Catalyzes the interconversion of ribulose-5-P and ribose-5-P. It has not isomerase activity towards D-allose 6-phosphate. aldehydo-D-ribose 5-phosphate = D-ribulose 5-phosphate Weakly inhibited by phosphate and completely inhibited by iodoacetate. kcat is 120 sec(-1) for D-ribose 5-phosphate. Carbohydrate degradation; pentose phosphate pathway; D-ribose 5-phosphate from D-ribulose 5-phosphate (non-oxidative stage): step 1/1. Homodimer. In mycobacterial enzymes, the usual proton acceptor is not a cysteine, but is remplaced by a glutamate. Belongs to the LacAB/RpiB family.