Electron Mitochondrion

The processes of electron transport and oxidative phosphorylation are membrane-associated. Bacteria are the simplest life form, and bacterial cells typically consist of a single cellular compartment surrounded by a plasma membrane and a more rigid cell wall. In such a system, the conversion of energy from NADH and [FADHg] to the energy of ATP via electron transport and oxidative phosphorylation is carried out at (and across) the plasma membrane. In eukaryotic cells, electron transport and oxidative phosphorylation are localized in mitochondria, which are also the sites of TCA cycle activity and (as we shall see in Chapter 24) fatty acid oxidation. Mammalian cells contain from 800 to 2500 mitochondria other types of cells may have as few as one or two or as many as half a million mitochondria. Human erythrocytes, whose purpose is simply to transport oxygen to tissues, contain no mitochondria at all. The typical mitochondrion is about 0.5 0.3 microns in diameter and from 0.5 micron to several microns long its overall shape is sensitive to metabolic conditions in the cell. 

FIGURE 21.1 (a) All electron micrograph of a mitochondrion, (b) A drawing of a mitochondrion with components labelled, (a, B. King/BPS)... 

The link with the final electron acceptor, O2, is the enzyme cytochrome c oxidase which spans the inner membrane of the mitochondrion. It consists of cytochromes a and a3 along with two, or possibly three, Cu atoms. The details of its action are not fully established but the overall reaction catalysed by the enzyme is ... 

Rotenone is a complex flavonoid found in the plant Derris ellyptica. It acts by inhibiting electron transport in the mitochondrion. Derris powder is an insecticidal preparation made from the plant, which is highly toxic to hsh. 

Rotenone A complex flavonoid produced by the plant Denis ellyptica. It has insecticidal activity due to its ability to inhibit electron transport in the mitochondrion. 

FIG. 4. Ultrastructure of vascular smooth muscle of the rabbit inferior vena cava revealed with electron microscopy. Serial cross-sections of VSMCs are shown in series 1 (panel A—D) and series 2 (panel E—G). Series 1 illustrates the close spatial apposition between the superficial SR sheet and the PM with the apices of the caveolae perforating through the superficial SR sheets to come into contact with the bulk cytoplasm. The membranes of the PM (dotted line) and the SR (solid line) in panel A-D are outlined to the right of the respective panels. The close apposition between the superficial SR sheet, the PM and the neck region of the caveolae creates a narrow and expansive restricted space. Series 2 illustrates the perpendicular sheets of SR, which appear to arise from the superficial SR sheets. Mitochondria also come into close contact with the perpendicular SR sheets. Panel H contains a stylized illustration of the close association between the superficial SR sheet, which is continuous with the perpendicular sheet, the perforating caveolae (C), the PM and a mitochondrion (M). Panel I shows calyculin-A mediated dissociation of the superficial SR sheets from the PM (see arrows). The black scale bar indicated represents 200 nm of distance. 

Ered Sanger, a double Nobel Prize winner, sequenced the human mitochondrial genome back in 1981. This genome codes for 13 proteins and the mitochondrion possesses the genetic machinery needed to synthesize them. Thus, the mitochondria are a secondary site for protein synthesis in eukaryotic cells. It turns out that the 13 proteins coded for by the mitochondrial genome and synthesized in the mitochondria are critically important parts of the complexes of the electron transport chain, the site of ATP synthesis. The nuclear DNA codes for the remainder of the mitochondrial proteins and these are synthesized on ribosomes, and subsequently transported to the mitochondria. 

What do I mean by a proton concentration gradient Simply, there is a higher concentration of protons in the space between the inner and outer membranes of the mitochondrion than in the mitochondrial interior. The gradient is formed from the energy released in the transfer of electrons down the electron transport chain. Put another way, the released energy is employed to pump protons across the inner mitochondrial membrane into the intermembrane space. 

Under aerobic conditions, the hydrogen atoms of NtUDH are oxidised within the mitochondrion pyruvate is also oxidised in the mitochondrion (Figure 9.15). However, NADH cannot be transported across the inner mitochondrial membrane, and neither can the hydrogen atoms themselves. This problem is overcome by means of a substrate shuttle. In principle, this involves a reaction between NADH and an oxidised substrate to produce a reduced product in the cytosol, followed by transport of the reduced product into the mitochondrion, where it is oxidised to produce hydrogen atoms or electrons, for entry into the electron transfer chain. Finally, the oxidised compound is transported back into the cytosol. The principle of the shuttle is shown in Figure 9.16. 

As mentioned, although complexes I through V are all integrated into the inner membrane of the mitochondrion, they are not usually in contact with one another, since the electrons are transferred by ubiquinone and cytochrome c. With its long apolar side chain, ubiquinone is freely mobile within the membrane. Cytochrome c is water-soluble and is located on the outside of the inner membrane. 

Oxidizible substrates from glycolysis, fatty acid or protein catabolism enter the mitochondrion in the form of acetyl-CoA, or as other intermediaries of the Krebs cycle, which resides within the mitochondrial matrix. Reducing equivalents in the form of NADH and FADH pass electrons to complex I (NADH-ubiquinone oxidore-ductase) or complex II (succinate dehydrogenase) of the electron transport chain, respectively. Electrons pass from complex I and II to complex III (ubiquinol-cyto-chrome c oxidoreductase) and then to complex IV (cytochrome c oxidase) which accumulates four electrons and then tetravalently reduces O2 to water. Protons are pumped into the inner membrane space at complexes I, II and IV and then diffuse down their concentration gradient through complex V (FoFi-ATPase), where their potential energy is captured in the form of ATP. In this way, ATP formation is coupled to electron transport and the formation of water, a process termed oxidative phosphorylation (OXPHOS). 

F. The electrons that are generated from the first step in ethanol metabolism (catalyzed by alcohol dehydrogenase) are transported into the mitochondrion by these two shuttles. 

Preliminary evidence of a mitosome (relict mitochondrion) in Cryptosporidium was provided by an electron microscopic study that revealed the... 

Fig. 4 Transmission electron microscopy of a longitudinal section of the posterior end of a Cryptosporidium parvum sporozoite showing immunogold localization of pyruvate NADP+ oxidoreductase (CpPNO). The mitochondrion-like organelle ( ) is posterior to the nucleus, and lies between the nucleus and the CB. It is labeled by mitochondrion-specific 15-nm gold anti- particles. Small -nm gold goat anti-CpPFO particles (arrows) show the localization of CpPNO. There are no 6-nm particles localized within the mitochondrion-like organelle (reprinted from Fig. 12 of Ctrnacta et al. 2006 with permission of the publishers)...

Nasirudeen AM, Tan KS (2004) Isolation and characterization of the mitochondrion-like organelle from Blastocystis hominis.J Microbiol Methods 58 101-109 Painter HJ, Morrisey JM, Mather MW, Vaidya AB (2007) Specific role of mitochondrial electron transport in blood-stage Plasmodium falciparum. Nature 446 88-91 Perkins GA, Song JY, Tarsa L, Deerinck TJ, Ellisman MH, Frey TG (1998) Electron tomography of mitochondria from brown adipocytes reveals crista junctions. J Bioeneerg Biomembr 30 431-432... 

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