Lipopolysaccharide biosynthesis proteins, LPS:glycosyltransferases [Cell envelope biogenesis, outer membrane]

A4GalT_like proteins catalyze the addition of galactose or glucose residues to the lipooligosaccharide (LOS) or lipopolysaccharide (LPS) of the bacterial cell surface. The members of this family of glycosyltransferases catalyze the addition of galactose or glucose residues to the lipooligosaccharide (LOS) or lipopolysaccharide (LPS) of the bacterial cell surface. The enzymes exhibit broad substrate specificities. The known functions found in this family include: Alpha-1,4-galactosyltransferase, LOS-alpha-1,3-D-galactosyltransferase, UDP-glucose:(galactosyl) LPS alpha1,2-glucosyltransferase, UDP-galactose: (glucosyl) LPS alpha1,2-galactosyltransferase, and UDP-glucose:(glucosyl) LPS alpha1,2-glucosyltransferase. Alpha-1,4-galactosyltransferase from N. meningitidis adds an alpha-galactose from UDP-Gal (the donor) to a terminal lactose (the acceptor) of the LOS structure of outer membrane. LOSs are virulence factors that enable the organism to evade the immune system of host cells. In E. coli, the three alpha-1,2-glycosyltransferases, that are involved in the synthesis of the outer core region of the LPS, are all members of this family. The three enzymes share 40 % of sequence identity, but have different sugar donor or acceptor specificities, representing the structural diversity of LPS.

Glycosyl transferase family 8. This family includes enzymes that transfer sugar residues to donor molecules. Members of this family are involved in lipopolysaccharide biosynthesis and glycogen synthesis. This family includes Lipopolysaccharide galactosyltransferase, lipopolysaccharide glucosyltransferase 1, and glycogenin glucosyltransferase.

Glycogenin belongs the GT 8 family and initiates the biosynthesis of glycogen. Glycogenin initiates the biosynthesis of glycogen by incorporating glucose residues through a self-glucosylation reaction at a Tyr residue, and then acts as substrate for chain elongation by glycogen synthase and branching enzyme. It contains a conserved DxD motif and an N-terminal beta-alpha-beta Rossmann-like fold that are common to the nucleotide-binding domains of most glycosyltransferases. The DxD motif is essential for coordination of the catalytic divalent cation, most commonly Mn2+. Glycogenin can be classified as a retaining glycosyltransferase, based on the relative anomeric stereochemistry of the substrate and product in the reaction catalyzed. It is placed in glycosyltransferase family 8 which includes lipopolysaccharide glucose and galactose transferases and galactinol synthases.

galactinol synthase