Regulation of lipid synthesis. Phosphatidic acid phosphatase is the key regulatory enzyme in lipid synthesis. When active, PAP generates diacylglycerol (DAG), which can react with activated alcohols to form phospholipids or with fatty acyl CoA to form triacylglycerols. When PAP is inactive, phosphatidate is converted into CMP-DAG for the synthesis of different phospholipids. PAP also controls the amount of DAG and phosphatidate, both of which function as second messengers.