Hypoxia-inducible factor-proline dioxygenase (EC 1.14.11.29, HIF hydroxylase) is an enzyme with systematic name hypoxia-inducible factor-L-proline, 2-oxoglutarate:oxygen oxidoreductase (4-hydroxylating). This enzyme catalyses the following chemical reaction hypoxia-inducible factor-L-proline + 2-oxoglutarate + O2 hypoxia-inducible factor-trans-4-hydroxy-L-proline + succinate + CO2 Hypoxia-inducible factor-proline dioxygenase contains iron, and requires ascorbate.