Sphingomyelin and ceramide metabolism
Sphingolipids are a class of lipids which possesses sphingoid bases such as sphingosine as backbone. Like other membrane phospholipids, sphingolipids are ampipathic possessing hydrophilic heads and hydrophobic tails. Sphingolipids are important in various cell signalling processes as first and second messengers including cell proliferation, differentiation and apoptosis in higher eukaryotes. They are also important constituents of lipid rafts of cell membranes. The simplest sphingolipids is ceramide which has very limited hydrophilicity as it only has 2 hydroxyl groups and no other polar groups are present. In addition to its role in signalling processes, it is also the precursor for the biosynthesis of other complex sphingolipids . Animals possess two different types of these complex sphingolipids, which are sphiongophospholipids and glycosphingolipids respectively. The main sphingophospholipid in mammals is sphingomyelin. Sphingomyelin possesses a phosphocholine group attached to the ceramide backbone. The glycosphingolipids are molecules which have sugar residues attached to the ceramide backbone. The three different types of glycosphingolipids are cerebrosides (1 sugar moiety attached to hydroxyl-group in ceramide), globosides (more than 1 sugar groups attached to ceramide) and gangliosides (at least 1 N-acetylneuraminic acid residue is attached to the sugar chain). In contrast to animals, inositol phosphorylceramide is generated from ceramide in plants, fungi and kinetoplastids such as Trypanosoma and Leishmania .
The apicomplexan parasites Plasmodium falciparum and Toxoplasma gondii can de novo synthesise sphingomyelin and glucosylceramide. In addition, they also utilise ceramide derived from host sphingolipids via the action of the enzyme sphingomyelinase (sphingomyelin phosphodiesterase). There are biochemical evidences available for P. falciparum to suggest the importance of both de novo synthesis [3, 4] and salvage [4, 5] mechanisms. The depletion of glucosylceramides and inhibition of intra-erythrocyte parasite growth with ceramide glucosyltransferase (glucosylceramide synthase) inhibitor threo-PPMP in P. falciparum suggests the importance of glucosylceramide synthesis .
The analysis of Piroplasma genomes shows that they possess the enzymes for the de novo synthesis of sphingomyelin and glucosylceramide from sphinganine. The sphingolipid desaturase, the oxidoreductase enzyme in the ceramide synthesis is missing in the gene models as in P. falciparum. There are two different enzymes which catalyse the synthesis of sphingomyelin from ceramide are present among apicomplexans. They are sphingomyelin synthase and ceramide cholinephosphotransferase. Of these sphingomyelin synthase is present in Theileria parva and Theileria annulata and ceramide cholinephosphotransferase is present in T. annulata and Babesia bovis. The enzyme sphingomyelinase catalysing salvage of host sphingomyelin into ceramide is also present in these Piroplasma species. These suggest that Piroplasma species can synthesise sphingomyelin in addition to the salvage from host.
|Enzyme||EC Number||Gene id|
|Oxidoreductase||1.14.-.-||Missing in annotation|
Sources and fates of metabolites
|Substrate||Source pathways||Product||Fate pathways|
|CDP-Choline||Phosphatidylcholine metabolism||Choline phosphate||Phosphatidylcholine metabolism|
|Phosphatidylcholine||Phosphatidylcholine metabolism||1,2-Diacylglycerol||Phosphatidylethanolamine metabolism|