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 alpha-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 wholecell 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 (Delta p Delta z) 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.