Probing the necessity of the lipoate cofactor in Plasmodium

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Lipoate is an organosulfur cofactor that participates in several, well‐conserved metabolic pathways in malaria parasites. The mitochondrial proteins of Plasmodium are lipoylated through the uptake of lipoate from the red blood cell, where it is then imported into the mitochondrion and ligated to three proteins: the mitochondrial pyruvate dehydrogenase (mPDH), the alpha‐ketoglutarate dehydrogenase (KDH), and the H protein of the glycine cleavage system. When parasites are treated with a lipoate analog, (8‐bromooctanoate), parasite growth is significantly decreased, with concomitant decrease in lipoylation; this inhibition is reversed with the addition of supplemental lipoate. Expression of an exogenous, bacterial lipoamidase, which can cleave lipoate from lipoylated proteins, is not tolerated in the mitochondrion of the parasite, while it is tolerated elsewhere in the parasite. This body of evidence suggests that lipoylation, and not just lipoate, is crucial for parasite growth. Gene disruption in the human parasite, P.falciparum, and gene knockout in the mouse parasite, P.berghei, suggest that the PDH and KDH may not be individually essential for parasite growth. Thus, the H protein may be the only essential lipoylated protein in the parasite. In parallel, site directed mutagenesis has been used to strongly attenuate catalytic activity of the bacterial lipoamidase. Mutant lipoamidase enzymes will be expressed in both human and murine malaria parasites in an attempt to obtain intermediate phenotypes. Analysis of these parasites will identify the essential biochemical pathway that is responsible for lipoate auxotrophy in malaria parasites. Research support is provided by funding from the NIAID, the JHMRI and the Sommer Scholarship.