Acetate/mevalonate

It is well known that the steroid hormones in mammals are biosynthesized from cholesterol. This compound is derived from the acetate-mevalonate pathway through the monoterpene geranyl pyrophosphate, which undergoes several enzymatic reactions to form the triterpene squalene. 

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]. 

There are diffent pathways by which all phenolic compounds are synthesized [6,7]. The shikimate/arogenate pathway leads, through phenylalanine, to the majority of plant phenolics, and therefore we shall centre the present revision on the detailed description of this pathway. The acetate/malonate pathway leads to some plant quinones but also to various side-chain-elongated phenylpropanoids (e.g. the group of flavonoids). Finally, the acetate/mevalonate pathway leads by dehydrogenation reactions to some aromatic terpenoids. 

The biosynthesis of monoterpenoids and camphor has been described by several authors (108-llU). Ruzicka (115,116) proposed a unified biogenetic scheme for terpenes. The biosynthetic building blocks for these terpenes are iso-prene units. The biosynthetically active isoprene units are isopentenyl pyrophosphate [l] and dimethyl allyl pyrophosphate [2] the compounds that are derived from acetate via mevalonic acid (Scheme V). Geranyl pyrophosphate [3] is the C-10 precursor for the terpenes (117). Banthorpe and Baxendale (ll8) confirmed the biosynthetic pathway of (iamphor via acetate mevalonate by conducting degradation study of camphor, biosynthesized from l c labelled mevalonic acid. The biosynthesis of camphor is summarised in Scheme VI. 

The red algal chamigrene sesquiterpene elatol (Structure 2.78) has been shown to deter feeding by reef fishes.93 Specimens of Laurencia elata from Southern Australia show a pronounced seasonal variation in elatol production. Incorporation studies using 14C acetate failed to confirm an acetate-mevalonate path for elatol production.125... 

Mitlin N. and Hedin P. A. (1974) Biosynthesis of grandlure, the pheromone of the boll weevil, Anthonomus grandis, from acetate, mevalonate, and glucose. J. Insect Physiol. 20, 1825-1831. 

The possibility that the biosynthesis of bile acids is regulated by a negative feedback mechanism was supported by early experiments by Thompson and Vars [206] and Eriksson [207], who showed that the rate of bile acid synthesis in rats increased about 10-fold when a bile fistula is made. Bergstrom and Danielsson demonstrated that duodenal infusion of taurochenodeoxycholic acid in bile fistula rats restored the increased synthesis to a normal rate [208]. Danielsson et al. [44] showed that the cholesterol 7a-hydroxylase activity increased in parallel with the bile acid synthesis after cannulation of the bile duct in rats. In a subsequent work by Mosbach et al., it was reported that the incorporation of isotope from labelled acetate, mevalonate and cholesterol but not from labelled 7a-hydroxycholesterol into bile acids was inhibited by duodenal infusion of taurocholate to bile fistula rats [209]. The incorporation of isotope from labelled acetate, mevalonate and cholesterol but not from labelled 7a-hydroxycholesterol was stimulated in perfused livers of cholestyramine-treated rabbits [210]. It was concluded that there are essentially no rate-limiting steps beyond 7a-hydroxycholesterol in the biosynthesis of bile acids from acetate. Since both cholesterol and bile acid biosynthesis was subjected to negative feedback inhibition by bile acids, it cannot be excluded that inhibition of cholesterol biosynthesis precedes inhibition of the bile acid biosynthesis, and that the latter inhibition is secondary to the former. 

Generally, terpenoids have been believed to be biosynthesized via acetate-mevalonate pathway. Aphidicolin, a tetracyclic diterpenoid isolated from moulds, Cephalosporium aphidicola [47] and Nigrospora sphaerica [48], was also proved to be biosynthesized from mevanonic acid [49]. In this pathway, isopentenyl pyrophosphate (IPP), geranylgeranyl diphosphate (GGPP), labdane-type and pimarane-type diterpenoids were proposed to be intermediate precursors of aphidicolin [50]. 

The first specific precursor for terpenoids in the cytoplasma is the Cg molecule mevalonic acid (MVA), which is built via the classical acetate/mevalonate pathway and converted by a series of phosphorylating and decarboxylation reactions into C5 isopentenyldiphosphate (IPP), the universal building block for chain elongation up to C20. In the chloroplasts, the biosynthesis of IPP starts from glyceraldehyde-3-phosphate and pyruvate to give l-deoxy-D-xylulose-5-phosphate (DOXP) via the non-mevalonate pathway as a recently detected alternative IPP route [19]. The reaction is catalyzed by the enzyme DOXP synthase and can be inhibited by a breakdown product of the herbicide clomazone [12]. 

Mosbach et al, (35) have used cholestyramine to confirm that the la-hydroxylation of cholesterol is the rate-limiting step in bile acid synthesis. Because of the loss of cholesterol from the enterohepatic circulation, there is a marked increase in cholesterol synthesis during administration of cholestyramine to rats (27) or man (36). Mosbach et aL, using perfused rabbit liver, showed that the biliary content of glycocholic acid rose from 0.34 to 3.3 mg, while the content of glycodeoxycholic acid fell from 7.4 to 3.7 mg. The conversion of radioactive acetate, mevalonate, or cholesterol to bile acids was increased from five- to twentyfold, but the conversion rate of 7a-hy-droxycholesterol to cholic acid was unchanged. The formation of 7a-hy-droxycholesterol from cholesterol is enhanced by treatment with cholestyramine (37,38). 

The initiation step consists in the formation of IPP (12), and its isomer DMAPP (13). The conventional metabolic pathway to form these two molecules is called the acetate/mevalonate pathway (MVA pathway) in which three molecules of acetyl-CoA (3) condense successively to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) (5), which leads to a key intermediate molecule, namely mevalonic acid (MVA) (6). The latter is further phosphorylated and decarboxylated to form the IPP molecule (12). In Hevea brasiliensis, this cytosolic pathway was described by Lynen and Lebras" " in the early 1960s and reviewed more recently by Kekwick and Ohya." Most experimental validations were obtained by observing the incorporation of radioactive tracers, such as [2- C] MVA and [3- C]HMG-CoA. The incorporation of [ ClIPP into rubber was found to be much faster than that of [2- C] MVA. This was assumed to be due to slow conversion of MVA into IPP." Another explanation might be that the MVA pathway was not exclusive for IPP biosynthesis. Indeed less than 10 years ago, a new, mevalonate-independent, IPP biosynthesis pathway was discovered by Rohmer." This plastidic DXP-MEP pathway initiates with a... 

However, failing incoporations of C-labeled aeetate and sueeessful ones of Relabeled glycerol as well as pyruvate in hopanes and ubiquinones showed isopen-tenyldiphosphate (IPP) to originate not only from the acetate mevalonate pathway, but also from activated acetaldehyde (C2, by reaction of pyruvate and thiamine diphosphate) and glyceraldehyde-3-phosphate (C3) R. In this way, 1-deoxy-pentulose-5-phosphate is generated as the first unbranched C5 preeursor of IPP. 

The biosynthesis of simple monoterpenes has not been studied extensively. Acetate, mevalonate, and geranylpyro-phosphate are incorporated into actinidine. However, neither loganin (2) nor actinidine (8) serves as a precursor for p-skytanthine (9) or its derivatives in Tecoma stans (Bigno-niaceae) (Fig. 36.2). These compounds appear to be derived from an intermediate preceding loganin in the biosynthetic pathway. 

According to earlier studies, acetate, mevalonate, and cholesterol (5) as well as cycloartenol (and lanosterol) (294) are significantly incorporated into tomatidine, solasodine, solanidine, solanocapsine, and/or spirostanols. Cholest-4-en-3-one and 26-hydroxycholesteroI were shown to be the first products of cholesterol metabolism in potato plants (295). The (25/ )- and... 

Similar findings were reported for the acidic part in the PA strigosine [69] and for angeloylic and tigloylic acid [70]. In the case of the acidic part in the PA monocrotaline, it was shown that not acetate, mevalonate, orglutarate (as reported earlier) are involved but it is only formed via isoleucine [71], Stereospecific aspects were also studied in the case of senecic and isatinecic acid [72, 73]. The biosynthesis of trichodesmic acid was studied and it was shown that one part of the Cio acid was formed by (25)-isoleucine or its biosynthetic precursor (25)-threonine and the other C5 unit from (25)-leucine or (25)-valine [74]. The complete labeling pattern of senecic acid (the acidic part in the PAs rosmarinine and senecionine) was studied by NMR experiments and it was stated that the biosynthesis of this acid is processed via two molecules of isoleucine (Scheme 13.2) [75]. 

Fig. 95.1 Terpene biosynthesis pathways and their subcellular localization in the plants. Different classes of terpenes are respectively formed in the cytosol or the plastid by two independent pathways in the plants, that is, acetate-mevalonate pathway (MEV) (cytosol) and methylerythritol 4-phosphate (MEP) or deoxyxylulose 5-phosphate pathway (DXP) (plastid). Mraioterpcmes, diterpenes, and tetraterpenes are derived from IPP and DMAPP Irran the plastidial MEP ot DXP pathway. Sesquiterpenes and triterpenes are biosynthesized from IPP and DMAPP from the cytosol pathway. Black square with a white question mark suggests a possible transport of IPP (isopentenylpyrophosphate) from the plastid to the cytosol. Other metabolites involved in the different steps are DMAPP dimethylallylpyrophosphate, FPP famesylpyrophosphate, GASP D- glyceraldehyde- 3-phosphate, GPP geranylpyrophosphate, GGPP geranylgeranylpyro-phosphate. TPSs in the circle correspond to terpene synthases. Broken arrows show several enzymatic steps (Adapted from Aharoni et al. [8] and Sallaud et al. [154])...

Acetate-mevalonate (Ac-MEV), 2943 Acetate pathway, 2314 Acetic acid (ACE), 851, 1608, 2884 Acetogenic bacteria, 2443 Acetone, 3371... 

Three different metabolic pathways are known to be involved in the synthesis of different classes of phenolic compounds, namely, (1) (Ce — C3) phenylpropanoid derivatives produced by the shikimate/chorismate pathway (2) side chain elongated phenylpropanoids, flavonoids (Ce - C3 - Cg), and few quinones synthesized by the acetate/malOTiate or polyketide pathway and (3) the aromatic terpenoids synthesized throu the acetate/mevalonate pathway. 

Figure 1. C-labelling pattern of B-carotene, phytol and nona-prenyl chain of plastoquinone-9 from [l- C]glucose as found in Lemna gibba and in two green algae (black circles C-enrichment e.g 3% versus 1% at unlabelled positions new IPP pathway). Open circles This labelling was expected if the compounds would have been formed via the classical acetate/mevalonate pathway of IPP formation.

In Chlorella cells grown on [l- C]glucose the sterol carbon skeleton is not labelled according to the acetate/mevalonate pathway. Chondrillasterol, 22,23-dihydrochondrillasterol and ergost-7-enol of Chlorella were labelled in a different way... 

In contrast to chondrillasterol, in the higher plant Lemna gibba, which was also grown on [l- C]glucose (6), sitosterol was labelled as expected via the acetate/mevalonate pathway (Fig. 1). [1- C]Glucose essentially labelled all carbon positions of sitosterol which correspond to C-2, C-4 and C-5 of IPP (Fig. 2). Some minor but significant C-enrichment was also found at carbon positions which correspond to C-I of IPP. This can be interpreted in the frame of the incorporation of C02 (liberated from [l- C]glucose via the oxidative pentose phosphate cycle) into IPP possibly via the leucine bypass or the mevalonate shunt. 

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