Succinate-CoQ reductase

FADH2 is reoxidized to FAD by donating two electrons to succinate-CoQ reductase (complex II), a protein complex that contains FeS clusters. It passes the electrons on to ubiquinone in the main electron transport chain where their further transport leads to the formation of an H+ gradient and ATP synthesis. However succinate-CoQ reductase does not itself pump H+ ions. 

Succinate-CoQ Reductase (Complex II) Succinate dehydrogenase, the enzyme that oxidizes a molecule of succinate to fumarate in the citric acid cycle, is an integral component of the succinate-CoQ reductase complex. The two electrons released in conversion of succinate to fumarate are transferred first to FAD, then to an iron-sulfur cluster, and finally to CoQ (see Figure 8-17). The overall reaction catalyzed by this complex is... 

Although the AG° for this reaction is negative, the released energy is insufficient for proton pumping. Thus no protons are translocated across the membrane by the succinate-CoQ reductase complex, and no proton-motive force is generated in this part of the respiratory chain. 

Current evidence suggests that a total of 10 protons are transported from the matrix space across the inner mitochondrial membrane for every electron pair that is transferred from NADH to O2 (see Figure 8-17). Since the succinate-CoQ reductase complex does not transport protons, only six protons are transported across the membrane for every electron pair that is transferred from succinate (or FADH2) to O2. Relatively little is known about the coupling of electron flow and proton translocation by the NADH-CoQ reductase complex. More is known about operation of the cytochrome c oxidase complex, which we discuss here. The coupled electron and proton movements mediated by the CoQH2-cytochrome c reductase complex, which involves a unique mechanism, are described separately. 

C0QH2 is generated both by the NADH-CoQ reductase and succinate-CoQ reductase complexes and, as we shall see, by the CoQH2-cytochrome c reductase complex Itself. In all cases, a molecule from the pool of reduced C0QH2 in the membrane binds to the Qo site on the intermembrane... 

The major components of the mitochondrial respiratory chain are four inner membrane multiprotein complexes NADH-CoQ reductase (I), succinate-CoQ reductase (II), CoQH2-cytochrome c reductase (III), and cytochrome c oxidase (IV). The last complex transfers electrons to O2 to form H2O. 

Rotenone NADH-CoQ reductase Blocks oxidation of NADH (site I). NADH will become reduced Substrates such as succinate that enter via FADH will still be oxidized and make 2 ATPs/mol. 

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. 

Two reactions of the reverse electron transfer are of physiological significance. 1 mean (i) oxidation of succinate (redox potential, +0.03 V) by NAD (redox potential, -0.32 V) and (ii) oxidation of NADH by NADPH responsible for maintenance of [NADPH]/NADP ] [NADH]/[NAD ] in spite of the fact that redox potential of the NADPH/NADH pair is almost equal to that of NADH/NAD pair. The former process includes a reversal of NADH-CoQ reductase (Complex I of the respiratory chain). Usually it operates as a A 1h+ generator catalysing the downhill electron transfer from NADH to CoQ. However, when NAD is reduced by succinate, the same complex acts as a ApH consumer carrying out the uphill transfer of electrons from C0QH2 to NAD" [5]. 

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 ... 

The function of the enzymes of the mitochondrial respiratory chain is to transform the energy of redox reactions into an electrochemical proton gradient across the hydrophobic barrier of a coupling membrane. Isolated oligoenzyme complexes of the respiratory chain of mitochondria, cytochrome c oxidase, succinate ytochrome c reductase, and NADH CoQ reductase, are able to catalyze charge transfer in model systems, e.g., at a water/octane interface, which can be followed by a change in the interfacial potential at this interface [20-... 

Isolated oligoenzyne complexes of the respiratory chain of mitochondria - cytochrome oxidase, succinate-cytochrome c reductase, and NADH-CoQ reductase -catalyze the transfer of charges between water and octane that can be recorded from the change in the potential shift at the octane/water phase separation boundary by the vibrating plate method. A necessary condition for the appearance of this effect has proved [18,62] to be the presence of the corresponding enzymes in the oxidation substrates in the aqueous phase and also the presence of a charge acceptor in the octane phase [10, 18, 59]. 

The half-saturation of the system with cytochrome c was achieved at a concentration of about 10 M (Fig. 15a, b). In all cases, the effect was reversed and prevented by antimycin - an inhibitor of the succinate-cytochrome c reductase activity of this enzyme complex - but was not suppressed by cyanide. In concluding this series of experiments, we investigated the isolated NADH-CoQ reductase complex of the respiratory chain of mitochondria. As can be seen from the results given in Fig. 16, an increase in the concentration of the enzyme complex was accompanied by an increase in the negative charge of the octane phase. 

Fig. 18. Catalysis of the transfer of positive charges from water into octane by NADH-CoQ reductase (1) and by succinate-cytochrome c reductase (2) as a function of the concentration of DNP. Incubation medium l)20mU tris-HCl (pH 7.4) 0.2 mM NADH and 25 ig of NADH-CoQ reductase protein per ml 2)20 mM tris-HCl (pH 4.7), 7 mM succinate, 0.2 mM cytochrome c, and 40 pg of succinate-cytochrome c reductase protein per ml. In both experiments, the medium contained 1 mM ferricyanide [18]...

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. 

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