Catalyzes the reversible isomerization-deamination of glucosamine 6-phosphate (GlcN6P) to form fructose 6-phosphate (Fru6P) and ammonium ion. Involved in chitin degradation. alpha-D-glucosamine 6-phosphate + H2O = beta-D-fructose 6-phosphate + NH4(+) kcat is 97.5 sec(-1) with GlcN6P as substrate. kcat is 44.2 sec(-1) with Fru6P as substrate. kcat is 39.6 sec(-1) with ammonia as substrate. Optimum pH is 8.0-8.5. Optimum temperature is 95-100 degrees Celsius. Glycan degradation; chitin degradation. Homodimer. Expression is induced by diacetylchitobiose (GlcNAc2) but not by chitobiose or maltose.