FIGURE 1.

PtdEtn synthesis pathways in Plasmodium and yeast. PtdEtn synthesis in Plasmodium occurs through two major pathways, the serine decarboxylase-CDP-ethanolamine pathway and the PtdSer decarboxylation pathway. The major enzymes executing reactions within these pathways are shown in boxes. In the serine decarboxylase-CDP-ethanolamine pathway, serine is decarboxylated by an unidentified serine decarboxylase (SD) to form ethanolamine. Serine decarboxylation is unique to malarial parasites and plants. Ethanolamine formed through this reaction is sequentially converted into phosphoethanolamine (P-Etn), CDP-ethanolamine (CDP-Etn), and PtdEtn by ethanolamine kinase (EK), ethanolamine-phosphate cytidylyltransferase (ECT), and ethanolamine phosphotransferase (EPT). In Plasmodium, P-Etn can be sequentially methylated to form phosphomonomethylethanolamine (P-MMe), phosphodimethylethanolamine (P-DMe), and phosphocholine (P-Cho) by a single phosphoethanolamine methyltransferase (PMT). The resultant phosphocholine is further metabolized by a choline-phosphate cytidylytransferase (CCT) and choline phosphotransferase (CPT) to produce CDP-Cho and PtdCho, respectively. In the PtdSer decarboxylation pathway, serine is incorporated into phosphatidylserine (PtdSer) by PtdSer synthase (PSS) and subsequently decarboxylated to form PtdEtn by PtdSer decarboxylase (PSD). In contrast to Plasmodium, yeast sequentially methylate PtdEtn via the action of the phospholipid methyltransferases, PEM1 and PEM2, to produce PtdCho. The black arrows show the parasite pathways, and the gray arrows show the yeast pathways. The enzymes catalyzing each step of the pathway are enclosed by boxes.