Polyketides are a large group of secondary metabolites which either contain alternating carbonyl and methylene groups (-CO-CH2-), or are derived from precursors which contain such alternating groups. 
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 (e.g., an acetoacetyl group; this is why the ultimate product is called a polyketide). 
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 (as in fatty acid synthesis), 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 (often macrolides produced by multimodular megasynthases), type II polyketides (often aromatic molecules produced by the iterative action of dissociated enzymes), and type III polyketides (often small aromatic molecules produced by fungal species). Polyketide antibiotics, antifungals, cytostatics, anticholesteremic, antiparasitics, coccidiostats, animal growth promoters and natural insecticides are in commercial use.
- The antibiotic agent doxycycline
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