Experiments like the one depicted in Figure 8-19 have shown that four protons are translocated across the membrane per electron pair transported from CoQH2 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 evolutionarily conserved mechanism, called the Q cycle, accounts for the two-for-one transport of protons and electrons by the CoQH2-cytochrome c reductase complex.
CoQH2 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 CoQH2 in the membrane binds to the Qo site on the intermembrane space (outer) side of the CoQH2-cytochrome c reductase complex (Figure 8-21, step 1). Once bound to this site, CoQH2 releases two protons into the intermembrane space (step [2a); these represent two of the four protons pumped per pair of electrons transferred. Operation of the Q cycle results in pumping of the other two protons. To understand this cycle, we first need to focus on the fates of the two electrons released by CoQH2 at the Q0 site. One of the two
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