Figure 2.

Biosynthetic pathways of aminoglycerophospholipids. (A) Biosynthesis of aminoglycerophospholipids in yeast. Formation of PS is accomplished in the ER and catalyzed in a CDP-DAG-dependent reaction by the PS synthase, Pss1p. Decarboxylation of PS yielding PE occurs in mitochondria (via Psd1p) and a Golgi/vacuolar compartment (via Psd2p). Three-step methylation of PE in the ER catalyzed by Cho2p/Pem1p and Opi3p/Pem2p leads to formation of PC with S-adenosine methionine (SAM) as methyl donor. PC and PE can be formed also by the CDP-ethanolamine and CDP-choline branches of the so-called Kennedy pathway making use of exogenous or endogenous choline (Cho) and ethanolamine (Etn), respectively. SL, sphingolipids. (B) Aminoglycerophospholipid biosynthesis in mammalian cells. The major mechanism of PS production in mammals is base exchange with PC (PSS1) and PE (PSS2) as substrates. A major route of PE and PC production are the CDP-ethanolamine and CDP-choline pathways. In mammalian cells, PE can also be produced by decarboxylation of PS. In hepatocytes, a single methyltransferase catalyzes methylation of PE to PC. (C) The Land’s cycle describes a sequence of deacylation and reacylation. PE and PC are converted to lyso-PE and lyso-PC and vice versa.