ABSTRACT
Ubiquinone (coenzyme Q or Q10) is an important redox component of cellular membranes. Its structure consists of two parts: a redox-active benzoquinone nu cleus, and a hydrophobic polyprenyl tail comprised of isoprene units linked with a trans configuration (Fig. 1). Q is detected in all organisms except gram-positive bacteria and cyanobacteria, and the length of the polyprenyl tail is species specific (1,2). The readily reversible redox chemistry of the benzoquinone head group allows Q to undergo sequential, one-electron reduction to form the ubisemiqui none (QH ) (or the semiquinone anion, Q ~) and the hydroquinone (QH2). As a membrane-soluble molecule, Q is an essential component of the eukaryotic respiratory chain located in the inner mitochondrial membrane of eukaryotes and in the plasma membrane of prokaryotes (3). Linear dichroism, NMR, fluores cence, and EPR studies indicate that ubiquinone is oriented in the membrane bilayer midplane with the tail perpendicular to the lipid chains, allowing the quinone head group to fluidly move through the lipids and react with the respiratory complexes and other molecules at the membrane surface (4-6). Membraneanchoring of the tail is important for maximizing Q reactivity since Q isoforms with shorter prenylated tails are less able to reduce cytochrome-c or ferricyanide
SchultzandClarke
Figure 1Reversible reduction and oxidation of Q. n designates the number of isoprene units in the polyprenyl tail which can vary from 6 (Q6,Saccharomyces cerevisiae), to 8 (Q8,Escherichia coli), 9(Q9,rat), or 10 (Q10,human) (1,2).
