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 Fo-c subunit (proteolipid subunit) are present in N. caninum genome as in T. gondii and P. falciparum. The genes for Fo-a and Fo-b are not identified in these species. For more genomic/biochemical evidences on apicomplexan Cox and ATP synthase, refer to the T. gondii page.

 

Enzyme EC Number Gene id Mitochondrial Complex
Glycerol-3-phosphate dehydrogenase 1.1.1.8 NCLIV_001180  
Glycerol-3-phosphate dehydrogenase 1.1.1.8 NCLIV_043550  
Glycerol-3-phosphate dehydrogenase 1.1.5.3 NCLIV_024220  
Malalte:quinone oxidoreductase

1.1.99.16

(Entry changed to 1.1.5.4)

NCLIV_040970  
Ubiquinol cytochrome c oxidoreductase bc1 complex 1.10.2.2 NCLIV_010020 Cytochrome bc1 complex (Complex III)
Ubiquinol cytochrome c oxidoreductase bc1 complex 1.10.2.2 NCLIV_010110 Cytochrome bc1 complex (Complex III)
Ubiquinol cytochrome c oxidoreductase bc1 complex 1.10.2.2 NCLIV_041210 Cytochrome bc1 complex (Complex III)
Flavoprotein subunit of succinate dehydrogenase 1.3.5.1 NCLIV_052500

Succinate dehydrogenase (ubiquinone) complex

(Complex II)

Flavoprotein subunit of succinate dehydrogenase 1.3.5.1 NCLIV_068970

Succinate dehydrogenase (ubiquinone) complex

(Complex II)

Dihydroorotate dehydrogenase 1.3.5.2 NCLIV_012040  
Iron-sulfur centres of succinate dehydrogenase 1.3.99.1 NCLIV_052230

Succinate dehydrogenase (ubiquinone) complex

(Complex II)

NAD(P)H dehydrogenase 1.6.5.3 NCLIV_003560  
NAD(P)H dehydrogenase 1.6.5.3 NCLIV_041240  
Cox4 1.9.3.1 NCLIV_003650 Cytochrome c oxidase (Complex IV)
Cox19 1.9.3.1 NCLIV_008520 Cytochrome c oxidase (Complex IV)
Cox17 1.9.3.1 NCLIV_016750 Cytochrome c oxidase (Complex IV)
Cox11 1.9.3.1 NCLIV_022110 Cytochrome c oxidase (Complex IV)
Cox10 1.9.3.1 NCLIV_044090 Cytochrome c oxidase (Complex IV)
Cox12 1.9.3.1 NCLIV_044600 Cytochrome c oxidase (Complex IV)
Cox15 1.9.3.1 NCLIV_045250 Cytochrome c oxidase (Complex IV)
Cox2 1.9.3.1 NCLIV_046390 Cytochrome c oxidase (Complex IV)
Cox2 1.9.3.1 NCLIV_054550 Cytochrome c oxidase (Complex IV)
Cox1 1.9.3.1 Present in mitochondrial genome Cytochrome c oxidase (Complex IV)
Cox3 1.9.3.1 Present in mitochondrial genome Cytochrome c oxidase (Complex IV)
ATP synthase subunit O 3.6.3.14 NCLIV_015180 ATP synthase (Complex V)
ATP synthase alpha chain 3.6.3.14 NCLIV_020840 ATP synthase (Complex V)
ATP synthase beta chain 3.6.3.14 NCLIV_025670 ATP synthase (Complex V)
ATP synthase gamma chain 3.6.3.14 NCLIV_031920 ATP synthase (Complex V)
ATP synthase delta subunit 3.6.3.14 NCLIV_046830 ATP synthase (Complex V)
ATP synthase epsilon chain 3.6.3.14 NCLIV_057710 ATP synthase (Complex V)
ATP synthase lipid-binding protein (Fo-c subunit) 3.6.3.14 NCLIV_065850 ATP synthase (Complex V)
ATP synthase subunit H 3.6.3.14 NCLIV_066570 ATP synthase (Complex V)
ATP synthase Fo-a subunit 3.6.3.14 Missing in annotation ATP synthase (Complex V)
ATP synthase Fo-b subunit 3.6.3.14 Missing in annotation ATP synthase (Complex V)
Cytochrome c none NCLIV_030340  
Cytochrome c none NCLIV_060860  
Cytochrome c1 none NCLIV_063330 Cytochrome bc1 complex (Complex III)
Cytochrome b none Present in mitochondrial genome Cytochrome bc1 complex (Complex III)

 

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Sources and fates of metabolites

 

Substrate Source pathways Product Fate pathways
Malate Tricarboxylic acid (TCA) cycle Oxaloacetate Tricarboxylic acid (TCA) cycle
L-dihydroorotate Pyrimidine metabolism Orotate Pyrimidine metabolism
Succinate Tricarboxylic acid (TCA) cycle Fumarate Tricarboxylic acid (TCA) cycle