Fig. 3.

Generic reaction scheme for FASs and PKSs. The starter substrate for FASs typically is an acetyl moiety and the chain extender a malonyl moiety, whereas for PKSs, the starter is typically either acetyl or propionyl and the extender malonyl or methylmalonyl. In the FASs the β-ketoacyl moiety produced following each chain-extension reaction is completely reduced to a saturated carbon prior to the next condensation. In the modular PKSs, the β-ketoacyl moiety may undergo complete or partial reduction, leading to the formation of products that retain keto, hydroxyl or enoyl groups along the acyl chain.