Terpenoids terpene synthases

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. 

Fig. 1.4 Outline of terpenoid biosynthesis from isopentenyl diphosphate (IPP) via dimethylallyl diphosphate (DMAPP), gcranyl diphosphate (GPP), famesyl diphosphate (FPP) and geranylgcranyl diphosphate (GGPP). These reactions are catalyzed by isoprenyl diphosphate synthases and terpene synthases. The major products of the monoterpene, sesquiterpene, and diterpene pathways that constitute the oleoresm of Picea abies are listed. The general precursor IPP is derived either from the plastidial methylerythritol phosphate (MEP) pathway or the cytosolic mevalonaic pathway.

Terpene synthases constitute a large class of enzymes that are able to convert just a handful of linear isoprenoid precursors, made from IPP and DMAPP by prenyltransferases, into a large diversity of thousands of terpenoids. As most of the enzyme products are of (poly)cyclic nature, these enzymes are often called terpene cyclases. Two types of terpene synthases can be mechanistically distinguished by their formation of a reactive carbocationic species. Class I terpene synthases contain a highly conserved DDXXD motif as also found in a-type prenyltransferases and perform substrate ionization via metal-triggered abstraction... 

As presented in this chapter, today, much is known about the process of terpene biosynthesis. The accumulated knowledge includes a detailed picture about the biosynthesis of the terpenoid monomers IPP and DMAPP either via the mevalonate or the DXP route and their interconversion by isomerases. Also, the stereochemical courses and enzyme mechanisms of all transformations have been largely elucidated. Especially the recently obtained structural data of prenyltransferases and various kinds of terpene synthases resulted in an evolutionary model that involves six domains (a, P, 7,8, e, and Q for the biosynthesis of linear polyisoprenoids from IPP and DMAPP and their subsequent transformation into (poly)cyclic terpenes. All these insights may open up new chances in controlling terpene biosynthesis, e.g., by directed evolution of terpene cyclases or domain swaps in multi-domain enzymes for the production of new terpenes, reconstitution of terpene biosynthetic pathways in heterologous hosts for production optimization, or targeted inhibitirm of pathways in pathogens for disease control. 

Dewick M (2002) The biosynthesis of C5-C25 terpenoid compounds. Nat Prod Rep 19 181 Tholl D (2006) Terpene synthases and the regulation, diversity and biological roles of terpene metabolism. CurrOpin Plant Biol 9 297... 

During the last decade, microbial platforms for industrial production of plant terpenoids have been developed the biotechnological production of artemisinin precursors in yeast and . coli is a relevant milestone [186,187]. Thanks to these technologies, even terpenes occurring in low amounts in plants can be produced at commercial levels, provided that terpene synthases that perform well in the chosen heterologous host can be found. 

Microbial cell factories [194] have recently been developed for the production of terpenoids such as sclareol [195], and, as was shown in the previous section, valencene and nootkatone. The genes responsible for the biosynthesis of the desired compound, such as prenyltransferases, terpene synthases, and additional transformation enzymes, are selected from a natural source and transferred into a host, usually S. cerevisiae or . coli, suitably engineered to overproduce IPP and DMAPP. High productivity values for a commercially exploitable production are achieved by further metabolic and bioprocess engineering improvements of the microbial system. 

Another example of successful engineering of terpene biosynthesis is the constitutive overexpression of the gene encoding the first-step enzyme 1-deoxy-D-xylulose-5-phosphate synthase (DXPS) in the DXP pathway in bacteria and Arabidopsis. In both cases, increased enzyme activity caused increased accumulation of downstream terpenoids, suggesting that DXPS is rate-limiting [8]. 

All terpenoid indole alkaloids are derived from tryptophan and the iridoid terpene secologanin (Fig. 2b). Tryptophan decarboxylase, a pyridoxal-dependent enzyme, converts tryptophan to tryptamine (62, 63). The enzyme strictosidine synthase catalyzes a stereoselective Pictet-Spengler condensation between tryptamine and secologanin to yield strictosidine. Strictosidine synthase (64) has been cloned from the plants C. roseus (65), Rauwolfla serpentine (66), and, recently, Ophiorrhiza pumila (67). A crystal structure of strictosidine synthase from R. serpentina has been reported (68, 69), and the substrate specificity of the enzyme can be modulated (70). 

Our recent research suggests organ-, tissue-, and cell-specific localization of constitutive and induced terpenoid defense pathways in conifers. For example, linalool synthase (PaTPS-Lin) seems to be preferentially expressed in needles of Norway spruce and Sitka spruce with little or no expression in sterns. ft is also likely that expression of PaTPS-Lin in spruce needles is not associated with resin ducts but could reside in other cells involved with induced terpenoid emission. In contrast, we can speculate that most other mono-TPS and di-TPS are associated with epithelial cells of constitutive and induced resin ducts. The possible localization of conifer sesqui-TPS is difficult to predict. Furthermore, the exact spatial and temporal patterns of terpenoid pathway gene expression associated with traumatic resin duct development in the cambium zone and outer xylem remain to be studied at the tissue and cell level. In situ hybridization and immuno-localization of TPS will address these open questions. These methods have worked well in identifying cell type specific gene and protein expression of alkaloid formation in opium poppy Papaver somniferum) As the biochemistry of induced terpene defenses and the development of traumatic resin ducts have been well described in spruce, this system is ideal for future studies of tissue- and cell-specific localization of transcripts and proteins associated with oleoresin defense and induced volatile emissions in conifers. In addition, the advent of laser dissection microscopy techniques presents a fascinating means by which to further address RNA and protein analysis in a tissue-and cell-specific manner. These techniques, when applied to the cambium zone, xylem mother cells, and the epithelial cells that surround traumatic resin ducts, and will allow a temporal and spatial analysis of cellular functions occurring in the traumatic resin response. 

The terpene cyclase or synthase is another class of enzyme that is capable of synthesizing new C—C bonds. Terpenoids is one of the most diverse classes of natural products. Their syntheses involve the cyclization of linear polyisoprene precursors by terpene... 

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