PS synthase

Figure 1. Synthetic pathway for PS and PE in mammalian cells. The major steps occuring in the synthesis and interconversion of PS and PE are shown. The PS synthases condense serine with a phosphatidyl moiety derived from PC and PE. The nascent PS can be converted to PE by decarboxylation. PE can also be formed by transfer of a phosphoethanolamine moiety from CDP-ethanolamine to diacylglycerol via the Kennedy pathway. The abbreviations used are PC, phosphatidylcholine PS, phosphatidylserine PE, phosphatidylethanolamine DG, diacylglycerol PSD, phosphatidylserine decarboxylase PSS, PS synthase.

Fig. 3. Molecular models for the conversion of di-DHA phosphatidyl ethanolamine or di-DPA (n-6) phosphatidylethanolamine (PE) into phosphatidylserine (PS) by base exchange catalyzed by PS synthase. Molecular models were computed by energy minimization of the phospholipid structure given the di-DHA PE structure.

Although PS accounts for less than 5% of Plasmodium lipids, it is a key intermediate in the synthesis of PE, which serves as a precursor to PC (see above). PS is synthesized by a condensation of serine with CDP-DAG catalyzed by PS synthase (PSS). Initially, PSS was characterized biochemically in P. knowlesi and later P. falciparum (Elabbadi and Vial, personal communication). Two putative genes that possibly encode PSS have been identified in the P. falciparum genome base and a recombinant protein has been made. In P. falciparum, the putative PSS gene is transcribed during the ring and trophozoite stages. PS decarboxylation via PS decarboxylase (PSD) was first demonstrated in P. knowlesi-miected red cells where it was found that radiolabeled serine was incorporated into Etn (Elabbadi et ah, 1997). 

P-D-glucoside). Treatment with arbutin (an MDO substrate analog) causes a 7-fold increase in the rate of PtdGro synthesis without a concomitant increase in PtdGroP synthase proteins or significant changes in membrane phospholipid composition. Thus, PS synthase... 

Fig. 7. Pathways for the biosynthesis of phosphatidylethanolamine (PE) and phosphatidylseiine (PS) in animals. The numbers indicate the enzymes. 1, ethanolamine kinase 2, CTP phosphoethanolamine cytidylyltransferase 3, CDP-ethanolamine l,2-diacylglycerol ethanolaminephosphotransferase 4, PS synthase-2 5, PS synthase-1 6, PS decarboxylase 7, acyl-CoA lyso-PE acyltransferase. PC, phosphatidylcholine.

Mammalian cDNAs encoding the two isoforms of PS synthase were subsequently cloned. The amino acid sequences of PS synthase-1 and PS synthase-2 are -30% identical and each protein is predicted to contain multiple membrane-spanning domains. As is the case for many lipid biosynthetic enzymes, the serine-exchange activity in mammalian cells is located on microsomal membranes. However, the majority of this activity is not in the bulk of the ER but is highly enriched in MAM, a domain of the ER that has been implicated in the import of PS into mitochondria (J.E. Vance, 2000) (Chapter 16). Although little is known about the active site of either PS synthase, several amino acids that are crucial for serine-exchange activity have been identified (M. Nishijima, 2004). Over-expression of PS synthase-2 in PS synthase-1-deficient CHO cells eliminated the requirement for exogenously added PS. Thus, in CHO cells PS synthase-2 can substitute for PS synthase-1. 

Successful isolation of these specialized ER structures, now called the MAM, indicated that they are selectively enriched in a subset of lipid synthetic enzymes, especially PS synthase (J.E. Vance, 1990). In mammalian cells, pulse-chase experiments coupled with subcellular fractionation have now established that the PS destined for the mitochondria must transit through the MAM and some step of this process requires ATP (Y. Shiao, 1995). A MAM structure has also been identified and isolated from yeast cells (G. Daum, 1997). 

Fig. 10. Genetic analysis of aminoglycerophospholipid transport in eukaryotes. The transport of PS synthesized in the ER is regulated by PSTA and PSTB genes. The acronym stands for PS transport (either A or B pathways). Likewise, the transport of PE synthesized in the mitochondria or Golgi/vacuole is proposed to be regulated by PEEA and PEEB genes. The acronyms stand for PE export (either A or B pathways). Both known and proposed mutations along the metabolic and transport pathways appear in lower case italics. The table summarizes the mutants, genes, and proteins that have been identified in Chinese hamster ovary (CHO) cells and yeast. Other abbreviations pss, PS synthase psd, PS decarboxylase pern, PE methyltransferase Cho, choline Etn, ethanolamine Ser, serine.

The main difference between animals/plants and yeast is the PS synthesis pathway. In the former, PS species are synthesized from PC and PE species through serine exchange with choline and ethanolamine by PS synthase 1 and 2, respectively, in... 

PS species in the yeasts are mainly synthesized by the CDP-DAG pathway and not by PS synthase from PE species. 

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