Catalyzes the isomerization of glucose-6-P to fructose-6-P. alpha-D-glucose 6-phosphate = beta-D-fructose 6-phosphate Competively inhibited by 6-phosphogluconate and erythrose 4-phosphate. Optimum pH is 6.3. 50% remaining activity is observed at pH 5.3 and pH 7. Optimum temperature is 89 degrees Celsius. It does not lose activity upon incubation at 80 degrees Celsius for about 120 min and still has a half-life at 95 degrees Celsius of 40 min. At 100 degrees Celsius, an almost complete loss of activity is observed after 30 min. Carbohydrate biosynthesis; gluconeogenesis. Carbohydrate degradation; glycolysis; D-glyceraldehyde 3-phosphate and glycerone phosphate from D-glucose: step 2/4. Homodimer. Belongs to the GPI family.