A structure graphic of the entire dimer, showing in color the three proteins that form the functional core of the enzyme complex: cytochrome b(green) and cytochrome c1(blue) are colored in one half, and the Rieske protein (purple) containing an Fe2S2 iron–sulfur cluster (red and yellow) is colored in the other. These three protein subunits interact across the two halves.
Transfer of electrons through cytochrome c reductase to the small, soluble carrier protein cytochrome c. Electrons entering from ubiquinol near the matrix side of the membrane are captured by the iron–sulfur cluster of the Rieske protein, which moves its iron–sulfur group back and forth to transfer these electrons to heme c(red). Heme c then transfers them to the carrier molecule cytochrome c.
As detailed in Figure 14–23, only one of the two electrons from each ubiquinol is transferred through this path. To increase proton pumping, the second ubiquinol electron is passed to a molecule of ubiquinone bound to cytochrome c reductase on the opposite side of the membrane—near the matrix. (C) This shows the symbol for cytochrome c reductase used throughout this chapter. (PDB code: 1EZV.) redox loop mechanism used is called the Q cycle because while one of the electrons received from each QH2 molecule istransferred from ubiquinone through the complex to the carrier protein cytochrome c, the other electron is recycled back into the quinone pool. Through the mechanism illustrated in Figure 14–23, the Q cycle increases the total amount of redox energy that can be stored in the electrochemical proton gradient. As a result, two protons are pumped across the crista membrane for every electron that is transferred from NADH dehydrogenase to cytochrome c.
The Cytochrome c Oxidase Complex Pumps Protons and Reduces O2 Using a Catalytic Iron–Copper Center
The final link in the mitochondrial electron-transport chain is cytochrome c oxidase. The cytochrome c oxidase complex accepts electrons from the soluble electron carrier cytochrome c, and it uses yet a different, third mechanism to pump protons across the inner mitochondrial membrane. The structure of the mammalian complex is illustrated in Figure 14–24. The atomic-resolution structures, combined with studies of the effect of mutations introduced into the enzyme by genetic engineering of the yeast and bacterial proteins, have revealed the detailed mechanisms of this electron-driven proton pump.
Because oxygen has a high affinity for electrons, it can release a large amount of free energy when it is reduced to form water. Thus, the evolution of cellular respiration, in which O2 is converted to water, enabled organisms to harness much more energy than can be derived from anaerobic metabolism. As we discuss later, the availability of the large amount of energy released by the reduction of molecular oxygen to form water is thought to have been essential to the emergence of multicellular life: this would explain why all large organisms respire. The ability of biological systems to use O2 in this way, however, requires sophisticated chemistry. Once a molecule of O2 has picked up one electron, it forms a superoxide radical anion (O2•–) that is dangerously reactive and rapidly takes up an additional three electrons wherever it can get them, with destructive effects on its immediate environment. We can tolerate oxygen in the air we breathe only because the uptake of the first electron by the O2 molecule is slow, allowing cells to use enzymes to control electron uptake by oxygen. Thus, cytochrome c oxidase holds
Figure 14–23 The two-step mechanism of the cytochrome c reductase Q-cycle.