Promotes the synthesis of starch in leaves. alpha-D-glucose 6-phosphate = beta-D-fructose 6-phosphate Inhibited by glycerol-3-P (G3P). Carbohydrate degradation; glycolysis; D-glyceraldehyde 3-phosphate and glycerone phosphate from D-glucose: step 2/4. Carbohydrate biosynthesis; gluconeogenesis. Belongs to the GPI family.