BASIC METABOLIC PATHWAYS AND THE ORIGIN OF SECONDARY METABOLITES 165
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The 1-deoxy-d-xylulose (triose/pyruvate) pathway. Following its discovery in 1956 mevalonic acid came to be considered the essential precursor for all isoprenoid syntheses. However, in 1993 M. Rohmer et al., (Biochem. J., 1993, 295, 517) showed that a non- mevalonate pathway existed for the formation of hopane-type triter- penoids in bacteria. The novel putative precursor was identified as 1-deoxy-d-xylulose-5-phosphate, formed from glucose via condensation of pyruvate and glyceraldehyde-3-phosphate. Subsequent steps includ- ing a skeletal rearrangement afford isopentenyl pyrophosphate—the same methyl-branched isoprenoid building block as formed by the MVA route.
It was soon demonstrated that this novel route to IPP was also opera- tive in the formation of monoterpenes (Mentha piperita, Thymus vul- garis), diterpenes (Ginkgo biloba, Taxus chinensis) and carotenoids (Daucus carota). This raised the question of to what extent the two pathways co-existed in the plant and it was hypothesized that the classi- cal acetate/mevalonate pathway was a feature of cytoplasmic reactions whereas the triose/pyruvate sequence was a characteristic of the plas- tids. This did not exclude either the movement of plastid-synthesized IPP and DMAPP from the organelle to the cytoplasm or the transloca- tion of a suitable C5-acceptor to the plastid. Evidence accumulating indicates a cooperative involvement of both pathways. Indeed recent work on the biosynthesis of the isoprene units of chamomile sesquit- erpenes (K.-P. Adam and J. Zapp, Phytochemistry, 1998, 48, 953) has shown that for the three C5 units of both bisaboloxide A and chamazu- lene, two were mainly formed by the non-mevalonate pathway and the third was of mixed origin.
The deoxyxylulose (DOX) pathway has helped explain the previ- ously reported rather poor incorporations of MVA into certain iso- prenoids. Thus V. Stanjek et al. (Phytochemistry, 1999, 50, 1141) have obtained a good incorporation of labelled deoxy-d-xylulose into the prenylated segment of furanocoumarins of Apium graveolens leaves, suggesting this to be the preferred intermediate.
Fig. 18.21 illustrates how [1-13C]-glucose, when fed to plants, can be used to differentiate IPP and subsequent metabolites, formed either by the MVA pathway or the DOX route.
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Recently, considerable attention has been directed to the possible ecological implications of secondary metabolites not only in relation to plant–plant interaction but also concerning the interrelationship of plants and animals. Various insects sequester specific alkaloids, iri- doids, lactones and flavonoids which serve as defensive agents or are converted to male pheromones. The literature concerning chemical ecology is regularly reviewed and the volatile isoprenoids that control insect behaviour and development have been reported on (J. A. Pickett, Nat. Prod. Rep., 1999, 16, 39). The enzymology associated with sec- ondary metabolism is now receiving considerable attention and, with respect to alkaloid formation, a number of enzymes associated with the biosynthesis of the tropane, isoquinoline and indole groups have been prepared. The biosynthetic origins of those metabolites of medicinal interest are considered in more detail in Part 5, which is arranged principally according to biogenetic groups.
Although a number of the biogenetic groups are characterized by particular skeletal structures, the actual chemical properties of particu- lar compounds are determined by the acquisition of functional groups.
Thus, terpenes may occur as alcohols (menthol), ethers (cineole), ketones (carvone), etc., and as such have similar chemical properties to non-terpenoid compounds possessing the same group; aldehydes, as an example of a functional group, may be of aliphatic origin (citronel- lal), aromatic (cinnamic aldehyde), steroidal (some cardioactive gly- cosides); and resulting from the introduction of a heterocyclic system one biogenetic group may possess some of the chemical properties of another (e.g. steroidal alkaloids).
A particular group of compounds may also involve different bio- genetic entities; thus, the complex indole alkaloids contain moieties derived from both the shikimate and isoprenoid pathways. In contrast, the same structure, as it occurs in different compounds, may arise from different pathways, as has been previously indicated with the forma- tion of aromatic systems.
Stress compounds
These are compounds which accumulate in the plant to a higher than normal level as a result of some form of injury, or disturbance to
CHO CH CH2OP HO
CH3 CO COO−
* O
HO
HO OH
OH OH
*
CH3 C SCoA
O
*
CH3 CO CH3 CO SCoA
*
*
*
* CH3 CO C CH CH2OP
OH H
HO
C CH2 H2C H3C
CH* 2OPP
*
C CH2 H2C H3C
CH2OPP
*
*
CH2 C CH2 CH2OH
* HOOC *
OH CH3
*
[1-13C] -Glucose
Acetyl-CoA
Acetylacetyl-CoA
Mevalonic acid
Isopentenyl diphosphate
Isopentenyl diphosphate 1-Deoxyxylulose-5-P
Pyruvate Glyceraldehyde-3-P
Thiamine diphosphate participation
*
Fig. 18.21 *
The incorporation of [1-13C]-glucose into isopentenyl diphosphate: left, via the mevalonic acid pathway; right, via the deoxyxyulose pathway.
BASIC METABOLIC PATHWAYS AND THE ORIGIN OF SECONDARY METABOLITES 167
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the metabolism; they may be products of either primary or second- ary metabolism. Common reactions involved in their formation are the polymerization, oxidation or hydrolysis of naturally occurring substances; many however, are entirely secondary in their formation.
A number of environmental and biological factors promote the synthe- sis of stress compounds and these include mechanical wounding of the plant, exposure to frost, ultraviolet irradiation, dehydration, treatment with chemicals, and microbial infection (see phytoalexins below). The production of such compounds has also been observed in cell cultures subjected to antibiotic treatment and in cells immobilized or brought into contact with calcium alginate. Examples of the latter include the formation of acridone alkaloid epoxides by Ruta graveolens, and the increased production of echinatin and the novel formation of a preny- lated compound by Glycyrrhiza echinata cultures.
Stress compounds are of pharmaceutical interest in that they may be involved in various crude drugs formed pathologically (e.g. some gums and oleoresins) and potential drugs (gossypol); they are impli- cated in the toxicity of some diseased foodstuffs and they play a role in the defensive mechanism of the plant. In the latter area, the phy- toalexins have received considerable attention in recent years and can be regarded as antifungal compounds synthesized by a plant in greatly increased amounts after infection. The antifungal isoflavonoid ptero- carpans produced by many species of the Leguminosae are well known, see ‘Spiny restharrow’. Other phytoalexins produced in the same fam- ily are hydroxyflavanones, stilbenoids, benzofurans, chromones and furanoacetylenes. Sesquiterpene phytoalexins have been isolated from infected Ulmus glabra and Gossypium. In the vine (Vitis vinifera) the
fungus Botrytis cinerea acts as an elicitor for the production of the stilbenes resveratrol (q.v.) and pterostilbene.
Chemically, stress compounds are, in general, of extreme variabil- ity and include phenols, resins, carbohydrates, hydroxycinnamic acid derivatives, coumarins, bicyclic sesquiterpenes, triterpenes and steroidal compounds. For the promotion of stress compounds in cell cultures see Chapter 13 and Table 13.1.
Further reading
Facchini PJ, Huner-Allnach KL, Tari LW 2000 Plant aromatic l-amino acid decarboxylases: evolution, biochemistry, regulation and metabolic engineering applications. Phytochemistry 54(2): 121–138
Grayson DH 2000 Monoterpenoids (a review covering mid-1997 to mid- 1999). Natural Product Reports 17(4): 385
Knaggs AR 1999, 2000 The biosynthesis of shikimate metabolites. Natural Product Reports 16(4): 525–560; 17(3): 269–292
Kruger NT, Hill SA, Ratcliffe RG (eds) 1999 Regulation of primary metabolic pathways in plants. Kluwer, Dordrecht, Netherlands
Ramos-Valdivia AC, Van der Heijden R, Verpoorte R 1997 Isopentenyl diphosphate isomerase: a core enzyme in isoprenoid biosynthesis. Natural Product Reports 14(6): 591–604
Rohmer M 1999 The discovery of a mevalonate-independent pathway for isoprenoid biosynthesis in bacteria, algae and higher plants. Natural Product Reports 16(5): 565–574
Seigler DS 1998 Plant secondary metabolism. Kluwer, Dordrecht, Netherlands
Singh BK 1999 (ed) Plant amino acids—biochemistry and biotechnology.
Marcel Dekker Inc, New York
Wink M (ed) 1999 Biochemistry of plant secondary metabolism. Sheffield Academic Press
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