Overview of the coenzyme Q biosynthetic pathway. The key step in the pathway is the condensation reaction of the polyisoprenoid side-chain from the mevalonate pathway with 4-hydroxybenzoate, which is the product of a separate multi-step pathway starting from the precursors tyrosine or phenylalanine. Since these precursors are in excess compared to the polyisoprenoids, the rate of ubiquinol synthesis is determined by the availability of the polyisoprenoid: the conversion of Farnesyl-PP to polyprenyl-pp that is catalyzed by enzyme trans-prenyl transferase is the rate limiting step in the synthesis of ubiquinol. Ubiquinol consists of a modified benzoquinone ring attached to hydrophobic isoprenyl tail that contains 6-10 isoprenyl units depending on species. Doted lines represent additional enzymatic steps in the pathway.