Metabolic Fate of Fumarate, a Side Product of Purine Salvage Pathway in the Intraerythrocytic Stages of Plasmodium Falciparum

In aerobic respiration, the tricarboxylic acid (TCA) cycle is pivotal to the complete oxidation of carbohydrates, proteins and lipids to carbon dioxide and water. Plasmodium falciparum, the causative agent of human malaria, lacks a conventional TCA cycle and depends exclusively on glycolysis for ATP production. However, all the constituent enzymes of the TCA cycle are annotated in the genome of P. falciparum implicating that the pathway might be having important, yet unidentified biosynthetic function(s). Here we show that fumarate, a side product of the purine salvage pathway and a metabolic intermediate of the TCA cycle is not a metabolic waste but converted to aspartate through malate and oxaloacetate. P. falciparum infected erythrocytes and free parasites incorporated 14 C-2,3-fumarate into the nucleic acid and protein fractions. 13 CNMR of parasites incubated with 13 C-2,3fumarate showed the formation of malate, pyruvate, lactate and aspartate, but not citrate or succinate. Further, treatment of free parasites with atovaquone inhibited the conversion of fumarate to aspartate, thereby indicating this pathway to be an electron transport chain dependent process. This study therefore, provides biosynthetic function for fumarate hydratase, malate quinone oxidoreductase and aspartate aminotransferase of P. falciparum. INTRODUCTION Plasmodium falciparum is a parasitic protozoan, which causes the most severe form of malaria in humans. Due to the emergence of wide spread drug resistance in the parasite to most of the first line antimalarials, development of new drugs that target the parasite metabolism is needed. Over several years, malarial parasites have been studied to understand various novel biochemical features, which not only give opportunities for chemotherapeutic interventions, but also stimulate broader scientific interests. During the intraerythrocytic stages of P. falciparum, the tricarboxylic acid (TCA) cycle does not seem to function like a conventional TCA cycle for the following reasons: (a) Bulk of the glucose is metabolized to lactate by anaerobic glycolysis, which is then secreted by the parasite as a metabolic waste (1)(2). As a result, unlike aerobic cells, in P. falciparum, there is minimal carbon flow from the cytoplasm to the mitochondrial TCA cycle, (b) the multi-enzyme complex pyruvate dehydrogenase, which channels pyruvate into TCA cycle through acetyl-CoA has been found to localize on the apicoplast membrane in P. falciparum (3), which is unlike mammalian cells, where the enzyme is present on the inner mitochondrial membrane (4), (c) the enzyme isocitrate dehydrogenase generates NADPH rather than NADH (5), thereby implicating that its main role is probably to act as a redox sensor rather than donating reducing equivalents to the electron transport chain. http://www.jbc.org/cgi/doi/10.1074/jbc.M110.173328 The latest version is at JBC Papers in Press. Published on January 5, 2011 as Manuscript M110.173328 Copyright 2011 by The American Society for Biochemistry and Molecular Biology, Inc. by gest on July 1, 2017 hp://w w w .jb.org/ D ow nladed from