Ubiquinol production

Effect of Rotenone and Antimycin A on Electron Transfer Rotenone, a toxic natural product from plants, strongly inhibits NADH dehydrogenase of insect and fish mitochondria. Antimycin A, a toxic antibiotic, strongly inhibits the oxidation of ubiquinol. 

The photochemistry of the reaction center takes place one electron at a time. However, one of the products of the electron transfer process is a reduced ubiquinone, which has taken up two electrons as well as two protons. To form this species, the reaction center must turn over twice, with electrons entering the complex by donation of cytochrome ci with the oxidized special pair. The electrons accumulate in the quinone acceptors and protons are taken up from the surrounding medium. Finally, a fidly reduced ubiquinol is formed, which is released from the complex into the hydrocarbon portion of the membrane. The quinol is subsequently reoxidized at the cytochrome bc complex (described below). 

The property of NO of inhibiting mitochondrial electron transfer was first recognized in 1994 by two British research groups [14, 15] that reported the inhibition of brain and muscle cytochrome oxidase (complex IV) activity by low NO concentrations in a reversible and Oj-competitive manner. More related to the scope of this review is the NO inhibition of electron transfer at complex III, ubiquinol-cytochrome c reductase, the second NO-sensitive point in the respiratory chain, where inhibition of electron transfer between cytochromes b and c enhances mitochondrial H2O2 production [16]. Nitric oxide, produced by NO donors or by mitochondrial nitric oxide synthase (mtNOS), inhibits complex III electron transfer and increases Oy and H2O2 production in sub-mitochondrial particles and in mitochondria. Complex IV is more sensitive to NO inhibition (IC5o=O.l pM) than complex III (IC5o=O.2 pM). 

Cadenas E, Boveris A, Ragan CI, Stoppani AOM (1977) Production of superoxide radicals and hydrogen peroxide by NADH-ubiquinone reductase and ubiquinol-cytochrome c reductase from beef heart mitochondria. Arch Biochem Biophys 180 248-257 Han D, Williams E, Cadenas E (2001) Mitochondrial respiratory chain-dependent generation of superoxide anion and its release to the intermebrane space. Biochem J 353 411-416... 

These compounds often occur as acetate or succinate esters and, for a total analysis, these esters are cleaved by a KOH saponification. Vitamin E is obtained commercially by molecular distillation. This does not provide a pure product, but it does separate vitamin E from most of the other materials associated with it such as chlorophyll, xanthophylls, carotenes, ubiquinol, ubichromenol, steroids, and quinines, but not vitamin A and the beta carotenes. 

The complex catalyzes electron transfer from reduced UQ to cytochrome c, coupled to the translocation of protons by a mechanism known as the Q cycle [55-57]. This involves the diversion of half of the electrons available from ubiqui-nol oxidation and deprotonation at a site on the outside of the inner mitochondrial membrane (Qo site) to reduce and protonate UQ at a site on the inside of the membrane (Qi site). The pathway for electron transfer across the membrane is provided by the two haem centers (bt and bn) of the mitochondrial gene product cytochrome b. The remainder of the electrons from ubiquinol oxidation pass along the chain to reduce first the Rieske iron sulfur protein (ISP), then cytochrome Cl and then cytochrome c (Fig. 13.1.3). 

Strobilurins halt the production of ATP by blocking the electron transport at the level of the be,-complex, located within the mitochondrial Inner membrane, which separates the matrix from the intermembrane space (transmission electron microscope image). In a process, the so-called Q cycle, which was first proposed by the British biochemist Peter Dennis Mitchell (1920-1992), ubihydroquinone (also known as ubiquinol) is oxidized to ubiquinone, thereby transfering an electron to each, the Rieske iron-sulfur complex and the bi heme. While this cycle operates twice, four protons in total are pumped into the intermembrane space, and generate a proton gradient. 

Complex IV (cytochrome c oxidase) uses electrons from the product of Complex III, ferrocytochrome c, to reduce molecular oxygen (Q2) with the result of forming water. However, it is useful to realize that cytochrome c oxidase is actually part of a superfamily of heme-copper respiratory oxidases. Qne member of the superfamily is a ubiquinol oxidase from E. coli, In this protein-based machine ubiquinol, QH2, replaces ferrocytochrome c (cyt as an electron source. Furthermore, ubiquinol oxidase contains no binuclear Cua center that normally oxidizes ferrocytochrome c in Complex IV. 

This rate-detemiining step is followed by the rapid uptake of a further electron and a further proton yielding the final product, ubiquinol. The rate of consumption of UQ is therefore proportional to the hydrogen ion concentration of the buffer solution adjacent to the hpid monolayer times the concentration of the UQ radical anion this can be expressed as a function of the UQ concentration, [UQ], via the Nernst equation as applied to the first electron-transfer step in quasiequilibrium. The resulting differential equation can be integrated by separation of variables between time t = 0, when [UQ] = [UQ]in> and a given time t ... 

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