Ubiquinone is also known as coenzyme Q10. It is a 1,4-benzoquinone, where Q refers to the quinone chemical group, and 10 refers to the isoprenyl chemical subunits. Ubiquinone is a carrier of hydrogen atoms (protons plus electrons) and functions as an ubiquitous coenzyme in redox reactions, where it is first reduced to the enzyme-bound intermediate radical semiquinone and in a second reduction to ubiquinol (Dihydroquinone; CoQH2). Ubiquinone is not tightly bound or covalently linked to any known protein complex but is very mobile. In eukaryotes ubiquinones were found in the inner mito-chondrial membrane and in other membranes such as the endoplasmic reticulum, Golgi vesicles, lysosomes and peroxisomes. The benzoquinone portion of Coenzyme Q10 is synthesized from tyrosine, whereas the isoprene sidechain is synthesized from acetyl-CoA through the mevalonate pathway. The mevalonate pathway is also used for the first steps of cholesterol biosynthesis. The enzyme para-hydroxybenzoate polyprenyltransferase catalyzes the condensation of p-hydroxybenzoate with polyprenyl diphosphate to generate ubiquinone.