Engineering β-1,4-galactosyltransferase to enhance lacto-N-neotetraose production in Corynebacterium glutamicum
Lacto-N-neotetraose (LNnT), a pivotal oligosaccharide in human milk, plays a significant role in modulating the intestinal microbiota of infants, and exhibits potential applications in antiviral therapy. Corynebacterium glutamicum is widely used in industrial fermentation, but it is incapable of synthesizing LNnT under natural conditions. Previously, we constructed a recombinant C. glutamicum strain CL014 for LNnT production, but the yield was limited. In this study, the key enzyme β-1,4-galactosyltransferase LgtB was engineered to improve LNnT biosynthesis in CL014. Fourteen mutant strains were constructed by replacing the critical residue Arg24 of LgtB to different amino acids and screened for the optimal LNnT production. The highest LNnT production was obtained in the mutant CL014-T. CL014-T could produce 1.40 g/L LNnT in shake flask cultivation, which is a 33.3% increase compared with the control strain CL014. Molecular docking simulations demonstrated that the mutation in CL014-T promotes the formation of additional hydrogen bonds in the flexible loop region, enhancing conformational flexibility and optimizing substrate interactions. Further mechanistic insights obtained through molecular dynamics simulations uncovered that the increased dynamic flexibility in two functional regions and the formation of more core hydrogen bonds collectively contribute to enhanced binding stability and improved product yield in the mutant. After 96 h feed-batch fermentation, CL014-T produced 2.29 g/L LNnT. This work demonstrates that C. glutamicum has the potential to be engineered as an efficient cell factory for human milk oligosaccharide production.