Triterpene synthases

MORITA, M., SHIBUYA, M., KUSHIRO, T., MASUDA, K., EBIZUKA, Y., Molecular cloning and functional expression of triterpene synthases from pea (Pisum sativum), Eur. J. Biochem., 2000, 267, 3453-3460. 

Chimeric enzymes constructed fi"om two different angiosperms (Panax ginseng, Araliaceae, and Arabidopsis thaliana, Brassicaceae) yielded mixtures of triterpenoids, p-amirin and lupeol, at a composition depending on the particular chimera methyl scrambling was observed for lupeol only (Kushiro 1999). A few differences in the amino acids at the active site are responsible for these effects. This implies that the high variety of plant triterpenoids owes more to chimeric enzymes than product-specific triterpene synthases. It was proposed that these chimeric enzymes act as multifunctional triterpene synthases (Kushiro 1999). 

Kushiro, T. Shibuya, M. Ebizuka, Y. (1999) Chimaic triteq)aie synthase. A possible model for multifunctional triterpene synthase. J. Am. Chan. Soc., 121, 1208-16. 

Hayashi, H., Huang, R, Inoue, K., Hiraoka, N., Ikeshiro, Y., Yazaki, K., Tanaka, S., Kushiro, T., Shibuya, M. and Ebizuka, Y. (2001) Molecular cloning and characterization of isomultiflorenol synthase, a new triterpene synthase from Luffa cylindrica, involved in biosynthesis of bryonolic acid. Eur.. Biochem., 268, 6311-7. 

Terpene synthesis in nature is a complex process involving successive electrophilic additions followed by a variety of skeletal rearrangements, including those of the Wagner-Meerwein variety. These reactions are typically catalyzed by enzymes and are responsible for the wide array of structural diversity in these compounds, including 6-6-6-5 tetracycles, 6-6-6-6-5 pentacycles, 6-6-6-6-6 pentacycles, and the less abundant acyclic, monocyclic, bicyclic, tricyclic, and hexacyclic triterpenoids. Each of the more than 100 triterpene skeletons identified in nature are formed through the involvement of several multifunctional triterpene synthases. 

Xiang, T., Shibuya, M., Katsube, Y, Tsutsumi, T., Otsuka, M., Zhang, H., Masuda, K., and Ebizuka, Y. 2006. A new triterpene synthase from Arabidopsis thaliana produces a tricyclic triterpene with two hydroxyl groups. Org Lett 8 2835-2838. 

FIGURE 1.4 Proposed biosynthetic route for the biosynthesis of (A) squalene oxide (squalene-2,3-oxide) via the isoprenoid pathway and (B) triterpene saponins of the dammarane-type and oleanane-type from squalene oxide. PP, diphosphate group GPS, geranyl phosphate synthase FPS, farnesyl phosphate synthase NADPH, nicotinamide adenine dinucleotide phosphate. 

Lee MH et al (2004) Enhanced triterpene and phytosterol biosynthesis in Panax ginseng overexpressing squalene synthase gene. Plant CeU Physiol 45 976... 

In green plants, which contain little or no cholesterol, cydoartenol is the key intermediate in sterol biosynthesis.161-1623 As indicated in Fig. 22-6, step c, cydoartenol can be formed if the proton at C-9 is shifted (as a hydride ion) to displace the methyl group from C-8. A proton is lost from the adjacent methyl group to close the cyclopropane ring. There are still other ways in which squalene is cyclized,162/163/1633 including some that incorporate nitrogen atoms and form alkaloids.1631 One pathway leads to the hop-anoids. These triterpene derivatives function in bacterial membranes, probably much as cholesterol does in our membranes. The three-dimensional structure of a bacterial hopene synthase is known.164 1643 Like glucoamylase (Fig. 2-29) and farnesyl transferase, the enzyme has an (a,a)6-barrel structure in one domain and a somewhat similar barrel in a second domain. 

Kushiro, T., Shibuya, M. Ebizuka, Y. beta-Amyrin synthase Cloning of oxidosqualene cyclase that catalyzes die formation of the most popular triterpene among higher plants. Eur J Biochem 1998 256 238-244. 

Biosynthetically-patterned microbiological transformations exploit the substrate flexibility of enzymes involved in the biosynthesis of secondary metabohtes. These biotransformations are sometimes known as analogue biosynthesis or precursor-directed biosynthesis. This approach to biotransformation can be useful in preparing analogues of biologically-active microbial metabolites for structure-activity studies, a feature that has been exploited with penicillins using a cloned isopenicillin N synthase. The structures of the substances that are transformed and of their products can also shed light on the stereo-electronic constraints of enzymatic steps and on the nature of biosynthetic intermediates, a feature that has been exploited in studies on the cyclization of squalene to the triterpenes and steroids. 

Two molecules of famesyl pyrophosphate are joined head-to-head to form squalene, a triterpene, in the first dedicated step towards sterol biosynthesis (Fig. 8.4). Squalene is then converted to 2,3-oxidosqualene, which next can be cyclized to the 30 carbon, 4-ring structure cycloartenol by the enzyme cycloartenol synthase (EC 5.4.99.8). Cycloartenol can be further modified by reactions such as desaturation or demethylation to form the common sterol backbones such as... 

Bohhnann J, Meyer-Gauen G, Croteau R (1998) Plant terpenoid synthases molecular biology and phylogenetic analysis. Proc Natl Acad Sci USA 95 4126-4133 Sacchettini JC, Poulter CD (1997) Creating isoprenoid diversity. Science 277 1788-1789 Dereth RP, Jeanne MR, Bonnie B, Seiichi PTM (2006) Biosynthetic diversity in plant triterpene cyclization. Curr Opin Plant Biol 9 305-314... 

Class 11 terpene cyclases are known for the cycUzation of di-, sester-, and triterpenoids. Prominent examples of triterpene cyclases are the human lanosterol synthase (oxidosqualene cyclase, OSC) and the squalene/hopene cyclase (SHC) firom A. acidocaldarius [1]. Both enzymes have a py-domain architecture (Fig. 87.15a). The p-domain exhibits a highly regular Oe-Oe barrel structure with six inner and six outer helices surrounding a central cavity (Fig. 87.15b) [198,199, 208]. The y-domains may have originated from the p-domains by gene duplicatimi... 

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