Catalyzes the reversible conversion of ribose-5-phosphate to ribulose 5-phosphate. aldehydo-D-ribose 5-phosphate = D-ribulose 5-phosphate Carbohydrate degradation; pentose phosphate pathway; D-ribose 5-phosphate from D-ribulose 5-phosphate (non-oxidative stage): step 1/1. Homodimer. Belongs to the ribose 5-phosphate isomerase family.