The evolutionary origin of naturally occurring intermolecular Diels-Alderases from Morus alba

Biosynthetic enzymes evolutionarily gain novel functions, thereby expanding the structural diversity of natural products to the benefit of host organisms. Diels-Alderases (DAs), functionally unique enzymes catalysing [4 + 2] cycloaddition reactions, have received considerable research interest. However, their evolutionary mechanisms remain obscure. Here, we investigate the evolutionary origins of the intermolecular DAs in the biosynthesis of Moraceae plant-derived Diels-Alder-type secondary metabolites. Our findings suggest that these DAs have evolved from an ancestor functioning as a flavin adenine dinucleotide (FAD)-dependent oxidocyclase (OC), which catalyses the oxidative cyclisation reactions of isoprenoid-substituted phenolic compounds. Through crystal structure determination, computational calculations, and site-directed mutagenesis experiments, we identified several critical substitutions, including S348L, A357L, D389E and H418R that alter the substrate-binding mode and enable the OCs to gain intermolecular DA activity during evolution. This work provides mechanistic insights into the evolutionary rationale of DAs and paves the way for mining and engineering new DAs from other protein families.

Biosynthesis of Phomactin Platelet Activating Factor Antagonist Requires a Two-Enzyme Cascade

Phomactin diterpenoids possess a unique bicyclo[9.3.1]pentadecane skeleton with multiple oxidative modifications, and are good platelet-activating factor (PAF) antagonists that can inhibit PAF-induced platelet aggregation. In this study, we identified the gene cluster (phm) responsible for the biosynthesis of phomactins from a marine fungus, Phoma sp. ATCC 74077. Despite the complexity of their structures, phomactin biosynthesis only requires two enzymes: a type I diterpene cyclase PhmA and a P450 monooxygenase PhmC. PhmA was found to catalyze the formation of the phomactatriene, while PhmC sequentially catalyzes the oxidation of multiple sites, leading to the generation of structurally diverse phomactins. The rearrangement mechanism of the diterpene scaffold was investigated through isotope labeling experiments. Additionally, we obtained the crystal complex of PhmA with its substrate analogue FGGPP and elucidated the novel metal-ion-binding mode and enzymatic mechanism of PhmA through site-directed mutagenesis. This study provides the first insight into the biosynthesis of phomactins, laying the foundation for the efficient production of phomactin natural products using synthetic biology approaches.

Euphohelides A-C, ent-Abietane-Type Norditerpene Lactones from Euphorbia helioscopia and Their Anti-Inflammatory Activities

Three unreported ent-abietane-type norditerpene lactones, euphohelides A-C (1-3), and 11 known analogs (4-14) were isolated from the whole plants of Euphorbia helioscopia L. Euphohelide A (1) is an unprecedented 2-nor-ent-abietane lactone bearing a unique 5/6/6/5 tetracyclic system. Euphohelides B (2) and C (3) possess 2-nor-6/6/6/5 and 2,3-dinor-5/6/6/5 dilactone tetracyclic moieties, respectively. Their structures were established by spectroscopic methods, computational ECD, and X-ray crystallographic analyses. A biomimetic synthesis of 1 was achieved from precursor 4 based on the speculative biogenetic pathway. Compounds 1 and 5 significantly alleviated the release of LPS-induced NO with IC₅₀ values of 32.98 ± 1.13 and 33.82 ± 3.25 μM, which might be related to the regulation of the NF-κB signaling pathway.

Synthesis of (+)-Xylogiblactones B and C through a Kinetic Resolution of the Allenoate γ-Addition: Stereochemical Establishment

Concise syntheses of naturally occurring γ-butenolides (+)-xylogiblactones B and C have been achieved for the first time starting from commercial methyl crotonate in 5-8 steps. The synthetic course involves allenoate γ-addition to racemic aldehydes through a kinetic resolution to establish the required stereochemical framework as center and axial chirality and subsequent oxacyclization via gold catalysis to complete the (+)-xylogiblactone skeleton. Both key transformations proceed in a regio- and stereospecific manner. This outcome relies on finding an efficient synthetic method for racemic aldehydes as precursors for the kinetic resolution. Completion of the synthesis provides stereochemical clarification for (+)-xylogiblactones B and C.