A flavin-dependent monooxygenase involved in ubiquinone/ubiquinol biosynthesis, Coq6 (Heeringa et al. 2011) catalyses the C5-hydroxylation of 3-hexaprenyl-4-hydroxybenzoic acid (HHB) to 3,4-dihydroxy-5-hexaprenylbenzoic acid (DHHB) (Ozeir et al. 2011, Gin et al. 2003). Coq6 is a peripheral membrane protein that localizes to the matrix side of the inner mitochondrial membrane (Gin et al. 2003). Ferredoxin, Yah1 and the ferredoxin reductase, Arh1 may be the in vivo source of electrons for Coq6 and additionally required for this reaction (Ozeir et al. 2011).