Self-assembling protein scaffold-mediated enzymes' immobilization enhances in vitro d-tagatose production from lactose

As a rare low-calorie sugar with special medicinal value, d-tagatose is widely used in the field of food, beverages, medicine, and cosmetics. However, enzymatic d-tagatose production in vitro is commonly limited to low conversion efficiency and poor thermo-stability. Herein, taking advantage of the self-assembling property of protein scaffold EutM (ethanolamine bacterial microcompartments), Spy and Snoop peptide pairs was used to drive the linkage between the EutM and cargo proteins, β-galactosidase (BagB), and l-arabinose isomerase (TMAI) to construct a dual-enzymes cascade and realize the d-tagatose production from lactose. The optimal conditions of the cascade were shown to be pH of 8.0, temperature of 60°C, 100 g/L lactose as substrate with supplementing 5 mM Mn²⁺. When the ratio of immobilized enzymes to EutM scaffold reached 1:6, the d-tagatose productivity of the dual-enzymes cascade could reach 1.03 g/L/h, which was 1.24-fold higher than free enzymes. In addition, the EutM-based scaffold could efficiently improve the stability of immobilized enzymes, in which 45% of the activity remained after 12 h, 2.14-fold higher than the free one. Overall, an attractive EutM-based self-assembling platform immobilizing BagB and TMAI was developed, showing enhanced catalysis efficiency and enzyme thermo-stability for d-tagatose production.