Phosphatidic acid is a key precursor in lipid metabolism. A simplified outline of the major lipids originating from phosphatidic acid (PA) is shown. These include glycerophospholipids: CDP-diacylglycerol (CDP-DAG), phosphatidylserine (PS), phosphatidylethanolamine (PE), phosphatidylcholine (PC), phosphatidylinositol (PI), and phosphoinositol phosphates (PIPs); and neutral lipids: diacylglycerol (DAG) and triacylglycerol (TAG). PA is drawn in its deprotonated form carrying a charge of 2-. Its structure is composed of a phosphomonoester headgroup (shown in red) attached to a DAG backbone (highlighted in yellow). This DAG backbone is composed of glycerol with two acyl chains attached at its sn1 and sn2 positions (green numerals indicate the sn positions of glycerol, R represents the remaining structure of each acyl chain that is not shown). All glycerophospholipids have their headgroups attached to the DAG backbone at the sn3 position of glycerol. The headgroups of PS, PE, PC and PI are shown for comparison and the positions of phosphorylation of the inositol ring in PIPs are labeled (3,4,5). The actions of DAG kinase (DGK) and phospholipase D (PLD) in regulating PA levels are also indicated (shown in blue italics).