Extreme promiscuity of a bacterial and a plant diterpene synthase enables combinatorial biosynthesis

Releasing the potential power of terpene synthases by a robust precursor supply platform

Terpenoids represent the largest family of natural products. Their structural diversity is largely due to variable skeletons generated by terpene synthases. However, terpene skeletons found in nature are much more than those generated from known terpene synthases. Most promiscuous terpene synthases (i.e. those that can generate more than one product) have not been comprehensively characterised. Here, we first demonstrated that the promiscuous terpene synthases can produce more variable terpenoids in vivo by converting precursor polyisoprenoid diphosphates of different lengths (C₁₀, C₁₅, C₂₀, C₂₅). To release the synthetic potential of these enzymes, we integrated the engineered MVA pathway, combinatorial biosynthesis, and point mutagenesis to depict the comprehensive product profiles. In total, eight new terpenoids were characterised by NMR and three new skeletons were revealed. This work highlights the key role of metabolic engineering for natural product discovery.

Total biosynthesis of the cyclic AMP booster forskolin from Coleus forskohlii

Forskolin is a unique structurally complex labdane-type diterpenoid used in the treatment of glaucoma and heart failure based on its activity as a cyclic AMP booster. Commercial production of forskolin relies exclusively on extraction from its only known natural source, the plant Coleus forskohlii, in which forskolin accumulates in the root cork. Here, we report the discovery of five cytochrome P450s and two acetyltransferases which catalyze a cascade of reactions converting the forskolin precursor 13R-manoyl oxide into forskolin and a diverse array of additional labdane-type diterpenoids. A minimal set of three P450s in combination with a single acetyl transferase was identified that catalyzes the conversion of 13R-manoyl oxide into forskolin as demonstrated by transient expression in Nicotiana benthamiana. The entire pathway for forskolin production from glucose encompassing expression of nine genes was stably integrated into Saccharomyces cerevisiae and afforded forskolin titers of 40 mg/L.

A (-)-kolavenyl diphosphate synthase catalyzes the first step of salvinorin A biosynthesis in Salvia divinorum

Salvia divinorum (Lamiaceae) is an annual herb used by indigenous cultures of Mexico for medicinal and ritual purposes. The biosynthesis of salvinorin A, its major bioactive neo-clerodane diterpenoid, remains virtually unknown. This investigation aimed to identify the enzyme that catalyzes the first reaction of salvinorin A biosynthesis, the formation of (-)-kolavenyl diphosphate [(-)-KPP], which is subsequently dephosphorylated to afford (-)-kolavenol. Peltate glandular trichomes were identified as the major and perhaps exclusive site of salvinorin accumulation in S. divinorum. The trichome-specific transcriptome was used to identify candidate diterpene synthases (diTPSs). In vitro and in planta characterization of a class II diTPS designated as SdKPS confirmed its activity as (-)-KPP synthase and its involvement in salvinorin A biosynthesis. Mutation of a phenylalanine into histidine in the active site of SdKPS completely converts the product from (-)-KPP into ent-copalyl diphosphate. Structural elements were identified that mediate the natural formation of the neo-clerodane backbone by this enzyme and suggest how SdKPS and other diTPSs may have evolved from ent-copalyl diphosphate synthase.