SERVIDOR WEB

As indicated earlier in Fig. 18.2, the basic metabolic pathways con­

stitute the origins of secondary plant metabolism and give rise to a vast array of compounds; some of these are responsible for the char­

acteristic odours, pungencies and colours of plants, others give a par­

ticular plant its culinary, medicinal or poisonous virtues and by far the greatest number are, on current knowledge, of obscure value to the plant (and to the human race). However, a number of modern authors suggest that secondary metabolites, rather than constituting waste products of metabolism, are biosynthesized to aid the plant’s survival. Notwithstanding, it may be that in some instances the pur­

pose for which these compounds were produced no longer exists but the biosynthetic pathway has survived. For an interesting discussion and references, see E. J. Buenz and S. J. Schepple, J. Ethnopharmacol, 2007, 114, 279. The possible functions in the plant of one large group of secondary metabolites, alkaloids, are discussed in Chapter 26.

Monoterpenes, C₁₀

Stereospecific reactions in terpenoid biogenesis. A, Formation from mevalonic acid (MVA) of isopentenyl pyrophosphate (IPP) by trans elimination; isomerization to dimethylallyl pyrophosphate (DMAPP).

B, Association of the two 5-C units with trans elimination of hydrogen.

C, As B but involving formation of cis double bonds as found in rubber.

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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 non­terpenoid 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⁻

* HO O

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.

18

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, Huner­Allnach KL, Tari LW 2000 Plant aromatic l­amino acid decarboxylases: evolution, biochemistry, regulation and metabolic engineering applications. Phytochemistry 54(2): 121–138

Grayson DH 2000 Monoterpenoids (a review covering 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

Ramos­Valdivia 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 mevalonate­independent 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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