8885ac19690-7503/1/0411:32 AM Page704mac76mac76:385 704 Chapter 19 Oxidative Phosphorylation and Photophosphorylation Plant mitochondria have Alternative mechanisms Complex Iv, cytochrome oxidase. This for Oxidizing NADH copper-containing enzyme, which also contains cytochromes a and a3, accumulates electrons Plant mitochondria supply the cell with ATP during then passes them to O2, reducing it to H2O periods of low illumination or darkness by mechanisms entirely analogous to those used by nonphotosynthetic a Some electrons enter this chain of carriers organisms. In the light, the principal source of mito- through alternative paths. Succinate is oxidized chondrial NADH is a reaction in which glycine, produced by succinate dehydrogenase(Complex D) by a process known as photorespiration, is converted to which contains a flavoprotein that passes serine(see Fig 20-21) electrons through several Fe-s centers to ubiquinone. Electrons derived from the 2 Glycine+ NAD- serine CO2+ NHs NADH +H oxidation of fatty acids pass to ubiquinone via For reasons discussed in Chapter 20, plants must carry he electron-transferring flavoprotein out this reaction even when they do not need NADH for a Plants also have an alternative, cyanide-resistant ATP production. To regenerate NAD from unneeded NADH oxidation pathway. NADH, plant mitochondria transfer electrons from NADH directly to ubiquinone and from ubiquinone directly to O2, bypassing Complexes Ill and Iv and their proton 19.2 ATP Synthesis pumps. In this process the energy in NADH is dissipated How is a concentration gradient of protons transformed as heat, which can sometimes be of value to the plant into atp? We have seen that electron transfer releases, (Box 19-1). Unlike cytochrome oxidase(Complex Iv) and the proton-motive force conserves, more than the alternative QH2 oxidase is not inhibited by cyanide. enough free energy(about 200 kJ)per"mole"of Cyanide-resistant NADH oxidation is therefore the hall- tron pairs to drive the formation of a mole of ATP, which requires about 50 kJ(see Box 13-1). Mi- SUMMARY 19. 1 Electron-Transfer reactions tochondrial oxidative phospho- in mitochondria rylation therefore poses no thermodynamic problem. But a Chemiosmotic theory provides the intellectual what is the chemical mechanism framework for understanding many biological that couples proton flux with energy transductions, including oxidative phosphorylation? phosphorylation and photophosphorylation. The chemiosmotic model The mechanism of energy coupling is similar in proposed by Peter Mitchell, is both cases: the energy of electron flow is the paradigm for this mecha- conserved by the concomitant pumping of nism. According to the model Peter Mitchell, protons across the membrane, producing an (Fig. 19-17, the electrochemi 1920-1992 electrochemical gradient, the proton-motive cal energy inherent in the difference in proton concen- force tration and separation of charge across the inner mito- a In mitochondria, hydride ions removed from chondrial membrane-the proton- motive force--drives substrates by NAD-linked dehydrogenases the synthesis of ATP as protons flow passively back into donate electrons to the respiratory the matrix through a proton pore associated with ATP (electron-transfer) chain, which transfers the synthase. To emphasize this crucial role of the proton electrons to molecular O2, reducing it to HO motive force, the equation for ATP synthesis is some- a Shuttle systems convey reducing equivalents times written from cytosolic NADH to mitochondrial NADH. ADP+P1+nH→→ATP+H2O+nH(19-10) Reducing equivalents from all NAD-linked Mitchell used"chemiosmotic to describe enzymatic re- dehydrogenations are transferred to mito- actions that involve, simultaneously, a chemical reaction chondrial NAdh dehydrogenase(Complex D) and a transport process. The operational definition of a Reducing equivalents are then passed through coupling "is shown in Figure 19-18. When isolated mi- a series of Fe-s centers to ubiquinone, which tochondria are suspended in a buffer containing ADP transfers the electrons to cytochrome b, the Pi, and an oxidizable substrate such as succinate, three first carrier in Complex I. In this complex, easily measured processes occur: (1)the substrate is electrons take two separate paths through two oxidized ( succinate yields fumarate), (2)O2 is consumed, b-type cytochromes and cytochrome ci to an and (3) ATP is synthesized. Oxygen consumption and Fe-S center. The Fe-S center passes electrons ATP synthesis depend on the presence of an oxidizable one at a time through cytochrome c and into substrate(succinate in this case) as well as ADP and PPlant Mitochondria Have Alternative Mechanisms for Oxidizing NADH Plant mitochondria supply the cell with ATP during periods of low illumination or darkness by mechanisms entirely analogous to those used by nonphotosynthetic organisms. In the light, the principal source of mito￾chondrial NADH is a reaction in which glycine, produced by a process known as photorespiration, is converted to serine (see Fig. 20–21): 2 Glycine
NAD
88n serine
CO2
NH3
NADH
H
For reasons discussed in Chapter 20, plants must carry out this reaction even when they do not need NADH for ATP production. To regenerate NAD
from unneeded NADH, plant mitochondria transfer electrons from NADH directly to ubiquinone and from ubiquinone directly to O2, bypassing Complexes III and IV and their proton pumps. In this process the energy in NADH is dissipated as heat, which can sometimes be of value to the plant (Box 19–1). Unlike cytochrome oxidase (Complex IV), the alternative QH2 oxidase is not inhibited by cyanide. Cyanide-resistant NADH oxidation is therefore the hall￾mark of this unique plant electron-transfer pathway. SUMMARY 19.1 Electron-Transfer Reactions in Mitochondria ■ Chemiosmotic theory provides the intellectual framework for understanding many biological energy transductions, including oxidative phosphorylation and photophosphorylation. The mechanism of energy coupling is similar in both cases: the energy of electron flow is conserved by the concomitant pumping of protons across the membrane, producing an electrochemical gradient, the proton-motive force. ■ In mitochondria, hydride ions removed from substrates by NAD-linked dehydrogenases donate electrons to the respiratory (electron-transfer) chain, which transfers the electrons to molecular O2, reducing it to H2O. ■ Shuttle systems convey reducing equivalents from cytosolic NADH to mitochondrial NADH. Reducing equivalents from all NAD-linked dehydrogenations are transferred to mito￾chondrial NADH dehydrogenase (Complex I). ■ Reducing equivalents are then passed through a series of Fe-S centers to ubiquinone, which transfers the electrons to cytochrome b, the first carrier in Complex III. In this complex, electrons take two separate paths through two b-type cytochromes and cytochrome c1 to an Fe-S center. The Fe-S center passes electrons, one at a time, through cytochrome c and into Complex IV, cytochrome oxidase. This copper-containing enzyme, which also contains cytochromes a and a3, accumulates electrons, then passes them to O2, reducing it to H2O. ■ Some electrons enter this chain of carriers through alternative paths. Succinate is oxidized by succinate dehydrogenase (Complex II), which contains a flavoprotein that passes electrons through several Fe-S centers to ubiquinone. Electrons derived from the oxidation of fatty acids pass to ubiquinone via the electron-transferring flavoprotein. ■ Plants also have an alternative, cyanide-resistant NADH oxidation pathway. 19.2 ATP Synthesis How is a concentration gradient of protons transformed into ATP? We have seen that electron transfer releases, and the proton-motive force conserves, more than enough free energy (about 200 kJ) per “mole” of elec￾tron pairs to drive the formation of a mole of ATP, which requires about 50 kJ (see Box 13–1). Mi￾tochondrial oxidative phospho￾rylation therefore poses no thermodynamic problem. But what is the chemical mechanism that couples proton flux with phosphorylation? The chemiosmotic model, proposed by Peter Mitchell, is the paradigm for this mecha￾nism. According to the model (Fig. 19–17), the electrochemi￾cal energy inherent in the difference in proton concen￾tration and separation of charge across the inner mito￾chondrial membrane—the proton-motive force—drives the synthesis of ATP as protons flow passively back into the matrix through a proton pore associated with ATP synthase. To emphasize this crucial role of the proton￾motive force, the equation for ATP synthesis is some￾times written ADP
Pi
nH
P On ATP
H2O
nH
N (19–10) Mitchell used “chemiosmotic” to describe enzymatic re￾actions that involve, simultaneously, a chemical reaction and a transport process. The operational definition of “coupling” is shown in Figure 19–18. When isolated mi￾tochondria are suspended in a buffer containing ADP, Pi , and an oxidizable substrate such as succinate, three easily measured processes occur: (1) the substrate is oxidized (succinate yields fumarate), (2) O2 is consumed, and (3) ATP is synthesized. Oxygen consumption and ATP synthesis depend on the presence of an oxidizable substrate (succinate in this case) as well as ADP and Pi . 704 Chapter 19 Oxidative Phosphorylation and Photophosphorylation Peter Mitchell, 1920–1992 8885d_c19_690-750 3/1/04 11:32 AM Page 704 mac76 mac76:385_reb: