Electron transport chain

A continuous supply of energy in the form of ATP is essential to the maintenance of life. In most eukaryotes, it is achieved by oxygen-dependent energy production and mitochondrial electron transport chain plays central role in ATP production. In higher eukaryotes, electron transport chain comprises four integral membrane protein complexes namely, NADH:ubiquinone oxidoreductase (complex I), succinate:ubiquinone oxidoreductase (complex II), ubiquinol:cytochrome c oxidoreductase/ cytochrome bc1 complex (complex III) and cytochrome oxidase (complex IV).  The electrons are transferred from NADH and succinate to oxygen through these series of enzymatic complexes of the inner mitochondrial membrane and oxygen is reduced to water. This releases energy and generates a proton gradient across mitochondrial membrane by pumping protons into intermembrane space. The energy of oxidation of hydrogen is used to phosphorylate ADP into ATP. This ATP generation is catalysed by ATP synthase complex (complex V).

The conventional NADH:ubiquinone oxidoreductase multiprotein complex is absent in the Neospora caninum genome as in the genomes of Plasmodium falciparum and Toxoplasma gondii. However, an alternative single gene NAD(P)H dehydrogenase enzyme homologous to the peripheral membrane NADH dehydrogenase in yeast, plants and fungi was identified in P. falciparum genome. These alternative NADH dehydrogenases of mitochondrial membrane are insensitive to rotenone, an inhibitor of complex I [1]. The presence of oxidation of exogenous NADH and its insensitivity to rotenone has been experimentally shown in P. falciparum and P. yoelii yoelii. This confirms the presence of alternative NADH dehydrogenase in these species [2, 3, 4]. The analysis of N. caninum genome has showed that there are two NAD(P)H dehydrogenases which are orthologous to the T. gondii alternative NAD(P)H dehydrogenase. The analysis of the genome of N. caninum shows the presence of only two subunits of succinate dehydrogenase (flavoprotein subunit and iron-sulphur protein subunits). The same was observed in T. gondii as well. The homologues of the membrane anchor subunits have not been identified in N. caninum and T. gondii genomes. The complex III of P. falciparum, T. gondii and N. caninum are similar to mammalian enzymes. For more biochemical information on this complex, refer to the T. gondii page.

The analysis of Neospora genome has led to identification of orthologs of all cytochrome oxidase proteins present in the T. gondii genome such as two subunits of Cox2, the accessory protein Cox4 and the assembly proteins Cox10, Cox11, Cox12, Cox15, Cox17 and Cox19. The genes for all the F₁ subunits and the F_o-c subunit (proteolipid subunit) are present in N. caninum genome as in T. gondii and P. falciparum. The genes for F_o-a and F_o-b are not identified in these species. For more genomic/biochemical evidences on apicomplexan Cox and ATP synthase, refer to the T. gondii page.

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Sources and fates of metabolites