Alternative Respiration in Plants and Fungi: Some Aspects of Its Biological Role
Electron transfer from NADH to molecular oxygen in animals within the mitochondrion proceeds via at least three respiratory complexes (Fig. 1): NADH:ubiquinone oxidoreductase (complex I), ubiquinohcytochrome c oxidoreductase (complex III) and cytochrome c
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Alternative Respiration in Plants and Fungi: Some Aspects of Its Biological Role Heike Rohr and Ulf Stahl
1 Introduction Electron tr ansfer from NADH to molecular oxygen in animals within the mitochondrion proceeds via at least three respiratory complexes (Fig. 1): NADH:ubiquinone oxidoreductase (complex 1), ubiquinol:cytochrome c oxidoreductase (complex Ill) and cytochrome c oxidase (complex IV). These "standard" respiratory enzymes form a linear respiratory chain, and electron transport of each is coupled to proton translocation out of the mitochondrion. In this way, a proton motive force is generated which is subsequently used for ATP synthesis by the ATPase/ ATP synthase. In addition, electron transfer also occurs from succinate dehydrogenase (complex II) to ubiquinone (Q), but it is not coupled to proton translocation. In contrast, plants and many fungi may use a branched respiratory chain with additional alternative components (Fig. 1; for reviews, see Exter nal NADH dehydrogenase \
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Internal NA~H dehydrogenase
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/
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NAD"d" :dro'~~I Succinate dehydrogenase
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Alternative oxidase
Ubiq uinol:cytoehro me c oxidoreductase
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III
IV
Fig. 1. The branched resp irato ry chain in plants and fungi. Electrons are tran sferred from the ubiquinon e pool either to the alterna tive oxidase or to the ubiqu inol:cytochrome c oxidoredu ctase and cytochrome c oxidase, both ways resulting in reducing molecular oxygen to water. Gray stars indicate where enzyme activity is linked to proton tra nslocation. Q Ubiquinone pool; I NADH dehydro genase; II succinate dehydrogenase; III ubiqu inol:cytochrome c oxidoredu ctase; IV cytochr ome c oxidase
Progress in Botan y, Vo!. 64 © Springer-Verlag Berlin Heidelberg 2003
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Genetics
Lambers 1982; Siedow and Berthold 1986; Moore and Siedow 1991; McIntosh 1994; Ioseph-Horne et al. 2001). 1. There are at least two alternative NADH dehydrogenases: one faces
the intermembrane space oxidizing cytoplasmic (external) NADH, the other one faces the matrix space of mitochondria and catalyzes oxidation of endogenously (internally) generated NADH (Promper et al. 1993). The latter is thought to be the counterpart of complex I (De Vries et al. 1992). 2. Apart from the cytochrome c oxidase, the mitochondria of plants and fungi also contain an alternative oxidase that directly accepts electrons from ubiquinol and catalyzes the reduction of molecular oxygen to water. Nevertheless, the participation of alternative components in the respiratory system, which are all non -energy conserving, seems to ensure proton translocation through at least one site during electron transfer from NADH to oxygen (Joseph-Horne et al. 2001). The biological function of alternative respiration, particularly the mode of action of the alternative oxidase, is still an open question. Since alternative respiratory enzymes are known to have a significantly lower affinity to NADH and ubiquinol than their counterparts in the cytochrome c pathway, earlier investigations hypothesized that alternative re
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