The essential role of the mitochondrial electron transport chain in Plasmodium falciparum

Malaria is a vector-borne parasitic disease spread by the Anopheles mosquito that accounts for 300-500 million cases each year and over one million deaths, mainly in sub-Saharan Africa. Of the four species that can infect humans, Plasmodium falciparum is responsible for the majority of morbidity and mortality. While the P. falciparum mitochondrion is a proven target for anti-malarial chemotherapy, mitochondrial pathways have been largely underexploited by drug discovery efforts. Here we describe three projects that address this concern. By using a bioinformatics approach to compile a list of high-confidence predicted mitochondrial genes, we identified three distinct expression profiles not previously observed from in vitro cultures. These results suggest that the mitochondrion may have a more active biochemical role in patient samples than under in vitro conditions, which may have implications for disease manifestation and drug treatment strategies. In an effort to characterize predicted mitochondrial drug targets, we examined the function of P. falciparum alternative NADH dehydrogenase (NDH2). Our work has shown that previously described NDH2 inhibitors, dibenziodolium chloride (DPI), diphenyliodonium chloride (IDP), and 1-hydroxy-2-dodecyl-4(1H)quinolone (HDQ), are not effective against the purified P. falciparum enzyme. Furthermore, we demonstrated that HDQ likely targets another P. falciparum mitochondrial enzyme, dihydroorotate dehydrogenase (pfDHOD), which is essential for de novo pyrimidine biosynthesis. Using a whole cell screening approach, we identified a novel, highly potent anti-malarial chemotype, tetracyclic benzothiazepines (BTZs), as potent inhibitors of mitochondrial electron chain transport (ETC) activity. Further examination into the ETC revealed the BTZ compounds to be specific inhibitors of cytochrome bc 1 activity, likely via binding to the Q 0 site of cytochrome b. The BTZ chemotype represents a promising alternative to existing mitochondrial inhibitors such as atovaquone. The P. falciparum mitochondrion is a unique organelle. While this poses challenges for bioinformatic, genomic, and biochemical study, further understanding of the mitochondrion may prove invaluable for the identification of novel anti-malarial drug targets.