Ubiquinone = Coenzyme

Meganathan, R., Biosynthesis of menaquinone (vitamin Kj) and ubiquinone (coenzyme Q) a perspective on enzymatic mechanism. Vitamins Hormones, 61, 173, 2001. 

Ubiquinones (coenzymes Q) Q9 and Qi0 are essential cofactors (electron carriers) in the mitochondrial electron transport chain. They play a key role shuttling electrons from NADH and succinate dehydrogenases to the cytochrome b-c1 complex in the inner mitochondrial membrane. Ubiquinones are lipid-soluble compounds containing a redox active quinoid ring and a tail of 50 (Qio) or 45 (Q9) carbon atoms (Figure 29.10). The predominant ubiquinone in humans is Qio while in rodents it is Q9. Ubiquinones are especially abundant in the mitochondrial respiratory chain where their concentration is about 100 times higher than that of other electron carriers. Ubihydroquinone Q10 is also found in LDL where it supposedly exhibits the antioxidant activity (see Chapter 23). 

Isoprene chains are sometimes used as lipid anchors to fix molecules to membranes (see p. 214). Chlorophyll has a phytyl residue (1 = 4) as a lipid anchor. Coenzymes with isoprenoid anchors of various lengths include ubiquinone (coenzyme Q 1 = 6-10), plastoqui-none (1 = 9), and menaquinone (vitamin K 1 = 4-6). Proteins can also be anchored to membranes by isoprenylation. 

The role of ubiquinone (coenzyme Q, 4) in transferring reducing equivalents in the respiratory chain is discussed on p. 140. During reduction, the quinone is converted into the hydroquinone (ubiquinol). The isoprenoid side chain of ubiquinone can have various lengths. It holds the molecule in the membrane, where it is freely mobile. Similar coenzymes are also found in photosynthesis (plastoquinone see p. 132). Vitamins E and K (see p. 52) also belong to the quinone/hydroquinone systems. 

Since mitochondria are essential to cell health, mitochondrial diseases tend to be severe but, thankfully, relatively uncommon. Accordingly, the medicinal chemistry of mitochondrial disorders is still in its infancy. There are no truly effective drug therapies for mitochondrial disorders, but several agents have been reported to be of some benefit in some individuals. These agents include ubiquinone (coenzyme QIO), carnitine, and riboflavin. These compounds may assist the ailing mitochondria to better complete their metabolic tasks. However, mitochondrial medicinal chemistry is an area of research in need of additional attention. 

In this chapter, mitochondrial dysfunctions will be briefly reviewed, but the neuroprotective effects of coenzyme Q10 in neurodegenerative disease will be discussed in detail. For additional documentations, the readers should consult the two volumes on mitochondrial ubiquinone (coenzyme Q10) biochemical, functional, medical, and therapeutic aspects in human health and disease (Ebadi et al., 2001). 

Ebadi, M., Marwah, J., and Chopra, R., Mitochondrial Ubiquinone (Coenzyme Q10) Biochemical, Functional, Medical, and Therapeutic Aspects in Human Health and Disease, Vol. 1, 2, Prominent Press, Scottsdale, AZ, 2001. 

Ubiquinone (coenzyme Q) Redox coenzyme in mitochondrial electron transport chain... 

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