In enzymology, an imidazoleacetate 4-monooxygenase () is an enzyme that catalyzes the chemical reaction
The 4 substrates of this enzyme are 4-imidazoleacetate, NADH, H<sup>+</sup>, and O<sub>2</sub>, whereas its 3 products are 5-hydroxy-4-imidazoleacetate, NAD<sup>+</sup>, and H<sub>2</sub>O.
The enzyme is a flavin-containing monooxygenase that uses molecular oxygen as oxidant and incorporates one of its atoms into the starting material. The systematic name of this enzyme class is 4-imidazoleacetate,NADH:oxygen oxidoreductase (5-hydroxylating). Other names in common use include imidazoleacetic hydroxylase, imidazoleacetate hydroxylase, and imidazoleacetic monooxygenase. This enzyme participates in histidine metabolism. It employs one cofactor, FAD.