Ribulose-phosphate 3-epimerase (<db_xref db="EC" dbkey="5.1.3.1"/>) (also known as pentose-5-phosphate 3-epimerase or PPE) is the enzyme that converts D-ribulose 5-phosphate into D-xylulose 5-phosphate in Calvin's reductive pentose phosphate cycle. In <taxon tax_id="106590">Ralstonia eutropha</taxon> (Alcaligenes eutrophus) two copies of the gene coding for PPE are known [<cite idref="PUB00002204"/>], one is chromosomally encoded <db_xref db="SWISSPROT" dbkey="P40117"/>, the other one is on a plasmid <db_xref db="SWISSPROT" dbkey="Q04539"/>. PPE has been found in a wide range of bacteria, archaebacteria, fungi and plants. All the proteins have from 209 to 241 amino acid residues. The enzyme has a TIM barrel structure.