FAD-dependent monooxygenase required for the C5-ring hydroxylation during ubiquinone biosynthesis. Catalyzes the hydroxylation of 3-hexaprenyl-4-hydroxybenzoic acid (HHB) to 3-hexaprenyl-4,5-dihydroxybenzoic acid (DHHB). The electrons required for the hydroxylation reaction may be funneled indirectly from NADPH via ferredoxin (YAH1) and ferredoxin reductase (ARH1) to COQ6 (PubMed:21944752). Can also convert 3-hexaprenyl-4-aminobenzoic acid (HAB), a COQ2-prenylated pABA, to DHHB in a two step process. HAB is first hydroxylated at C5 to yield 3-hexaprenyl-4-amino-5-hydroxybenzoic acid (HHAB) which is further deaminated at C4 by COQ6 to produce DHHB (PubMed:26260787). 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] 4-amino-3-all-trans-hexaprenylbenzoate + 2 H(+) + O2 + 2 reduced [2Fe-2S]-[ferredoxin] = 4-amino-5-hydroxy-3-all-trans-hexaprenylbenzoate + H2O + 2 oxidized [2Fe-2S]-[ferredoxin] 4-amino-5-hydroxy-3-all-trans-hexaprenylbenzoate + 5 H(+) + O2 + 4 reduced [2Fe-2S]-[ferredoxin] = 3,4-dihydroxy-5-all-trans-hexaprenylbenzoate + H2O + NH4(+) + 4 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. Present with 2940 molecules/cell in log phase SD medium. Belongs to the UbiH/COQ6 family. Based on current literature, utilization of para-aminobenzoic acid (pABA) involving C4-deamination seems not to occur in bacteria, plants and mammals where only C5 hydroxylation of HHB has been shown, even if human COQ6 is able to perform the deamination reaction in the absence of COQ9.