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.