FAD-dependent monooxygenase required for the C5-ring hydroxylation during ubiquinone biosynthesis. Catalyzes the hydroxylation of 3-polyprenyl-4-hydroxybenzoic acid to 3-polyprenyl-4,5-dihydroxybenzoic acid. The electrons required for the hydroxylation reaction may be funneled indirectly from NADPH via a ferredoxin/ferredoxin reductase system to COQ6. 4-hydroxy-3-all-trans-hexaprenylbenzoate + 2 H(+) + O2 + 2 reduced [2Fe-2S]-[ferredoxin] = 3,4-dihydroxy-5-all-trans-hexaprenylbenzoate + H2O + 2 oxidized [2Fe-2S]-[ferredoxin] Cofactor biosynthesis; ubiquinone biosynthesis. Component of a multi-subunit COQ enzyme complex, composed of at least COQ3, COQ4, COQ5, COQ6, COQ7 and COQ9. Interacts with COQ8B and COQ7. Localizes to cell processes and Golgi apparatus in podocytes. In the kidney, expressed almost exclusively in glomerular podocytes. In the inner ear, expressed in the spiral ganglion, as well as in stria vascularis and spiral ligament cells. Belongs to the UbiH/COQ6 family.