SUMMARY

• 20.1 Oxidative Phosphorylation in Eukaryotes Takes Place in Mitochondria

  Mitochondria generate most of the ATP required by aerobic cells through a joint endeavor of the reactions of the citric acid cycle, which take place in the mitochondrial matrix, and oxidative phosphorylation, which takes place in the inner mitochondrial membrane.

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• 20.2 Oxidative Phosphorylation Depends on Electron Transfer

  In oxidative phosphorylation, the synthesis of ATP is coupled to the flow of electrons from NADH or FADH₂ to O₂ by a proton gradient across the inner mitochondrial membrane. Electron flow through three asymmetrically oriented transmembrane complexes results in the pumping of protons out of the mitochondrial matrix and the generation of a membrane potential. ATP is synthesized when protons flow back to the matrix.

• 20.3 The Respiratory Chain Consists of Proton Pumps and a Physical Link to the Citric Acid Cycle

  The electron carriers in the respiratory assembly of the inner mitochondrial membrane are quinones, flavins, iron–sulfur complexes, heme groups of cytochromes, and copper ions. Electrons from NADH are transferred to the FMN prosthetic group of NADH-Q oxidoreductase (Complex I), the first of four complexes. This oxidoreductase also contains Fe-S centers. The electrons emerge in QH₂, the reduced form of ubiquinone (Q). The citric acid cycle enzyme succinate dehydrogenase is a component of the succinate-Q reductase complex (Complex II), which donates electrons from FADH₂ to Q to form QH₂. This hydrophobic carrier transfers its electrons to Q-cytochrome c oxidoreductase (Complex III), a complex that contains cytochromes b and c₁ and an Fe-S center. This complex reduces cytochrome c, a water-soluble peripheral membrane protein. Cytochrome c, like Q, is a mobile carrier of electrons, which it then transfers to cytochrome c oxidase (Complex IV). This complex contains cytochromes a and a₃ and three copper ions. A heme iron ion and a copper ion in this oxidase transfer electrons to O₂, the ultimate acceptor, to form H₂O.