Pinoresinol‐lariciresinol reductase: Substrate versatility, enantiospecificity, and kinetic properties

Resolution and Absolute Configuration of Fargesin Enantiomers

Fargesin is an important bioactive furofuran lignan isolated from different plant species. Despite presenting potent biological activities, its stereochemical characterization has relied mostly on empirical correlations of optical rotation, an approach considered risky that commonly leads to misassignments and error propagation. Additionally, the enantiomeric purity of fargesin isolates used for biological assays has not been previously investigated. Herein, we report the enantioresolution of a scalemic mixture of fargesin isolated from Aristolochia warmingii along with the first unambiguous determination of the absolute configuration of each enantiomer by means of optical rotatory dispersion, as well as electronic and vibrational circular dichroism aided by quantum-chemical calculations.

Comparative Assessment of Lignan, Tocopherol, Tocotrienol and Carotenoids in 40 Selected Varieties of Flaxseed (Linum usitatissimum L.)

Multiple varieties of flaxseeds have been identified in the world, yet the relationship between these varieties, their agronomic traits, and their seeds' quality remains unclear. This study aimed to determine the level of lignan, vitamins and carotenoids in 40 selected flaxseed varieties, and the relationship between varieties, agronomic traits, and seed quality was investigated. In this study, notably, fiber flax variety No. 225 exhibited the highest lignan content among all tested seeds. Additionally, oil variety No. 167 demonstrated the highest level of α-tocotrienol (α-T3), β-tocopherol (β-T), γ-tocotrienol (γ-T3), and β-carotene (β-Car.). Conversely, intermediate flax variety No. 16 displayed the highest content of α-tocopherol (α-T), but lowest content of lutein (Lut.), zeaxanthin (Zea.), β-carotene (β-Car.), and total carotenoids (Total Car.). Furthermore, a correlation was observed between petal color with the lignan, while a strong correlation has been explored in seed yield, seed type, plant natural height, and fiber content in straw. Nevertheless, further investigation is required to elucidate the internal relationship between varieties with compositions.

Dirigent protein subfamily function and structure in terrestrial plant phenol metabolism

Aquatic plant transition to land, and subsequent terrestrial plant species diversification, was accompanied by the emergence and massive elaboration of plant phenol chemo-diversity. Concomitantly, dirigent protein (DP) and dirigent-like protein subfamilies, derived from large multigene families, emerged and became extensively diversified. DP biochemical functions as gateway entry points into new and diverse plant phenol skeletal types then markedly expanded. DPs have at least eight non-uniformly distributed subfamilies, with different DP subfamily members of known biochemical/physiological function now implicated as gateway entries to lignan, lignin, aromatic diterpenoid, pterocarpan and isoflavene pathways. While some other DP subfamily members have jacalin domains, both these and indeed the majority of DPs throughout the plant kingdom await discovery of their biochemical roles. Methods and approaches were developed to discover DP biochemical function as gateway entry points to distinct plant phenol skeletal types in land plants. Various DP 3D X-ray structural determinations enabled structure-based comparative sequence analysis and modeling to understand similarities and differences among the different DP subfamilies. We consider that the core DP β-barrel fold and associated characteristics are likely common to all DPs, with several residues conserved and nearly invariant. There is also considerable variation in residue composition and topography of the putative substrate binding pockets, as well as substantial differences in several loops, such as the β1-β2 loop. All DPs likely bind and stabilize quinone methide intermediates, while guiding distinctive regio- and/or stereo-chemical entry into Nature's chemo-diverse land plant phenol metabolic classes.

RNAi Modulation of Chlorogenic Acid and Lignin Deposition in Nicotiana tabacum and Insufficient Compensatory Metabolic Cross-Talk

Chlorogenic acid (CGA) and guaiacyl/syringyl (G/S) lignin formation involves hydroxycinnamoyl ester intermediacy, the latter formed via hydroxycinnamoyl CoA:shikimate hydroxycinnamoyl transferase (HCT) and hydroxycinnamoyl CoA:quinate hydroxycinnamoyl transferase (HQT) activities. HQT and HCT RNAi silencing of a commercial tobacco (Nicotiana tabacum) K326 line was examined herein. NtHQT gene silencing gave relatively normal plant phenotypes, with CGA levels reduced (down to 1% of wild type) with no effects on lignin. RNAi NtHCT silencing had markedly adverse phenotypes (e.g., stunted, multiple stems, delayed flowering, with senescence delayed by several months). Lignin contents were partially lowered, with a small increase in cleavable p-hydroxyphenyl (H) monomers; those plants had no detectable CGA level differences relative to wild type. In vitro NtHCT kinetic parameters revealed preferential p-coumaroyl CoA and shikimate esterification, as compared to other structurally related potential acyl group donors and acceptors. In the presence of coenzyme A, NtHCT catalyzed the reverse reaction. Site-directed mutagenesis of NtHCT (His153Ala) abolished enzymatic activity. NtHQT, by comparison, catalyzed preferential conversion of p-coumaroyl CoA and quinic acid to form p-coumaroyl quinate, the presumed CGA precursor. In sum, metabolic pathways to CGA and lignins appear to be fully independent, and previous conflicting reports of substrate versatilities and metabolic cross-talk are resolved.

Assembly of Plant Enzymes in E. coli for the Production of the Valuable (-)-Podophyllotoxin Precursor (-)-Pluviatolide

Lignans are plant secondary metabolites with a wide range of reported health-promoting bioactivities. Traditional routes toward these natural products involve, among others, the extraction from plant sources and chemical synthesis. However, the availability of the sources and the complex chemical structures of lignans often limit the feasibility of these approaches. In this work, we introduce a newly assembled biosynthetic route in E. coli for the efficient conversion of the common higher-lignan precursor (+)-pinoresinol to the noncommercially available (-)-pluviatolide via three intermediates. (-)-Pluviatolide is considered a crossroad compound in lignan biosynthesis, because the methylenedioxy bridge in its structure, resulting from the oxidation of (-)-matairesinol, channels the biosynthetic pathway toward the microtubule depolymerizer (-)-podophyllotoxin. This oxidation reaction is catalyzed with high regio- and enantioselectivity by a cytochrome P450 monooxygenase from Sinopodophyllum hexandrum (CYP719A23), which was expressed and optimized regarding redox partners in E. coli. Pinoresinol-lariciresinol reductase from Forsythia intermedia (FiPLR), secoisolariciresinol dehydrogenase from Podophyllum pleianthum (PpSDH), and CYP719A23 were coexpressed together with a suitable NADPH-dependent reductase to ensure P450 activity, allowing for four sequential biotransformations without intermediate isolation. By using an E. coli strain coexpressing the enzymes originating from four plants, (+)-pinoresinol was efficiently converted, allowing the isolation of enantiopure (-)-pluviatolide at a concentration of 137 mg/L (ee ≥99% with 76% isolated yield).