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 a ferredoxin/ferredoxin reductase system to COQ6 (By similarity).Is able to perform the deamination reaction at C4 of 3-hexaprenyl-4-amino-5-hydroxybenzoic acid (HHAB) to produce DHHB when expressed in yeast cells lacking COQ9, even if 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 (PubMed:26260787).