the biosynthesis of unusual extender units described ..
carboxylase in the biosynthesis of the extender units.
(2R)-Methoxymalonyl-ACP, (2R)-hydroxymalonyl-ACP, and (2S)-aminomalonyl-ACP elongate a polyketide chain with methoxyacetyl, glycolyl, or glycyl units, respectively, with the substituents at the α-carbons (methoxyl, hydroxyl, and amino groups) conferring functionalities and hydrogen-bonding potential not available through the use of the CoA-linked extender units., While it has not been described as a PKS extender unit, another ACP-linked PKS precursor worthy of discussion is glyceryl-ACP, which incorporates a glycerol-derived three-carbon unit into a tetronate ring of polyketides known as acyltetronic acids. The occurrence and biosynthesis of these four precursors is discussed below.
Biosynthesis Polyketides are ..
The next class of PKS extender units differs from the CoA-linked extender units discussed above in that they are covalently tethered to the 4′-Ppant groups of holo-ACPs (). Also, unlike malonyl-CoA, (2S)-methylmalonyl-CoA, and (2S)-ethylmalonyl-CoA, these precursors are unique to polyketide biosynthesis, with the genes coding for their biosynthesis found within or flanking the associated biosynthetic gene clusters. Although they are not nearly as common as malonyl-CoA or (2S)-methylmalonyl-CoA extender units, they are becoming more prevalent as more natural product biosynthetic pathways are elucidated. Currently, they are all found in pathways involving modular type I PKSs, although future natural product discoveries may reveal their presence in other types of PKSs as well.