5. Conclusion
Herein, we successfully created the NADP+-dependentPd PDHD36A/I37R and constructed the NADPH recycling module. By employing the constructed GS-linker system in vitro to assemble the oxidoreductases, the D-tagatose yield from lactose (0.378 g/g) was 9.28-fold higher than the initial yield. Furthermore, whey, a by-product in cheese production, was used as the sole material for producing D-tagatose, bioethanol, and microbial protein with an integrated bioprocess combined with enzyme cascade and anaerobic fermentation. Finally, 266.5 gD-tagatose, 371.3 g bioethanol, and 215.5 g dry yeast (including 38% protein) were obtained from 1 kg WP (including 810 g lactose). The economic feasibility was improved owing to the expensive price of D-tagatose, compared with those of bioethanol and microbial protein. Moreover, the risk of food safety was avoided owing to non-GMO technology. This bioprocess in which the dairy waste was entirely utilized helps reduce the net emission of carbon dioxide and drives sustainable development.
Acknowledgements
The authors would like to thank Dr. Sen Wang from State Key Laboratory of Microbial Technology of Shandong University for help and guidance in transmission electron microscopy. This work was supported by National Key R&D Program of China (No. 2018YFA0901700), National Natural Science Foundation of China (No. 32271526).