The NADH dehydrogenase complex (often referred to as Complex I) is the largest of these respiratory enzyme complexes. It accepts electrons from NADH and passes them through a flavin mononucleotide and eight iron–sulfur clusters to the lipid-soluble electron carrier ubiquinone. The reduced ubiquinol then transfers its electrons to cytochrome c reductase.
2.
The cytochrome c reductase (also called the cytochrome b-c1 complex) is a large membrane protein assembly that functions as a dimer. Each monomer contains three cytochrome hemes and an iron–sulfur cluster. The complex accepts electrons from ubiquinol and passes them on to the small, soluble protein cytochrome c, which is located in the crista space and carries electrons one at a time to cytochrome c oxidase.
3.
The cytochrome c oxidase complex contains two cytochrome hemes and three copper atoms. The complex accepts electrons one at a time from cytochrome c and passes them to molecular oxygen. In total, four electrons and four protons are needed to convert one molecule of oxygen to water.
We have previously discussed how the redox potential reflects electron affinities. Figure 14–19 presents an outline of the redox potentials measured along the respiratory chain. These potentials change in three large steps, one across each proton-translocating respiratory complex. The change in redox potential between any two electron carriers is directly proportional to the free energy released when an electron transfers between them. Each complex acts as an energy-conversion device by harnessing some of this free-energy change to pump H+ across the inner membrane, thereby creating an electrochemical proton gradient as electrons pass along the chain.
Figure 14–17 Quinone electron carriers. Ubiquinone in the lipid bilayer picks up one h+ (red) from the aqueous environment for each electron (blue) it accepts, in two steps, from respiratory-chain complexes. the first step involves the acquisition of a proton and an electron and converts the ubiquinone into an unstable ubisemiquinone radical. In the second step, it becomes a fully reduced ubiquinone (called ubiquinol), which is freely mobile as an electron carrier in the lipid bilayer of the membrane. When the ubiquinol donates its electrons to the next complex in the chain, the two protons are released. the long hydrophobic tail (green) that confines ubiquinone to the membrane consists of 6–10 five-carbon isoprene units, depending on the organism. the corresponding electron carrier in the photosynthetic membranes of chloroplasts is plastoquinone, which has almost the same structure and works in the same way. For simplicity, we refer to both ubiquinone and plastoquinone in this chapter as quinone (abbreviated as Q).
Figure 14–18 The path of electrons through the three respiratory-chain proton pumps. (Movie 14.3) the approximate size and shape of each complex is shown. During the transfer of electrons from NaDh to oxygen (blue arrows), ubiquinone and cytochrome c serve as mobile carriers that ferry electrons from one complex to the next. During the electron-transfer reactions, protons are pumped across the membrane by each of the respiratory enzyme complexes, as indicated (red arrows).
For historical reasons, the three proton pumps in the respiratory chain are sometimes denoted as Complex I, Complex III, and Complex IV, according to the order in which electrons pass through them from NaDh. Electrons from the oxidation of succinate by succinate dehydrogenase (designated as Complex II) are fed into the electron-transport chain in the form of reduced ubiquinone. although embedded in the crista membrane, succinate dehydrogenase does not pump protons and thus does not contribute to the proton-motive force; it is therefore not considered to be an integral part of the respiratory chain.