Identification of the First Stand-Alone Intermolecular Diels–Alderase

Key words

Diels–Alder reaction - Diels–Alderase - [4+2] cycloaddition - chalcomoracin - flavonoids

Significance

The groups of Lei, Dai, and Huang report the discovery and characterization of the first naturally occurring stand-alone intermolecular Diels–Alderase. Isolated from mulberry trees by using a biosynthetic intermediate probe-based target identification strategy, the Morus alba Diels–Alderase (MaDA) was shown to catalyze a [4+2]-cycloaddition between a moracin-derived diene and moralchalcone to complete the biosynthesis of the isoprenylated flavonoid chalcomoracin. The reaction proceeds with high efficiency as well as with excellent diastereo- and enantioselectivity. Notably, the enzyme was shown to be promiscuous towards both dienes and dienophiles thus providing access to other natural products and nonnatural derivatives containing the methylcyclohexene motif. The broad applicability of MaDA was showcased in chemoenzymatic total syntheses of five other natural products found in Morus species.

Comment

The majority of enzymes capable of catalyzing Diels–Alder reactions are multifunctional or act through a stepwise rather than the characteristic concerted mechanism. Almost a decade after the discovery of the first stand-alone intramolecular [4+2]-cyclase, SpnF (H. K. Kim, M. W. Ruszczycky, S-h. Choi, Y-n. Liu, H-w. Liu Nature 2011, 473, 109), the authors have identified an intermolecular [4+2] cycloaddition catalyzed by a flavin adenine dinucleotide (FAD)-dependent Diels–Alderase. Density functional theory calculations and kinetic isotope effect studies suggest that the reaction proceeds by a concerted asynchronous pathway. Control experiments indicated that the presence of the FAD cofactor is crucial for the catalytic activity of the enzyme. The authors propose that the function of the FAD cofactor might be related to sustaining substrate binding through hydrogen-bonding interactions. Identification of the exact role of the FAD cofactor is keenly anticipated.

Publication History

Article published online:
18 August 2020