Catalyzes the conversion of the nucleoside breakdown products ribose-1-phosphate and deoxyribose-1-phosphate to the corresponding 5-phosphopentoses.Catalyzes the reversible isomerization of alpha-D-glucose 1-phosphate to alpha-D-glucose 6-phosphate but with a lower catalytic efficiency.The mechanism proceeds via the intermediate compound alpha-D-glucose 1,6-bisphosphate.In vitro, has also a low glucose 1,6-bisphosphate synthase activity which is most probably not physiologically relevant.