There are two types of fatty acid synthase systems. The type I system is found in metazoans and is carried outby a multifunctional polypeptide with multiple active sites. In contrast, the type II system found in bacteria and plantsconsists of a set of discrete monofunctional proteins, each encoded by a separate gene. ACP1 is central to both of thesepathways because it functions to ferry the pathway intermediates between active site centres or enzymes. ACPs are alsocritical to the function of other metabolic pathways such as polyketide synthases. The type II fatty acid synthase ACPs are abundant, small, acidic proteins that carry the acyl intermediates attached as thioesters to the terminus of the 4'-phosphopantetheine prosthetic group. This prosthetic group is added post-translationally to apoACP by holo-(acyl carrier protein) synthase (AcpS), which transfers the 4'-phosphopantetheine moiety of CoA to a serine reidue of apoACP.The crystal structures of a number of the type II fatty acid synthase ACPs have been determined. The structures reveal a novel trimeric arrangement of molecules resulting in three active sites [, ].
