Metabolic Engineering of Escherichia coli for Lacto-N-triose II Production with High Productivity

Lacto-N-triose II (LNT II), a core structural unit of human milk oligosaccharides (HMOs), has attracted substantial attention for its nutraceutical potentials and applications in the production of complex HMOs. In this study, Escherichia coli was metabolically engineered to efficiently produce LNT II using glycerol as a carbon source and lactose as a substrate. The UDP-N-acetylglucosamine (UDP-GlcNAc) biosynthesis pathway was strengthened, and β-1,3-N-acetylglucosaminyltransferase (LgtA) was introduced to construct an LNT II-producing base strain. To increase the titer and yield of LNT II, combinatorial optimization of the copy number and the ribosomal binding site sequence was performed to tune the gene expression strength and translation rates of the pathway enzymes. Next, multipoint mutations were introduced to glucosamine-6-phosphatesynthase (GlmS) to relieve the feedback inhibition. Then, a series of engineered strains were constructed by blocking the futile pathways by the deletion of the relevant genes. Finally, the culture conditions were optimized. LNT II titer was improved step-by-step from 0.53 to 5.52 g/L in shake-flask cultivations. Fed-batch culture of the final engineered strain produced 46.2 g/L of LNT II, with an LNT II productivity and content of 0.77 g/(L·h) and 0.95 g/g dry cell weight, respectively.