the enzyme that converts D-ribulose 5-phosphate into D-xylulose 5-phosphate in Calvin's reductive pentose phosphate cycle . D-ribulose 5-phosphate D-xylulose 5-phosphate Hence, this enzyme has one substrate , D-ribulose 5-phosphate , and one product , D-xylulose 5-phosphate . This enzyme belongs to the family of isomerases , specifically those racemases and epimerases