one of the proteins of the complex in cytoplasm that contains all of the enzymes required to convert acetyl-CoA (and, in certain cases, butyryl-CoA or propionyl-CoA) and malonyl-CoA to palmitic acid. This complex is tightly bound together in mammalian tissues and in yeast, but that from Escherichia coli is readily dissociated. The ACP thus isolated is a heat-stable protein with a molecular weight of about 10,000. It contains a free -SH that binds the acyl intermediates in the synthesis of fatty acids as thioesters. This -SH group is part of a 4′-phosphopantetheine, added to the apoprotein by ACP phosphodiesterase, which thus plays the same role that it does in coenzyme A. ACP is involved in every step of the fatty acid synthetic process.