Catalyzes the conversion of glucosamine-6-phosphate to glucosamine-1-phosphate. alpha-D-glucosamine 1-phosphate = D-glucosamine 6-phosphate Binds 1 Mg(2+) ion per subunit. Activated by phosphorylation. Can autophosphorylate in vitro using ATP. Disruption results in a drastically reduced methicillin resistance. Belongs to the phosphohexose mutase family.