Background: Type III polyketide synthases (PKSs) contribute to the synthesis of many economically important natural products, which are typically produced by direct extraction from plants or synthesized chemically. For example, humulone and lupulone (Fig. 1a) in hops (Humulus lupulus) account for the characteristic bitter taste of beer and display multiple pharmacological effects. 4-Hydroxy-6-methyl-2-pyrone is a precursor of parasorboside contributing to insect and disease resistance of plant Gerbera hybrida, and was recently demonstrated to be a potential platform chemical. Fig. 1 Examples of phloroglucinols (a) and 2-pyrones (b) synthesized by type III PKS. PIBP phlorisobutyrophenone; PIVP phlorisovalerophenone; TAL 4-hydroxy-6-methyl-2-pyrone (triacetic acid lactone); HIPP 4-hydroxy-6-isopropyl-2-pyrone; HIBP 4-hydroxy-6-isobutyl-2-pyrone
Results: In this study, we achieved simultaneous biosynthesis of phlorisovalerophenone, a key intermediate of humulone biosynthesis and 4-hydroxy-6-isobutyl-2-pyrone in Escherichia coli from glucose. First, we constructed a biosynthetic pathway of isovaleryl-CoA via hydroxy-3-methylglutaryl CoA followed by dehydration, decarboxylation and reduction in E. coli. Subsequently, the type III PKSs valerophenone synthase or chalcone synthase from plants were introduced into the above E. coli strain, to produce phlorisovalerophenone and 4-hydroxy-6-isobutyl-2-pyrone at the highest titers of 6.4 or 66.5 mg/L, respectively.
Conclusions: The report of biosynthesis of phlorisovalerophenone and 4-hydroxy-6-isobutyl-2-pyrone in E. coli adds a new example to the list of valuable compounds synthesized in E. coli from renewable carbon resources by type III PKSs.
Keywords: 4-Hydroxy-6-isobutyl-2-pyrone; Escherichia coli; Isovaleryl-CoA; Phlorisovalerophenone; Valerophenone synthase.