Malbrancheamide is a dichlorinated fungal indole alkaloid isolated from both Malbranchea aurantiaca and Malbranchea graminicola that belongs to a family of natural products containing a characteristic bicyclo[2.2.2]diazaoctane core. The introduction of chlorine atoms on the indole ring of malbrancheamide differentiates it from other members of this family and contributes significantly to its biological activity. In this study, we characterized the two flavin-dependent halogenases involved in the late-stage halogenation of malbrancheamide in two different fungal strains. MalA and MalA’ catalyze the iterative dichlorination and monobromination of the free substrate premalbrancheamide as the final steps in the malbrancheamide biosynthetic pathway. Two unnatural bromo-chloro-malbrancheamide analogs were generated through MalA-mediated chemoenzymatic synthesis. Structural analysis and computational studies of MalA’ in complex with three substrates revealed that the enzyme represents a new class of zinc-binding flavin-dependent halogenases, and provides new insights into a potentially unique reaction mechanism.
Fungal bicyclo[2.2.2]diazaoctane indole alkaloids represent an important family of natural products with a wide-spectrum of biological activities. Although biomimetic total syntheses have been completed for representative compounds, the details of their biogenesis, especially the mechanisms for assembly of diastereomerically distinct and enantiomerically antipodal metabolites, have remained largely uncharacterized. Brevianamide A represents the most basic form of the sub-family bearing a dioxopiperazine core and a rare 3-spiro-ψ-indoxyl skeleton. Here, we identified the Brevianamide A biosynthetic gene cluster from Penicillium brevicompactum NRRL 864 and fully elucidated the metabolic pathway by gene disruption, heterologous expression, precursor incorporation experiments, and in vitro biochemical analysis. In particular, we determined BvnE as a cofactor-independent isomerase/pinacolase that is essential for selective production of Brevianamide A. Structural elucidation, molecular modeling, and mutational analysis of BvnE, and quantum chemical calculations provided critical mechanistic insights into the diastereoselective formation of the 3-spiro-ψ-indoxyl moiety in Brevianamide A. This occurs through a BvnE-controlled semi-pinacol rearrangement and a subsequent spontaneous intramolecular [4+2] hetero-Diels-Alder cycloaddition. Resolution of this 50-year old mechanistic mystery together with our recent characterization of the Diels-Alderase-mediated biogenesis of monooxopiperazines highlight the diversified biosynthetic strategies deployed by fungi for creating structurally diverse spiro-cyclized indole alkaloids.Fungal indole alkaloids bearing the unusual bicyclo[2.2.2]diazaoctane core have drawn considerable attention from natural product, synthetic and biological chemists for decades. A wealth of studies on the discovery of analogs (including semi-synthetic, synthetic and natural), biological activities and biosynthetic mechanisms have been
Various fungi of the genera Aspergillus, Penicillium, and Malbranchea produce prenylated indole alkaloids possessing a bicyclo[2.2.2]diazaoctane ring system. After the discovery of distinct enantiomers of the natural alkaloids stephacidin A and notoamide B, from A. protuberus MF297-2 and A. amoenus NRRL 35660, another fungi, A. taichungensis, was found to produce their diastereomers, 6-epi-stephacidin A and versicolamide B, as major metabolites. Distinct enantiomers of stephacidin A and 6-epi-stephacidin A may be derived from a common precursor, notoamide S, by enzymes that form a bicyclo[2.2.2] diazaoctane core via a putative intramolecular hetero-Diels–Alder cycloaddition. This review provides our current understanding of the structural and stereochemical homologies and disparities of these alkaloids. Through the deployment of biomimetic syntheses, whole-genome sequencing, and biochemical studies, a unified biogenesis of both the dioxopiperazine and the monooxopiperazine families of prenylated indole alkaloids constituted of bicyclo[2.2.2]diazaoctane ring systems is presented.
The paraherquamides are potent anthelmintic natural products with complex heptacyclic scaffolds.One key feature of these molecules is the spiro-oxindole moiety that lends a strained threedimensional architecture to these structures. The flavin monooxygenase PhqK was found to catalyze spirocycle formation through two parallel pathways in the biosynthesis of paraherquamides A and G. Two new paraherquamides (K and L) were isolated from a ΔphqK strain of Penicillium simplicissimum, and subsequent enzymatic reactions with these compounds generated two additional metabolites paraherquamides M and N. Crystal structures of PhqK in complex with various substrates provided a foundation for mechanistic analyses and computational studies. While it is evident that PhqK can react with various substrates, reaction kinetics and molecular dynamics simulations indicated that the dioxepin-containing paraherquamide L was the favored substrate. Through this effort, we have elucidated a key step in the biosynthesis of the paraherquamides, and provided a rationale for the selective spirocyclization of these powerful anthelmintic agents.
Prenylated indole alkaloids isolated from various fungi possess great structural diversity and pharmaceutical utility. Among them are the calmodulin inhibitory malbrancheamides and paraherquamides, used as anthelmintics in animal health. Herein, we report complete elucidation of the malbrancheamide biosynthetic pathway accomplished through complementary approaches. These include a biomimetic total synthesis to access the natural alkaloid and biosynthetic intermediates in racemic form, and in vitro enzymatic reconstitution that provides access to the natural antipode (+)-malbrancheamide. Reductive cleavage of a L-Pro-L-Trp dipeptide from the MalG nonribosomal peptide synthetase (NRPS) followed by reverse prenylation and a cascade of post-NRPS reactions culminates in an intramolecular [4+2] hetero-Diels-Alder (IMDA) cyclization to furnish the bicyclo[2.2.2]diazaoctane scaffold. Enzymatic assembly of optically pure (+)-premalbrancheamide involves an unexpected zwitterionic intermediate where MalC catalyzes enantioselective cycloaddition as a bifunctional NADPHdependent reductase/Diels-Alderase. Crystal structures of substrate and product complexes together with site-directed mutagenesis and molecular dynamics simulations demonstrated how MalC and PhqE, its homolog from the paraherquamide pathway, catalyze diastereo-and enantioselective cyclization in the construction of this fascinating class of secondary metabolites.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.