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A
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Secondary metabolites differ from primary metabolites (amino acids, nucleotides,
sugars, acyl lipids) in having a restricted distribution in the plant kingdom
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A schematic to illustrate biosynthetic relationships between principal primary
and secondary metabolites (circled)
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• They serve as attractants for pollinators and seed dispersing animals and as agents
of plant–plant competition
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They have a wide range of chemical structures and biological activities
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Primary metabolism

– The biological reactions are essential to maintain life in living organisms and are known as
primary metabolism
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– These carbohydrates are stored and used for differentiation and formation of plant tissues
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– The products formed by secondary metabolism are called secondary metabolites
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g
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Secondary Metabolites Are Divided into Three Major Groups
Plant secondary metabolites can be divided into three chemically distinct
groups: terpenes, phenolics, and nitrogen-containing compounds
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It is generally insoluble in water
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 The terpenes are among the most widespread and chemically diverse groups
of natural products
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 Steroids and sterols are tetracyclic triterpenoids
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 Polyterpenes include the carotenoid pigments and natural rubber
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The basic structural elements of terpenes are sometimes called isoprene units
because terpenes can decompose at high temperatures to give isoprene: Thus all
terpenes are occasionally referred to as isoprenoids
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Larger terpenes include triterpenes (30 carbons), tetraterpenes (40
carbons), and polyterpenoids ([C5] n carbons, where n > 8)
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They are
(1) mevalonic acid pathway and (2) methylerythritol phosphate (MEP) pathway
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This key six-carbon intermediate is then
pyrophosphorylated, decarboxylated, and dehydrated to yield isopentenyl
diphosphate (IPP2)
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Outline of terpene biosynthesis
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Isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP)
combined to yield geranyl pyrophosphate (GPP), leading to monoterpenes
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The function of terpenes in plants is generally considered to be both ecological and
physiological
– Allelophathy
– Insecticidal
– Insect pollinators
– Plant hormone (Abscisic acid, gibberellin)
Among the non-volatile terpene anti-herbivore compounds are the limonoids, a group
of triterpenes (C30) well known as bitter substances in citrus fruit
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For example, the monoterpene esters called pyrethroids that occur in the leaves and
flowers of Chrysanthemum species show very striking insecticidal activity
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Many plants contain mixtures of volatile monoterpenes and sesquiterpenes, called
essential oils that lend a characteristic odor to their foliage
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The chief monoterpene constituent of peppermint oil is menthol; that of lemon oil is
limonene
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Triterpenes that are active against vertebrate herbivores include cardenolides and
saponins
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In humans, they have dramatic effects on the heart muscle through their influence on
Na+/K+-activated ATPases
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Cardenolides extracted from species of foxglove (Digitalis) are prescribed
to millions of patients for the treatment of heart disease
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It was named because of their soaplike properties
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The toxicity of saponins is due to the ability to form complexes with sterols
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The principal role of saponins appears to be as a preformed defense against attack by
fungi
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When the saponins react with sterols in the
membranes of invading fungal hyphae, the result is a loss of membrane integrity
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PHENOLIC COMPOUNDS
Phenylalanine Is an Intermediate in the Biosynthesis of Most Plant Phenolics

Two basic pathways are involved: the shikimic acid pathway and the malonic acid
pathway
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The shikimic acid pathway converts simple carbohydrate precursors derived from
glycolysis and the pentose phosphate pathway to the aromatic amino acids
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The well-known, broad-spectrum herbicide glyphosate (available commercially as
Roundup) kills plants by blocking a step in this pathway
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Shikimic acid is a key intermediate in the synthesis of both aromatic amino acids and
phenylpropanoids
The malonic acid pathway, although an important source of phenolic secondary
products in fungi and bacteria, is of less significance in higher plants
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This reaction is catalyzed by phenylalanine ammonia lyase
(PAL),
PAL is the most studied enzyme in plant secondary metabolism
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The activity of PAL is increased by environmental factors, such as low nutrient levels,
light (through its effect on phytochrome), and fungal infection
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Fungal invasion, for example, triggers the transcription of messenger RNA that codes
for PAL, thus increasing the amount of PAL in the plant, which then stimulates the
synthesis of phenolic compounds
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Phenylpropanoids are important
building blocks of the more complex phenolic compounds

Fig: - Outline of phenolic biosynthesis from phenylalanine onward

FIGURE :- The shikimic acid pathway for biosynthesis of aromatic amino acids in plants
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Enzymes indicated as 1, 2, and 3 are subject to feedback inhibition and thus are
important regulatory enzymes in the pathway
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The enzyme EPSP synthase (EPSPS), which catalyzes the second of three reactions in
the conversion of shikimate to chorismate, is inhibited by the herbicide glyphosate
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The formation of many plant phenolics, including simple phenylpropanoids, coumarins,
benzoic acid derivatives, lignin, anthocyanins, isoflavones, condensed tannins, and
other flavonoids, begins with phenylalanine
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Salicylic acid is involved in establishing the hypersensitive reaction to insect herbivory
and pathogen infection
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FIGURE:- The gene-for-gene model predicts that incompatibility between a pathogen
and host requires complementary dominant avirulence and resistance genes in the
pathogen and host, respectively
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Flavonoids:-

The flavonoids are one of the largest classes of plant phenolics
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They are anthocyanins, the flavones, the
flavonols, and the isoflavones
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Anthocyanins are
glycosides that have sugars at position 3 (generally)
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Isoflavonoids Have Antimicrobial Activity
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FIGURE:- Basic flavonoid carbon skeleton
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Positions on the flavonoid
ring system are numbered as shown
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Nitrogen-containing secondary metabolites, include alkaloids, cyanogenic glycosides,
glucosinolates, and nonprotein amino acids
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Alkaloids are noted primarily for their pharmacological properties and medical
applications
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One of the oldest known is opium, an exudate
obtained from the immature seed capsule of the opium poppy (Papaver somniferon)
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Codeine, morphine, and heroin are structurally related alkaloids
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Heroin is a semisynthetic alkaloid produced by acetylation of morphine
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Codeine is commonly used as a cough suppressant and local anesthetic
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Compounds such as alkaloids, cyanogenic glycosides, glucosinolates, nonprotein
amino acids, and proteinase inhibitors protect plants from a variety of herbivorous
animals
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Phytoalexin production appears to be a common mechanism of resistance to
pathogenic microbes in a wide range of plants
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For example, isoflavonoids are common phytoalexins in the legume family, whereas
in plants of the potato family (Solanaceae), such as potato, tobacco, and tomato,
various sesquiterpenes are produced as phytoalexins
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The point of control is usually the initiation of gene
transcription
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SUMMARY
Primary metabolites such as protein, lipid, carbohydrate, and nucleic acids comprise
the basic metabolic machinery of all cells
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Secondary metabolites, on the other hand, may be found only in specific tissues or at
particular stages of development and have no obvious role in the development or
survival of the organism
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The mevalonic acid and MEP pathways give rise to two 5-carbon compounds,
isopentenyl pyrophosphate and dimethylallyl pyrophosphate that form the basis for the
terpenoid family
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Secondary products include a range of 5-carbon hemiterpenoids through 40-carbon
tetraterpenes and the polyterpenes, rubber
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Deamination of phenylalanine to cinnamic acid, catalyzed by the enzyme
phenylalanine ammonia lyase (PAL), effectively diverts carbon from primary
metabolism into the synthesis of a wide range of secondary metabolites—coumarins,
lignin, tannins, flavonoids, and isoflavonoids—based on simple phenolic acids
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The gene-for-gene model predicts that no infection will occur
when the pathogen and host plant carry dominant avirulence and response genes,
respectively
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The alkaloids are a heterogeneous group of nitrogenous compounds with significant
pharmacological properties
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Question from CSIR-UGC NET

1
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Shikimic acid pathway
2
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Mevalonic acid pathway
4
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Following are certain statements regarding terpene class of secondary

metabolites in plants:
A
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B
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C
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D
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Which one of the following combination of above statements is correct?
1
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A, B and D
3
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A, C and D
(2015-DECEMBER, SET-A, Q-101)
3
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Secondary metabolites are diverse array of organic compounds in plants
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They protect plants against being eaten by herbivores and against being
infected by microbial pathogens
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Terpenes, the largest class of secondary metabolites are synthesized by
methyl erythritol phosphate(MEP) pathway and shikimic acid pathway
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The most abundant classes of phenolic compounds in plants are derived
from phenylalanine
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Alkaloids are nitrogen containing secondary metabolites in plants
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A, B and C
2
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A, C and D
4

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