Hypericin, a naphthodianthrone present in the medicinal plant Hypericum perforatum (Hypericaceae), was proposed to be biosynthesized via the acetate-malonate (polyketide) pathway. One molecule of acetyl-CoA condenses with seven molecules of malonyl-CoA, yielding a linear unstable octaketide, which undergoes cyclization and decarboxylation to produce atrochrysone and/or emodin anthrone; hypericin precursors. This reaction is believed to be catalyzed in vivo by an octaketide synthase (OKS) either alone or together with tailoring enzyme/s, as octaketide cyclase (OKC). OKS-encoding transcripts from H. perforatum [1], Aloe arborescens [2] and Polygonum cuspidatum (unpublished) were cloned and heterologously expressed in Escherichia coli. All three recombinant OKSs formed a linear octaketide which, was incorrectly folded and cyclized to give two aromatic octaketides (SEK4 and SEK4b) under in vitro conditions [Fig. 1
]. Conversely, we detected the functional biosynthesis of anthranoids in cell-free protein extracts from yeast-extract-treated Cassia bicapsularis (Fabaceae) cell cultures without shunt products formation [3].
Our aim is to study at the biochemical and molecular biological levels the OKS enzyme and its interaction with accessory factor/s (tailoring enzyme/s) in the anthranoid scaffold biosynthesis, as exemplified by cannabinoid biosynthesis [4].
We transiently expressed H. perforatum OKS in Nicotiana benthamiana. Furthermore, a number of putative tailoring enzymes required for anthranoid scaffold formation were cloned from H. perforatum. Co-expression of HpOKS and these recently cloned enzymes in N. benthamiana is under investigation. Using three publicly available Cassia transcriptomes, we were able to identify a number of PKSs that are undergoing cloning and functional analysis to test them as potentially anthranoid-forming enzymes.
Fig. 1 In vitro and in vivo products resulting from the OKS reaction.
