CoQH2-cytochrome c reductase

CoQH2-Cytochrome c Reductase (Complex III) A C0QH2 generated either by complex I or complex II donates two electrons to the CoQH2-cytochrome c reductase complex, regenerating oxidized CoQ. Concomitantly it releases two protons picked up on the cytosolic face into the Intermembrane space, generating part of the proton-motive force. Within complex III, the released electrons first are transferred to an iron-sulfur cluster within complex III and then to... 

Cytochrome C Oxidase (Complex IV) Cytochrome c, after being reduced by the CoQH2-cytochrome c reductase complex, transports electrons, one at a time, to the cytochrome c oxidase complex (Figure 8-18). Within this complex, electrons are transferred, again one at a time, first to a pair of copper ions called Cu, then to cytochrome a, next to a... 

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. 

The Q Cycle Increases the Number of Protons Translocated as Electrons Flow Through the CoQH2-Cytochrome c Reductase Complex... 

Experiments like the one depicted In Figure 8-19 have shown that four protons are translocated across the membrane per electron pair transported from C0QH2 through the CoQH2-cytochrome c reductase complex. Thus this complex transports two protons per electron transferred, whereas the cytochrome c oxidase complex transports only one proton per electron transferred. An evolutlonarlly conserved mechanism, called the Q cycle, accounts for the two-for-one transport of protons and electrons by the CoQH2-cytochrome c reductase complex. 

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. 

The Q cycle allows four protons (rather than two) to be translocated per pair of electrons moving through the CoQH2-cytochrome c reductase complex (see Figure 8-21). 

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... 

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