In the N-terminal section; belongs to the glucosamine/galactosamine-6-phosphate isomerase family. NagB subfamily. Lacks the two conserved aspartate and the histidine residues in position 106, 177 and 179 respectively, that are involved in the active site of the protein in orthologs; they are replaced by a tyrosine, a valine and an asparagine residue respectively.