5-Demethoxyubiquinone hydroxylase COQ7 catalyzes the hydroxylation of 6-methoxy-3-methyl-2-decaprenyl-1,4-benzoquinol (DMQ10H2, DMQ) to 3-demethylubiquinol-10 (DeMQ10H2, DeMQ), using NADH and oxygen. COQ7 forms a heterooctameric complex with ubiquinone biosynthesis protein COQ9, a lipid-binding protein presenting the substrate to COQ7 activity. In this complex, COQ7 binds two iron ions and forms a heterodimer with COQ9, with four of these dimers forming an octameric cage (Vajo et al., 1999; Lohman et al., 2014; Manicki et al., 2022). This reaction was first studied in yeast (Marbois & Clarke, 1996; Tran et al., 2006). Both COQ8A and COQ8B bind to COQ7, presumably during formation of a hypothetical multienzyme COQ complex. COQ8A,B are required for the assembly of the complex but may not be stably incorporated into it, so only the components identified by Floyd et al. are annotated (Ashraf et al., 2013; Floyd et al., 2016; reviewed in Hojabri et al., 2023; Liang et al., 2023).