Quinol, vitamin

Ubiquinone or Q (coenjyme Q) (Figure 12-5) finks the flavoproteins to cytochrome h, the member of the cytochrome chain of lowest redox potential. Q exists in the oxidized quinone or reduced quinol form under aerobic or anaerobic conditions, respectively. The structure of Q is very similar to that of vitamin K and vitamin E (Chapter 45) and of plastoquinone, found in chloroplasts. Q acts as a mobile component of the respiratory chain that collects reducing equivalents from the more fixed flavoprotein complexes and passes them on to the cytochromes. 

The reaction has been applied for the synthesis of polyprenyl quinol natural product ubiquinone and vitamin K. 

In the reduction or oxidation of quinone/ quinol systems, free radicals also appear as intermediate steps, but these are less reactive than flavin radicals. Vitamin E, another qui-none-type redox system (see p.l04), even functions as a radical scavenger, by delocalizing unpaired electrons so effectively that they can no longer react with other molecules. 

Quinols and ends a-Tocopherol (vitamin E) Ubiquinol (coenzyme Q) Ascorbic acid (vitamin C)... 

It has been reported that vitamin Kj and several of the vitamin K2 homologues are capable of restoring electron transport in solvent-extracted or irradiated bacterial and mitochondrial preparations. Other reports suggest that vitamin K is concerned with the phosphorylation reactions accompanying oxidative phosphorylation The capacity of these compounds to exist m several forms, e.g., quinone, quinol. chromanol, etc., appears to strengthen the proposal that links them to oxidative phosphorylation. Information has suggested that vitamin K acts to induce prothrombin synthesis. Since prothrombin has been shown to be synthesized only by liver parenchymal cells m the dog, it would appear that the proposed role for vitamin K is not specific for only prothrombin synthesis, but applicable to other proteins. 

Many reasons for studying the chemistry of ortko-quinol derivatives lie in their implications in the biomechanisms of naturally occurring compounds [6]. Their putative involvement in the metabolism of a-tocopherol (i.e. vitamin E) [52-54], and in the mode of action of the in-... 

Ubiquinone functions as a carrier in the mitochondrial electron transport chain it is responsible for the proton pumping associated with complex I (Brandt, 1999) and is directly reduced by the citric acid cycle enzyme succinate dehydrogenase (Lancaster, 2002). As shown in Figure 14.8, it undergoes two single-electron reduction reactions to form the relatively stable semiquinone radical, then the fully reduced quinol. In addition to its role in the electron transport chain, it has been implicated as a coantioxidant in membranes and plasma lipoproteins, acting together with vitamin E (Section 4.3.1 Thomas etal., 1995, 1999). 

Quinones are firmly established in photosynthesis models. But how about vitamins E and K How do quinones work in animals First of all they transport electrons in a similar way as in photosynthesis (Metzler, 1977 Voet, 1990). Second, tocopherol is known to act as an antioxidant or radical quencher. The radical chain starting with the decomposition of unsaturated lipid peroxides, for example, is inhibited by tocopherol, which produces long-lived semiquinone radicals (Scheme 7.2.10). Vitamin E prevents, for example, sterility in rats fed rancid lipids. Vitamin E in connection with carotenes is also used as a stabilizer for diet margarines containing large amounts of essential fatty acids. Another possible activity of tocopherol is participation in oxidative phosphorylation a hydroquinone is mono-esterified with phosphoric acid to form a quinol phosphate and then oxidized to the quinone. The cationic quinone-phosphate adduct then reacts with anionic phosphate to form pyrophosphate (Scheme 7.2.10), (Wang, 1969 Breslow, 1980 Isler and Brubacher, 1982). 

Chemistry. A new method has been described for the regio- and stereo-controlled polyprenylation of quinones by means of trimethyltin derivatives. The polyprenyl trimethylstannanes (200) were prepared from the polyprenyl halides (201) and trimethylstannyl-lithium, and were then coupled with the required benzo-quinone (202) or naphthoquinone (203) in the presence of BFs-ether to give a quinol which was reduced with Ag20 to the ubiquinone (204) or vitamin K... 

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