Catalyzes the hydroxylation of 2-polyprenyl-3-methyl-6-methoxy-1,4-benzoquinol (DMQH2) during ubiquinone biosynthesis. 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 (PubMed:19478076). 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. 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. Interacts with COQ8B and COQ6. Interacts with COQ9. Highly expressed in tissues with high energy demand such as heart, muscle, liver, and kidney. Mice lacking Coq7 start to die after E8. Heterozygous mutant reveal that the reduction of Coq7 levels in these animals profoundly alters their mitochondrial function despite the fact that ubiquinone production is unaffected. The mitochondria of young mutants heterozygous are dysfunctional, exhibiting reduced energy metabolism and a substantial increase in oxidative stress. In life-span analysis, transgenic expression reverted the extended life span of clk-1 to the comparable level with wild-type control. Belongs to the COQ7 family.