Phosphopentose epimerase (also known as ribulose-phosphate 3-epimerase and ribulose 5-phosphate 3-epimerase) encoded by the RPE gene is a metalloprotein that catalyzes the interconversion between D-ribulose 5-phosphate and D-xylulose 5-phosphate. This reversible conversion is required for carbon fixation in plants – through the Calvin cycle – and for the nonoxidative phase of the pentose phosphate pathway. This enzyme has also been implicated in additional pentose and glucuronate interconversions.