Terpene synthases

ThoU D (2006) Terpene synthases and the regulation, diversity and biological roles of terpene metaboUsm. Curr Opin Plant Biol 9 297-304... 

Chen F, Ro DK, Petri J, Gershenzon J, Bohhnann J, Pichersky E, Tholl D (2004) Characterization of a root-specific Arabidopsis terpene synthase responsible for the formation of the volatile monoterpene 1,8-cineole. Plant Physiol 135 1956-1966... 

Degenhardt J, Gershenzon J (2000) Demonstration and characterization of ( )-nerolidol synthase from maize a herbivore-inducible terpene synthase participating in (3- )-4, 8-dimethyl-1,3,7-nonatriene biosynthesis. Planta 210 815-822... 

Schnee C, Kollner TG, Gershenzon J, Degenhardt J (2002) The maize gene terpene synthase 1 encodes a sesquiterpene synthase catalyzing the formation of ( )-P-farnesene, ( )-nerolidol, and ( , )-famesol iter herbivore damage. Plant Physiol 130 2049-2060... 

Nagegowda DA, Gutensohn M, WBkerson CG, Dudareva N (2008) Two nearly identical terpene synthases catalyze the formation of neroUdol and hnalool in snapdragon flowers. Plant J 55 224-239... 

Chang YJ, Song SH, Park SH, Kim SU. (2000) Amorpha-4,11-diene synthase of Artemisia annua cDNA Isolation and bacterial expression of a terpene synthase involved in artemisinin biosynthesis. Arch Biochem Biophys 3S3 178-184. 

Most of the compounds shown in Figure 22-4 are derived from the C15 famesyl diphosphate. There are more than 300 known cyclic structures among these sesquiterpenes, and many sesquiterpene synthases have been characterized.91/91a Aristolochene is formed by the action of a 38-kDa cyclase that has been isolated from species of Penicillium and Aspergillus,92"4 Notice that the synthesis must involve two cyclization steps and migration of a methyl group. Three-dimensional structures are known for at least two terpene synthases,95 96 and comparison of gene sequences suggests that many others have similar structures. 

Figure 6.15 Hypothetical alternative late stages of enantiospecific de novo biosynthesis of ipsenol and ipsdienol in male Ips paraconfusus and Ips pini. Biosynthesis may proceed from geranyl diphosphate to myrcene as catalyzed by a sex-specific monoterpene synthase. Terpene synthases (including a myrcene synthase) have been characterized from conifers (Bohlmann et al. 1997, 1998). Alternatively, biosynthesis may proceed from geranyl diphosphate to 5-hydroxygeranyl diphosphate (W. Francke, personal communication).

Figure 10.7 All terpenes are derived from allylic diphosphates which are polymers of repeating isopentyl units (IPP) put together by the action of prenyltransferases. In plants, IPP can be derived from the mevalonate biosynthetic pathway (a cytoplasmic pathway) or the methyl erythritol phosphate pathway (a plastidic pathway). Monoterpenes are then derived from the CIO precursor geranyl diphosphate (GPP), sesquiterpenes from the C15 precursor famesyl diphosphate (FPP), and diterpenes from the C20 precursor geranylgeranyl diphosphate (GGPP) by the action of terpene synthases or cyclases, which divert carbon into the specific branch pathways.

C) Terpene synthases Abies grandis Bohimannefo/, (1997) Bohimann et al. (1998a) Steele et al. (1998a) Vogel et al. (1996)... 

Terpene synthases, also known as terpene cyclases because most of their products are cyclic, utilize a carbocationic reaction mechanism very similar to that employed by the prenyltransferases. Numerous experiments with inhibitors, substrate analogues and chemical model systems (Croteau, 1987 Cane, 1990, 1998) have revealed that the reaction usually begins with the divalent metal ion-assisted cleavage of the diphosphate moiety (Fig. 5.6). The resulting allylic carbocation may then cyclize by addition of the resonance-stabilized cationic centre to one of the other carbon-carbon double bonds in the substrate. The cyclization is followed by a series of rearrangements that may include hydride shifts, alkyl shifts, deprotonation, reprotonation and additional cyclizations, all mediated through enzyme-bound carbocationic intermed iates. The reaction cascade terminates by deprotonation of the cation to an olefin or capture by a nucleophile, such as water. Since the native substrates of terpene synthases are all configured with trans (E) double bonds, they are unable to cyclize directly to many of the carbon skeletons found in nature. In such cases, the cyclization process is preceded by isomerization of the initial carbocation to an intermediate capable of cyclization. 

Figure 5.6 Proposed mechanism for the cyclization of geranyl diphosphate to sabinene and sabinene hydrate under catalysis by monoterpene synthases the reaction begins with the hydrolysis of the diphosphate moiety to generate a resonance-stabilized carbocation (1) the carbocation then isomerizes to an intermediate capable of cyclization by return of the diphosphate (2) and rotation around a single bond (3) after a second diphosphate hydrolysis (4) the resulting carbocation undergoes a cyclization (5) a hydride shift (6) and a second cyclization (7) before the reaction terminates by deprotonation (8) or capture of the cation by water (9). Cyclizations, hydride shifts and a variety of other rearrangements of carbocationic intermediates are a characteristic of the mechanisms of terpene synthases. No known terpene synthase actually produces both sabinene and sabinene hydrate these are shown to indicate the possibilities for reaction termination. PP indicates a diphosphate moiety.

Not all terpene synthases catalyse complex reactions. Isoprene synthase converts DMAPP to the hemiterpene (G5), isoprene (Fig. 5.1), a comparatively simple process involving the ionization of the diphosphate group, followed by double-bond migration and proton elimination (Silver and Fall, 1991). Present in chloroplasts in both stromal and thylakoid-bound forms, isoprene synthase is a homodimer that differs from other terpene synthases in many properties, such as subunit architecture, optimum pH and kinetic parameters... 

Figure 5.7 Proposed mechanism for the cyclization of geranylgeranyl diphosphate (GGPP) to the diterpene copalyl diphosphate, an example of terpene synthase-catalysed cyclization initiated by double-bond protonation, rather than by hydrolysis of the diphosphate ester. PP indicates a diphosphate moiety.

Bohlmann, J. and Croteau, R. (1999) Diversity and variability of terpenoid defences in conifers molecular genetics, biochemistry and evolution of the terpene synthase gene family in grand fir (Abies grandis). Novartis Found Symp., 223,132-49. 

Martin DM, Ealdt J, Bohlmann J. Eunctional characterization of nine Norway spruce TPS genes and evolution of gym-nosperm terpene synthases of the TPS-d subfamily. Plant Physiol. 2004 135 1908-1927. 

Martin DM, Bohlmann J. Identification of Vitis vinifera (-)-alpha-terpineol synthase by in silico screening of full-length cDNA ESTs and functional characterization of recombinant terpene synthase. Phytochemistry 2004 65 1223-1229. 

In contrast to the above-ground organs of plants, roots represent an unexplored area of terpene biosynthesis and function. To date, just a small number of terpene synthases have been identified in plant roots. In Arabidopsis, the terpene synthase genes that encode 1,8-cineole synthase and (Z)-y-bisabolene synthase are expressed differentially in the stele of younger root growth zones and in the cortex and epidermis of older roots (reviewed... 

Type III polyketide synthases (PKSs) generate a diverse array of natural products by condensing multiple acetyl units from malonyl-CoA to specific starter substrates (1,2). The homodimeric enzymes orchestrate a series of acyltransferase, decarboxylation, condensation, cyclization, and aromatizatidn reactions at two functionally independent active sites. Due to their ability to vary either the starter molecule or the type of cyclization and the number of condensations, they are, along with terpene synthases, one of the major generators of carbon skeleton diversity in plant secondary metabolites (i). Among the starter substrates used, benzoyl-CoA is a rare starter molecule. It is utilized by bacterial type I PKSs to form soraphen A, enterocin and the wailupemycins (4). in plants, benzoyl-CoA is the starter unit for two type III PKSs, benzophenone synthase (BPS) and biphenyl synthase (BIS), both of which were cloned in our laboratory. 

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