Catalyzes the incorporation of an atom of dioxygen into p-hydroxybenzoate (p-OHB) to form 3,4-dihydroxybenzoate (3,4DOHB). The reaction occurs in two parts: reduction of the flavin adenine dinucleotide (FAD) in the enzyme by reduced nicotinamide adenine dinucleotide phosphate (NADPH) in response to binding p-hydroxybenzoate to the enzyme and oxidation of reduced FAD with oxygen to form a hydroperoxide, which then oxygenates p-hydroxybenzoate. 4-hydroxybenzoate + H(+) + NADPH + O2 = 3,4-dihydroxybenzoate + H2O + NADP(+) Binds 1 FAD per subunit. Aromatic compound metabolism; benzoate degradation via hydroxylation; 3,4-dihydroxybenzoate from benzoate: step 2/2. Homodimer. Controlled catalysis is achieved by movement of the flavin and protein between three conformations: in, out and open. The open conformation is important for substrate binding and product release, the in conformation for reaction with oxygen and hydroxylation, and the out conformation for the reduction of FAD by NADPH. Belongs to the aromatic-ring hydroxylase family.