Catalyzes the reversible isomerization of glucose-6-phosphate to fructose-6-phosphate. Does not have phosphomannose isomerase (PMI) activity. Isomerization proceeds through a cis-enediol-based mechanism. alpha-D-glucose 6-phosphate = beta-D-fructose 6-phosphate Optimal activity is achieved when His-310 and Lys-422 are both present to open the sugar ring, and Glu-281 catalyzes the isomerization step (PubMed:33422670). Activity is decreased in the presence of the PGI inhibitor 6-phosphogluconate (PubMed:33422670). kcat is 17.4 sec(-1) with fructose 6-phosphate as substrate. 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.