Enzymatic Cycloaddition Reactions in Natural Product Biosynthesis

Enzymatic cycloaddition reactions form a cornerstone of natural product biosynthesis by enabling the rapid construction of complex ring systems with precise regio- and stereocontrol. These pericyclic processes, most notably the [4+2] Diels–Alder cycloaddition, are catalysed by specialised enzymes, often termed pericyclases or Diels–Alderases, that bring reactive diene and dienophile substrates into close proximity within a tailored active site. The resulting scaffolds underpin the biological activities of diverse secondary metabolites, including antibiotics, antifungals and anticancer agents. Recent advances have revealed that such enzymes are more widespread than previously appreciated, encompassing intermolecular as well as intramolecular cycloadditions across polyketide, terpene and hybrid pathways. Structural, mechanistic and evolutionary investigations have begun to uncover how simple ancestral enzymes evolve novel binding cavities and catalytic residues to stabilise transient pericyclic transition states. Understanding these principles opens avenues for genome mining, enzyme engineering and the development of sustainable biocatalytic routes to high-value cyclic molecules.

Research from Nature Portfolio

Recent studies have elucidated the evolutionary emergence and mechanistic principles of natural Diels–Alderases. Work on plant-derived enzymes demonstrated how a flavin-dependent oxidocyclase ancestor acquired key substrate-binding substitutions to catalyse intermolecular [4+2] cycloadditions in Morus alba, revealing the structural basis for new enzymatic functions. Complementing this, full reconstitution of a fungal terpene pathway identified a norbornene-forming synthase that uses oxidative tailoring to activate a hydrocarbon precursor for intramolecular cycloaddition. High-resolution structures of the enzyme–substrate complex showed how the protein accelerates ring closure to suppress competing redox side-reactions. Together, these findings define common strategies by which pericyclases co-opt existing enzyme scaffolds to extend the chemical repertoire of natural products.

Enzymatic Cycloaddition Reactions in Natural Product Biosynthesis publication trend

The graph below shows the total number of articles in enzymatic cycloaddition reactions in natural product biosynthesis across all publications each year (not limited to Nature Index journals).

Technical terms

Cycloaddition reaction: A pericyclic process in which two unsaturated reactants form a cyclic adduct by simultaneous bonding; exemplified by [4+2] Diels–Alder reactions.

Diels–Alderase: An enzyme that catalyses [4+2] cycloaddition reactions, typically between a conjugated diene and a dienophile, to form six-membered rings.

Pericyclase: A class of enzymes that catalyse pericyclic reactions, encompassing Diels–Alderases and other cycloaddition or rearrangement catalysts.

Diene: An organic molecule containing two conjugated double bonds that serves as one reactant in [4+2] cycloadditions.

Dienophile: An electron-deficient unsaturated compound that reacts with a diene in [4+2] cycloadditions.

Polyketide synthase (PKS): A multi-domain enzyme complex that assembles polyketide backbones through successive condensations of simple acyl building blocks.

Spirotetronate skeleton: A bicyclic motif featuring a tetrahydrofuran fused to a spirocyclic lactone, commonly found in antibiotic natural products.

References

1. The evolutionary origin of naturally occurring intermolecular Diels-Alderases from Morus alba. Nature Communications (2024).
2. Discovery and characterization of a terpene biosynthetic pathway featuring a norbornene-forming Diels-Alderase. Nature Communications (2022).
3. Delineation of the complete reaction cycle of a natural Diels–Alderase. Chemical Science (2024).
4. Crystal structure of the putative cyclase IdmH from the indanomycin nonribosomal peptide synthase/polyketide synthase. IUCrJ (2019).
5. Discovery and investigation of natural Diels–Alderases. Journal of Natural Medicines (2021).