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; dimer of pentamers. Belongs to the RbsD / FucU family. RbsD subfamily. Was originally thought (PubMed:7921236 and PubMed:9353933) to be a high affinity ribose transport protein.