Terpene synthases are responsible for the large diversity of terpene carbon skeletons found in plants. The unique, carbocationic reaction mechanism of these enzymes can form multiple products from a single prenyl diphosphate substrate. Two maize genes were isolated that encode very similar sesquiterpene synthases, TPS6 and TPS11, which both produce ␤-bisabolene, a common monocyclic sesquiterpene, and ␤-macrocarpene, an uncommon bicyclic olefin. Investigation of the reaction mechanism showed that the formation of ␤-macrocarpene proceeds via a neutral ␤-bisabolene intermediate and requires reprotonation by a proton that may ultimately be abstracted from water. This reprotonation is dependent on the pH and the presence of a Mg 2؉ cofactor. Mutational analysis of the enzyme demonstrated that a highly conserved tyrosine residue in the active center of the enzymes is important for the protonation process. TPS6 and TPS11 are transcribed both in leaves and roots of maize, but the respective terpene products were only detected in roots. The expression in roots was up-regulated by herbivore damage to the leaves, suggesting a long distance signal transduction cascade between leaves and roots.Terpenes form the largest group of plant secondary metabolites and are known to have a plethora of different functions both within the plant and in communication with other organisms. Most of the structural diversity among terpenes can be attributed to the terpene synthases, a large enzyme class that catalyzes the conversion of the ubiquitous prenyldiphosphates geranyldiphosphate (GPP), 2 farnesyldiphosphate (FPP), or geranylgeranyldiphosphate (GGPP) into a large number of basic terpene skeletons (1). A unique feature of terpene synthases is their capability to produce multiple products with different carbon skeletons from a single prenyldiphosphate substrate (2, 3). For example, ␦-selinene synthase and ␥-humulene synthase from Abies grandis synthesize 34 and 52 different sesquiterpenes from their farnesyl diphosphate substrate, respectively (4). This unusual behavior is due to an electrophilic reaction mechanism common to all terpene synthases, which is initiated by elimination of the allylic diphosphate from the prenyl diphosphate substrate. The resulting highly reactive carbocationic intermediate undergoes a series of cyclizations, hydride shifts, and other rearrangements until the reaction is terminated by proton loss or the addition of a nucleophile (5).In addition to highly active, carbocationic intermediates, sesquiterpene synthases like 5-epi-aristolochene synthase (TEAS) from tobacco were also shown to produce stable, enzymebound intermediates (6, 7). The structural elucidation of this sesquiterpene synthase provided insight into the reaction mechanism and structure-function relationships in the active center (8). The reaction starts with the dephosphorylation and ionization of the (E,E)-FPP substrate to form a farnesyl cation, which undergoes a 1,10-cyclization to form a stable, enzymebound germacrene A intermediate (3). Base...
On the defense: Sesterterpenoid 1 and its 11‐dehydroxy derivative, which possess a unique C25 skeleton, have been isolated from the glandular trichomes of Leucosceptrum canum. They have potent antifeedant activities and inhibitory effects against pathogenic fungi, which suggest a role of sesterterpenoids in plant defense.
Maoecrystal V (1), a novel C(19) diterpenoid possessing a unique 6,7-seco-6-nor-15(8-->9)-abeo-5,8-epoxy-ent-kaurane skeleton, was isolated from the leaves of a Chinese medicinal herb, Isodon eriocalyx. Its structure was determined by comprehensive NMR and MS spectroscopic analysis and confirmed by single-crystal X-ray diffraction study. Compound 1 showed remarkable inhibitory activity toward HeLa cells with IC(50) = 0.02 microg/mL (cis-platin: IC(50) = 0.99 microg/mL).
Three new ent-kaurane diterpenoids laxiflorin E (1), laxiflorin H (6) and laxiflorin I (8) were isolated from the leaves of Isodon eriocalyx var. laxiflora, along with nine known diterpenoids, eriocalyxin A (2), laxiflorin C (3), laxiflorin D (4), laxiflorin A (5), maoecrystal S (7), maoecrystal Q (9), eriocalyxin B (10), maoecrystal C (11) and enmelol (12). The structures of the new compounds were determined by spectroscopic methods. Compounds 1-5 are 6,7-seco-ent-kaurane-7,20-olide diterpenoids, and 6-12 belong to 7,20-epoxy-ent-kauranoids. The spectral data of enmelol (12) are reported here for the first time. Ten of these compounds were tested for their cytotoxicity toward K562 and T24 human tumor cells. Compounds 1, 3 and 10 showed significant inhibitory effects on K562 cells with IC50 values 0.077, 0.569 and 0.373 microg/mL, and compounds 1 and 10 also demonstrated significant inhibitory activities toward T24 cells with IC50 values 0.709 and 0.087 microg/mL. Compounds 8 and 11 also displayed inhibitory effect on both two kinds of cells with IC 50 value less than 6.5 microg/mL.
A new class of unique sesterterpenoids, colquhounoids A-C (1-3), were identified from the peltate glandular trichomes of Colquhounia coccinea var. mollis (Lamiaceae) through precise laser-microdissection coupled with UPLC/MS/MS and spectroscopic analyses and X-ray diffraction. Very interestingly, their structural features and defensive function are closely related to leucosceptroid-class sesterterpenoids harbored by the glandular trichomes of another Lamiaceae taxon, Leucosceptrum canum, even though this is morphologically distinct and taxonomically distant.
Stone cells (sclereids) in Norway spruce (Picea abies) bark have been reported to be highly lignified tissues that are important in physical defence against bark beetle invasion. Microchemical analyses of the low-molecular weight compounds in the stone cells of Norway spruce were carried out using laser microdissection in combination with cryogenic nuclear magnetic resonance and mass spectrometry (LMD/NMR/MS). Two phenolic compounds, the stilbene astringin and the dihydroflavonol dihydroxyquercetin 3'-O-beta-D-glucopyranoside, were identified indicating that stone cells are more than just repositories for lignin. Both of these compounds were also found to be present in other phloem tissue at a higher level than in the stone cells based on quantification by cryogenic 1H NMR. Our results suggest that stone cells may be involved in chemical as well as physical defense against bark beetles and their associated microorganisms. This paper reports on the identification of secondary plant metabolites from a single laser-microdissected population of plant cells offering a sensitive new way to determine the chemical profile of specific plant cell types with a high degree of precision.
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