Catalyzes the hydroxylation of 2-polyprenyl-3-methyl-6-methoxy-1,4-benzoquinol (DMQH2) during ubiquinone biosynthesis (By similarity). Has also a structural role in the COQ enzyme complex, stabilizing other COQ polypeptides (By similarity). Involved in lifespan determination in a ubiquinone-independent manner (By similarity). Plays a role in modulating mitochondrial stress responses, acting in the nucleus, perhaps via regulating gene expression, independent of its characterized mitochondrial function in ubiquinone biosynthesis (PubMed:25961505). a 6-methoxy-3-methyl-2-all-trans-polyprenyl-1,4-benzoquinol + AH2 + O2 = A + a 3-demethylubiquinol + H2O Binds 2 iron ions per subunit. 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 COQ6 (PubMed:24270420). Interacts with COQ9 (PubMed:25339443). Expressed dominantly in heart and skeletal muscle. The disease is caused by variants affecting the gene represented in this entry. Belongs to the COQ7 family.