Advances in understanding glycosyltransferases from a structural perspective

Abstract

Glycosyltransferases (GTs), the enzymes that catalyse glycosidic bond formation, create a diverse range of saccharides and glycoconjugates in nature. Understanding GTs at the molecular level, through structural and kinetic studies, is important for gaining insights into their function. In addition, this understanding can help identify those enzymes which are involved in diseases, or that could be engineered to synthesize biologically or medically relevant molecules. This review describes how structural data, obtained in the last 3-4 years, have contributed to our understanding of the mechanisms of action and specificity of GTs. Particular highlights include the structure of a bacterial oligosaccharyltransferase, which provides insights into N-linked glycosylation, the structure of the human O-GlcNAc transferase, and the structure of a bacterial integral membrane protein complex that catalyses the synthesis of cellulose, the most abundant organic molecule in the biosphere.

Figure 1

Glycosyltransferases; representative folds and trends in structure solution. Representative structures of (a) GT-A, (b) GT-B, and (c) GT-C glycosyltransferase folds. In each case helices are shown in red, beta-strands in yellow and loops in green. Metal ions, where present, are shown as magenta spheres, and ligands in cyan ball-and-stick representation. The GT-A structure is SpsA from Bacillus subtilis in complex with UDP and magnesium (PDB code 1QGS [58]). The GT-B structure is the T4 phage β-glucosyltransferase in complex with UDP (PDB code 2BGU [59]). The GT-C structure is the oligosaccharyltransferase from Campylobacter lari in complex with magnesium and peptide substrate (PDB code 3RCE [10^••]). (d) Graphical representation of the number of non-redundant GT genes curated in the CAZy database (filled circles; left y axis) and the number of redundant GT structures (open circles; right y axis). Note the number of structures is over-representative of the number of novel GT structures as these figures include ligand complexes, mutants etc. of the same enzyme.

Figure 2

The bacterial oligosaccharyltransferase. (a) Overall structure (in 2 orientations) of the bacterial oligosaccharyltransferase PglB (PDB code 3RCE [10^••]). The transmembrane domain is shown in green cartoon and the periplasmic domain in blue cartoon, with the surface in beige. The magnesium ion is shown as a magenta sphere and peptide substrate in yellow ball-and-stick representation. (b) Active site of PglB (in the same colouring as (a)) showing the DXD motif coordinating the metal ion, and the catalytically important residues Asp56 and Glu319. (c) Overall structure of the archaeal oligosaccharyltransferase AglB (PDB code 3WAJ [15^•]). The transmembrane domain is shown in cyan cartoon and the periplasmic domain in yellow cartoon, with the surface in beige. The zinc ion is shown as a magenta sphere.

Figure 3

The human O-GlcNAc transferase. (a) Overall structure of the human O-GlcNAc transferase (PDB code 3PE4 [23^••]). The truncated tetratricopeptide repeat domain (4.5 of the 11.5 repeats were present in the structure) is shown in red cartoon, the N-terminal catalytic domain in cyan cartoon, the intervening domain in green cartoon and the C-terminal catalytic domain in yellow cartoon, with the surface in beige. UDP is shown in magenta ball-and-stick representation and the peptide substrate in green. (b) Overlap of OGT ternary complex (with UDP-5SGlcNAc and peptide acceptor; green; PDB code 4GYY [26^•]) and product complex (with UDP and 5SGlcNAc-glycopeptide; cyan; PDB code 4GZ3 [26^•]). (c) Structure of OGT (with colouring as in (a)) in complex with UDP-5SGlcNAc in magenta ball-and-stick representation and residues 1-26 of HCF-1 peptide in green (PDB code 4N3B [31^••]).

Figure 4

Bacterial cellulose synthase. (a) Overall structure (in 2 orientations) of the bacterial cellulose synthase complex (PDB code 4HG6 [32^••]). The transmembrane domain of BcsA is shown in green cartoon, the GT domain in yellow cartoon, and the C-terminal domain in red cartoon; BcsB is shown in blue. The overall surface is shown in beige. UDP is shown in magenta ball-and-stick representation and the cellulose fragment in cyan. (b) Overlap of cellulose synthase in the presence of UDP (cyan cartoon/ball-and-stick; PDB code 4HG6 [32^••]) and presence of cyclic-di-GMP (green cartoon/ball-and-stick; PDB code 4P02 [35]).