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). 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 (By similarity). Interacts with COQ8B and COQ7 (PubMed:24270420). Localizes to cell processes and Golgi apparatus in podocytes. Widely expressed. The disease is caused by variants affecting the gene represented in this entry. Mutations in COQ6 may play a role in susceptibility to Schwannomatosis, a cancer predisposition syndrome in which patients develop multiple non-vestibular schwannomas, benign neoplasms that arise from Schwann cells of the cranial, peripheral, and autonomic nerves. Belongs to the UbiH/COQ6 family. Based on current literature, utilization of para-aminobenzoic acid (pABA) involving C4-deamination is still unclear, and 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 support C4-deamination in yeast cells lacking COQ9.