Catalyzes the reversible isomerization of glucose-6-phosphate to fructose-6-phosphate. alpha-D-glucose 6-phosphate = beta-D-fructose 6-phosphate Activity is decreased in the presence of the PGI inhibitor 6-phosphogluconate (PubMed:16212940, PubMed:17126561). Does not require mono- or divalent cations for activity (PubMed:16212940, PubMed:17126561). Optimum pH is 9.0. Optimum temperature is from 20 to 40 degrees Celsius. Retains full activity up to 50 degrees Celsius (PubMed:16212940). Retains full activity up to 45 degrees Celsius (for six-histidine-tag protein) (PubMed:17126561). 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.