A Novel β-1,4-Galactosyltransferase from Histophilus somni Enables Efficient Biosynthesis of Lacto-N-Neotetraose via Both Enzymatic and Cell Factory Approaches

Human milk oligosaccharides (HMOs) attract particular attention because of their health benefits for infants. Lacto-N-neotetraose (LNnT) is one of the most abundant neutral core structures of HMOs. Bacterial β-1,4-galactosyltransferase (β-1,4-GalT) displays an irreplaceable role in the practical application of LNnT biosynthesis. In this study, a novel β-1,4-GalT from Histophilus somni was identified to efficiently synthesize LNnT from UDP-Gal and lacto-N-triose II (LNT II). The optimum pH and temperature were determined to be pH 6.0 and 30 °C, respectively. The enzyme showed both transgalactosylation and hydrolysis activity, with a specific activity of 3.7 and 6.6 U/mg, respectively. LNnT was synthesized using H. somni β-1,4-GalT via both enzymatic and cell factory approaches, and both approaches provided an LNnT ratio with the remaining LNT II at approximately 1:2 when reactions attained a balance. These findings indicated that H. somni β-1,4-GalT has a potential in biosynthesis of LNnT and derivatives in future.