Ubiquinone

Ubiquinone
• Ubiquinone is 2,3-dimethoxy-5-methyl-6-multiprenyl-1, 4-
benzoquinone, also known as CoEnzyme Q (CoQ10, Q). The reduced
form (QH2) is called ubiquinol and the partially reduced free-radical
form is called semiquinone. The quinone group allows CoEnzyme Q to
function as an electron-carrier, while the highly hydrophobic tail of
isoprene units helps to confine CoEnzyme Q to lipid-rich areas of cells.
In most species the number of isoprene units is between 6 and 10.
CoEnzyme Q in mammals either has 9 (CoQ9) or 10 (CoQ10) isoprene
units. In mouse and rat nearly 90% of ubiquinone is CoQ9, whereas
rabbit, pig, goat, sheep, cow and horse have predominantly CoQ10,
with 4% or less of CoQ9. CoQ10 is also predominant in guinea pigs
and humans.

• CoQ is the only component of the electron transport chain that is lipid rather than protein, and CoQ is the only
component that is not anchored to the inner mitochondrial membrane. CoQ is so hydrophobic, in fact, that it
usually shuttles back and forth laterally in the middle of the phospholipid bilayer without getting close to the polar
phosphate groups on the edges of the membrane. CoQ picks up reducing equivalents from protein Complex I and
protein Complex II and shuttles these "electrons" (as CoQH2 = QH2) to protein Complex III and then returns (as
CoQ = Q) to get more reducing equivalents (the "Q cycle").
• Normally most of the CoEnzyme Q is in reduced form (ubiquinol, QH2), which is the form that is most effective as
an anti-oxidant. The ubiquinol can neutralize a lipid peroxyl radical by donating one of its hydrogen atoms to
become the free-radical semiquinone, which is then restored to a non-free-radical state by the respiratory chain Q
cycle. Vitamin E is actually more effective as a mitochondrial membrane anti-oxidant than is CoEnzyme Q because
Vitamin E neutralizes lipid peroxyl radicals far more readily and because Vitamin E is less hydrophobic, allowing it to
more freely move throughout the mitochondrial membrane. CoEnzyme Q is more confined to the center of the
phospholipid bilayer. But Vitamin E tocopheroxyl radical should be neutralized to prevent it from causing oxidative
damage. And unlike CoEnzyme Q, which is synthesized in the cell, Vitamin E must be obtained from the diet.