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. aldehydo-D-ribose 5-phosphate = D-ribulose 5-phosphate Inhibited by D-4-phosphoerythronic acid. Optimum pH is 6 (at 50 degrees Celsius). Optimum temperature is over 95 degrees Celsius. Carbohydrate degradation; pentose phosphate pathway; D-ribose 5-phosphate from D-ribulose 5-phosphate (non-oxidative stage): step 1/1. Homotetramer. Belongs to the ribose 5-phosphate isomerase family.