ApicomplexansApicomplexans are unicellular and spore forming
ApicomplexansApicomplexans are unicellular and spore-forming obligate intracellular parasites that occupy diverse host niches (Mogi and Kita 2010). They have remodeled mitochondrial carbon and energy metabolism through reductive evolution. The development of novel drugs is now a very serious challenge in the face of the increasing problem of the multidrug resistance of Plasmodium that causes malaria in humans. The function of the Plasmodium mitochondria is unclear because it is widely accepted that the majority of the energetic demand of the parasite is provided by glycolysis. In addition, it has been suggested for a long time that the Plasmodium mitochondria cannot conduct oxidative phosphorylation, because they lack the membrane anchor subunits for ATP synthase. However, it has been shown that the mitochondria can generate a large transmembrane potential (Biagini et al. 2006). Ten subunits of Plasmodium FoF1-ATP synthase, including membrane anchor subunits a and b, were finally identified, although they are highly divergent from their eukaryotic and bacterial counterparts (Kawahara et al. 2009). Thus, the Plasmodium mitochondria appear to be capable of oxidative phosphorylation. However, it is more likely that the mitochondria of these malaria parasites are engaged in cellular functions other than ATP synthesis, such as calcium homeostasis maintenance or pyrimidine synthesis (Stocks et al. 2014). There is no evidence for Complex I presence in the P. falciparum mitochondrial ap-1 transport chain, but bioinformatic analysis has identified the type II NADH dehydrogenase (PfNDH2) (Gardner et al. 2002). Analysis of the PfNDH2 gene indicates that the PfNDH2 protein is a single 52kDa monomer and possesses two regions, one likely responsible for the noncovalent attachment of the flavin nucleotide cofactor and the second probably responsible for NADH-binding (Fisher et al. 2007). The mode of interaction with UQ has not yet been elucidated, but it has been suggested that the enzyme kinetics follows a ping-pong (nonsequential) mechanism. Clustal analysis of PfNDH2 with EF-hand-containing alternative NADH dehydrogenases does not indicate the presence of conserved EF-hand domains. In addition, Ca2+ dependence has not been determined experimentally. Kawahara et al. (2009) reported that PfNDH2 reoxidizes NADH in the mitochondrial matrix and can also use NADPH as a substrate. Importantly, the PfNDH2 activity is essential for the generation of mitochondrial transmembrane potential and the inhibition of the dehydrogenase is lethal to P. falciparum (Biagini et al. 2006). It is likely that PfNDH2 serves as a “choke point” in the mitochondrial electron transport chain, enabling electron transfer through the other respiratory chain complexes. However, in the P. falciparum mitochondria, the proton circuit is different from typical mitochondria, since the primary generators of the electrochemical gradient are Complexes III and IV (Fisher et al. 2007). Although necessary subunits of the ATP synthase are present and assemble correctly in the P. falciparum mitochondria, the contribution of the enzyme to ATP generation is imperceptible (Balabaskaran et al. 2011). Because a proton electrochemical gradient exists across the P. falciparum inner mitochondrial membrane, there must be a proton leak in the membrane that is sufficient to complete the proton circuit (Fisher et al. 2007). It remains unclear whether it is the basal proton conductance of the membrane, a specific uncoupling protein, or a minimal ATP flux through ATP synthase. It has been hypothesized that PfNDH2 is an evolutionary adaptation to a microaerophilic lifestyle of the parasites enabling the (proton) uncoupled oxidation of NADH and a reduction in mitochondrial superoxide generation (Fisher et al. 2007). Similar to P. falciparum, there is no evidence of the presence of Complex I in Plasmodium yoelii mitochondria, but an alternative NADH dehydrogenase with an approximately 65kDa molecular mass exists in the inner mitochondrial membrane (PyNDH2) (Uyemura et al. 2004). In addition, in these malarial parasites, oxidative phosphorylation is considered minimal if it exists at all. In contrast to P. falciparum and other known parasites, the presence of the AOX has been excluded in the P. yoelii mitochondria (Uyemura et al. 2004). However, a fatty acid-induced GTP-inhibited mitochondrial uncoupling has been observed, suggesting that the activity of the uncoupling protein (UCP) could account for the closing of the proton circuit in the P. yoelii inner mitochondrial membrane (Uyemura et al. 2004).