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704 Chapter 19 Oxidative Phosphorylation and Photophosphorylation

Plant 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 mitochondrial 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+-> 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 hallmark 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 electron 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 mechanism. According to the model (Fig. 19-17), the electrochemical energy inherent in the difference in proton concentration 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 protonmotive force, the equation for ATP synthesis is sometimes written

Peter Mitchell, 1920-1992

Mitchell used "chemiosmotic" to describe enzymatic reactions that involve, simultaneously, a chemical reaction and a transport process. The operational definition of "coupling" is shown in Figure 19-18. When isolated mitochondria 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.

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