Catalyzes the interconversion of beta-pyran and beta-furan forms of D-ribose. beta-D-ribopyranose = beta-D-ribofuranose Carbohydrate metabolism; D-ribose degradation; D-ribose 5-phosphate from beta-D-ribopyranose: step 1/2. Homodecamer. Belongs to the RbsD / FucU family. RbsD subfamily.