The length of the polyisoprenoid chain of ubiquinone aka coenzyme Q (Q), varies depending on the species involved: it is 6 in budding yeast, Saccharomyces cerevisiae, (Q6) and 10 in humans (Q10). Most ubiquinone is naturally reduced to ubiquinol (Q10H2 in humans) and it is this form which dominates in most human tissues. It functions as an ubiquitous coenzyme in redox reactions and its ability both to associate with complexes I and III of the electron transport chain and to move freely in the inner mitochondrial membrane are central to its role in electron transport. In eukaryotes ubiquinones/ubiquinols are also found in other membranes such as the endoplasmic reticulum, Golgi vesicles, lysosomes and peroxisomes.
Ubiquinol/ubiquinone is synthesized in the following way. Initially, the polyisoprenoid tail is assembled by a polyprenyl diphosphate synthase. Next, 4 hydroxybenzoate polyprenyltransferase (COQ2) catalyses the formation of the covalent linkage between the benzoquinone head group and the tail to produce 4 hydroxy 3 polyprenyl benzoic acid intermediate (DHB, 3 decaprenyl 4 hydroxybenzoic acid in humans). There follows modifications of the aromatic ring starting with hydroxylation, followed by O methylation, and decarboxylation to form the 2 methoxy 6 polyprenyl phenol intermediate (DMPhOH ; 2 methoxy 6 decaprenylphenol in humans). Following this, two additional methylations, one C methylation, and one O methylation step, finally generate the fully substituted hydroquinone, ubiquinol (Szkopinska 2000, Kagan & Quinn 2000, Tran & Clarke 2007, Kawamukai 2009).