Complex scaffold remodeling in plant triterpene biosynthesis

Abstract

Triterpenes with complex scaffold modifications are widespread in the plant kingdom. Limonoids are an exemplary family that are responsible for the bitter taste in citrus (e.g., limonin) and the active constituents of neem oil, a widely used bioinsecticide (e.g., azadirachtin). Despite the commercial value of limonoids, a complete biosynthetic route has not been described. We report the discovery of 22 enzymes, including a pair of neofunctionalized sterol isomerases, that catalyze 12 distinct reactions in the total biosynthesis of kihadalactone A and azadirone, products that bear the signature limonoid furan. These results enable access to valuable limonoids and provide a template for discovery and reconstitution of triterpene biosynthetic pathways in plants that require multiple skeletal rearrangements and oxidations.

Post-Cyclization Skeletal Rearrangements in Plant Triterpenoid Biosynthesis by a Pair of Branchpoint Isomerases

“…The fact that the two related enzymes ISM1 and ISM2 generate different rearrangement products from the same epoxide substrate also suggests that protein engineering is highly promising to harness these isomerases for even further skeletal modifications in the future. Recent data additionally supports the role of these enzymes in limonoid biosynthesis …”

A comprehensive review of azadirachtin: physicochemical properties, bioactivities, production, and biosynthesis

“…This remarkable progress in enzyme identification was further advanced by Lian and Wang in 2020, who identified MtOSC1 from China and AiOSC1, AiCYP71CD2, and AiCYP71BQ5 from neem, respectively [13][14] . In 2023, Osbourn et al [18] and his team achieved another milestone by identifying a set of candidate genes related to citrus biosynthesis. They employed a systematic approach that combined transcriptome and genome mining, co-expression analysis, phylogenetics, and comparative studies across citrus and neem datasets.…”

“…Through systematic transcriptome and genome exploration, coupled with co-expression, phylogenetic, and comparative analysis, Osbourn et al achieved a transformative breakthrough by successfully converting the intermediate skeletal compound from 2,3-oxidized squalene to tirucalla-7,24-dien-3β-ol [17] . This complex process involves ring-opening rearrangements, C4 bond cleavage, furan ring formation, and oxidative modifications common to limonoid compounds, ultimately culminating in the formation of a core C26 scaffold and completing the synthesis of azadirone and kihadalactone A [17][18] . Bioinformatics related to azadirachta and the azadirachtin metabolic pathway have advanced significantly, providing greater clarity.…”

Nomilin and its analogue obacunone alleviate NASH and hepatic fibrosis in mice via enhancing antioxidant and anti‐inflammation capacity

“…19 Both obacunone (OBA) and nomilin (NML) are limonoids enriched in citrus fruits such as lemons, oranges, and grapefruits as well as traditional Chinese medicines made from the part of citrus fruits with various bioactivities such as anti-aging, anti-cancer, anti-inflammation, anti-oxidation and neuroprotection. [20][21][22] It has been reported that both obacunone and nomilin may regulate the Keap1-Nrf2 signaling pathway to protect tissue from oxidative injury. [23][24][25] Obacunone has been found to alleviate CCl 4 -induced liver fibrosis via enhancing glutathionperoxidase-4 signal.…”

“…Limonoids are natural tetranortriterpenoids in Rutaceae and Meliaceae families 19 . Both obacunone (OBA) and nomilin (NML) are limonoids enriched in citrus fruits such as lemons, oranges, and grapefruits as well as traditional Chinese medicines made from the part of citrus fruits with various bioactivities such as anti‐aging, anti‐cancer, anti‐inflammation, anti‐oxidation and neuroprotection 20–22 . It has been reported that both obacunone and nomilin may regulate the Keap1‐Nrf2 signaling pathway to protect tissue from oxidative injury 23–25 .…”