Structural characterization of the O-GlcNAc cycling enzymes: insights into substrate recognition and catalytic mechanisms

•O-GlcNAc modification of nuclear and cytoplasmic proteins is critical in metazoan biology.•The enzymes OGT and OGA attach and remove O-GlcNAc, respectively.•OGT also cleaves an essential cell cycle regulator using the same active site as it does for protein glycosylation.•Structures of OGT and OGA help explain catalytic mechanisms and promiscuous substrate recognition, and show how inhibitors bind.•OGA substrates bind in two different backbone orientations, but adopt a similar V-shaped binding conformation.