Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Electron Transport, Oxidative Phosphorylation, and the Regulation of ATP Synthesis
Ubiquinone is a fat-soluble quinone

The next link in the chain of reducing equivalent carriers is ubiquinone, or coenzyme Q (named after the first letter of the word quinone). The name "ubiquinone" reflects the ubiquitous distribution of this coenzyme, as it has been found in virtually all Cells. Ubiquinone is a lipid-soluble quinone featuring a very long isoprenoid side chain (Fig. 17-10). Most mammalian Tissues contain a ubiquinone with a side chain consisting of ten five-carbon isoprene units, designated as Q₁₀ or CoQ₁₀. The tissues of other animals utilize ubiquinones with side chains containing only six or eight isoprene units (Q₆ or Q₈). When reducing equivalents are transferred from reduced NADH dehydrogenase (E-FMNH₂) via iron-sulfur centers to ubiquinone, the latter is reduced to ubiquinol, or QH₂ (Fig. 17-10), with the simultaneous regeneration of the oxidized form of NADH dehydrogenase:

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Fig. 17-10. Ubiquinone, or coenzyme Q; n represents the number of isoprene units in the side chain (see text). The groups highlighted against a red Background participate in hydrogen atom transfer. Note that the reduction of ubiquinone to ubiquinol is also accompanied by a shift in THE POSITION OF the double bonds within the ring.

Ubiquinone molecules, which are substantially longer than the phospholipid molecules of The inner mitochondrial membrane, exist both in a free form and bound to Proteins. Ubiquinone acts as a collector, gathering reducing equivalents not only from NADH dehydrogenase, but also from other flavin-dependent dehydrogenases located within the Cell/35.html">Mitochondria (see Fig. 17-7), specifically succinate dehydrogenase and acyl-CoA dehydrogenase, which is involved in Fatty acid oxidation (Chapter 18).