Polyketides are a large group of secondary metabolites which either contain alternating carbonyl groups and methylene groups, or are derived from precursors which contain such alternating groups. Many polyketides have antimicrobial and immunosuppressive properties. Many mycotoxins produced by fungi are polyketides.
Biosynthesis
Polyketides are synthesized in bacteria, fungi, plants, and certain marine animals by the stepwise condensation of acetyl-CoA or propionyl-CoA with malonyl-CoA or methylmalonyl-CoA extender units. Each condensation reaction is driven by the decarboxylation of the extender unit and yields a beta-keto functional group. from polyketide intermediate. The polyketide chains produced by a minimal polyketide synthase are often further derivatized and modified into bioactivenatural products. Palmitic acid is an example of a polyketide since it is formed by the condensation of one acetyl-CoA primer and seven malonyl-CoA extender units. Following each condensation reaction, the new beta-keto group may be reduced, dehydrated, and reduced again as with fatty acids and therefore the final product may not contain any actual ketide subunits itself. Polyketides are synthesized by multienzyme polypeptides that resemble eukaryotic fatty acid synthase but are often much larger. They include acyl-carrier domains plus an assortment of enzymatic units that can function in an iterative fashion, repeating the same elongation/modification steps, or in a sequential fashion so as to generate more heterogeneous types of polyketides. Polyketides are structurally a very diverse family of natural products with diverse biological activities and pharmacological properties. They are broadly divided into three classes: type I polyketides, type II polyketides, and type III polyketides. Polyketide antibiotics, antifungals, cytostatics, anticholesteremic, antiparasitics, coccidiostats, animal growth promoters and natural insecticides are in commercial use.
