Terpenes mevalonic acid intermediate

Strategy Problem 6 A labelled compound for biosynthetic studies. Mevaloitic acid (TM 418) is an intermediate in the biosynthesis of terpenes and steroids (Tedder, volume 4, p.217 ff). To study exactly what happens to each carbon atom during its transformation into, say, hmonene (418A), we need separate samples of mevalonic acid labelled with in each carbon atom in the molecule. This turns our normal strategy on its head since we must now look for one carbon discoimections. You can use reagents like Na CN, and... 

A considerable number of mycotoxins that show high toxicity to vertebrates and/ or invertebrates are produced by organisms associated with crop plants (Flannigan 1991). There are many known cases of human poisoning caused by such compounds. There are three broad categories of mycotoxins represented here, based on the structures of the intermediates from which these secondary metabolites are derived. They are (1) compounds derived from polyketides, (2) terpenes derived from mevalonic acid, and (3) cyclic peptides and derivatives thereof. 

Until 1993, all terpenes were considered to be derived from the classical acetate/mevalonate pathway involving the condensation of three units of acetyl CoA to 3-hydroxy-3-methylglutaryl CoA, reduction of this intermediate to mevalonic acid and the conversion of the latter to the essential, biological isoprenoid unit, isopentenyl diphosphate (IPP) [17,18,15]. Recently, a totally different IPP biosynthesis was found to operate in certain eubacteria, green algae and higher plants. In this new pathway glyceradehyde-3-phosphate (GAP) and pyruvate are precursurs of isopentenyl diphosphate, but not acetyl-CoA and mevalonate [19,20]. So, an isoprene unit is derived from isopentenyl diphosphate, and can be formed via two alternative pathways, the mevalonate pathway (in eukaryotes) and the deoxyxylulose pathway in prokaryotes and plant plastids [16,19]. 

Daphniphyllum macropodum contains a great variety of related alkaloids whose structures are quite complex and novel (Section II). It is structurally evident that these alkaloids with an isopropyl or a potential isopropyl group are regarded as a terpene alkaloid. Bio-genetieally, these Daphniphyllum alkaloids, particularly C22 alkaloids, have been proposed to be derived from four molecules of mevalonic acid (MVA) and one acetate unit (15, 39). However, the recent tracer experiments showed that these alkaloids could be biosynthesized from six MVA molecules through a squalene-like intermediate. Accordingly, they should be included in a group of triterpene alkaloid. 

In fact, this is not correct Isoprene is not an intermediate, and the discovery of the true pathway started when acetate was, rather surprisingly, found to be the original precursor for all terpenes. The key intermediate is mevalonic acid, formed from three acetate units and usually isolated as its lactone. 

The major steps in the mechanisms of terpene and polyisoprene formation in plants and trees are known, and this knowledge should help improve the natural production of terpene hydrocarbons (Fig. 3.4). Mevalonic acid (1), a key intermediate derived from plant sugars via acetylcoenzyme A, is succes-... 

Mevalonic acid (MVA) (69) is a very important biosynthetic intermediate used by us to make steroids and by plants to make terpenes. Tracing these biosynthetic pathways required radioactively labelled MVA and J. W. Cornforth has published many such syntheses. MVA has two 1,3-dicarbonyl relationships (70), one of which can be disconnected to (71) and (72). If the OH and CO2H groups are both protected as esters, we can use the Reformatsky method to activate (72) and finally remove both protecting groups in one step. 

Steroids are members of a large class of lipid compounds called terpenes. Using acetate as a starting material, a variety of organisms produce terpenes by essentially the same biosynthetic scheme (Fig. 8). The self-condensation of two molecules of acetyl coenzyme A (CoA) forms acetoacetyl CoA. Condensation of acetoacetyl CoA with a third molecule of acetyl CoA, then followed by an NADPH-mediated reduction of the thioester moiety produces mevalonic acid [150-97-0] (72). Phosphorylation of (72) followed by concomitant decarboxylation and dehydration processes produce isopentenyl pyrophosphate. Isopentenyl pyrophosphate isomerase establishes an equilibrium between isopentenyl pyrophosphate and 3,3-dimethylallyl pyrophosphate (73). The head-to-tail addition of these isoprene units forms geranyl pyrophosphate. The addition of another isopentenyl pyrophosphate unit results in the sesquiterpene (C15) famesyl pyrophosphate (74). Both of these head-to-tail additions are catalyzed by prenyl transferase. Squalene synthetase catalyzes the head-to-head addition of two achiral molecules of famesyl pyrophosphate, through a chiral cyclopropane intermediate, to form the achiral triterpene, squalene (75). 

Mevalonic acid is required to form isopentenyl pyrophosphate, an intermediate in terpene synthesis. It has the following structure. What is its lUPAC name ... 

The usual method of study is to suggest a possible precursor and to feed it to the biosynthesizing system. The precursor has to be labelled in some way to trace it through the sequence of reactions, and that is usually by some isotopic element. It may be a radio-active isotope, such as H, " 0, or that can be followed by its radiation or it can be a stable heavy isotope, such as H, C, N, or 0, that can be traced by mass spectrometry or nuclear magnetic resonance (NMR) spectroscopy (Table 5.1). Another possible way is to use mutant strains of an organism that lack the enzymes to complete a particular synthesis, or to add a specific enzyme inhibitor, so that intermediates accumulate and can be identified. A mutant strain of yeast was important in discovering mevalonic acid and its place in terpene biosynthesis (Chapter 6) and a number of mutants of the bacterium Escherichia coli helped to understand the shikimic acid pathway (Chapter 8). 

In 1937 it was shown that labelled acetate gave labelled terpenes, but evidently by a different route from the fatty acids and acetogenins. In 1956 it was accidentally found that a substance called mevalonic acid (Figure 6.2) was an intermediate between acetate and terpenes, and this gave the clue needed to study their biosynthesis. 

Mevalonic acid, the key intermediate in solving the terpene biosynthesis pathway is an oily liquid, in solution it is in equilibrium with mevalonolactone (Figure 6.5), a crystalline solid, so the latter is used in biosynthetic studies. Fluoromevalonolactone, and the corresponding acid, are powerful inhibitors of terpene formation. If addition of fluoromevalonate to a biosynthesizing system blocks the formation of a compound, then it can be concluded that that compound has a terpene origin. 

More than half of the reported secondary metabolites from macroalgae are isoprenoids. Terpenes, steroids, carotenoids, prenylated quinines, and hydroqui-nones make up the isoprenoid class, which is understood to derive from either the classical mevalonate pathway, or the mevalonate-independent pathway (Stratmann et al. 1992). Melavonic acid (MVA) (Fig. 1.2) is the first committed metabolite of the terpene pathway. Dimethylallyl (dl meth al lal) pyrophosphate (DMAPP) (Fig. 1.3) and its isomer isopentenyl pyrophosphate (IPP, Fig. 1.3) are intermediates of the MVA pathway and exist in nearly all life forms (Humphrey and Beale 2006). Geranyl (ja ran al) (C10) and famesyl (C15) units are generated by head-to-tail (Fig. 1.3) condensation of two (for C10) or three (for C15) 5-carbon DMA-like isoprene units, identifiable in final products by the characteristic fish-tail repeating units, as traced over the structure of a sesquiterpene in Fig. 1.3 (Humphrey and Beale 2006). Additional IPP condensation with famesyl pyrophosphate (FPP)... 

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