3-hydroxybenzoate 4-monooxygenase () is an enzyme that catalyzes the chemical reaction
The foursubstrates of this enzyme are 3-hydroxybenzoic acid, reduced nicotinamide adenine dinucleotide phosphate (NADPH), oxygen, and a proton. Its products are protocatechuic acid, oxidised NADP<sup>+</sup>, and water.
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 3-hydroxybenzoate,NADPH:oxygen oxidoreductase (4-hydroxylating). It is also called 3-hydroxybenzoate 4-hydroxylase. It participates in benzoate degradation via hydroxylation and uses flavin adenine dinucleotide as a cofactor.
As of late 2007, two structures have been solved for this class of enzymes, with PDB accession codes and .