In plants complex II includes homologs of these subunits but additionally four extra proteins of unknown function ( Millar et al., 2004 Huang and Millar, 2013). Complex II is composed of four subunits in bacteria and mitochondria of animals and fungi. The function of this additional domain is currently unclear but it has been suggested to be important in the context of an inner-cellular CO 2 transfer mechanism to provide mitochondrial CO 2 for carbon fixation in chloroplasts ( Braun and Zabaleta, 2007 Zabaleta et al., 2012). Compared to its homologs from bacteria and other eukaryotic lineages it has an extra domain which includes carbonic anhydrase-like proteins. Complex I is especially large in plant mitochondria and includes nearly 50 different subunits ( Braun et al., 2014). The “classical” oxidoreductase complexes of the respiratory chain (given in dark blue in Figure 1) resemble their homologues in animal mitochondria but at the same time have some clear distinctive features (reviewed in Millar et al., 2008, 2011 Rasmusson and Moller, 2011 van Dongen et al., 2011 Jacoby et al., 2012). In this review we aim to integrate current knowledge on the ETC system in plants with respect to its components, electron transport pathways and supramolecular structure. This is especially true for the plant ETC system, which is highly branched. However, electrons can enter and leave the ETC at several alternative points. In its classically described form, cellular respiration is based on a linear ETC (from NADH via complexes I, III, and IV to molecular oxygen). As a result, a proton gradient is formed which can be used by the ATP synthase complex (complex V) for the phosphorylation of ADP. Three of the four oxidoreductase complexes (complexes I, III and IV) couple their electron transfer reactions with proton translocation across the inner mitochondrial membrane. Overall, electrons are transferred from the coenzymes NADH or FADH 2 onto molecular oxygen which is reduced to water. Its core consists of four oxidoreductase complexes, the NADH dehydrogenase (complex I), the succinate dehydrogenase (complex II), the cytochrome c reductase (complex III) and the cytochrome c oxidase (complex IV), as well as of two mobile electron transporters, cytochrome c, and the lipid ubiquinone. The respiratory electron transport chain (ETC) of mitochondria is at the center of this process. This mini review aims to summarize recent findings on respiratory electron transfer pathways in plants and on the involved components and supramolecular assemblies.ĭuring cellular respiration, organic compounds are oxidized to generate usable chemical energy in the form of ATP. Entry of electrons into the system occurs via numerous pathways which are dynamically regulated in response to the metabolic state of a plant cell as well as environmental factors. Besides the “classical” oxidoreductase complexes (complex I–IV) and the mobile electron transporters cytochrome c and ubiquinone, it comprises numerous “alternative oxidoreductases.” Furthermore, several dehydrogenases localized in the mitochondrial matrix and the mitochondrial intermembrane space directly or indirectly provide electrons for the ETC. In plants, the ETC is especially intricate. The resulting proton gradient is used by the ATP synthase complex for ATP formation. The respiratory electron transport chain (ETC) couples electron transfer from organic substrates onto molecular oxygen with proton translocation across the inner mitochondrial membrane. Abteilung Pflanzenproteomik, Institut für Pflanzengenetik, Leibniz Universität Hannover, Hannover, Germany.
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