Ubiquinones, function

Ubiquinones function within the mitochondria of cells to mediate the respiration process in which electrons are transported from the biological reducing agent NADH to molecular oxygen. Through a complex series of steps, the ultimate result is a cycle whereby NADH is oxidized to NAD+, O2 is reduced to water, and energy is produced. Ubiquinone acts only as an intermediary and is itself unchanged. 

Ubiquinones function as electron transport agents within the inner mitochondrial membranes496 and also within the reaction centers of the photosynthetic membranes of bacteria (Eq. 23-32).484/488/494 The plasto-quinones also function in electron transport within... 

Most of the components involved in electron transport in mitochondria are contained in four supramolecular protein complexes that traverse the inner mitochondrial membrane. Complex I, which contains FMN and various iron-sulfur clusters as active sites, transfers electrons from NADH to ubiquinone (Fig. 6-8). Complex II, which contains FAD, various iron-sulfur clusters, and a Cyt >, transfers electrons from succinate also to a ubiquinone. Ubiquinone functions as a pool of two-electron carriers, analogous to the function of plastoquinone A in the lamellar membranes of chloroplasts, which accepts electrons from Complexes I and II and delivers them to the... 

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). 

Emster L. and DaUner, G., Biochemical, physiological and medical aspects of ubiquinone function, Biochim. Biophys. Acta, 1271,195,1995. 

There exist several hypotheses as to how the ubiquinone functions in the membrane. The most common one is that coenzyme Q floats freely in the membrane37 or forms a mobile Q pool27 or Q cycle38,39 Ubiquinones differ in the length of the side isoprenoid chain, consisting usually of 6-10 carbon atoms. 

Ubiquinones have a long, isoprene-derived side chain (see Spedal Topic E in WileyPLUS and Section 23.3). Ten isoprene units are present in the side chain of human ubiquinones. This part of their structure is highly nonpolar, and it serves to solubilize the ubiquinones within the hydrophobic bilayer of the mitochondrial inner membrane. Solubility in the membrane environment facilitates their lateral diffusion from one component of the electron transport chain to another. In the electron transport chain, ubiquinones function by accepting two electrons and two hydrogen atoms to become a hydroquinone. The hydroquinone form carries the two electrons to the next acceptor in the chain ... 

There is evidence that specific ubiquinones function in... 

There is evidence that speciTc ubiquinones function in the remission I prevention I nl some of the symptoms of vitamin E defidency. IAIsq see COENZYME D.l... 

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