In 1893, Collie isolated the first polyketide (PK), orcinol, and originally proposed the term polyketide to represent these natural products derived from CH₂-CO building blocks ¹. Since then, over 10,000 PKs consisting of macrolides, polyphenols, enediynes, polyethers and polyenes have been discovered from nature. Most PKs have intriguing pharmaceutical activities, and thus have become important sources of novel therapeutics ², of which more than 20 are commercial drugs with annual sales reaching over 20 billion dollars ³.

In nature, PKs are synthesized by polyketide synthase (PKS), and so far, three different types of PKSs have been discovered and classified as type I, II and III ². No matter what type of PKS is used, the formation of core PK backbones always relies on iterative decarboxylative Claisen condensations catalysed by the core KS domain, with potential modifications catalysed by other modifying domains ⁴. Nevertheless, such PKSs-mediated catalysis still suffer from a series of drawbacks, including complicated architecture, energy inefficiencies, tight regulation, and competition with growth-essential metabolic pathways (e.g. phospholipids biosynthesis) for extender unit malonyl-CoA (C3). Moreover, even though the activated C3 malonyl thioester is used as extender during condensation, only an acetyl (C2) unit is incorporated into the PK backbone, with the remaining C1 unit lost in the form of greenhouse gas of CO₂ (Fig. 1). This obviously compromises the atom economy and can potentially cause an environmental issue in industrial biomanufacturing.

Therefore, a question that always drew our attention was, could we conceive a new non-decarboxylative Claisen condensation platform for PK backbone formation? Specifically, in the new platform, the C2 acetyl unit will be directly incorporated into the PK backbone, cutting out the middleman malonyl-CoA.

Based on this idea, we next searched for potential qualified candidates. Thiolases are deemed as near relatives of KSs, which from prior studies were known to reversibly condense acetyl-CoA with fatty acyl-CoA to form 3-oxoacyl-CoA (type I), or condense two units of acetyl-CoA to form acetoacetyl CoA (type II) ⁵. We hypothesized, if certain thiolases are capable of recognizing polyketoacyl-CoAs (e.g. acetoacetyl CoA or 3-oxoacyl-CoA) as starters to iteratively catalyse condensations with acetyl-CoA units, the expected PK backbones will be formed. We termed such thiolases polyketoacyl-CoA thiolase (PKT) (Fig. 1).

Fig. 1 The PK backbones formed by PKS and PKT

In our experimental work, we successfully demonstrated the feasibility of using PKT to catalyse the formation of polyketoacyl-CoA backbones through the synthesis of representative PKs such as lactones (triacetic acid lactone), alkylresorcinolic acids (orsellinic acid), alkylresorcinols (orcinol), hydroxybenzoic acid (6-MSA) and alkylphenol (m-cresol). Compared with PKS, the PKT platform saves ATP and carbon, and also reduces the number of catalytic steps from central metabolites to the final PKs. More importantly, as the PKT platform operates on the CoA-bound system, the AT and ACP domains in natural PKSs become dispensable, which greatly simplify the organization and architecture for PK synthesis through PKTs in comparison to PKSs.

Our study not only conceived a new platform for the formation of PK backbones, but also serves as a good example of exploring familiar enzymes in unfamiliar roles. We also hope to highlight that, even highly evolved proteins can still be exploid for performing novel, non-native biochemical functions.

If you are interested in our work, please find our article at "A polyketoacyl-CoA thiolase-dependent pathway for the synthesis of polyketide backbones" in Nature Catalysis (https://www.nature.com/articles/s41929-020-0471-8). 

References

1. Collie, N. & Myers, W. VII.—The formation of orcinol and other condensation products from dehydracetic acid. Journal of the Chemical Society, Transactions 63, 122-128 (1893).
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4. Nivina, A., Yuet, K.P., Hsu, J. & Khosla, C. Evolution and Diversity of Assembly-Line Polyketide Synthases. Chem Rev 119, 12524-12547 (2019).
5. Jiang, C., Kim, S.Y. & Suh, D.Y. Divergent evolution of the thiolase superfamily and chalcone synthase family. Molecular phylogenetics and evolution 49, 691-701 (2008).