In enzymology, a 4-hydroxyproline epimerase is an enzyme that catalyzes the chemical reaction trans-4-hydroxy-L-proline ⇌ {\displaystyle \rightleftharpoons } cis-4-hydroxy-D-prolineHence, this enzyme has one substrate, trans-4-hydroxy-L-proline, and one product, cis-4-hydroxy-D-proline. Wikipedia