Designer biocatalysts for direct incorporation of exogenous galactose into globotriose

Single-mutations at the galactose-binding site of enzymes GalK, GalU, and LgtC enable the efficient synthesis of UDP-6-azido-6-deoxy-d-galactose and azido-functionalized Gb3 analogs

Lysosomal accumulation of the glycosphingolipid globotriaosylceramide Gb3 is linked to the deficient activity of the α-galactosidase A in the Anderson-Fabry disease and an elevated level of deacylated Gb3 is a hallmark of this condition. Localization of Gb3 in the plasma membrane is critical for studying how the membrane organization and its dynamics are affected in this genetic disorder. Gb3 analogs containing a terminal 6-azido-functionalized galactose in its head group globotriose (αGal1, 4βGal1, and 4Glc) are attractive chemical reporters for bioimaging, as the azido-group may act as a chemical tag for bio-orthogonal click chemistry. We report here the production of azido-Gb3 analogs employing mutants of galactokinase, UTP-glucose-1-phosphate uridylyltransferase, and α-1,4-galactosyltransferase LgtC, which participate in the synthesis of the sugar motif globotriose. Variants of enzymes galactokinase/UTP-glucose-1-phosphate uridylyltransferase generate UDP-6-azido-6-deoxy-d-galactose, which is the galactosyl-donor used by LgtC for transferring the terminal galactose moiety to lactosyl-acceptors. Residues at the galactose-binding site of the 3 enzymes were modified to facilitate the accommodation of azido-functionalized substrates and variants outperforming the wild-type enzymes were characterized. Synthesis of 6-azido-6-deoxy-d-galactose-1-phosphate, UDP-6-azido-6-deoxy-d-galactose, and azido-Gb3 analogs by variants GalK-E37S, GalU-D133V, and LgtC-Q187S, respectively, is 3-6-fold that of their wild-type counterparts. Coupled reactions with these variants permit the production of the pricy, unnatural galactosyl-donor UDP-6-azido-6-deoxy-d-galactose with ~90% conversion yields, and products azido-globotriose and lyso-AzGb3 with substrate conversion of up to 70%. AzGb3 analogs could serve as precursors for the synthesis of other tagged glycosphingolipids of the globo-series.

Pathway Optimization and Uridine 5'-Triphosphate Regeneration for Enhancing Lacto-N-Tetraose Biosynthesis in Engineered Escherichia coli

Recently, human milk oligosaccharides (HMOs) have attracted increasing attention and display great commercial importance, especially for the infant formula industry. Lacto-N-tetraose (LNT) is an important neutral HMO commercially added in infant formula and a core structure for synthesizing complex HMOs. Previously, a novel LNT-generating β-1,3-galactosyltransferase from Pseudogulbenkiania ferrooxidans was identified and used for construction of an LNT-producing engineered Escherichia coli. In this work, LNT biosynthesis was further enhanced by pathway optimization and uridine 5'-triphosphate (UTP) regeneration. The main strategies included genomic integration of UDP-glucose 4-epimerase-encoding gene, fine-tuning of the LNT pathway-related genes, blocking of competitive pathways related to UDP-galactose, and overexpression of UTP supply related genes. The maximal LNT titer reached 6.16 and 57.5 g/L by shake-flask and fed-batch fermentation, respectively.

Physiological effects, biosynthesis, and derivatization of key human milk tetrasaccharides, lacto-N-tetraose, and lacto-N-neotetraose

Human milk oligosaccharides (HMOs) have recently attracted ever-increasing interest because of their versatile physiological functions. In HMOs, two tetrasaccharides, lacto-N-tetraose (LNT) and lacto-N-neotetraose (LNnT), constitute the essential components, each accounting 6% (w/w) of total HMOs. Also, they serve as core structures for fucosylation and sialylation, generating functional derivatives and elongation generating longer chains of core structures. LNT, LNnT, and their fucosylated and/or sialylated derivatives account for more than 30% (w/w) of total HMOs. For derivatization, LNT and LNnT can be modified into a series of complex fucosylated and/or sialylated HMOs by transferring fucose residues at α1,2-, α1,3-, and α1,3/4-linkage and/or sialic acid residues at α2,3- and α2,6-linkage. Such structural diversity allows these HMOs to possess great commercial value and an application potential in the food and pharmaceutical industries. In this review, we first elaborate the physiological functions of these tetrasaccharides and derivatives. Next, we extensively review recent developments in the biosynthesis of LNT, LNnT, and their derivatives in vitro and in vivo by employing advanced enzymatic reaction systems and metabolic engineering strategies. Finally, future perspectives in the synthesis of these HMOs using enzymatic and metabolic engineering approaches are presented.