Hyperforin is the prototype molecule of polycyclic polyprenylated acylphloroglucinols (PPAPs) with a bicyclo[3.3.1]nonane core. PPAPs comprise bridged bicyclic compounds and caged tri/tetracyclic substances. They exhibit intriguing pharmacological activities. However, low natural abundance and difficult chemical synthesis limit their preclinical research.
A promising alternative is the reconstruction of PPAP biosyntheses in microorganisms, once the pathways are elucidated. A phloroglucinol intermediate, formed via the type III polyketide route, undergoes stepwise prenylation and concomitant cyclization reactions. The prenyltransferases involved are membrane-integrated UbiA superfamily members. They feature 7-9 transmembrane helices and share two conserved aspartate-rich motifs.
While the acylphloroglucinol scaffold can be produced in both bacteria and yeast, only the latter can serve as host for expression of the membrane-integrated prenyltransferases, preferably as transit peptide-truncated enzymes and fusion proteins. For example, three consecutive prenyltransferases which catalyze geminal diprenylation and reverse prenylation in xanthone metabolism were co-expressed in the yeast strain AE9G, engineered for terpene production, i.e. increased prenyl donor availability. Upon feeding of 1,3,6,7-tetrahydroxyxanthone, antibacterial hyperixanthone A was formed as intracellular product. The pathway providing the xanthone as prenyl acceptor is being established.
Tailored microbial cell factories expressing the reconstructed plant pathways hold promise for sustainable production of complex PPAPs for preclinical development.