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(also benzoquinone, coenzyme Q), any of a series of derivatives of 5-methyl-2,3-dimethoxyquinone having a variable terpenoid side chain attached to the carbon atom in position six. Ubiquinones have from six to ten isoprenoid units (C5H8). The structural formula is

Ubiquinones figure in reactions that supply energy to cells. They are designated by the symbols UQn or CoQn; in the first, n represents the number of carbon atoms, and in the second, the number of isoprenoid units. For example, UQ30 will correspond to CoQ6. Ubiquinones occur as colorless, crystalline substances whose absorption maximum is at 270 micrometers. They are insoluble in water and soluble in organic solvents.

Ubiquinones are present in all organisms; UQ50 predominates in mammals, UQ30–50 in fungi, UQ40 in bacteria, UQ40–45 in protozoa, and UQ45–50 in insects. Plant mitochondria contain UQ40–50 in small quantities, and the chloroplasts in plants contain plastoquinones, which are similar to ubiquinones. Various naph-thoquinones occur in certain bacteria.

The biological role of ubiquinones is based on their ability to participate in reversible oxidation-reduction conversions. Ubiquinones are located in the cytoplasmic membranes of bacteria and in the inner membranes of mitochondria. They participate in the transfer of electrons and hydrogen between flavoproteins and cytochrome b in the respiratory chain.


Skulachev, V. P. Transformatsiia energii v biomembranakh. Moscow, 1972.
Morton, R. A. “Ubiquinones, Plastoquinones, and Vitamins K.” Biological Reviews of the Cambridge Philosophical Society, 1971, vol. 46, no. 1.


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Photosystems I and II of chloroplast, and complex I, ubiquinone and complex III of electron transport chain (ETC) in mitochondria are the major sites of their synthesis (Gill and