A bifunctional type III polyketide synthase from raspberry (Rubus idaeus L.) with both chalcone synthase and benzalacetone synthase activity

Raspberry ketone accounts for the characteristic aroma of the raspberry fruit. A bifunctional enzyme with both chalcone synthase (CHS) and benzalacetone synthase (BAS) activity is thought to play a crucial role in the synthesis of p-hydroxybenzalacetone, yet the in vitro enzymatic properties and reaction products of the CHS/BAS recombinant enzyme from raspberry have not been characterized. In this work, a type III polyketide synthase (PKS) gene (RinPKS1) and its corresponding cDNA were isolated from raspberry. Sequence and phylogenetic analyses demonstrated that RinPKS1 is a CHS. However, functional and enzymatic analyses showed that recombinant RinPKS1 is a bifunctional enzyme with both CHS and BAS activity. RinPKS1 showed some interesting characteristics: (1) no traces of bis-noryangonin and 4-coumaroyltriacetic acid lactone could be detected in the enzyme reaction mixture at different pH values; and (2) recombinant RinPKS1 overexpressed in Escherichia coli effectively yielded p-hydroxybenzalacetone as a dominant product at high pH; however, it effectively yielded naringenin as a dominant product at low pH. Furthermore, 4-coumaroyl-CoA and feruloyl-CoA were the only cinnamoyl-CoA derivatives accepted as starter substrates. RinPKS1 did not accept isobutyryl-CoA, isovaleryl-CoA or acetyl-CoA as substrates.