along the mitochondrial electron _400 transport chain. the redox potential _300 (designated Eʹ0) increases as electrons flow down the respiratory chain to oxygen. the _200 standard free-energy change in kilojoules, _100 ∆G°, for the transfer of each of the two can be obtained from the left-hand ordinate [∆G° = –n(0.096) ∆Eʹ0, where n is the number of electrons transferred across a redox potential change of ∆Eʹ0 mV]. Electrons flow through a respiratory through the multiple electron carriers in 500 each complex (blue arrows). as indicated, part of the favorable free-energy change 600 is harnessed by each enzyme complex to 700 pump h+ across the inner mitochondrial membrane (red arrows). the NaDh 800 dehydrogenase pumps up to four h+ per electron, the cytochrome c reductase complex pumps two, whereas the direction of electron fow
X-ray crystallography has elucidated the structure of each of the three respiratory-chain complexes in great detail, and we next examine each of them in turn to see how they work.
the NaDh Dehydrogenase Complex Contains Separate Modules for Electron transport and proton pumping
The NADH dehydrogenase complex is a massive assembly of membrane and nonmembrane proteins that receives electrons from NADH and passes them to ubiquinone. In animal mitochondria, it consists of more than 40 different protein subunits, with a molecular mass of nearly a million daltons. The x-ray structures of the NADH dehydrogenase complex from fungi and bacteria show that it is L-shaped, with both a hydrophobic membrane arm and a hydrophilic arm that projects into the mitochondrial matrix (Figure 14–20).
Electron transfer and proton pumping are physically separated in the NADH dehydrogenase complex, with electron transfer occurring in the matrix arm and proton pumping in the membrane arm. The NADH docks near the tip of the matrix arm, where it transfers its electrons via a bound flavin mononucleotide to a string of iron–sulfur clusters that runs down the arm, acting like a wire to carry electrons to a protein-bound molecule of ubiquinone. Electron transfer to the quinone is thought to trigger proton translocation in a set of proton pumps in the membrane arm, and for this to happen the two processes must be energetically and mechanically linked. A mechanical link is thought to be provided by a 6-nm long, amphipathic α helix that runs parallel to the membrane surface on the matrix side of the membrane arm. This helix may act like the connecting rod in a steam engine to generate a mechanical, energy-transducing power stroke that links the quinone-binding site to the proton-translocating modules in the membrane (see Figure 14–20).
The reduction of each quinone by the transfer of two electrons can cause four protons to be pumped out of the matrix into the crista space. In this way, NADH dehydrogenase generates roughly half of the total proton-motive force in mitochondria.
Cytochrome c reductase takes Up and releases protons on the Opposite Side of the Crista Membrane, thereby pumping protons
As described previously, when a quinone molecule (Q) accepts its two electrons, it also takes up two protons to form a quinol (QH2; see Figure 14–17). In cytochrome c oxidase complex pumps one per electron.
Note that NaDh is not the only source of electrons for the respiratory chain. the flavin FaDh2, which is generated by fatty acid oxidation (see Figure 2–56) and by succinate dehydrogenase in the citric acid cycle (see Figure 2–57), also contributes. Its two electrons are passed directly to ubiquinone, bypassing NaDh dehydrogenase.