Abstract
Natural products are valuable sources for obtaining bioactive molecules. Conventionally, a tedious extraction, isolation, and purification process is required to identify compounds with a desired bioactivity. However, it has become more and more challenging to obtain structurally or functionally novel compounds, with an increasing number of molecules found. Therefore, new methodologies to discover and derivatize new natural products become urgently required. In the past decade, methodologies, such as biosynthetic engineering, genome mining, and heterologous expression, have become feasible and efficient approaches to overcome the drawbacks of traditional approaches, with an increase in publicly available bioinformatic data and progress in biological techniques nowadays.Polyketides and nonribosomal peptides are two of the major classes of natural products, often featuring highly complicated molecular skeletons, which exhibit great pharmaceutical importance. Representative polyketides include the antiparasitic avermectin, the antibiotic tetracycline, and the cholesterol-lowering medication lovastatin, whereas the antibiotics penicillins and the immunosuppressant agents cyclosporines are clinically important nonribosomal peptides.
Fungal polyketide-nonribosomal peptide hybrids (PK-NRPs) are an abundant source of bioactive compounds, which contribute to the structural complexification and diversification of natural products. Through the genome mining approach, a unique gene cluster encoding a polyketide synthase (PKS) and a nonribosomal peptide synthetase (NRPS) was characterized from the fungus Aspergillus duricaulis CBS 481.65. Utilizing the heterologous expression approach, it was found that a heterodimeric depside CJ-20,557, composed of 3-methylorsellinic acid and 3,5-dimethylorsellinic acid, was synthesized by the PKS DrcA and then transformed into a duricamidepside, a depside conjugated with an amino acid, by the NRPS DrcB. In a detailed investigation, it was determined that depside bond formation was catalyzed by the starter-unit acyltransferase (SAT) domain of DrcA, providing the first example of SAT playing this role in polyketide biosynthesis.
PK-NRPs are often biosynthesized by hybrid enzymes consisting of PKSs and NRPSs. Although PKS-NPRS hybrids are widespread in fungi, NRPS-PKSs do not occur at the same frequency as PKS-NPRSs in fungi. Up to date, only a few NRPS-PKSs have been identified and characterized. An orphan gene cluster encoding the NRPS-PKS hybrid AcdB was found in the fungus Aspergillus candidus CBS 102.13, and an alkaloid aspcandine with a new molecular skeleton was discovered. Further investigation shows that the above NRPS-PKS AcdB unusually accepts 3-hydroxy-L-kynurenine, and then a single round of chain elongation is conducted, which is followed by apparently nonenzymatic chain release to give the products.
Collectively, two unusual megasynthases were identified and characterized, with two novel metabolites from fungi successfully discovered. Therefore, the methodology utilized in this work was quite valuable and efficient in finding diversified natural products, thus accelerating the expansion of natural product diversity.
| Date of Award | 23 Aug 2023 |
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| Original language | English |
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| Supervisor | Yudai MATSUDA (Supervisor) |