Despite the substantial differences between these two classes of natural products, each is synthesized biologically under the control of exceptionally large, multifunctional proteins termed polyketide synthases (PKSs) and non-ribosomal
peptide synthetases (NRPSs) that contain repeated, coordinated groups of active sites called modules, in which each module is responsible for catalysis of one complete cycle of polyketide or polypeptide chain elongation and associated functional group modifications.