School of Botany - Theses
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ItemExploring how fatty acids synthesized by malaria parasites are incorporated into lipids: Characterization of the Plasmodium apicoplast glycerol-3-phosphate acyltransferase( 2015)The Plasmodium parasites responsible for malaria synthesize fatty acids in a reduced plastid organelle known as the apicoplast. There has been enormous interest in the apicoplast fatty acid synthesis pathway as a potential drug target, but comparatively little research into its role in parasite lipid metabolism. The fatty acid synthesis pathway is essential only at certain stages of the parasite life cycle, and appears to be required to support membrane lipid biosynthesis. However, the nature of the lipid species reliant on the apicoplast pathway and the intermediate steps in their synthesis have largely not been explored. The majority of parasite membrane lipid species can be synthesized from a phosphatidic acid precursor. Phosphatidic acid is composed of two fatty acids linked to glycerol-3-phosphate, and it is synthesized in two steps by a pair of acyltransferases. Plasmodium parasites were initially predicted to have two complete phosphatidic acid synthesis pathways located in the apicoplast and endoplasmic reticulum. Recent discoveries in the P. yoelii rodent model have demonstrated the apicoplast pathway is essential for parasites in the liver stage, consistent with the requirement for fatty acid synthesis in that species. However, it also indicated the apicoplast pathway may be incomplete, suggesting its function was instead to incorporate newly-made fatty acids into the intermediate lysophosphatidic acid. This thesis investigates the glycerol-3-phosphate acyltransferase of the P. falciparum and P. berghei apicoplast lysophosphatidic acid synthesis pathway to gain greater insight its role in lipid metabolism. The localization of the P. falciparum enzyme is confirmed and its activity demonstrated and explored through complementation, site-directed mutagenesis and structure homology modeling. The P. falciparum glycerol-3-phosphate acyltransferase is shown to be non-essential in the blood stage in standard culturing conditions, consistent with the dispensability of fatty acid synthesis at this stage. Unexpectedly, the P. falciparum enzyme is also found to be dispensable in lipid-depleted media conditions that induce fatty acid synthesis, suggesting the fatty acids made by parasites in this environment are used via other pathways. The P. berghei homologue is demonstrated to be essential in the liver stage, and its loss to closely mirror the phenotype of fatty acid synthesis knockouts in rodent models, implicating the pathway as a major route for newly-synthesized fatty acids to be incorporated into membrane lipid precursors. However, loss of the P. berghei enzyme does not impact expression of a key merozoite surface protein to the extent seen in the fatty acid synthesis knockouts, providing novel insight into how newly-made fatty acids might contribute to the synthesis of the lipid anchor of this protein. These findings provide information about the apicoplast lysophosphatidic acid synthesis pathway in two further Plasmodium species, and contribute towards understanding how newly-synthesized fatty acids are incorporated into precursors for membrane lipid synthesis.