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 the mitochondrial electron transport chain plays a central role in ATP production. In higher eukaryotes, the 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 the 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 genomes of both 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 T. gondii genome has showed that there are two NAD(P)H dehydrogenases that are orthologous to the Plasmodium alternative NAD(P)H dehydrogenase. The inhibition of T. gondii NAD(P)H dehydrogenase with 1-hydroxy-2-dodecyl-4(1H)quinolone has led to the collapse of mitochondrial inner membrane potential and depletion of ATP [5] suggesting the importance of this enzyme in mitochondrial electron transport and its role in ATP production. In addition, studies have suggested that both the isoforms of NADH dehydrogenase are essential for the growth of T. gondii and even single knockout mutants displayed a decreased replication rate and also decreased mitochondrial membrane potential [6].

 

The analysis of the genome of T. gondii shows the presence of only two subunits of succinate dehydrogenase (flavoprotein subunit and iron-sulphur protein subunits). The orthologs of the membrane anchor subunits have not been identified in the T. gondii genome. It was also the case with P. falciparum genome. Purification and molecular characterisation of the succinate dehydrogenase in P. falciparum has identified the above mentioned two subunits (flavoprotein subunit and iron-sulphur protein subunits)  and demonstrated significant activity. It is also suggested that this enzyme may be a peripheral complex [4, 7, 8]. The complex III of P. falciparum and T. gondii are similar to mammalian enzymes. The mammalian complex III inhibitors such as myxothiazol and antimycin A can inhibit Plasmodium complex III activity [4]. The differences in the ubiquinol binding regions of Plasmodium cytochrome b to the mammalian protein has enabled the use of atovaquone as anti-malarial. Its action in complex III was confirmed by the study showing the resistance to atovoquone in malarial parasites with mutations in the ubiquinol binding domains of cytochrome b [9]. The use of related drugs to treat East coast fever, an infection of another apicomplexan, Theileria parva [10] may suggest the conservation of ubiquinol binding domain of cytochrome b across all apicomplexan species including Toxoplasma.

 

The analysis of Toxoplasma genome has led to identification of two subunits of Cox2, the accessory protein Cox4 and the assembly proteins Cox10, Cox11, Cox12, Cox15, Cox17 and Cox19. The orthologs of all these genes are present in the P. falciparum genome (PlasmoDB/MPMP). The Cox1 and Cox3 are present in the mitochondrial genome which has been reported in P. falciparum and P. yoelii [11, 12] . The Plasmodium mitochondrial genome available in PlasmoDB also possess the genes for Cox1 and Cox3 and reconstructed in MPMP pathway. The activity of Cox complex has been evident from the study in the malaria parasite P. berghei [13].

 

The genomes of both P. falciparum and T. gondii possess the genes for all the F1 subunits and the Fo-c subunit (proteolipid subunit). The genes for Fo-a and Fo-b are not identified in these species [4, 10]. It has been demonstrated in intra-erythrocytic stages of P. falciparum that electron transport chain does not play a role in ATP synthesis and it is only important for regeneration of ubiquinone as an electron acceptor for dihydroorotate dehydrogenase. It has also been proposed that the hydrolysis of ATP by matrix localised ATP synthase and transport of ADP for ATP will generate net negative charge and establishes membrane potential [14]. Although Fo-a and Fo-b subunits of ATP synthase are also missing in T. gondii gene models, it has been demonstrated that electron transport chain mediated generation of ATP is essential for T. gondii. The 30% reduction of ATP levels with inhibition of NAD(P)H dehydrogenase with 1-hydroxy-2-dodecyl-4(1H)quinolone within one hour and 70% reduction within 24 hours, 70% reduction in ATP levels with Fo subunit inhibitor oligomycin and oligomycin leading to stabilisation of membrane potential in the presence of 1-hydroxy-2-dodecyl-4(1H)quinolone suggests the importance of Fo subunits of ATP synthase. This also suggests the importance of electron transport and oxidative phosphorylation in ATP generation in T. gondii. These differences  in electron transport chain might have been due to the differences in life styles of Plasmodium and Toxoplasma [5]. In addition, the mitochondrial respiratory chain has also been proved to be essential in Plasmodium yoeli as the inhibition of electron transport and mitochondrial depolarization with atovaquone treatment caused cellular damage and death [15]. The same effect was also observed with atovaquone in T. gondii [16]. All these evidence suggest that the respiratory chain and oxidative phosphorylation are functional in apicomplexans T. gondii and Plasmodium species, although the pathway differs in several aspects from that of the hosts [17].

 

Protein EC Number Gene id Mitochondrial Complex
Glycerol-3-phosphate dehydrogenase 1.1.1.8 TGME49_307570  
Glycerol-3-phosphate dehydrogenase 1.1.5.3 TGME49_263730  
Malate:quinone oxidoreductase

1.1.99.16

(Entry changed to 1.1.5.4)

TGME49_288500  
Ubiquinol cytochrome c oxidoreductase bc1 complex 1.10.2.2 TGME49_288750 Cytochrome bc1 complex (Complex III)
Ubiquinol cytochrome c oxidoreductase bc1 complex 1.10.2.2 TGME49_320140 Cytochrome bc1 complex (Complex III)
Ubiquinol cytochrome c oxidoreductase bc1 complex 1.10.2.2 TGME49_320220 Cytochrome bc1 complex (Complex III)
Flavoprotein subunit of succinate dehydrogenase 1.3.5.1 TGME49_215590 Succinate dehydrogenase (ubiquinone) complex (Complex II)
Dihydroorotate dehydrogenase 1.3.5.2 TGME49_210790  
Iron-sulfur centres of succinate dehydrogenase 1.3.99.1 TGME49_215280 Succinate dehydrogenase (ubiquinone) complex (Complex II)
NAD(P)H dehydrogenase 1.6.5.3 TGME49_209150  
NAD(P)H dehydrogenase 1.6.5.3 TGME49_288830  
Cox11 1.9.3.1 TGME49_202800 Cytochrome c oxidase (Complex IV)
Cox10 1.9.3.1 TGME49_206310 Cytochrome c oxidase (Complex IV)
Cox4 1.9.3.1 TGME49_209260 Cytochrome c oxidase (Complex IV)
Cox2 1.9.3.1 TGME49_226590 Cytochrome c oxidase (Complex IV)
Cox15 1.9.3.1 TGME49_228180 Cytochrome c oxidase (Complex IV)
Cox17 1.9.3.1 TGME49_240550 Cytochrome c oxidase (Complex IV)
Cox19 1.9.3.1 TGME49_254260 Cytochrome c oxidase (Complex IV)
Cox12 1.9.3.1 TGME49_306390 Cytochrome c oxidase (Complex IV)
Cox2 1.9.3.1 TGME49_310470 Cytochrome c oxidase (Complex IV)
Cox1 1.9.3.1 Present in mitochondrial genome (see text above) Cytochrome c oxidase (Complex IV)
Cox3 1.9.3.1 Present in mitochondrial genome (see text above) Cytochrome c oxidase (Complex IV)
ATP synthase alpha chain 3.6.3.14 TGME49_204400 ATP synthase (Complex V)
ATP synthase delta subunit 3.6.3.14 TGME49_226000 ATP synthase (Complex V)
ATP synthase gamma chain 3.6.3.14 TGME49_231910 ATP synthase (Complex V)
ATP synthase lipid-binding protein (Fo-c subunit) 3.6.3.14 TGME49_249720 ATP synthase (Complex V)
ATP synthase subunit H 3.6.3.14 TGME49_251470 ATP synthase (Complex V)
ATP synthase beta chain 3.6.3.14 TGME49_261950 ATP synthase (Complex V)
ATP synthase subunit O 3.6.3.14 TGME49_284540 ATP synthase (Complex V)
ATP synthase epsilon chain 3.6.3.14 TGME49_314820 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 TGME49_219750  
Cytochrome c none TGME49_229420  
Cytochrome c1 none TGME49_246540 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