A full description of the range of plant compounds is outside the scope of this chapter but may be found in a number of recent texts such as Mann (1987) and Luckner (1990). An overview of those classes most commonly found is presented here. The dividing line between primary and secondary metabolism is unclear. The two are connected in that primary metabolites provide the starting material for secondary metabolites which are largely formed from three principal starting materials (Mann, 1987):
1. Shikimic acid, leading to aromatic acids, amino acids, phenols and some alkaloids.
2. Amino acids, leading to alkaloids, amines, glucosinolates and cyano-compounds.
3. Acetate, leading to fatty acids and their derivatives (e.g. polyacetylenes and polyketides), polyphenols and the terpenoids (isoprenoids) (terpenes, steroids and carotenoids) via two pathways: the malonate and mevalonate pathways.
The structures of many compounds are derived from subunits from at least two metabolic pathways. Most of these compounds are relatively rare but some, such as flavonoids, are widespread (Mann, 1987).
Secondary metabolic pathways are inter-related with the pathways of primary metabolism as shown in Fig. 2.1. These pathways, either singly or in combination, give rise to the major classes of secondary metabolites, that is the terpenoids (isoprenoids), the phenolics (including flavonoids, tannins and quinones), nitrogen compounds (particularly the alkaloids), and fatty acids and their derivatives, such as the polyacetylenes. In addition, there are other significant groups, such as the heterogeneous cyanogenic compounds and glucosinolates, some polyketides, and a range of polymeric material, such as structural carbohydrates, lignans, etc. It would appear that the groups that have evolved as secondary compounds have done so because of the availability of precursors and because of their chemical reactivity which has allowed them to be modified into many different shapes (stereoisomeric configurations) which in turn affect their biological properties. As an example, the compound geraniol, a volatile terpenoid which can be considered as an intermediate in the biosynthesis of cholesterol and is widely distributed in flower perfumes (Knudsen et al., 1993) can be oxidised to produce carvone which in one configuration gives caraway its characteristic odour and in another the smells of spearmint. Larger terpenoids are less volatile but the more shapes that can be produced the larger the molecule and so the variety of biological properties increases so that tens of thousands of terpenoids are known to exist, although most remain untested as to their usefulness to humans. A brief description of these classes follows. (For a detailed description of their biosynthesis and full structural range see Bell and Charlwood, 1980; Vickery and Vickery, 1981; Mann, 1987; Luckner, 1990.)
The terpenoids (isoprenoids) The terpenoids or isoprenoids are the most varied group of natural products in their structure, distribution and function. They occur in both plants and animals and act as regulators of reproduction, growth and development, electron transport chains, cell transport mechanisms, membrane constituents, and as attractants and repellents for other organisms. Although they include a diverse range of structures, from small volatile molecules to large non-volatile molecules consisting of interlocking carbon rings (di- and tri-terpenes, including the steroids) or open chains (the carotenoid pigments), they can be considered as arising from two or more units of a 5-carbon building block, the isoprene unit.
The so-called monoterpenes are made up of two, the sesquiterpenes of three, the diterpenes of four, the triterpenes of six and the carotenoids of eight to ten isoprene units, but the actual biogenic isoprene unit is not isoprene itself, which is rare in nature, but isopentenyl pyrophosphate formed from mevalonic acid pyrophosphate (Vickery and Vickery, 1981).
Animals can synthesise only a limited number of terpenoids and may depend on an adequate dietary intake to survive; for example, insects rely on plants for sterols as hormone precursors and many phyla require an adequate intake of carotenoids for both protective colouration and as part of their visual apparatus, but seed-bearing plants can synthesise the whole range.
The simpler terpenoids (mono-, sesqui- and di-) are common in seed-bearing plants, but rare in other orders. All green plants can synthesise linear isoprenoids, including the phytyl side chain which confers biological activity on the chlorophyll molecule and thereby permitting photosynthesis. The monoterpenes are responsible for the fragrance of many plants. Usually many related structures are present and the composition of the mixture, rather than the individual components, is important in the attraction or repulsion of insects, or as germination inhibitors in competing species (Swain, 1974). Gershenzon and Croteau (1991) provide a recent review of the ecological role of terpenoids.
Phenolics, including flavonoids, tannins and related structures Plant phenolics include a wide variety of structures from the simple phenols and their derivatives to complex tannins. The flavonoids comprise a range of compounds derived from flavone, which has a gamma-pyrone ring with ether-linked oxygen joined to two aromatic rings, designated A and B. Most are water soluble but some are highly ether soluble and occur in waxes and pigments. They are ubiquitous in plants, occurring in all angiosperms, gymnosperms, ferns, mosses and liverworts. With some exceptions they are absent from algae, fungi and bacteria (Harborne, 1991). They usually occur in living cells as glycosides. Most widespread are those having 3'4'-dihydroxy substituents on the B ring. Biosynthetically they are of mixed origin: the B ring is derived from shikimic acid and the A ring from acetate. Putatively more advanced families such as the Leguminosae contain representatives of all structural types, but more primitive taxa contain a fewer number. Some features of flavonoid structure can be discerned as more advanced than others from their present-day distribution. The most primitive flavonoid-containing division of the plant kingdom is the Chlorophyta, the more advanced members of which contain C-glycosyl derivatives of the flavone apigenin. C-glycosides are found in mosses, liverworts, ferns and some less advanced angiosperms and their presence is considered to be a primitive feature. In the more advanced angiosperms they are superseded by the synthesis of the equivalent O-glycosides and hydroxylation of the oxygen-containing ring, which has enabled them to synthesise a greater range of flavonoid compounds.
Many flavonoids are highly coloured and are components of flower pigments. Many apparently colourless flavonoids are visible to insects, and play a part in insect pollination. They are highly absorptive in the ultraviolet (UV) and near ultraviolet regions of the spectrum and may have served as protectants from excess UV radiation as plants moved onto land. They are also antioxidants and it was suggested as early as 1969 that they may serve to protect lipids and polyacetylenes in plants from oxidative damage (McClure, 1975). The majority are non-toxic to mammals but have been shown capable of exerting a range of physiological effects, including anti-viral and immune-modulatory activity. Some iso-flavonoids have oestrogenic activity and the group known as rotenones are toxic to insects and fish (Harborne, 1988, 1991). Many phytoalexines, anti-fungal compounds only produced when the plant is damaged or attacked, are isoflavonoids, for example the pterocarpans medicarpin, pisatin and phaseollin produced by species of Leguminosae (Harborne, 1988; Luckner, 1990).
Tannins are high molecular weight phenolic compounds which have an astringent taste and have been used to tan animal skins to produce leather, because they form cross-linkages with the protein in the hide and prevent deterioration. They can be divided into two types: hydrolysable and condensed. The hydrolysable type can be hydrolysed with hot dilute acid, the condensed cannot. The condensed tannins are polymers of flavonoids: two flavonoids condense to form a dimer linked by C—C bonds known as a proanthocyanidin. This dimer then polymerises further, possibly by a non-enzymic mechanism, to give the condensed tannins. Hydrolysable tannins are not flavonoid derivatives. The most common type are sugars with the hydroxy groups substituted by phenolic acids (Vickery and Vickery, 1981; Swain, 1979). Tannins can act as defensive chemicals in plants (Hagerman and Butler, 1991).
Alkaloids are a heterogeneous collection of nitrogen-containing compounds, most of which show some degree of toxicity to mammals and at low doses exert disturbing effects on metabolism (Hartmann, 1991). Over 10000 have now been described (Southon and Buckingham, 1989). Many are the active ingredients in traditional remedies and/or are used in a pure form as drugs. They probably represent the most effective protectants against mammalian herbivory to have evolved. Mostly bitter tasting, they are frequently accumulated by insects to deter bird and mammal predators.
So-called 'true' alkaloids contain nitrogen as part of a heterocyclic ring structure and are synthesised from protein common amino acids, either ornithine or lysine, or the aromatic phenylalanine, tryptophan and tyrosine. They are absent from ferns and gymnosperms. In addition, a number of 'pseudoalkaloids' are known, which are compounds formed from a higher terpene with nitrogen added at a late stage in the biosynthesis; also 'protoalkaloids' which do not contain heterocyclic nitrogen, but are derived from aromatic amines.
Alkaloids are mostly found in fungi or the higher vascular plants. A few are accumulated by insects from feeding on alkaloid-containing plants while others are synthesised de novo in some insects and amphibia (Harborne, 1988). They are rare in bacteria and cyanobacteria (Swain, 1974; Vickery and Vickery, 1981; Leete, 1980; Fodor, 1980).
Non-protein amino acids, that is amino acids which are not found as constituents of proteins, are widely distributed in plants, bacteria and fungi but are particularly common and varied in the Leguminosae. They are formed by changing pre-formed protein amino acids, by modifications of pathways to protein amino acids and sometimes by other routes. Many of those of higher plants have been shown to be toxic or deterrent to some animals (Bell, 1980b; Rosenthal, 1991). A variety of amines also occur in higher plants (Smith, 1980).
Many higher plants and fungi contain compounds which release hydrogen cyanide when they are crushed or treated with dilute acids. In higher plants the precursors are mostly glycosides of alpha-hydroxynitriles, but cyanogenic lipids are also known. Hydrogen cyanide production has obvious defensive properties (Conn, 1980; Nahrstedt, 1988; Seigler, 1991). Glucosinolates are a closely related group of compounds which can be hydrolysed by the enzyme myrosinase to yield glucose and an isothiocyanate (mustard oil) as the major product. These have so far only been found in higher plants, in some dicotyledon families, and are known to have ecological roles as both attractants and repellents of other organisms (Underhill, 1980; Chew, 1988; Louda and Mole, 1991).
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