-added value through a cascade reaction. During this process, the thermal stability and catalytic efficiency of rate-limiting enzymes during cascade reactions are important issues that must be addressed in industrial applications. In this study, we prepared and validated PDA/SiO2 microspheres as highly loaded immobilization carriers by oxidative self-polymerization of polydopamine (PDA) on silica (SiO2) microspheres. D-xylose was successfully catalyzed by the co-immobilized D-5-phosphate ribose isomerase B (RpiB) and D-tagatose-3-epimerase (DTE), denoted PDA/SiO2-RpiB-DTE, to produce D-ribulose and D-xylulose. After immobilization, the K-m of co-immobilized RpiB and DTE decreased, which suggested increased substrate affinity. The half-life of the co-immobilized enzymes was 3.5-8 fold higher than that of the free enzymes at 50-70 degrees C, and the inactivation rate constant was reduced, demonstrating that immobilization improved significantly the thermal stability. The conversion of D-xylose at 10 mM was 29.98% under optimal conditions, which contained 9.85% D-ribulose and 20.13% D-xylulose. PDA/SiO2-RpiB-DTE showed up to 89.10% initial enzyme activity after 3 recycle runs. The thermal stability and recovery aspects were improved by immobilization, which facilitates the use of biomass to produce high-value-added products and further exploitation.