New insights into apicoplast metabolism in blood-stage malaria parasites

Abstract

The apicoplast of Plasmodium falciparum is the only source of essential isoprenoid precursors and Coenzyme A (CoA) in the parasite. Isoprenoid precursor synthesis relies on the iron-sulfur cluster (FeS) cofactors produced within the apicoplast, rendering FeS synthesis an essential function of this organelle. Recent reports provide important insights into the roles of FeS cofactors and the use of isoprenoid precursors and CoA both inside and outside the apicoplast. Here, we review the recent insights into the roles of these metabolites in blood-stage malaria parasites and discuss new questions that have been raised in light of these discoveries.

Figure 1.. Isoprenoid precursor biosynthesis and Coenzyme A (CoA) biosynthesis pathways of Plasmodium falciparum and the utilization of their end products.

Isoprenoid precursors isopentenyl pyrophosphate (IPP) and its regioisomer dimethylallyl pyrophosphate (DMAPP) generated by apicoplast methylerythritol phosphate (MEP) pathway are utilized both within and outside the apicoplast. Within the organelle, isoprenoid precursors are used for tRNA modification and the synthesis of long-chain polyprenyls, whereas outside the organelle they are used for protein prenylation in cytosol (black outlined box), ubiquinone synthesis in the mitochondrion (orange outlined box), and dolichol synthesis in the endoplasmic reticulum (blue outlined box). Iron-sulfur clusters (FeS) generated by the FeS synthesis pathway are essential for the activity of two terminal enzymes of MEP pathway. DXPR is the target of fosmidomycin (Fos). The CoA synthesis pathway is shared between the cytosol and apicoplast, where the final product CoA is produced in the apicoplast. Within the organelle, CoA is used for fatty acid biosynthesis (FASII) while outside the organelle it is used in the TCA cycle and acetyl CoA synthesis in the mitochondrion (orange outlined box). Boxes filled in grey are non-essential pathways/processes in the blood-stage parasites. GA3P, glyceraldehyde-3 phosphate; DOXP, 1-deoxy-D-xylulose 5-phosphate; DXPS, DOXP synthase; DXPR, DOXP-reductoisomerase; IspD, 2C-methyl-D-erythritol 4-phosphate cytidyltransferase; IspE, 4-diphosphocytidyl-2C-methyl-D-erythritol (CDP-ME) kinase; CDP-MEP, CDP-ME phosphate; IspF, 2C-methyl-D-erythritol-2,4-cyclodiphosphate (MEcPP) synthase; IspG, hydroxylmethylbutenyl diphosphate (HMB-PP) synthase; IspH, HMB-PP reductase; PPS, polyprenyl synthase; MiaA, tRNA isopentenyltransferase; MiaB, tRNA-i⁶A37 methylthiotransferase; FASII, type II fatty acid synthesis pathway; CoA, Coenzyme A, ACP, acyl carrier protein; ACPS, ACP synthase; DPCK, dephospho-CoA kinase; Pan, pantothenate; GPP, geranyl pyrophosphate; FPP, farnesyl pyrophosphate; GGPP, geranylgeranyl pyrophosphate; FPP/GGPPS, farnesyl/geranylgeranyl pyrophosphate synthase; PPRD, polyprenol reductase.

Figure 2.. Inhibition of apicoplast housekeeping functions causes delayed death.

Inside the apicoplast (green outlined box), isoprenoid precursors IPP and DMAPP are important for apicoplast maintenance and have a possible role in membrane fluidity. Outside the organelle, they are required for protein prenylation in the cytosol (necessary for vesicular trafficking), ubiquinone synthesis in the mitochondrion (necessary for the electron transport chain), and dolichol synthesis in the endoplasmic reticulum (necessary for GPI anchor synthesis and N-linked glycosylation). Several classes of antibiotics inhibit housekeeping functions (replication, transcription, or translation of the apicoplast genome), causing apicoplast disruption in the second growth cycle. Loss of the apicoplast genome prevents FeS synthesis and the activity of FeS-dependent MEP pathway enzymes. Parasites fail to grow in the second cycle (delayed death) due to lack of protein prenylation (Left panels). Second cycle defects caused by a reduction of isoprenoid products and decreased protein prenylation can be rescued with geranylgeraniol (GGOH) supplementation (green arrow). However, the delayed death phenotype cannot be rescued with GGOH beyond second cycle, presumably due to depletion of ubiquinone and/or dolichols (Right panels).

Figure 3.. Active pathways in apicoplast-disrupted parasites with isopentenyl pyrophosphate (IPP) supplementation.

In apicoplast-disrupted parasites, protein prenylation in the cytosol (black outlined box), ubiquinone synthesis in the mitochondrion (orange outlined box), and dolichol synthesis in the endoplasmic reticulum (blue outlined box) can remain functional as long as the parasites are exogenously supplemented with IPP. The dephospho-CoA kinase (DPCK) enzyme of the Coenzyme A (CoA) synthesis pathway remains active in the vesicles (green box) seen in apicoplast-disrupted parasites which provide CoA for mitochondrial processes (orange outlined boxes). Boxes shaded grey are non-essential pathways/processes in blood-stage parasites. Pan, pantothenate; DMAPP, dimethylallyl pyrophosphate; GPP, geranyl pyrophosphate; FPP, farnesyl pyrophosphate; GGPP, geranylgeranyl pyrophosphate; FPP/GGPPS, farnesyl/geranylgeranyl pyrophosphate synthase; PPRD, polyprenol reductase.