English: Type II PKS ACPs are small, helical proteins that are activated upon installation of an 18-Å phosphopantetheine arm. A, the Ppant arm is derived from coenzyme A and installed onto a conserved serine at the Nterminus of helix II via a phosphopantetheinyl transferase. B, the helical structure is a conserved feature of type II PKS ACPs (shown here is the holo-ACP from the actinorhodin biosynthetic pathway; PDB: 2K0X). Helices I-IV are depicted (I: dark blue, II: light blue, III: gray, IV: purple along with loops I-II in teal with the Ppant arm colored by the element). ACP, acyl carrier protein; Ppant, phosphopantetheine; PKS, polyketide synthase.