Catalyzes the reversible isomerization-deamination of glucosamine 6-phosphate (GlcN6P) to form fructose 6-phosphate (Fru6P) and ammonium ion. alpha-D-glucosamine 6-phosphate + H2O = beta-D-fructose 6-phosphate + NH4(+) Allosterically activated by N-acetylglucosamine 6-phosphate (GlcNAc6P). Amino-sugar metabolism; N-acetylneuraminate degradation; D-fructose 6-phosphate from N-acetylneuraminate: step 5/5. Belongs to the glucosamine/galactosamine-6-phosphate isomerase family. NagB subfamily.