8885ac19690-7503/1/0411:32 AM Page691mac76mac76:385 19.1 Electron-Transfer Reactions in Mitochondria processes involve the flow of electrons through a chain ATP synthase Outer membrane of membrane-bound carriers. (2) The free energy made available by this"downhill"(exergonic) electron flow Freely permeable to Cristae small molecules and ions is coupled to the "uphill"transport of protons across a proton-impermeable membrane, conserving the free energy of fuel oxidation as a transmembrane electro- chemical potential (p. 391).3)The transmembrane brane rmeable to most flow of protons down their concentration gradient all molecules and ions through specific protein channels provides the free including H energy for synthesis of ATP, catalyzed by a membrane protein complex(ATP synthase) that couples proton Respiratory electron flow to phosphorylation of ADP. carriers( Complexes I-IV) we begin this chapter with oxidative phosphoryla- ADP-atP translocase tion. We first describe the components of the electron- ATP synthase(FoF1 transfer chain, their organization into large functional Other membrane complexes in the inner mitochondrial membrane, the path of electron flow through them, and the proton movements that accompany this flow. we then consider Matrix the remarkable enzyme complex that, by " rotational Contains: atalysis, captures the energy of proton flow in ATP, nd the regulatory mechanisms that coordinate oxida- tive phosphorylation with the many catabolic pathways complex by which fuels are oxidized. with this understanding of mitochondrial oxidative phosphorylation, we turn to photophosphorylation, looking first at the absorption of Fatty acid light by photosynthetic pigments, then at the light- driven flow of electrons from h,o to nadp and the Amino acid molecular basis for coupling electron and proton flow We also consider the similarities of structure and mech anism between the AtP synthases of chloroplasts and DNA. ribosomes mitochondria, and the evolutionary basis for this con- Porin channels servation of mechanism ATP,ADP, P, Mg2+, Ca2+,K+ Many soluble metabolic OXIDATIVE PHOSPHORYLATION 19. 1 Electron-Transfer Reactions FIGURE 19-1 Biochemical anatomy of a mitochondrion. The convo- lutions(cristae) of the inner membrane provide a very large surfac in Mitochondria area. The inner membrane of a single liver mitochondrion may have more than 10,000 sets of electron-transfer systems(respiratory chains) The discovery in 1948 by Eugene Kennedy and Albert and ATP synthase molecules, distributed over the membrane surface Lehninger that mitochondria are the site of oxidative Heart mitochondria, which have more profuse cristae and thus a much phosphorylation in eukaryotes marked the beginning larger area of inner membrane, contain more than three times as many f the modern phase of studies sets of electron-transfer systems as liver mitochondria. The mitochon- in biological energy transduc- drial pool of coenzymes and intermediates is functionally separate from tions Mitochondria, like gram- the cytosolic pool. The mitochondria of invertebrates, plants, and mi- negative bacteria, have two crobial eukaryotes are similar to those shown here, but with much vari- membranes (Fig. 19-1). The ation in size, shape, and degree of convolution of the inner membrane. outer mitochondrial membrane is readily permeable to small molecules (M <5, 000) and molecules and ions, including protons (; the only ions, which move freely species that cross this membrane do so through specific through transmembrane chan- transporters. The inner membrane bears the compo- nels formed by a family of inte- nents of the respiratory chain and the ATP synthase gral membrane proteins called The mitochondrial matrix, enclosed by the inner porins. The inner membrane is membrane, contains the pyruvate dehydrogenase com- 1917-1986 impermeable to most small plex and the enzymes of the citric acid cycle, the fattyprocesses involve the flow of electrons through a chain of membrane-bound carriers. (2) The free energy made available by this “downhill” (exergonic) electron flow is coupled to the “uphill” transport of protons across a proton-impermeable membrane, conserving the free energy of fuel oxidation as a transmembrane electro￾chemical potential (p. 391). (3) The transmembrane flow of protons down their concentration gradient through specific protein channels provides the free energy for synthesis of ATP, catalyzed by a membrane protein complex (ATP synthase) that couples proton flow to phosphorylation of ADP. We begin this chapter with oxidative phosphoryla￾tion. We first describe the components of the electron￾transfer chain, their organization into large functional complexes in the inner mitochondrial membrane, the path of electron flow through them, and the proton movements that accompany this flow. We then consider the remarkable enzyme complex that, by “rotational catalysis,” captures the energy of proton flow in ATP, and the regulatory mechanisms that coordinate oxida￾tive phosphorylation with the many catabolic pathways by which fuels are oxidized. With this understanding of mitochondrial oxidative phosphorylation, we turn to photophosphorylation, looking first at the absorption of light by photosynthetic pigments, then at the light￾driven flow of electrons from H2O to NADP
and the molecular basis for coupling electron and proton flow. We also consider the similarities of structure and mech￾anism between the ATP synthases of chloroplasts and mitochondria, and the evolutionary basis for this con￾servation of mechanism. OXIDATIVE PHOSPHORYLATION 19.1 Electron-Transfer Reactions in Mitochondria The discovery in 1948 by Eugene Kennedy and Albert Lehninger that mitochondria are the site of oxidative phosphorylation in eukaryotes marked the beginning of the modern phase of studies in biological energy transduc￾tions. Mitochondria, like gram￾negative bacteria, have two membranes (Fig. 19–1). The outer mitochondrial membrane is readily permeable to small molecules (Mr 5,000) and ions, which move freely through transmembrane chan￾nels formed by a family of inte￾gral membrane proteins called porins. The inner membrane is impermeable to most small molecules and ions, including protons (H
); the only species that cross this membrane do so through specific transporters. The inner membrane bears the compo￾nents of the respiratory chain and the ATP synthase. The mitochondrial matrix, enclosed by the inner membrane, contains the pyruvate dehydrogenase com￾plex and the enzymes of the citric acid cycle, the fatty 19.1 Electron-Transfer Reactions in Mitochondria 691 Outer membrane Freely permeable to small molecules and ions ATP synthase (FoF1) Cristae Impermeable to most small molecules and ions, including H
Contains: Contains: Ribosomes Porin channels • Respiratory electron carriers (Complexes I–IV) • ADP-ATP translocase • ATP synthase (FoF1) • Other membrane transporters • Pyruvate dehydrogenase complex • Citric acid cycle enzymes • Amino acid oxidation enzymes • DNA, ribosomes • Many other enzymes • ATP, ADP, Pi , Mg2
, Ca2
, K
• Many soluble metabolic intermediates Inner membrane Matrix • Fatty acid -oxidation enzymes Albert L. Lehninger, 1917–1986 FIGURE 19–1 Biochemical anatomy of a mitochondrion. The convo￾lutions (cristae) of the inner membrane provide a very large surface area. The inner membrane of a single liver mitochondrion may have more than 10,000 sets of electron-transfer systems (respiratory chains) and ATP synthase molecules, distributed over the membrane surface. Heart mitochondria, which have more profuse cristae and thus a much larger area of inner membrane, contain more than three times as many sets of electron-transfer systems as liver mitochondria. The mitochon￾drial pool of coenzymes and intermediates is functionally separate from the cytosolic pool. The mitochondria of invertebrates, plants, and mi￾crobial eukaryotes are similar to those shown here, but with much vari￾ation in size, shape, and degree of convolution of the inner membrane. 8885d_c19_690-750 3/1/04 11:32 AM Page 691 mac76 mac76:385_reb: