In enzymology, a 4-methoxybenzoate monooxygenase (O-demethylating) (EC 1.14.99.15) is an enzyme that catalyzes the chemical reaction

4-methoxybenzoate + AH₂ + O₂ ${\displaystyle \rightleftharpoons }$ 4-hydroxybenzoate + formaldehyde + A + H₂O

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The 3 substrates of this enzyme are 4-methoxybenzoate, an electron acceptor AH₂, and O₂, whereas its 4 products are 4-hydroxybenzoate, formaldehyde, the reduction product A, and H₂O.

This enzyme belongs to the family of oxidoreductases, specifically those acting on paired donors, with O2 as oxidant and incorporation or reduction of oxygen. The oxygen incorporated need not be derive from O miscellaneous. The systematic name of this enzyme class is 4-methoxybenzoate,hydrogen-donor:oxygen oxidoreductase (O-demethylating). Other names in common use include 4-methoxybenzoate 4-monooxygenase (O-demethylating), 4-methoxybenzoate O-demethylase, p-anisic O-demethylase, and piperonylate-4-O-demethylase. This enzyme participates in 2,4-dichlorobenzoate degradation.