Enhanced biosynthesis of d-tagatose from maltodextrin through modular pathway engineering of recombinant Escherichia coli

In our previous study, a whole-cell biocatalytic system involving α-glucan phosphorylase, phosphoglucomutase, glucose 6-phosphate isomerase, tagatose 1,6-bisphosphate aldolase, and phosphoglycolate phosphatase, was constructed to produce d-tagatose from maltodextrin. However, the biotransformation efficiency of this whole-cell catalyst was low because of the unbalanced ratio of each enzyme and metabolic flux of the intermediates. Therefore, in this study, the biosynthesis of d-tagatose was enhanced by optimizing vectors and improving the expression of rate-limiting enzymes through the construction of multi-copy genes to regulate expression levels. The conversion rate increased from 20.8% after 24 h to 25.2% after 3 h using 10 g/L maltodextrin as the substrate. Furthermore, the genes in the bypass pathways in Escherichia coli, pfka and zwf, were deleted to increase the accumulation of intermediates. The strain ER-2GatZ (ΔpΔz) produced 3.383 g/L d-tagatose using 10 g/L substrate after 3 h, which was 1.34 times that of the wild strain. This work exemplifies the use of modular engineering to enhance d-tagatose production using whole-cell catalysts.