Coenzyme Q. reduced

Coenzyme Q (oxidized form) Coenzyme Q (reduced form)... 

Oxidized form of coenzyme Q Reduced form of coenzyme Q... 

In the third complex of the electron transport chain, reduced coenzyme Q (UQHg) passes its electrons to cytochrome c via a unique redox pathway known as the Q cycle. UQ cytochrome c reductase (UQ-cyt c reductase), as this complex is known, involves three different cytochromes and an Fe-S protein. In the cytochromes of these and similar complexes, the iron atom at the center of the porphyrin ring cycles between the reduced Fe (ferrous) and oxidized Fe (ferric) states. 

FIGURE 24.11 The acyl-CoA dehydrogenase reaction. The two electrons removed in this oxidation reaction are delivered to the electron transport chain in the form of reduced coenzyme Q (UQH9). 

Abnormalities of the respiratoiy chain. These are increasingly identified as the hallmark of mitochondrial diseases or mitochondrial encephalomyopathies [13]. They can be identified on the basis of polarographic studies showing differential impairment in the ability of isolated intact mitochondria to use different substrates. For example, defective respiration with NAD-dependent substrates, such as pyruvate and malate, but normal respiration with FAD-dependent substrates, such as succinate, suggests an isolated defect of complex I (Fig. 42-3). However, defective respiration with both types of substrates in the presence of normal cytochrome c oxidase activity, also termed complex IV, localizes the lesions to complex III (Fig. 42-3). Because frozen muscle is much more commonly available than fresh tissue, electron transport is usually measured through discrete portions of the respiratory chain. Thus, isolated defects of NADH-cytochrome c reductase, or NADH-coenzyme Q (CoQ) reductase suggest a problem within complex I, while a simultaneous defect of NADH and succinate-cytochrome c reductase activities points to a biochemical error in complex III (Fig. 42-3). Isolated defects of complex III can be confirmed by measuring reduced CoQ-cytochrome c reductase activity. 

Most compounds oxidized by the electron transport chain donate hydrogen to NAD+, and then NADH is reoxidized in a reaction coupled to reduction of a flavoprotein. During this transformation, sufficient energy is released to enable synthesis of ATP from ADP. The reduced flavoprotein is reoxidized via reduction of coenzyme Q subsequent redox reactions then involve cytochromes and electron transfer processes rather than hydrogen transfer. In two of these cytochrome redox reactions, there is sufficient energy release to allow ATP synthesis. In... 

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. 

CoQ Coenzyme Q FADH- Radical form of reduced flavin adenine dinucleotide... 

In addition to NAD and flavoproteins, three other types of electron-carrying molecules function in the respiratory chain a hydrophobic quinone (ubiquinone) and two different types of iron-containing proteins (cytochromes and iron-sulfur proteins). Ubiquinone (also called coenzyme Q, or simply Q) is a lipid-soluble ben-zoquinone with a long isoprenoid side chain (Fig. 19-2). The closely related compounds plastoquinone (of plant chloroplasts) and menaquinone (of bacteria) play roles analogous to that of ubiquinone, carrying electrons in membrane-associated electron-transfer chains. Ubiquinone can accept one electron to become the semi-quinone radical ( QH) or two electrons to form ubiquinol (QH2) (Fig. 19-2) and, like flavoprotein carriers, it can act at the junction between a two-electron donor and a one-electron acceptor. Because ubiquinone is both small and hydrophobic, it is freely diffusible within the lipid bilayer of the inner mitochondrial membrane and can shuttle reducing equivalents between other, less mobile electron carriers in the membrane. And because it carries both electrons and protons, it plays a central role in coupling electron flow to proton movement. 

Free energy is released as electrons are transferred along the electron transport chain from an electron donor (reducing agent or reduc-tant) to an electron acceptor (oxidizing agent or oxidant). The (electrons can be transferred in different forms, for example, as hydride ions ( FT) to NAD+, as hydrogen atoms (-H) to FMN, coenzyme Q, and FAD, or as electrons (-e ) to cytochromes. 

Correct answer = D. Thirteen of the approximately 100 polypeptides required for oxidative phosphorylation are coded for by mitochondrial DNA, including the electron transport components cytochrome c and coenzyme Q. Oxygen directly oxidizes cytochrome oxidase. Succinate dehydrogenase directly reduces FAD. Cyanide inhibits electron flow, proton pumping, and ATP synthesis. 

The reduced coenzymes NADH and FADH2 each donate a pair of electrons to a specialized set of electron carriers, consisting of FMN, coenzyme Q, and a series of cytochromes, col lectively called the electron transport chain. This pathway is present in the inner mito chondrial membrane, and is the final common pathway by which electrons derived from different fuels of the body flow to oxygen. The terminal cyctochrome, cytochrome a+ 83, is the only cytochrome able to bind oxygen. 

In 1955, R. A. Morton and associates in Liverpool announced the isolation of a quinone which they named ubiquinone for its ubiquitous occurrence.484 485 It was characterized as a derivative of benzoquinone attached to an unsaturated polyprenyl (isoprenoid) side chain (Fig. 15-24). In fact, there is a family of ubiquinones that from bacteria typically contains six prenyl units in its side chain, while most ubiquinones from mammalian mitochondria contain ten. Ubiquinone was also isolated by F. L. Crane and associates using isooctane extraction of mitochondria. These workers proposed that the new quinone, which they called coenzyme Q, might participate in electron transport. As is described in Chapter 18, this function has been fully established. Both the name ubiquinone and the abbreviation Q are in general use. A subscript indicates the number of prenyl units, e.g., Q10. Ubiquinones can be reversibly reduced to the hydro-quinone forms (Fig. 15-24), providing a basis for their function in electron transport within mitochondria and chloroplasts.486 490... 

MPTP decreases glutathione levels and increases the levels of reactive oxygen species and the degree of lipid peroxidation in mouse brain slices in vitro and increases the levels of reactive oxygen species in mouse brain in vivo. MPTP neurotoxicity in vitro is reduced by glutathione. In vitro studies have shown that MPP neurotoxicity can be reduced by vitamin E, vitamin C, coenzyme Q, and mannitol (but not by superoxide dismutase, catalase, allopurinol, or dimethyl sulfoxide). P-Carotene, vitamin C, and /V-acctylcystcine partially protect against the neurotoxic effects of MPTP in mice, as do nicotinamide, coenzyme Q, and the free-radical spin trap A-tert-butyl-a-(sulfophenyl) nitrone. 

NADH, reduced nicotinamide-adenine dinueleotide Q, coenzyme Q FNR, fumarate nitrate reduction SoxR, superoxide activated regulatory protein IRP, iron regulatory protein NIFU, nitrogen fixation gene U product. 

Several reactions in metabolism are oxidation-reduction (or redox) reactions. Two of the principal redox carriers are nicotinamide adenine dinucleotide (NAD+) and coenzyme Q. Remember that we live in an oxidizing world, so species that are in the reduced form are frequently high-energy compounds that react exothermically with oxygen. Also recall that organic molecules are reduced by adding bonds to hydrogen. 

Figure 12.36 shows the reduction reaction of coenzyme Q into the reduced form, QFI2. The added hydrogens are shown in blue. QF12 is the energized form that can feed into the electron transport chain to synthesize ATP. 

CoQH2 reduced coenzyme Q (ubiquinol) GPI glycosyl phosphatidylinositol... 

---