To evaluate the putative sugar binding site, the sequence of the GNE kinase domain was aligned with that of E. coli glucokinase complexed with glucose (PDB: 1sz2) [23] , which is the closest homologous structure containing a bound substrate currently available in the PDB data bank. The five residues involved in sugar binding are conserved in GNE (N516, D517, E566, H569, E588, GNE numbering). These five residues are arranged to accommodate the sugar substrate ( Figure 6 ). Two residues, H569 and E588, are located in the ROK family zinc-binding signature motif and H569 directly coordinates the zinc ion. This finding suggests that zinc may play a catalytic role in sugar substrate binding, as well as a structural role.