Flavonoids are polyphenolic natural products predominantly isolated from plants and exhibit a diverse array of biological activities. Because they serve as important pharmaceuticals and nutraceuticals, there is a strong demand for their sustainable supply. Chlorflavonin is a rare fungal flavonoid with potent antitubercular activity. While its biosynthetic enzymes are expected to be valuable tools for application in fungal production of flavonoids, the biosynthetic pathway remains unknown. Here, we elucidate the complete biosynthetic pathway for chlorflavonin through detailed functional analysis of each biosynthetic enzyme. Previously, stepwise and straightforward introduction of 3-, 7-, and 8-methoxy; 2'-hydroxy; and 3'-chloro functionalities have been proposed. In contrast to this proposal, we uncovered an intricate biosynthetic route involving a dynamic interconversion between the 6- and 8-methoxy forms of flavonoid skeletons mediated by the chalcone isomerase CfvF and the flavin-dependent oxygenase CfvI. CfvF interconverted the 6- and 8-methoxyflavanones, likely via a chalcone intermediate. CfvI oxidized the chemically inert 2,3-double bond of flavanone, giving the hemiacetal product. Although these enzymes generate products existing in equilibrium states, respective downstream enzymes catalyze selective conversion of one specific species, facilitating smooth progression to the final product. We also solved the crystal structure of CfvK, the dehydratase that selectively converts the 8-methoxy form of the CfvI product. Through analyzing the structure complexed with its substrate and product and site-directed mutagenesis, key residues determining the substrate selectivity were identified. Our comprehensive analysis established a rational framework for preparing 46 flavonoids, including unnatural ones, setting the stage for fungal production of structurally diverse flavonoids.
Sho Furumura, T. Ozaki, Kazuya Hasegawa et al.· Journal of the American Chem...· 0 citations
Many functional RNA motifs adopt structures that deviate from the canonical A-form helix and are emerging targets for RNA-directed therapeutics. The microtubule-associated protein tau (MAPT) A-bulge motif (5′-GCAGU/5′-ACGU) is one such motif. Because its structure is stabilized by a delicate balance of local interactions, its accurate modeling remains a major challenge for molecular dynamics (MD) simulations. The experimentally determined nuclear magnetic resonance (NMR) structure of the MAPT A-bulge motif provides a stringent test of whether RNA force fields can accurately reproduce the experimentally observed conformation. Most current AMBER-family RNA force-field models have incorrectly favored a non-native base-triple state of the MAPT A-bulge motif over the experimentally observed stacked state. Structural comparison of the stacked and base-triple conformations revealed that overly favorable NH□–N hydrogen bonds between the bulged adenosine and an adjacent Watson–Crick base pair were the primary source of this imbalance. We developed gHBfix-18Ab, an 18-component hydrogen-bond correction that distinguishes NH and NH□ donors. gHBfix-18Ab was combined with the previously developed OL3CP and NBfix0BPh corrections to generate the composite model gHBfix-18Ab*. This model restored the experimentally observed stacked state as the global minimum in the calculated free-energy profile and improved agreement with NMR-derived distance data for the A-bulge region. Importantly, gHBfix-18Ab* did not produce marked structural destabilization of the cUUCGg tetraloop, a widely used benchmark for RNA force-field validation, suggesting that the refinement preserves the stability of the unrelated RNA motif. These results demonstrate that targeted refinement of hydrogen-bond interactions provides a practical strategy for systematic improvement of RNA force fields toward more accurate modeling of noncanonical RNA motifs. Graphical Summary
