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

Abstract

In aerobic respiration, the tricarboxylic acid 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 tricarboxylic acid cycle and depends exclusively on glycolysis for ATP production. However, all of the constituent enzymes of the tricarboxylic acid cycle are annotated in the genome of P. Plasmodium falciparum is a parasitic protozoan, which causes the most severe form of malaria in humans. Because of the emergence in the parasite of widespread drug resistance to most of the first-line antimalarials, the 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 gives opportunities for chemotherapeutic interventions but also stimulates broader scientific interests.During the intraerythrocytic stages of P. falciparum, the tricarboxylic acid cycle does not seem to function like a conventional tricarboxylic acid cycle for the following reasons. (a) The 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 tricarboxylic acid cycle. (b) The multi-enzyme complex pyruvate dehydrogenase, which channels pyruvate into the tricarboxylic acid cycle through acetyl-CoA, has been found to localize on the apicoplast membrane in P. falciparum (3), 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 implying that its main role is probably to act as a redox sensor rather than donating reducing equivalents to the electron transport chain. In addition, P. falciparum synthesizes ATP mainly by substrate level phosphorylation through glycolysis and not through oxidative phosphorylation (6), suggesting that the primary function of the tricarboxylic acid cycle in generating reducing equivalents for the electron transport chain might be dispensable for the parasite. However, genes for all of the enzymes of the tricarboxylic acid cycle are present and are expressed in P. falciparum (7,8) during the intraerythrocytic stages, thereby implying that the pathway probably has important, yet unidentified biosynthetic functions.Unlike its human host, P. falciparum lacks the de novo purine biosynthetic pathway and is hence completely dependent on the purine salvage pathway to meet its purine nucleotide requirements (9). In the purine salvage pathway, hypoxanthine salvaged from the host is phosphoribosylated to IMP, which then branches out to form AMP and GMP (Fig. 1). IMP is converted to AMP in two steps, catalyzed by two distinct enzymes: (a) adenylosuccinate synthetase, which adds a molecule of aspartate to the 6-oxo position of IMP with a concomitan...

Apicomplexan Energy Metabolism: Carbon Source Promiscuity and the Quiescence Hyperbole

“…Furthermore, the b-subunit of ATP synthase is also required in blood stages of P. falciparum, although this is not the case in P. berghei [38][ 1 4 _ T D $ D I F F ] (Figure 1). It is possible that the classic 'primary' roles of these enzymes are not responsible for these phenotypes, but rather their roles in cytosolic purine salvage (FH [45]) and, perhaps, mitochondrial structure (MQO and ATP synthase [36,38]). …”

“…Fumarate Hydratase (FH) FH converts fumarate to malate and exists in nature in two forms: class I, as found in Apicomplexa, which contains a required Fe-S cluster [45], and the Fe-S cluster-free class II, as found in mammals [58,59]. Apicomplexan FH can contribute to either (mitochondrial) TCA metabolism or (cytosolic) purine salvage, depending on its localization.…”

“…Apicomplexan FH can contribute to either (mitochondrial) TCA metabolism or (cytosolic) purine salvage, depending on its localization. Initial studies suggest that P. falciparum FH is mitochondrial [45], although this localization, as for other Apicomplexa, is not entirely clear. FH is one of only two TCA enzymes essential to the asexual stages of P. falciparum [36], and this, coupled with the apparent disposability of the TCA, suggest that a role in cytosolic purine salvage is of paramount importance in these stages.…”

“…In Plasmodium spp., however, only one MDH is present, and this enzyme is suggested to be exclusively cytosolic [60,61]. This isoform is implicated in reactions of purine salvage [45] and phosphoenolpyruvate (PEP) carboxylation (see below) [62]. Instead of mMDH, Plasmodium spp.…”

“…Therefore, TCA-derived malate may contribute to purine salvage, and likely explains the retention of cMDH in Plasmodium, the essentiality of FH in asexual stages [36], and the ability to rescue P. falciparum PEP carboxylation mutants with exogenous malate [62]. However, it should be noted that the fumarate/OAA transporter(s) required for transport of fumarate into the mitochondrion (where FH appears to be localized [45]) have not yet been identified in Plasmodium. Moreover, labeling studies suggest that PEP carboxylation to oxaloacetate and conversion to malate may have an anaplerotic role, albeit minor, in TCA metabolism [62].…”