1. Introduction
D-tagatose is widely used in the food and pharmaceutical industry and is receiving increasing attention for its disease-modifying properties in controlling diabetes and obesity, as a blood metabolism regulator, and for its health-promoting effects.[1–3] D-tagatose is an ideal substitute for sucrose owing to its high sweetness and low calorific value (92% as sweet and 30% of the energy of an equivalent amount of sucrose).[3] It is an epimer ofD-fructose and a ketose isomer ofD-galactose that is classified as a “rare sugar” because of its limited occurrence in nature.[1]
D-tagatose can be chemically synthesized using calcium catalysts and strong acids. The process is energy-intensive and requires complex purification steps.[1,4]Therefore, environmentally friendly biosynthesis technology for producing tagatose should be developed. D-tagatose has been produced using L-arabinose isomerase in vitro .[5–7] However, the theoretical conversion rate of the isomerase reaction is less than 0.5 mol/mol galactose at room temperature because of the unfavorable thermodynamic equilibrium (ΔG o = 4.96 kJ mol–1 at 298.15 K).[8,9]
Recently, Sha et al. reported that the conversion rate of tagatose from galactitol reached 91% through oxidation.[10] Liu et al. constructed an oxidoreductive reaction with aldose reductase (XR) and galactitol dehydrogenase (GDH) in Saccharomyces cerevisiaefor producing tagatose from lactose.[11] The theoretical conversion rate of the oxidoreductive pathway is higher than that of the isomerase reaction because of the lack of a reverse reaction in the oxidoreductive pathway for tagatose production. However, XR and GDH exhibit coenzyme dependence on NADP+ and NAD+, respectively; this difference in the coenzyme dependence of XR and GDH limited the target production yield.
Moreover, improving the economics of the production process in industrial production is important. Many scientists are looking for cheap materials as an alternative to galactose or galactitol. Whey, a by-product of the dairy industry, is abundant and inexpensive. Global production is estimated to be approximately 160 million tons per year.[12] Whey is an ideal substrate forD-tagatose production because it is lactose-rich. Simultaneously, whey contains proteins, fats, trace elements, and other nutrients that facilitate microbial fermentation. Thus, an efficient bioprocess should be developed to produce D-tagatose and utilize the nutrients in whey powder (WP).
Herein, D-tagatose was synthesized using lactose from WP through oxidoreductive pathways. Subsequently, the coenzyme dependence of polyol dehydrogenase from Paracoccus denitrificans(Pd PDH) in the oxidoreductive pathways was modified from NAD+ to NADP+ using rational design for improving the tagatose yield. Moreover, the reaction process was accelerated using an enzyme assembly with a glycine–serine linker (GS-linker). Furthermore, the efficient coproduction ofD-tagatose, bioethanol, and microbial protein from WP was achieved. This study provides a promising application forD-tagatose biosynthesis while improving the economics of WP processing.