Electron transport chain complex

Complexes of the Mitochondrial Electron-Transport Chain Complex I (NADH Ubiquinone Oxidoreductase)... 

Boxes indicate electron-transport chain complexes, whereas ovals represent the electron transporters UQ, RQ and cytochrome c. The open boxes represent complexes involved in the classical aerobic respiratory chain, whereas grey boxes represent complexes involved in malate dismutation. The vertical bar represents a scale for the standard redox potentials in mV. Translocation of protons by the complexes is indicated by H+ +. Abbreviations Cl, Clll and CIV, complexes I, III and IV of the respiratory chain cyt c, cytochrome c FRD, fumarate reductase Fum, fumarate SDH, succinate dehydrogenase Succ, succinate RQ, rhodoquinone UQ, ubiquinone. 

Accompanying electron flow in mitochondria, H+ is transported from the matrix side of the inner membrane to the lumen between the limiting membranes, i.e., within the cristae (Figs. 1-9 and 6-9). Certain electron flow components are situated in the membranes such that they can carry out this vectorial movement. Protein Complex I, which oxidizes NADH, apparently transfers four H+ s across the inner membrane per pair of electrons from NADH. Complex II, which oxidizes FADH2 and leads to the reduction of a ubiquinone whose two electrons move to Complex III, apparently causes no H+ s to move from the matrix to the lumen. Transport of four H+ s from the matrix to the lumen side most likely occurs through protein Complex III per pair of electrons traversing the electron transport chain. Complex IV (cytochrome oxidase) may also transport four H+ s (Fig. 6-9 summarizes these possibilities). We also note that two H+ s are necessary for the reduction of 02 to H20, and these protons can also be taken up on the matrix side (Fig. 6-9). 

Jung C, Higgins CM, Xu Z (2002) Mitochondrial electron transport chain complex dysfunction in a transgenic mouse model for amyotrophic lateral sclerosis. J Neurochem 83 535-545... 

Deficiencies of electron transport In cells, complete transfer of electrons from NADH and FAD(2H) through the chain to O2 is necessary for ATP generation. Impaired transfer through any complex can have pathologic consequences. Fatigue can result from iron-defeciency anemia, which decreases Fe for Fe-S centers and cytochromes Cytochrome Cj oxidase, which contains the O2 binding site, is inhibited by cyanide Mitochondrial DNA (mtDNA), which is maternally inherited, encodes some of the subunits of the electron transport chain complexes and ATP synthase. Oxphos diseases are caused by mutations in nuclear DNA or mtDNA that decrease mitochondrial capacity for oxidative phosphorylation. 

Figure 11.2 Electron transport in the respiratory chain. The diagram details the flow of electrons from the Krebs cycle intermediates malate and succinate via the electron transport chain (complexes 1, II, III and IV) to oxygen. 

Rossi SC, Wetterhahn KE (1989) Chromium (V) is produced upon reduction of chromate by mitochondrial electron transport chain complexes. Carcinogenesis 10 913-921... 

It should be taken into account that the state 3 respiration rate is eontroUed by (1) the aetivity of the reactions involved in the oxidation of the substrates and in the produetion of membrane potential, including the activities of electron transport chain complexes and (2) the activity of reactions that use the membrane potential for the synthesis and the export of ATP, ineluding ATP synthase activity. The absence of an age-related change in state 3 respiration, despite a reduction of the aetivity of each individual complex of the electron transport chain as well as of the ATP synthase eomplex, support the hypothesis that supramolecular assembly of respiratory ehain eomplexes into respirasomes (described in the previous chapter) can compensate for the eomplex being present at the lower levels and activities in old mitochondria. 

The Electron Transport Chain Can Be Isolated in Four Complexes... 

In the third complex of the electron transport chain, reduced coenzyme Q (UQHg) passes its electrons to cytochrome c via a unique redox pathway known as the Q cycle. UQ cytochrome c reductase (UQ-cyt c reductase), as this complex is known, involves three different cytochromes and an Fe-S protein. In the cytochromes of these and similar complexes, the iron atom at the center of the porphyrin ring cycles between the reduced Fe (ferrous) and oxidized Fe (ferric) states. 

Cytochrome c, like UQ is a mobile electron carrier. It associates loosely with the inner mitochondrial membrane (in the intermembrane space on the cytosolic side of the inner membrane) to acquire electrons from the Fe-S-cyt C aggregate of Complex 111, and then it migrates along the membrane surface in the reduced state, carrying electrons to cytochrome c oxidase, the fourth complex of the electron transport chain. 

It should be emphasized here that the four major complexes of the electron transport chain operate quite independently in the inner mitochondrial membrane. Each is a multiprotein aggregate maintained by numerous strong associations between peptides of the complex, but there is no evidence that the complexes associate with one another in the membrane. Measurements of the lateral diffusion rates of the four complexes, of coenzyme Q, and of cytochrome c in the inner mitochondrial membrane show that the rates differ considerably, indicating that these complexes do not move together in the membrane. Kinetic studies with reconstituted systems show that electron transport does not operate by means of connected sets of the four complexes. 

Although only two protons are pumped out of the matrix, two others from the matrix are consumed in the formation of H2O. There is therefore a net translocation of four positive charges out of the matrix which is equivalent to the extrusion of four protons. If four protons are required by the chemiosmotic mechanism to convert cytosolic ADP + Pj to ATP, then 0.5 mol ATP is made for the oxidation of one mol of ubiquinol and one mol ATP for the oxidation of 2 mols of reduced cytochrome c. These stoichiometries were obtained experimentally when ubiquinol was oxidized when complexes I, II, and IV were inhibited by rotenone, malonate, and cyanide, respectively, and when reduced cytochrome c was oxidized with complex III inhibited by antimycin (Hinkle et al., 1991). (In these experiments, of course, no protons were liberated in the matrix by substrate oxidation.) However, in the scheme illustrated in Figure 6, with the flow of two electrons through the complete electron transport chain from substrate to oxygen, it also appears valid to say that four protons are extmded by complex I, four by complex III, and two by complex 1. 

In contrast to common usage, the distinction between photosynthetic and respiratory Rieske proteins does not seem to make sense. The mitochondrial Rieske protein is closely related to that of photosynthetic purple bacteria, which represent the endosymbiotic ancestors of mitochondria (for a review, see also (99)). Moreover, during its evolution Rieske s protein appears to have existed prior to photosynthesis (100, 101), and the photosynthetic chain was probably built around a preexisting cytochrome be complex (99). The evolution of Rieske proteins from photosynthetic electron transport chains is therefore intricately intertwined with that of respiration, and a discussion of the photosynthetic representatives necessarily has to include excursions into nonphotosynthetic systems. 

Studies (see, e.g., (101)) indicate that photosynthesis originated after the development of respiratory electron transfer pathways (99, 143). The photosynthetic reaction center, in this scenario, would have been created in order to enhance the efficiency of the already existing electron transport chains, that is, by adding a light-driven cycle around the cytochrome be complex. The Rieske protein as the key subunit in cytochrome be complexes would in this picture have contributed the first iron-sulfur center involved in photosynthetic mechanisms (since on the basis of the present data, it seems likely to us that the first photosynthetic RC resembled RCII, i.e., was devoid of iron—sulfur clusters). 

Flutolanil is an inhibitor of succinate dehydrogenase complex (Complex II), in the mitochondrial respiratory electron transport chain. ... 

Unlike the photosynthetic apparatus of photosynthetic bacteria, that of cyanobacteria consits of two photosystems, PS I and II, connected by an electron transport chain. The only chlorophyll present is chlorophyll a, and, therefore, chlorophylls b—d are not of interest in this article. Chlorophyll a is the principal constituent of PS I. Twenty per cent of isolated pigment-protein complexes contain one P700 per 20—30 chlorophyll a molecules the other 80% contain only chlorophyll a20). The physical and chemical properties of chlorophyll a and its role in photosynthesis have recently been described by Meeks77), Mauzerall75), Hoch60), Butler10), and other authors of the Encyclopedia of Plant Physiology NS Vol. 5. 

Ubiquinones (coenzymes Q) Q9 and Qi0 are essential cofactors (electron carriers) in the mitochondrial electron transport chain. They play a key role shuttling electrons from NADH and succinate dehydrogenases to the cytochrome b-c1 complex in the inner mitochondrial membrane. Ubiquinones are lipid-soluble compounds containing a redox active quinoid ring and a tail of 50 (Qio) or 45 (Q9) carbon atoms (Figure 29.10). The predominant ubiquinone in humans is Qio while in rodents it is Q9. Ubiquinones are especially abundant in the mitochondrial respiratory chain where their concentration is about 100 times higher than that of other electron carriers. Ubihydroquinone Q10 is also found in LDL where it supposedly exhibits the antioxidant activity (see Chapter 23). 

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