Involved in the first step of the non-oxidative branch of the pentose phosphate pathway. It catalyzes the reversible conversion of ribose-5-phosphate to ribulose 5-phosphate. Can also act on D-ribose-5-diphosphate and D-ribose-5-triphosphate as substrate. aldehydo-D-ribose 5-phosphate = D-ribulose 5-phosphate kcat is 1072 sec(-1) for ribose-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.