Figure 1.

Synthetic pathways for polar lipids. The black arrows indicate the bacterial and eukaryotic pathway, and the red arrows denote the archaeal pathway. The red striped arrows show hypothetical steps in the archaeal pathway. The names or EC numbers in rectangles indicate the enzymes responsible for the corresponding steps. Experimentally identified archaeal enzymes are indicated by characters reverse-printed on red. Bacterial and eukaryotic enzymes belonging to the CDP alcohol phosphatidyltransferase family are shown in green rectangles. Enzymes in rectangles with blue frames are involved in the synthesis of lipids with choline. Abbreviations: DHAP = dihydroxyacetonephosphate; G3PDH = sn-glycerol-3-phosphate dehydrogenase; G1PDH = sn-glycerol-1-phosphate dehydrogenase; G3P = sn-glycerol-3-phosphate; G1P = sn-glycerol-1-phosphate; GGGPS, GGGP synthase; GGGP = 3-O-geranylgeranyl-sn-glycerol phosphate; DGGGPS, DGGGP synthase; PA, phosphatidic acid; DGGGP = 2,3-di-O-geranylgeranyl-sn-glycerol phosphate; PCS = phosphatidylcholine synthase; PSS = phosphatidylserine synthase; ASS = archaetidylserine synthase; PGS = phosphatidylglycerol synthase; AGS = archaetidylglycerol synthase; PIS = phosphatidylinositol synthase; AIS = archaetidylinositol synthase; PC = phosphatidylcholine; PS = phosphatidylserine; AS = archaetidylserine; PGP = phosphatidylglycerolphosphate; AGP = archaetidylglycerolphosphate; PI =phosphatidylinositol; AI = archaetidylinositol; PSD = phosphatidylserine decarboxylase; ASD = archaetidylserine decarboxylase; PGPP = PGP phosphatase; PE = phosphatidylethanolamine; AE = archaetidylethanolamine; PG = phosphatidylglycerol; and AG = archaetidylglycerol.