Succinate-coenzyme Q, reductase

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

Complex II (succinate dehydrogenase) - Complex II is not in the path traveled by electrons from Complex I (Figure 15.3). Instead, it is a point of entry of electrons from FADH2 produced by the enzyme succinate dehydrogenase in the citric acid cycle. Both complexes I and II donate their electrons to the same acceptor, coenzyme Q. Complex II, like complex I, contains iron-sulfur proteins, which participate in electron transfer. It is also called succinate-coenzyme Q reductase because its electrons reduce coenzyme Q. 

Succinate dehydrogenase (also called succinate-coenzyme Q reductase or Complex II) is an enzyme of the citric acid cycle and glyoxylate cycle that catalyzes the reaction below ... 

FIG. 4.2 Malate metabolism in mitochondria from body wall muscle of adult Ascaris smm. (1) Fumarase (2) malic enzyme (3) pyruvate dehydrogenase complex (4) complex I (5) succinate-coenzyme Q reductase (complex II, fumarate reductase) (6) acyl CoA transferase (7) methylmalonyl CoA mutase (8) methyl-malonyl CoA decarboxylase (9) propionyl CoA condensing enzyme (10) 2-methyl acetoacetyl CoA reductase (11) 2-methyl-3-oxo-acyl CoA hydratase (12) electron-transfer flavoprotein (13) 2-methyl branched-chain enoyl CoA reductase (14) acyl CoA transferase. 

Takamiya, S., Furushima, R. and Oya, H. (1986) Electron-transfer complexes of Ascaris suum muscle mitochondria. II. Succinate coenzyme Q reductase (complex II) associated with substrate-reducable cytochrome B g. Biochim. Biophys. Acta 848 99-107. 

Complex II, also known as succinate-coenzyme Q reductase, accepts electrons from succinate formed during the TCA cycle (see the previous chapter). Electrons flow from succinate to FAD (the flavin-adenine dinucleotide) coenzyme, through an iron-sulfur protein and a cytochrome bsso protein (the number refers to the wavelength where the protein absorbs), and to coenzyme Q. No protons are translocated by Complex II. Because translocated protons are the source of the energy for ATP synthesis, this means that the oxidation of a molecule of FADH2 inherently leads to less ATP synthesized than does the... 

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-... 

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. 

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 ... 

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... 

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