Catalyzes the reversible isomerization of glucose-6-phosphate to fructose-6-phosphate. alpha-D-glucose 6-phosphate = beta-D-fructose 6-phosphate Carbohydrate biosynthesis; gluconeogenesis. Carbohydrate degradation; glycolysis; D-glyceraldehyde 3-phosphate and glycerone phosphate from D-glucose: step 2/4. Increases acid tolerance in rpoS mutants, defective in glucose metabolism. Belongs to the GPI family.