The enzyme 4-hydroxybenzoate 3-monooxygenase, also commonly referred to as para-hydroxybenzoate hydroxylase (PHBH), is a flavoprotein belonging to the family of oxidoreductases.

Specifically, it is a hydroxylase, and is one of the most studied enzymes and catalyzes reactions involved in soil detoxification, metabolism, and other biosynthetic processes.

[3] The hydroxylase, 4-hydroxybenzoate 3-monooxygenase, proceeds through a catalytic process that begins with the entrance of NADPH and 4-hydroxybenzoate (the native substrate) into the active site of the enzyme.

NADP+ is lost and O2 enters into the complex, followed by oxidation of the flavin to form a hydroperoxide, which acts as the hydroxide transfer reagent.

Finally, the product exits from the complex and the hydroxy-flavin is dehydrated, regenerating FAD and allowing the process to repeat.

[2] The substrate binds in the active site of the enzyme via non-covalent interactions with proximal amino acid side chains.

The final step in the mechanism is dissociation of the product and water from FAD, causing the flavin to return to the open conformation.