2. Synthesis and assignment of membrane lipids 1) Synthesis of membrane lipids Because membrane lipids are extremely hydrophobic, they are synthesized in association with already existing cellular membranes rather than in the aqueous environment of the cytosol Most membrane lipids are synthesized in ER from water-soluble cytosolic precursors(PL, cholesterol are synthesized mainly in SER, although PL may be also synthesized in rER), the. major exceptions are sphingomyelin and glycolipids, whose synthesis begins in the ER and is completed in Golgi complex. The enzymes involved in the synthesis of PL are themselves integral proteins of ER membrane with their active sites facing the cytosol, Fatty acids are first transferred from Coenzyme A carriers to glycerol-3-phosphate by a membrane-bound acyltransferase, and the resulting phospholipid (phosphatidic acid) is inserted into the membrane. It is this step that enlarges the lipid bilayer; the later steps catalyse the addition of different polar head groups, forming PL molecules, PC, PS, PE and PI. So newly synthesized PLS are inserted into the leaflet facing the cytosol
2. Synthesis and assignment of membrane lipids 1) Synthesis of membrane lipids Because membrane lipids are extremely hydrophobic, they are synthesized in association with already existing cellular membranes rather than in the aqueous environment of the cytosol. Most membrane lipids are synthesized in ER from water-soluble cytosolic precursors(PL, cholesterol are synthesized mainly in sER, although PL may be also synthesized in rER), the major exceptions are sphingomyelin and glycolipids, whose synthesis begins in the ER and is completed in Golgi complex. The enzymes involved in the synthesis of PL are themselves integral proteins of ER membrane with their active sites facing the cytosol, Fatty acids are first transferred from Coenzyme A carriers to glycerol-3-phosphate by a membrane-bound acyltransferase, and the resulting phospholipid (phosphatidic acid) is inserted into the membrane. It is this step that enlarges the lipid bilayer; the later steps catalyse the addition of different polar head groups, forming PL molecules, PC, PS, PE and PI. So newly synthesized PLs are inserted into the leaflet facing the cytosol
Protein Sorting and Transport 361 Fatty acyI C po ( CDP-choline H2-CH2-N(CH35 CH2一N(H ER lumen X Phosphatidylinositol osphatidylethanolamine phosphatidylserine
CoA CoA H2 脂酰辅酶A3-磷酸甘油 CDP胆碱 酰基① ② 转移藤 磷酸酶 画碱 磷酸转移酶 细胞质基质 cH-CH一CH 2 CH2-CH-CH2 CH2-CH-CH2 磷脂酸 二酰基甘油 磷脂酰胆碱 ER腔面
Fatty acvI CoA acyl-transferase phosphatase phosphatidic acid Glycerol phosphate →→+ Diglyceride choline phosphotransferase PC CDP-choline
Fatty acyl CoA acyl-transferase phosphatase + phosphatidic acid Glycerol phosphate Diglyceride choline phosphotransferase + PC CDP-choline
-CH,-CH,-N(CH3h CH, C=O HCOH HCOH C-o Glycolipids