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 apicomplexan genomes. 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 Plasmodium falciparum genome. This enzyme is rotenone insensitive [1, 2, 3, 4]. The orthologs of this enzyme are present in the Piroplasma species such as Theileria parva, Theileria annulata and Babesia bovis. The analysis of Piroplasma genomes shows the presence of only two sub units of succinate dehydrogenase (flavoprotein subunit and iron-sulphur protein subunits). The orthologs of the membrane anchor subunits have not been identified in these genomes. It was also the case with P. falciparum and Toxoplasma gondii genomes. 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, 5, 6].

 

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 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 ubiquinol binding domains of cytochrome b [7]. The use of related drugs to treat East Coast Fever, an infection of T. parva [8] may suggest the conservation of ubiquinol binding domain of cytochrome b across Apicomplexa including these Piroplasma species. The genes for the two subunits of Cox2, the accessory protein Cox4 and the assembly proteins Cox10, Cox11, Cox12, Cox15, Cox17 and Cox19 are present in the nuclear genomes of these Piroplasma species. The Cox1 and Cox3 are present in the mitochondrial genomes of these species.

 

The Theileria and Babesia genomes possess the genes for all the F1 subunits and the Fo-c subunit (proteolipid subunit) of ATP synthase complex. The genes for Fo-a and Fo-b are not identified in these species as is the case with P. falciparum and T. gondii. 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 [9]. The presence of pyrimidine biosynthesis in Piroplasma suggests the role of this pathway in regeneration of ubiquinone. The role of this pathway in energy generation is yet to be confirmed in these species.

 

Enzyme EC Number Gene id Mitochondrial Complex
Glycerol-3-phosphate dehydrogenase 1.1.1.8 TA21330  
Glycerol-3-phosphate dehydrogenase 1.1.5.3 TA17925  
Malate:quinone oxidoreductase 1.1.5.4 TA18100  
Ubiquinol cytochrome c oxidoreductase bc1 complex 14kDa subunit 1.10.2.2 TA06055 Cytochrome bc1 complex (Complex III)
Ubiquinol cytochrome c oxidoreductase bc1 complex hinge protein 1.10.2.2 TA07960 Cytochrome bc1 complex (Complex III)
Ubiquinol cytochrome c oxidoreductase bc1 complex Fe-S subunit 1.10.2.2 TA10085 Cytochrome bc1 complex (Complex III)
Flavoprotein subunit of succinate dehydrogenase 1.3.5.1 TA03455 Succinate dehydrogenase (ubiquinone) complex (Complex II)
Dihydroorotate dehydrogenase 1.3.5.2 TA11695  
Iron-sulfur centres of succinate dehydrogenase 1.3.99.1 TA19430 Succinate dehydrogenase (ubiquinone) complex (Complex II)
NAD(P)H dehydrogenase 1.6.5.3 TA05115  
Cox2 1.9.3.1 TA02790 Cytochrome c oxidase (Complex IV)
Cox19 1.9.3.1 TA06305 Cytochrome c oxidase (Complex IV)
Cox10 1.9.3.1 TA07535 Cytochrome c oxidase (Complex IV)
Cox4 1.9.3.1 TA12915 Cytochrome c oxidase (Complex IV)
Cox15 1.9.3.1 TA18480 Cytochrome c oxidase (Complex IV)
Cox2 1.9.3.1 TA20025 Cytochrome c oxidase (Complex IV)
Cox11 1.9.3.1 TA20075 Cytochrome c oxidase (Complex IV)
Cox1 1.9.3.1 Tap370b08.q2ca38.01 (Mitochondrial genome) Cytochrome c oxidase (Complex IV)
Cox3 1.9.3.1 Tap370b08.q2ca38.02c (Mitochondrial genome) Cytochrome c oxidase (Complex IV)
ATP synthase subunit O 3.6.3.14 TA06450 ATP synthase (Complex V)
ATP synthase delta subunit 3.6.3.14 TA12155 ATP synthase (Complex V)
ATP synthase lipid-binding protein (Fo-c subunit) 3.6.3.14 TA15215 ATP synthase (Complex V)
ATP synthase alpha chain 3.6.3.14 TA18615 ATP synthase (Complex V)
ATP synthase gamma chain 3.6.3.14 TA19395 ATP synthase (Complex V)
ATP synthase beta chain 3.6.3.14 TA20945 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 c1 none TA08150 Cytochrome bc1 complex (Complex III)
Cytochrome c none TA10380  
Cytochrome c none TA12950  
Cytochrome b none Tap370b08.q2ca38.03c (Mitochondrial genome) Cytochrome bc1 complex (Complex III)

 

Open in a new window

 

 

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