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 Inhibited by arabinose-5-phosphate, D-erythrose-4-phosphate and D-erythronic acid. kcat is 2100 sec(-1) for ribose-5-phosphate. After incubation at 45 degrees Celsius for 30 minutes RpiA retains 90% of its original activities. Carbohydrate degradation; pentose phosphate pathway; D-ribose 5-phosphate from D-ribulose 5-phosphate (non-oxidative stage): step 1/1. Homodimer. Constitutively expressed. Belongs to the ribose 5-phosphate isomerase family. Extended N-terminus.