Branched tricarboxylic acid metabolism in Plasmodium falciparum

Abstract

A central hub of carbon metabolism is the tricarboxylic acid (TCA) cycle1, which serves to connect the processes of glycolysis, gluconeogenesis, respiration, amino acid synthesis and other biosynthetic pathways. The protozoan intracellular malaria parasites (Plasmodium spp.), however, have long been suspected of possessing a significantly streamlined carbon metabolic network in which TCA metabolism plays a minor role2. Blood-stage Plasmodium parasites rely almost entirely on glucose fermentation for energy and consume minimal amounts of oxygen3, yet the parasite genome encodes all of the enzymes necessary for a complete TCA cycle4. By tracing 13C-labeled compounds using mass spectrometry5 we show that TCA metabolism in the human malaria parasite P. falciparum is largely disconnected from glycolysis and is organized along a fundamentally different architecture than the canonical textbook pathway. We find that this pathway is not cyclic but rather a branched structure in which the major carbon sources are the amino acids glutamate and glutamine. As a consequence of this branched architecture, several reactions must run in the reverse of the standard direction thereby generating two-carbon units in the form of acetyl-coenzyme A (acetyl-CoA). We further show that glutamine-derived acetyl-CoA is used for histone acetylation while glucose-derived acetyl-CoA is used to acetylate aminosugars. Thus the parasite has evolved two independent acetyl-CoA-production mechanisms with different biological functions. These results significantly clarify our understanding of the Plasmodium metabolic network and highlight the ability of altered variants of central carbon metabolism to arise in response to unique environments.

ATP Synthase Complex of Plasmodium falciparum

“…The mitochondrion in malaria parasites is highly unusual in many respects; it has the smallest genome encoding just three proteins and rRNA genes in pieces (38 -41); it possesses a unique branched tricarboxylic acid metabolism with no contributions from a pyruvate dehydrogenase (33,42); and it does not seem to be a significant source of ATP production in blood stages of the parasite (6 -9). Yet the Plasmodium mitochondrion clearly has a functional electron transport chain that is a validated target for antimalarial drugs (43)(44)(45).…”

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

“…Aspartate feeds into synthesized proteins, and its carbon skeleton through pyrimidine nucleotides feeds into nucleic acids; with both of these processes being highly active in the trophozoite stage of the parasite, the steady state concentration of free 13 C-enriched aspartate would be expected to be low. Also, it should be noted that apart from labeling proteins and nucleic acids through pyrimidines, [ 13 (40) highlights the fact that glutamine feeds the tricarboxylic acid cycle intermediates through 2-oxoglutarate in P. falciparum. The authors (40) have shown that malate is secreted out and also fumarate, but the latter to a much lesser extent.…”

“…Also, it should be noted that apart from labeling proteins and nucleic acids through pyrimidines, [ 13 (40) highlights the fact that glutamine feeds the tricarboxylic acid cycle intermediates through 2-oxoglutarate in P. falciparum. The authors (40) have shown that malate is secreted out and also fumarate, but the latter to a much lesser extent. Indeed, our observations agree with the findings of Olszewski et al (40).…”

Glucose‐6‐phosphate metabolism in Plasmodium falciparum

“…1) (14), whereas different NADPH sources in the malaria parasite such as glutamate dehydrogenase and isocitrate dehydrogenase were suggested (34,35). According to the branched architecture of the tricarboxylic acid cycle in Plasmodium, isocitrate dehydrogenase most likely oxidizes rather than produces NADPH (36). A recent study showed that the overall production of NADPH by the three known glutamate dehydrogenases is very low.…”

“…Glycolysis is the main source of ATP in Plasmodium during the asexual blood stage, with the tricarboxylic acid cycle being largely disconnected from glycolysis (36). As malaria parasites do not store energy reserves, uptake and metabolism of glucose are critical for their survival.…”