Catalyzes the interconversion of ribulose-5-P and ribose-5-P. It probably also has activity on D-allose 6-phosphate. aldehydo-D-ribose 5-phosphate = D-ribulose 5-phosphate D-allose 6-phosphate = D-allulose 6-phosphate Inhibited by iodoacetate and glucose 6-phosphate. After incubation at 45 degrees Celsius for 30 minutes RpiB retains 65% of its original activities. At 60 degrees Celsius RpiB is rapidly inactivated. Carbohydrate degradation; pentose phosphate pathway; D-ribose 5-phosphate from D-ribulose 5-phosphate (non-oxidative stage): step 1/1. Homodimer, and homotetramer. Induced by ribose and repressed by RpiR. Belongs to the LacAB/RpiB family.