Coenzyme Q cytochrome c reductase

NADH coenzyme Q reductase defect (complex I) Succinate coenzyme Q reductase defect (complex II) Coenzyme Q cytochrome C reductase defect (complex III)... 

Rieske, J. S., D. H. Maclennan, and R. Coleman Isolation and properties of an iron-protein from the (reduced-coenzyme Q) -cytochrome c reductase complex of the respiratory chain. Biochem. Biophys. Res. Commun. 15, 338 (1964). 

Electron transport chain Present in the mitochondrial membrane, this linear array of redox active electron carriers consists of NADH dehydrogenase, coenzyme Q, cytochrome c reductase, cytochrome c, and cytochrome oxidase as well as ancillary iron sulfur proteins. The electron carriers are arrayed in order of decreasing reduction potential such that the last carrier has the most positive reduction potential and transfers electrons to oxygen. 

Complex III is also known as coenzyme Q-cytochrome c reductase. It accepts electrons from reduced coenzyme Q, moves them within the complex through two cytochromes b, an iron-sulfur protein, and cytochrome Ci. Electron flow through Complex II transfers proton(s) through the membrane into the intermembrane space. Again, this supplies energy for ATP synthesis. Complex III transfers its electrons to the heme group of a small, mobile electron transport protein, cytochrome c. 

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. 

Thenoyltrifluoroacetone and carboxin and its derivatives specifically block Complex II, the succinate-UQ reductase. Antimycin, an antibiotic produced by Streptomyees griseus inhibits the UQ-cytochrome c reductase by blocking electron transfer between bn and coenzyme Q in the Q site. Myxothiazol inhibits the same complex by acting at the site. 

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. 

Electrons enter the ETC at respiratory Complexes I and II. The electrons from NADH enter at respiratory Complex I (RC I, NADH dehydrogenase) with the concomitant oxidation of NADH to NAD+. The electrons carried by FADH2 are transferred to RC II (succinate dehydrogenase) as the FADH2 is oxidized to FAD and succinate is reduced to fumarate. These electrons from RC I and II are transferred to the quinone form of coenzyme Q (CoQ), which delivers them to RC III (UQ-cytochrome c reductase). Cytochrome c then accepts the electrons from RC III, and the reduced cytochrome c is reoxidized as it delivers the electrons to RC IV, cytochrome c oxidase. The electrons are then used by RC IV to reduce molecular oxygen to water. 

NADH and FADHg are produced as a result of substrate level dehydrogenations. Oxidation of these reduced coenzymes by oxygen is accomplished by the intervention of a series of electron carriers between the primary reductant and the terminal oxidant (Fig. 2). The electron-transport components represent redox couples of increasing redox potential and are therefore favored thermodynamically. The respiratory chain can be separated into four multienzyme complexes NADH-Q reductase (complex I), succinate-Q reductase (complex II), QH2"Cytochrome c reductase (complex III), and cytochrome c oxidase (complex IV). At each of these successive oxidation-reduction steps, a certain amount of free energy is available, the amount being determined by the difference in the oxidation-reduction potential of the two sequential components. The difference in the redox potential between... 

Complex III The third complex, CoQH2-cytochrome c oxidoreductase (also called cytochrome reductase), catalyzes the oxidation of reduced coenzyme Q (GoQHg). The electrons produced by this oxidation reaction are passed along to cytochrome c in a multistep process. The overall reaction is... 

Reflect and Apply Cytochrome oxidase and succinate-CoQ oxido-reductase are isolated from mitochondria and are incubated in the presence of oxygen, along with cytochrome c, coenzyme Q, and succinate. What is the overall oxidation-reduction reaction that can be expected to take place ... 

A protein complex capable of rapidly reducing coenzyme Q in the presence of succinate (succinic coenzyme Q reductase), NADH (NADH coenzyme Q reductase), and cytochrome c (cytochrome c coen-... 

Respiratory-chain disorders (with mitochondrial myopathy) Lactic Various, including cytochromes b, aa, cytochrome c oxidase, NADH-coenzyme Q reductase 15.6... 

---