Triterpenes, biosynthesis

GlycoUpids, Synthesis of Imaging Techniqnes Lipids Labeling Techniqnes Lipids Lipidomics Lipids, Semi-synthetic Steroid and Triterpene Biosynthesis PHYSICAL PROPERTIES PHASE BEHAVIOR AND PHASE TRANSITIONS... 

Steroid and Triterpene Biosynthesis Terpenes, Biosynthesis of Terpenoids in Plants ... 

Terpenoids, which are also known as isoprenoids, constitute the most abundant and structurally diverse group of plant secondary metabolites, consisting of more than 40,000 different chemical structures. The isoprenoid biosynthetic pathway generates both primary and secondary metabolites that are of great importance to plant growth and survival. Among the primary metabolites produced by this pathway are phytohormones, such as gibberellic acid (GA), abscisic acid (ABA), and cytokinins the carotenoids, such as chlorophylls and plastoquinones involved in photosynthesis the ubiquinones required for respiration and the sterols that influence membrane stmcture (see also Steroid and Triterpene Biosynthesis) (Fig. 1). Monoterpenoids (CIO), sesquiterpenoids (Cl5), diterpenoids (C20), and... 

Chemistry of Cytochrome P450 Monooxygenases, Glycosyl Transferases and Methyl Transferases Metabolic Engineering Steroid and Triterpene Biosynthesis Terpenoid Biosynthesis... 

Sterols include 4a-methylsterols, intermediates of sterols and triterpens biosynthesis. Analysis of the sterol fraction demostrated the constant presence of P-sitosterol, campesterol and stigmasterol, A -avenasterol, while A" -avenasterol may be absent or present in very low amount. P-sitosterol makes up the 75-90% of the total sterol fraction, the rest of sterols occur in minute quantities. 

The biosynthesis of squalene is similar to that of phy toene in tetraterpene biosynthesis, except that in the final step, a double bond is formed by elimination in tetraterpenes, whereas a hydride from NADPH is added in triterpene biosynthesis (Poulter, 1990). 

In the second step of phytoene formation, inorganic pyrophosphate is expelled and a proton is eliminated. Pyridine nucleotide cofactors are not involved, as is the case in triterpene biosynthesis (Fig. 26.3) (Poulter, 1990). 

Ebizuka, Y., Katsube, Y, Tsutsumi, T., Kushiro, T., and Shibuya, M. 2003. Functional genomics approach to the study of triterpene biosynthesis. Pure Appl Chem 75 369-374. 

Lodeiro, S., Xiong, Q., Wilson, W.K, Kolesnikova, M.D., Onak, C.S., and Matsuda, S.P.T. 2007. An oxidosqualene cyclase makes numerous products by diverse mechanisms A challenge to prevailing concepts of triterpene biosynthesis. J Am Chem Soc 129 11213-11222. 

DEVELOPMENTAL REGULATION OF STEROL AND PENTACYCLIC TRITERPENE BIOSYNTHESIS AND COMPOSITION A CORRELATION WITH SORGHUM FLORAL INITIATION... 

Table 1. Effect of Calmodulin Antagonists on Triterpene Biosynthesis...

Squalene is also an intermediate in the synthesis of cholesterol. StmcturaHy, chemically, and biogeneticaHy, many of the triterpenes have much in common with steroids (203). It has been verified experimentally that squalene is the precursor in the biosynthesis of all triterpenes through a series of cyclization and rearrangement reactions (203,204). Squalene is not used much in cosmetics and perfumery formulations because of its light, heat, and oxidative instabiUty however, its hydrogenated derivative, squalane, has a wide use as a fixative, a skin lubricant, and a carrier of Hpid-soluble dmgs. 

Despite the broad medical potentials reported so far, the total synthesis of triterpene QMs is yet to be reported. On the contrary, the biosynthesis of triterpene QMs has recently been validated as from the oxidosqualene 88 (Scheme 8.16) in the plants including Maytenus aquifolium and Salacia campestris.10S With the assistance of HPLC analysis and isotopic labeling, it was found that triterpene QMs 90 were formed only in the root of these plants from friedelin 89 and similar cyclized intermediates, which were synthesized in the leaves from oxidosqualene by cyclase. 

Biosynthesis of Sterol-Related Triterpenes - De Novo Sterol Biosynthesis... 

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... 

These processes are normally enzyme-catalyzed. Purefy ctemical processes are seldom encountered with carbon conq)ounds in nature. The few exceptions include the very act of phenol coupling (by which racemic compounds are obtained), cyclization reactions ofpolyprenyl compoimds (which benefit from the preferred conformation of the reaction partners, suitable for the cyclization, Wendt 2000), and Diels-Alder cycloadditions. The latter have been advocated for the biosynthesis of celastroidine A (= volubilide) from a lupane triterpene and an abietane diterpene in two different plants, Hippocratea celastroides Kunth from Mexico (Jimenez-Estrada 2000) and Hippocratea volubilis Linnaeus (Alvarenga 2000). 

Most animal steroids arise from cholesterol, which in turn is derived from squalene. This C30 triterpene, whose biosynthesis is described in Section B, is named after the dogfish Squalus in whose liver it accumulates as a result of blockage in oxidation to cholesterol. Squalene is also a prominent constituent of human skin lipids. Its conversion to cholesterol, which takes place in most animal tissues,117/154-156 is initiated by a microsomal enzyme system that utilized 02 and NAD-PH to form squalene 2,3-oxide (Fig. 22-6, step a). 

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. 

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