Succinic dehydrogenase localization

Where does the citric acid cycie take piace in the ceii Unlike glycolysis, which takes place in the cytosol, the citric acid cycle occurs in mitochondria. Most of the enzymes of the citric acid cycle are in the mitochondrial matrix. Succinate dehydrogenase, the sole exception, is localized in the inner mitochondrial membrane. 

Succinate is a very simple molecule, a dicarboxylic acid where the two -COOH groups are held together by -CHj-CHj-. There is no place for an electron to localize, unless a x-system is formed. In Complex II (succinate dehydrogenase), succinate is oxidized to fumarate (HOOC-CH=CH-COOH) in a series of complicated, not fully known processes. In these processes, quinone (Q) is reduced to quinol (QH2). The electrons are transported to Complex III through the membrane. The protons are transported perpendicular to the membrane to the intermembrane space, in order to build up a proton gradient for the production of ATP. 

Fig. 63. Model representation of how a respiratory group is localized in the inner mitochondrial membrane. Above, side view below, cross-section. D = NAD", FP == flavoprotein, SD = flavoprotein of succinate dehydrogenase, b = cytochrome B complex, c = cytochrome C543 -H C544, a-fa3 = cytochrome oxidase complex. X, Y, Z = links to oxidative phosphorylation (modified from Lehninger 1969).

Fig. 1 Enzymes localized to B. hominis mitochondrial-like organelle. The enzymes are 1 malic enzyme, 2 pyruvate NADP oxidoreductase, 3 acetate succinate CoA transferase, 4 succinate thiokinase, 5 a-ketoglutarate dehydrogenase, 6 isocitrate dehydrogenase, and 7 aconitase...

In metazoans, the electron transport chain consists of four integral membrane complexes localized to the inner mitochondrial membrane complex I (NADH-ubiquinone oxidoreductase), complex II (succinate-ubiquinone oxidoreductase), complex III (ubiquinol-cytochrome c oxidoreductase) and complex IV (cytochrome c oxidase), plus coenzyme Q (ubiquinone) and cytochrome c. As first shown by Fry and Beesley (1991), the plasmodial electron transport chain differs from the metazoan system in lacking complex I however, a single subunit NADH dehydrogenase is present and is homologous to that found in plants, bacteria and yeast but not in animals (Krungkrai, 2004 Vaidya, 2004,2005 van Dooren et al., 2006). 

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