Structure and active site residues of Pg1D, an N-acetyltransferase from the Bacillosamine synthetic pathway required for N-glycan synthesis in Campylobacter jejuni

Campylobacter jejuni is highly unusual among bacteria in forming N-linked glycoproteins. The heptasaccharide produced by its pgl system is attached to protein Asn through its terminal 2,4-diacetamido-2,4,6-trideoxy-D-Glc (QuiNAc4NAc or N,N'-diacetylbacillosamine) moiety. The crucial, last part of this sugar's synthesis is the acetylation of UDP-2-acetatnido-4-amino-2,4,6-trideoxy-D-Glc by the enzyme Pg1D, with acetylCoA as a cosubstrate. We have determined the crystal structures of Pg1D in CoA-bound and unbound forms, refined to 1.8 and 1.75 angstrom resolution, respectively. Pg1D is a trimer of subunits each comprised of two domains, an N-terminal alpha/beta-domain and a C-terminal left-handed beta-helix. Few structural differences accompany CoA binding, except in the C-terminal region following the beta-helix (residues 189-195), which adopts an extended structure in the unbound form and folds to extend the beta-helix upon binding CoA. Computational molecular docking suggests a different mode of nucleotide-sugar binding with respect to the acetyl-CoA donor, with the molecules arranged in an L-shape, compared with the in-line orientation in related enzymes. Modeling indicates that the oxyanion intermediate would be stabilized by the NH group of Gly143', with His125' the most likely residue to function as a general base, removing H+ from the amino group prior to nucleophilic attack at the carbonyl carbon of acetyl-CoA. Site-specific mutations of active site residues confirmed the importance of His125' Glu124', and Asn118. We conclude that Asn118 exerts its function by stabilizing the intricate hydrogen bonding network within the active site and that Glu124'-may function to increase the pK(a) of the putative general base, His125'.