N-acetyl-D-glucosamine (GlcNAc) is a major component of bacterial cell walls. Many organisms recycle GlcNAc from the cell wall or metabolise environmental GlcNAc. The first step in GlcNAc metabolism is phosphorylation to GlcNAc-6-phosphate. In bacteria, the ROK family kinase NagK performs this activity. Although ROK kinases have been studied extensively, no ternary complex showing the two substrates has yet been observed. Here, we solved the structure of NagK from the human pathogen Plesiomonas shigelloides in complex with GlcNAc and the ATP analogue AMP-PNP. Surprisingly, PsNagK showed two conformational changes associated with the binding of each substrate. Consistent with this, the enzyme showed a sequential random enzyme mechanism. This indicates that the enzyme acts as a coordinated unit responding to each interaction. Molecular dynamics modelling of catalytic ion binding confirmed the location of the essential catalytic metal. Site-directed mutagenesis confirmed the catalytic base, and that the metal coordinating residue is essential. Together, this study provides the most comprehensive insight into the activity of a ROK kinase.