FIG. 6.
Proposed mechanism for ZEA biosynthesis by ZEA1p and ZEA2p. For step 1, the first 10 carbon additions are catalyzed by ZEA2p, the reducing PKS. As shown here, the chain may be subsequently transferred to the ACP domain of ZEA1p. For step 2, the remaining three rounds of C2 additions are catalyzed by ZEA1p, the nonreducing PKS. For step 3, the unreduced ketones in the portion of the molecule are highly reactive and are spontaneously aromatized. For step 4, the lactone ring is completed with the subsequent release of ZEA1p to yield the completed ZEA molecule.