Whole-cell biosynthesis of D-tagatose from maltodextrin by engineered Escherichia coli with multi-enzyme co-expression system

D-tagatose is a functional sweetener that occurs in small quantity in nature. It is mainly produced through the isomerization of D-galactose by L-arabinose isomerase (L-AI; EC 5.3.1.4). However, the cost of D-galactose is much higher than those commonly used for the production of functional sweeteners such as glucose, maltodextrin, or starch. Here, a multi-enzyme catalytic system consists of five enzymes that utilizes maltodextrin as substrate to synthesize D-tagatose were co-expressed in E. coli, resulting in recombinant cells harboring the plasmids pET-Duet-agp-pgm and pCDFDuet-pgi-gatz-pgp. The activity of this whole-cell catalyst was optimal at 60 degrees C and pH 7.5, and 1 mM Mg2+ and 50 mM phosphate were the optimal cofactors for activity. Under the optimal reaction conditions, 2.08 and 3.2 g L-1 D-tagatose were produced by using 10 and 20 g L-1 maltodextrin as substrates with recombinant cells for 24 h. This co-expression system provides a one-pot synthesis approach for the production of D-tagatose using inexpensive substrate, avoiding enzymes purification steps and supplementation of expensive cofactors.