Lecture 1
June 17, 2005
Introduction:
 Phenolic metabolism
 Isoprenoid metabolism
 Special Nitrogen metabolism
These lectures will be based on
Plant Biochemistry
Edited by J.B.Harborne et.al
Academic press 1997
===================
What are phenols?
Phenols are compound composed of one or more aromatic benzene rings with one or more
hydroxyl groups (C-OH). This enormous class includes over 4000 compounds that are
chemically distinct from terpenes.
Although the essential oils are often classified as terpenes, many of the volatile chemicals are
actually phenolic compounds, such as eucalyptol from Eucalytus, citronellal from
Cymbopogon sps oil and thymol from Trachyspermum ,carvacrol thyme and eugenol from
Syzygium
Three different route leads to plant phenolics

Shikimate /arogenate pathway produces aromatic amino acids like phenylalanine,
tyrosine and tryptopan. The first one is very important as precursor of phenolics.
Several major classes of secondary metabolites such as, flavonoids, coumarins , lignan,
stilbenes and benzoic acid are derived from phenylalanine.
Some phenolics such as hydroxybenzoate galate are formed from dehydroshikimate are formed
from the intermediates
Dehydroshikimate
Some quinones are formed from chrosimate through succinylabenzoate path way

Acetate /malonate pathway(polyketide pathway)
This pathway leads to some quinones but also to various side chains that forms flavonoids and
isoflavonoids.

Actate mevlonate pathway: this pathway leads by dehydrogenation of some aromatic
terpenoids mostly monoterpenes.
Function of phenols
 Plant-Plant Interactions: Allelopathy of Juglans regia is due juglones present
 Mechanical Support: lignin and suberin provide hard ness to cell wall and wood
so much desired for stem.
 Attraction Pollinators (Pigments and Scent)
 Repellent:
 Resistance to Pathogen (Phytoalexin)
 Protect From Predators: herbivorous
Major classes of phenols
Number of
carbon
6
Carbon
Skelton
C6
Compound class
7
C 6 - C1
hydroxybenzoate
8
C 6- C2
Acetophenones
Simple phenols
Benzoquinones
Compound example/
structure
Hydroquinone
catechol
Plant where
they occur
Arbutin
family
Rosaceae
Ericaceae
4-hydroxybenzoates
Cell Wall
bound
4Picein in abies
hydroxyacetophenones and picea
9
C 6 - C3
phenyacetates
Hydroxycinnamte
Caffeate
Phenylpropenes
Eugenol
Taraxacum
Chlorogenate
in solanaceae
Is syzigium etc
Coumarins
Esculetein
Cichorium
Chromones
10
13
14
15
18
30
n
C 6 - C4
C6 _- C1C6
C6 _- C2C6
C6 _- C3C6
( C6-C3 )2
( C6-C3-C6
)2
( C6-C3 )n
( C6) n
(C6 - C3 C6)n
2-methyl-5hydroxy-7
methoxychromone
Naphthaquinones Juglones
Xanthones
1,3,6,7hydroxyxanthone
Stilbenes
Resveratrol
anthraquinones
emodin
flavonoids
quercetin
Lignan
pineresinol
Biflavonoids
amentoflavones
Catechol
melanins
Naphthalene polymers
Gallotannins
Hydrolizable
tannin
Condenssed
tannins
Summarized image of these compounds can be seen in page 389 of text
eugenia
Juglans
mangifera
Eucalyptus
Rheum
Rutin in
fagopyrum
Picea and
pinus
Many
gymnosperms
Ascomycetes
Rhus semilata
Lecture 2
June 19, 2005
Carbohydrate metabolism
Erythrose 4-phosphate
+
Phosphatenolatepyruvate[PEP]
Shikimate
Chorismate
Prephenate
Anthranilate
L-arogenate
L-tryptophan
L-tyrosine
L-phenyalanine
The sequence of reaction needs 11 enzymes
E 1-1
E 1-2
E 1-3
E 1-4
E 1-5
E 1-6
E 1-7
E 1-8
E 1-9
E 1-10
2-Dehydro-3-deoxyphosphoheptanoate aldolase ( =DAHP
synthatase)
3-dehydroquinate synthase
3-dehydroquinate dehydratase
Shikimate -3 hehydrogenase
Shikimate kinase
3-phosphoshikimate 1-carboxyvinylsynthatase (EPSP synthase)
Chorismate synthase
Chorismate mutase
Prephenate aminotransferase
Arogenate dehydrogenase (decarboxylating)
E 1-11
Arogenate dehydratase
( decarboxylating )
Shikimate pathway
Over all reaction for the pathway
D-Erythrose-4-Phosphate
H2O
Condensation
Pi
+
Phosphoenoylpyruvate
2-Dehydro-3deoxyphosphoheptanoate
aldolase ( =DAHP
synthatase)
7-Phospho-2-dehydro-3-deoxy-Darabinoheptulosonate [DAHP]
Β-elimination,a reduction
Intramolecular aldol
condensation
NAD+
NADH, H+, Pi
Co++
3-Dehydroquinate
synthase
3-Dehydroquinate
Stereospecific ciselimination of water
3-Dehydroquinate
dehydratase
H2O
3-Dehydroshikimate
NADPH, H+
Shikimate
dehydrogenase
Reduction
NADP+
------------------
Shikimate
ATP
Mg++
C-phosphorylation
ADP
Shikimate kinase
3-Phosphoshikimate
Phosphoenoylpyruvate
3-phosphoshikimate-1carboxyvinyl transferase
Pi
(EPSP synthase)
3-Phospho-5-enoylpyruvylshikimate
Chorismate synthase
Pi
Chorismate
Lecture 3
June 19,2005
Larsen PO, Onderka DK, Floss HG. [1975]
14C-labelled shikimic acid and double labelled shikimic acid tritiated stereospicifically at C-6 are
incorporated into 3-(3-carboxyphenyl) alanine, 3-(3-carboxy-4-hydroxyphenyl) alanine,
phenylalanine, and tyrosine in Reseda lutea L., Reseda odorata L., Iris x Hollandica cv. Prof.
Blauw, and Iris x hollandica cv. Wedgwood.
The experiments with 14C-labelled shikimic acid confirm that the aromatic carboxyl groups and
rings in 3-(3-carboxyphenyl) alanine and 3-(3-carbocy-4-hydroxyphenyl) alanine derive from the
carbocyl group and ring in shikimic acid whereas the experiments with double labelled shikimic
acid demonstrate that the pro-6S-hydrogen atom is retained and the pro-6R-hydrogen atom lost
in the biosynthesis of 3-(3-carboxyphenyl) alanine, phenylalanine, and tyrosine in the plants
used. 3H was located in the ortho-position in the aromatic rings of phenylalanine and tyrosine
but in a position para to the alanine side chain of 3- (3-carboxyphenly) alanine. No 3H was
found in 3- (3-carboxy-4-hydroxyphenyl) alanine. This supports a derivation of the last two
compounds from chorismic acid via isochorismic acid, isoprephenic acid, and 3'carboxyphenylpyruvic acid and 3'-carbocy-4'-hydroxyphenylphruvic acid. The 3H/14C ratio in 3(3-carboxyphenyl) alanine was found higher than in the precursor used. This isotope effect must
operate by competition between the pathways from isoprephenic acid to 3'carbocyphenylpyruvic acid and to 3'-carbocy-4'- hydroxyphenylpyruvic acic.
The proposed biosynthetic pathways for the two carboxy-substituted amino acids are in
agreement with their distribution patterns in the plant kingdom and suggest that they may derive
from minor changes of enzymes involved in the general pathways of aromatic biosynthesis.
Lecture 3 continued …..
June 21, 2005
Phenylalanine / hydroxycinnamate pathway
The enzymes involved in the pathway are
1. Phenylalanine ammonia lyase [PAL]
2. Cinnamate hydroxylase ie E-cinnamate,NADPH,Oxygen
oxidoreductase
3. 4-Coumarate3-hydroxylase
4. Ferulate 5- hydrxylase
5. Caffeate/5-hydroxyferulate methyltransferase
6. Hydroxycinnmoyal-CoA ligase
7. Hydroxycinnamate O-glucosyltransferases
+++ For details on PAL page 393
Mechanism:
The elimination of ammonia from -amino acids is a chemically difficult process. While
the non-acidic -proton has to be abstracted, the much more acidic ammonium protons
must remain untouched to maintain the leaving group ability of this positively charged
group.
Histidine and phenylalanine ammonia-lyases (HAL and PAL) possess a
catalytically essential electrophilic group which has been believed to be
dehydroalanine for 30 years.
Recently [2002], the X-ray structure of HAL has been solved. The electron density was
not consistent with dehydroalanine but showed the presence of methylidene
imidazolone (MIO) instead. The high electrophilicity of this prosthetic group as well
as the geometry at the active site support a previously proposed mechanism involving a
Friedel–Crafts-type attack at the aromatic ring of the substrate. Further biochemical
evidence for this unprecedented electrophile-assisted ammonia elimination is also
presented. Although no X-ray structure of PAL has been published as yet,
spectrophotometrical evidence for the presence of MIO has been provided. Based on
above, a chemical model for the PAL reaction has been proposed.
This part has been taken form www.usm.my/pharmacy/...
Lecture 4
June 24, 2005
In our previous lecture we talked on formation of hydroxycinnamates. This occurs in two
forms E-isomer and Z-isomer. Latter is formed either by photochemical or by enzymes.
Hydroxylases:
Cinnamate hydroxylase ie E-cinnamate,NADPH,Oxygen oxidoreductase &
4-Coumarate3-hydroxylase is monooxygenaes and introduces one molecule of oxygen
which split and single atom is introduced in the reaction and second atom is reduced to
H2O by a second oxidizable substrate like NADPH.
Hydroxylases are of two types: (1) the cytochrome bound P-450 dependent oxygenases
like
Cinnamate hydroxylase ie E-cinnamate,NADPH,Oxygen oxidoreductase
Ferulate 5- hydrxylase
(2) The soluble phenolase which catalyses introduction of 2nd hydroxyl group in a
phenol. Example of this class is 4-Coumarate3-hydroxylase
NIH shift: This is an intramolecular migration of the proton that is displaced to an
adjacent Ortho position. This is named after National Institute of Health [NIH] USA.
This proceeds via arene oxide intermediate.
The phenolase 4-coumarate hydroxylase introduces second hydroxyl ortho (3-position)
To the the hydroxyl group of 4-coumarate.The hydroxylases specifically accept
Hydroxycinnamte conjugate..
This can be seen in case of formation caffeate at the 4-coumaroyl moiety of
4-coumaroyl –CoA or of 5-O-(4-coumaroyl)-shikimate.
O-methyaltion of caffeate and hydroxyferulate yields ferulate and sinnapate resepectively.
Caffeate/5-hydroxyferulate methyltransferase involvoled uses S-adenosyl-L-methionine.
The hydroxycinnamates path ways are much important as is used by predominant type
Side chain:
1. Condensation (side chain elongation) sequential reaction with three malonylCoAs,leads to the formation of flavonoids.Here acetate unit is added and CO2 liberates.
2. Reduction (NADPH dependent) leads to lignin precursor, Hydroxycinnamyl
alcohol.
3. Degradation (side chain shortening) leads to Hydroxybenzoates.
4. Conjugation through attachment of Hydroxy or amino group bearing molecule
leads to esters or amides in rare cases attachment to phenolic hydroxyl group
leads formation of glycosides.
Lecture: 5
June 28, 2005
Hydroxybenzoates: Hydroxybenzoate (C6-C1) seem to be universally distributed in
Plants as 4-hydroxybenzoate, Protocatechulate, vanilate, Gallate, Syringate. They
may be soluble conjugated form as well as bound to cell wall. Trihydroxy derivative,
Gallate is
Often present as gallotanin.Another
hydroxybenzoate)
trihydroxybenzoate
is
salicylate
(
2-
This compound is major constituent of Gaultheria fragarantissima (oil of
wintergreen).
It is likely that there are several pathways leading to individual hydroxybenzoate
in plant.
One pathway is degradation of side chain by removal of acetate. The reaction is
Proposed to proceed via CoA –ester. The second pathway in fig above also can
Not be excluded.
The substitution pattern depends on the hydroxycinnamte precursor.
Hydroxylation and methylation may occur with hydroxybenzoates as in
Phenylalanine/ hydroxycinnamte pathway.
Postulated Biosynthetic pathways to Hydroxybenzoate
Hydroxybenzoates may also be formed by shikimate /arogenate pathway at 3hydroxycinnamte point.
Hydroxybenzoate is a precursor for a number of comounds like gallates. Hydroxycinnamate
through CoA-ester and may reduce to benzaldehyde which gives cocain.
Lecture 6
June 28,005
Falovonoids: C15 aglycone are occurs in various structural classes according to
oxidation state.
They have a central pyran ring.Structures within these classes are modified
Hydroxylation and
Methylation. They may also be glcosylated of acylate. The enzymes involved are
shown below.
Enzyme
1. Chalcone synthase
Examples
Malonyl-CoA ;4-coumaroyl-CoA malonyltransferase
2. Acetyl-CoA carboxylase
Acetyl-CoA ;CO2 ligase
3. Chalcone isomerase
4. Flavone synthase I (=2hydroxy flavonesynthase)
5. Flavone synthase II (=2hydroxy flavonesynthase)
6. Isoflavone synthase 1(=2hydroxy synthase)
7. Flavonone 3hydroxylase(deoxygenase)
8. Dihydroflavonol/dihydroflavo
none 4-reductase
9. Flavonol
synthase(dioxogenase)
10. Flavan-3,4-diol 4-reductase
Flavonone; 2-oxoglutarate; Oxygenoxidoreductase
(dioxogenase;dehydroreductase.
Flavonone;NADPH
;Oxygenoxidoreductase(dioxogenase;dehydroreductase.
Flavonone;NADPH
;Oxygenoxidoreductase(dioxogenase;dehydroreductase..
Naringenin; 2-oxoglutarate; Oxygenoxidoreductase.
Dihydroflavonol/dihydroflavone;NADPH;oxidoreductase
Dihydroflavonol; 2-oxoglutarate;Oxygenoxidoreductase
Flavan-3,4-diol;NADPH oxidoreductase
Chalcone synthase ( Malonyl-CoA ;4-coumaroyl-CoA malonyltransferase) is the rate
limiting enzyme in flavonoid syntheis .It is because this catalyses the formation of
basic C15 and channels Hydroxycinnamte to Flavonoid biosynthesis.
This enzyme Mr 78000-88000 is dimeric protein. pH optimum ranges from 7.5 to 8.5.
Naringen chalcone is the first intermediate monohydoxyl B –ring typically present in
all flavonois.
In this reaction Acetate unit is added stepwise to 4-coumaryl –CoA.
A random orientation of Acetate unit forms the “A” ring of the flavonoids. The
chalcone synthase uses CoA ester as immediate substrates. And malonyl-CoA is
supplied byATP dependent action acetyl CoA carboxylase.
Chalcone provides the basic skeleton for different class of Flavonoids.
Lecture 7 + 8
June 30, 2005
Substitution of flavanoids
Basic substitution patterns of flavanoids are partly determined by mode of action
and substrate preference of chalcone synthase (5, 7 hydroxyl group of A ring and
4’ hydroxyl group of B ring) and partly on enzymes of the aglycone pathway.
Further hydroxylation as well as methylations of Hydroxyl group especially on BRing is catalyzed by flavanoid specific hydroxyases and O-methyl transferases.
The 3’ and 5’ hydroxylation are catalyzed by cytochrome P-450 enzymes.
Distinct position specific SAM dependent O-methyltranseferases leads to some
common
B-Ring methoxylated flavanoids.
The common substitution patterns
anthocyanindins are shown below
of
naturally
occurring
flavnoids
and
Anthocyanidin
Flavonol
Substitution
Kaempferol
Quercetin
Myricetin
Isorhamnetin
Larycitrin
Syringetin
Pelargonidin
Cyaniding
Delphinidin
Peonidin
Petunidin
Malvidin
3’
H
OH
OH
OCH3
OCH3
OCH3
5’
H
H
OH
H
OH
OCH3
Methyltransferase activities also lead to some rare polymethoxylated flavanoids of
Saxifragaceae.
When sugar molecule is conjugated to a flavone molecule, it is called flavonoidal
glycosides. Two major types of glycosides 3-O-glycoside and 3-C –glycosides.
The rutin of fagopyrum esulentum is a 3-O-glycoside and the Vitexin is an 8-Cglucosides.
Tannins:
Water soluble plant polyphenols which cause protein precipitation from aqueous
solution are called tannins.
They are of two types Hydrolysable and condensed (non-Hydrolysable).
Non-hydrolysable phenols are also called pro-anthocyanidins because they give
anthocyanidin on acid hydrolysis.
For our purpose we consider condensed tannins only. They are present in areca
nut, catechu etc and are getting more attention as antioxidants.
Flavanoids may bind to each other and form dimmer as in case of amentoflavone.
The linkage is between C-5’ of B-ring and C-8 of A-ring. The carbonyl group at C4 is always free. They are rather lipophilic and are locate outside cell wall.
amentoflavone showing linkage between C-5’ of B-ring and C-8 of A-ring.
Oligo and polymeric flavanoids [condensed tannins] such as pro-anthocyanidins
always have bonds between C-4 and C-8.These products accumulates as higher
molecular weight [Mr 2000 to 7000].
The monomeric flavanoid subgroups involved anthocyanidin biosynthesis are 3,4diols, although 4-ols are also frequently found. They form polymerized flavan 3-ols
andflavan, respectively.
The flavan 3-ols units may occur in four isomeric forms although only two of them
are commonly found in nature.
The catechins are formed by 2, 3-trans pathway:
The pathway for epicatechins is not yet confirmed. It is thought that there is a
stereospecific capture of the intermediate carbocations or quinone methides by the
end products flavan-3-ols.
Quinones: The common natural quinones are
Some of the key enzymes leading to the naphthanoate intermediates are
described from bacterial sources.
Secondary benzoquinone are constituents of Hyphomyces and Basidiomyces
[Fungi].
An example of occurrence of benzoquinone in higher plant is arbutin
[Hydroquinone glucoside] in rosaceae and ericaceae.
Naphthoquinones are responsible for pigmentation of colored heartwood an
example of its presence is Juglans regia
Anthraquinones are present in Bacteria,Fungi,and lichens; in higher plants they are
present in Caesalpiniaceae, polygonaceae, Rhamnaceae and Rubiaceae.
There are at least two structural types of anthraquinones, one bearing hydroxyl
group in C-ring and the other bearing hydroxyl group in both A and C ring.
There are two pathway for quinoid structure [1] Polyketide pathway and [2] succinyl
benzoate pathway.
The occurrence of both these path way are exemplified by naphthaquinone
plumbagin from Plumbago europea / plumbago [?] anthraquinone emodin in
Rhamnus frangula are formed by polyketide [ Actate/Malonate derived quinones.
while naphthaquinone lawsone Impatiens balsamina and anthraquinone alizarin of
Rubia tinctoria are derived from succinylbenzoate pathway.
Succinylbenzoate pathway:
In the shikimate pathway [lecture 2] we have come across a compound
“chorismate”. The Chorismate is catalyzed by isochorismate hydroxymutase to
isochorismate.The latter in presence of 2-Oxoglutarate and thymine pyrophosphate
is converted to 2-succinylbenzoate.This reaction sequence constitutes hitherto
unprecedented aromatization process.
The initial steps in the succinylbenzoate pathway have been partially characterized
at enzyme level. The metabolite steps beyond SCoA ester is known only in case of
formation of Juglone which proceeds via 1, 4-Naphthaquinoate.
The third ring of alizarin is generated by dimethylallylpyrophosphate(DMAPP).
Lecture 9+10
July 2, 2005
ISOPRENOIDS are the compounds made up of isoprene units i.e. 5 C atoms;
They have a general formula of C5H10. The term “Terpenoid” is an alternative
generic name. Both these term are interchangeably used in chemical literature.
They are known since ancient times as ingredients of perfume, soap, flavoring and
food colorants.
They are widely distributed in all 94 orders of flowering plants.
Analytical tools such as chromatography and spectrophotometry has led to a
general understanding of isoprenoids structure, biosynthesis and properties.
Conolly and Hill [1991] has included 22000 structures in their “Dictionary of
Terpenoids”.
№ of
Carbo
n
atoms
10
15
Monoterpenoids
Sesquiterpenoids
GPP
FPP
20
25
30
Diterpenoids
Sesterterpenoids
Triterpenoids
GGPP
GFPP
Squalene
40
>40
Tetraterpenoids
Poluprenols, rubber
Phytoene
GGPP+ (C5)n
Name
Parent
isoprenoid
Subclasses
Iridoids
Abscisic
acid,Sesquterp
enoid lactones
Gibberllins
None
Phytosterols,
saponins,
Cardenoloides.
None
None
Nomenclature and Classification: Nomenclature of isoprenoids reflects number
of isoprene units present in it. Isoprene rule proposed by Wallach and latter
extended to the biogenetic Isoprene rule by Ruzicka states that each class of
Isoprenoid is derived from a single parent compound unique to that class.
These are formed by various cyclizations, rearrangments and even the loss or
addition of carbon units from linear arrangements of carbon units.
General Pathway of Isoprenoid biosynthesis:
Originally HMG-CoA sterol pathway was proposed to explain formation sterols in
animal kingdom. The side branches shown are seen in plants only where
terpenoids characterstics of a particular species. The key to this pathway was
discovery of Mevalonic acid (MVA).
Mevalonic acid (MVA) is formed from Acetyl-CoA.In this pathway
AcetoacetylCoAthiolase and hydroxymethylallyl pyrophosphate plays vital role.
The formation of mevalonic acid via mevaldic acid is catalyzed by hydroxymethylCoA reductase and requires two molecules of NADPH per molecule of mevalonic
acid formed.
The mevalonic acid in three steps, each requiring one mole of ATP per mole of
substrate and gives the biologically “active isoprene”Isopentenyl pyrophosphate
(IPP)
Dihydroxymethylallyl pyrophosphate (DMAPP) acts as prenyl donor to a molecule
of isopentenyl pyrophosphate (IPP) and produces geranyl pyrophosphate (GPP).
geranyl pyrophosphate (GPP) acts as prenyl donor to another isopentenyl
pyrophosphate to produce Farnesyl pyrophosphate (FPP). Farnesyl
pyrophosphate (FPP). Condenses with another molecule of isopentenyl
pyrophosphate (IPP) to produce higher prenyl pyrophosphate.Prenyl transferases
catalyzes the reaction that produces all –trans prenyl pyrophosphate.
In the chain shown below “chain lengthening” is by head to tail condensation
but the formation of tri and tetraterpenoids involves head to head condensation of
farnesyl pyrophosphate geranylgernyl pyrophosphate.. This yields compound like
squalene and phytoene.
Monoterpenoids are the simplest isoprenoids with C10
structure constructed
with two isoprene units.
They are probably everywhere in higher plant kingdom.
They can be classed into four broad structural classes.
Acyclic, cyclopentanoid, cyclohexanoid and irregular monoterpenes
BIOSYNTHESIS OF MONOTERPENOIDS: Geranyl pyrophosphate (GPP) is
the precursor of monoterpenoids.
Geranyl pyrophosphate (GPP) is efficiently converted to cyclic monoterpenoids.
Involved enzyme Geranyl pyrophosphate synthase( cyclases )has been purified
and characterized.
These cyclases (Synthases) are operationally soluble proteins Mr 50 to 100 kDa.
More than 20 monoterpenoid cyclases have been identified.
Monoterpene cyclases have similar mechanism as the prenyl transferases.. They
involve in an initial ionization of allylic pyrophosphate with an electrophilic attack of
allylic carboacation on a double bond.
The cyclic oxygenated monoterpenoids are formed from cyclization products by
oxidation and reduction which involves cytochrome P-450 dependent oxidases.
Most of them catalyze the formation of alkenes, but few yield oxygenated products.
Mentha spicata
Mentha piperita
The oxygenated compound is at C6
The oxygenated compound is at C3
It has been shown that (-)-(4S) limonene, the first cyclic terpene formed from
GGPP is hydroxylated at C-3 to form Trans isopiperitenol in pipermint and at C-6 to
form (-) Trans carveol in spearmint.
Limonene synthase is ( GPP: limonene cyclases is a soluble protein and has been
purified from both species of menth. Basic properties have been found to be the
same.
Biosynthesis of pinene has been studied in soluble extract of Salvia officinalis.
Three monterpene synthase ( Cyclase) help the formation of α-pinene and βpinene.
Biological activities of Monoterpenoids:
1. Monoterpenoids like α-pinene, β-pinene,limonene,citronellol inhibit the growth
of Amaranthus retroflexus under orange tree.
2. Monoterpenoids show antibacterial activity α-pinene, limonene and carvone
3. Monoterpenoids have many therapeutic uses.
Lecture 11
July 5, 2005
Sesquterpenoids are largest class of terpenoids and occur in the entire plant
kingdom. Over 100 skeletons of Sesquterpenoids are known with thousands of
known compounds.
They commonly occur with monoterpenoids.
Structure elucidation shows them to be derived from three isoprene units (C 15).Their
structural diversity is remarkable
They are formed by the condensation of IPP with GPP to yield all-trans-Farnesyl
pyrophosphate.
The biosynthesis has been proposed based on hypothetical routes with cationic
intermediates, with All-trans-FPP,2-cis,6-trans-FPP and nerolidyl pyrophosphate
(NPP) as precursor.
Cyclization, hydride shift, rearrangements etc governed by steric and electronic
considerations lead to the formation of sesquiterpenoids.
Sesquiterpenoid lactone: They are the bitter principles of several herbal
remedies.
Over 300 sesquiterpenoid lactones are characterized most of them contain αmethylene-γ-lactone group. Examples are
The epoxide function and carbonyl [lactones] cause irreversible alkylation of
enzymes and this property probably play an important role in plant’s interaction
with other organisms. These functions are also responsible for antibiotic shown by
antibiotics.
Abscisic acid pathway is one of the major course by which these sesquiterpene
lactones are formed.
Abscisic acid has structure of sesquiterpenoid and was expected to derive from 3
C4 units. Theoretically there are two routes for the conversion MVA to ABA.
1. Direct route via Farenesyl pyrophosphate [FPP] 2. Indirect route via cleavage
of a C40 carotenoids.
Since 1980’s after experimental evidences the indirect pathway is more accepted.
This path way is via 9-cis violaxanthin to xathoxin to ABA-aldehyde to ABA.
ABA is inactivated by its catabolism through two routes: Oxidation and conjugation.
The primary pathway in tomato is 8’-hydroxy ABA Phaseic acid.
Diterpenoids are C20 compounds, derived from GGPP, often with skeletal
rearrangements. They are found in higher plants and fungi and include the
gibberlins. Till date 79 gibberellins are known in higher plants and 27 in fungi.
Gibberellins are divided into
1. Those that retain the full C20 atoms of (-)-ent-kaurene (the C20 GAs example
2. Those that have lost one carbon atom (C20) with formation of a 19,10-γlactone.
================================================
Biosynthesis of GAs :






First step in the process is conversion of
GGPP to copalyl pyrophosphate.
In second step CPP is converted to entkaurene
This reaction is two step cyclization.
The enzyme here is ent.kaurene
synthetase
Ent-kaurene is converted to entkaurenoic acid in three sequential
oxiadation
The enzyme involved is Kaurene oxidase
Triterpenoids: they are C30 terpenoids formed by head to head condensation of
Farnesyl pyrophosphate
Over 4000 triterpenoids molecules have been isolated. These triterpenoids have
been found to contain over 40 skeleton types but the Pentacyclic ring is the most
common: example
β-amyrin
Phytosterols: Is a member of triterpenoids isoprenoids and are characterized by 3β-monohydroxy perhydro-1, 2-cyclopentanophenanathrene ring system.
Numbering system shown is as per IUPAC-IUB rules [1976] as shown
Only over 300 sterols are reported from plants. They are mostly alkylated at C24
Note the structure above both in stigmasterols and sitosterol C24 is alkylated.
It is accepted that the step from HMG-CoA to squalene is same in plant and animal
kingdom and is catalyzed by squalene synthetase .The epoxidation is facilitated by
squalene epoxidase.This reaction also requires O2 , NADPH,and a cytosolic
fraction.
Saponins: are Glycosidic triterpenoids found in the plant plant kingdom are soluble
in water and give stable foam [Sapindus]. They have three major classes.
a. Steroidal glycosides; b. Steroid alkaloids glycosides; and c. triterpenoid
glycoside. All of them have one or more linear or branched chain of carbohydrates
at C3
Carotenoids are yet important triterpenoids and contribute color and pigment to
plants parts such as leaf, petal, fruits. Over 150 carotenoids are known from plants.
The carotenoid Hydrocarbons are called carotenes
While those containing oxygen functios are called Xanthophylls examples are
lutein ,Violaxanthins and neoxanthins.
The carotenoids are of flower are divided into three main groups
1. Highly Oxygenated such asauroxanthin and falvoxanthins.
2. carotenes with high concentration such as β-carotenes in Narcissus
3. species specific example Crocetins of crocus.
Their biosynthesis is autonomous in most fruit and is responsible for change in color
of fruit ripening even after taken out from branches.
In fruits. They are of eight different groups
1.
2.
3.
4.
5.
insignificant amount ex strawberry
large amount chloroplast carotenoids ex Blue berry
Large amount of lycopene and its derivatives ex tomato
Large amount of β-carotene and its derivatives in ex peach
large amount of carotenoids epoxides ex carambola
6. unusual carotenoids ex capsanthin in red pepper.
7. prolycarpene in ex tangerine
8. apocarotenoids ex persicaxanthins.
Minor classes of carotenoids: They are degraded in vivo to yield nor- or apo –
products. Nor refers to loss of methyl group where as apo refers cleavage of larger
skeletal fragments.
Lecture 12+
July 10, 2005
Special Nitrogen Metabolism: Nitrogen compounds are important for
functioning of every plant. Besides, important compounds like purine, pyrimidines
and derived nucleosides and nucleotides plant produces a number of secondary
metabolites.



These secondary metabolites function as “alleclochemicals” and as
chemical of defense against herbivores and micro-organism. They also
function as attractant to insects and animals which help in fertilization as
well as dispersion of seed.
Evidences indicate that many secondary metabolites are important the
fitness of plants.
Nitrogen stores for seed during germination
Besides, 20-protein building amino acids other amino acids such as ornithine is the
precursor of these secondary metabolites. Example: Non-protein amino acids and
their structural analogue.
Protein amino acid
Non-protein amino acid
Biosynthesis of various amino acids:
These Non protein amino acids are formed from protein amino acids:
Example The Non protein amino acids of garlic and onion are derivatives of
cysteine and are not incorporated into protein, rather they are hydrolysed by alliin
lyase to allicin, and other product formed is propenylsullfenic acid which
spontaneously rearranges to syn-propanethiol S-oxide. Allicin and diallylsulphide
display strong antimicrobial activity. syn-propanethiol S-oxide functions as tear
promotion factor. syn-propanethiol S-oxide further hydrolyzes to propionaldehyde ,
sulfuric acid, and hydrogen sulfide.
Thus, these non-protein amino acids in Garlic and onion try to defend by release of
tear promoting compounds as well as antibacterial compounds.
Amines are yet another Nitrogenous compound. Amines in plants are either
aliphatic or aromatic amines. In higher plants aliphatic aldehydes are
predominantly aminated by an L-alanine –aldehyde aminotransferase which
dependent on pyridoxal phosphate.
Biosynthesis of some aromatic amine are shown below: principal reaction is
decarboxylation
Amino acid
Amine
Enzyme
Tyrosine decarboxylase
Phenylamine
decarboxylase
Aromatic amino acid
decarboxylase
Tryptophan decarboxylase
Histidine decarboxylase
Aromatic amine such as tyramine, Dopamine, Phenylethylamine, Hordeine, ephedrine
are widely distributed in nature
Example: Dopamine is present in mucuna pururiens , hordeine is present in Hordeum
and other members of poa.Ephedrine and nor epinephrine are present in Ephedra
gerardiana.
These aromatic amine some time mimic some important neuotransmitters such as
Dopamine and norepinephrine (noradrenaline) or hormone such as epinephrine.
Amines play important role of attracting insects which help in fertilization
Role of Amines in the flower Biology [Arum maculatum]
Monoamine, Diamines and polyamines in the aliphatic series play important role in
several plant families play important roles from attractant & repellent. Similarly a number
of aromatic amines play roles in plants. Stinging hair of Urtica dioica serotonin and
acetylcholine causes stinging repelling the intervening herbivores. The compounds
responsible for the stinging action are Histamine, Seratonin, and acetylcholine.
Stinging hair of Urtica dioica serotonin and acetylcholine causes stinging
Other Nitrogenous compound of prime importance is cynogenic glycosides, and
glucosinolates, alkaloids, auxins and cytokinin:
Here we will discuss the alkaloids as they are among the key compounds in some
plants traded from Nepal:
Earlier the term alkaloids were restricted to plant bases with heterocyclic Nitrogen atom.
The exocyclic nitrogen was termed as pseudoalakloids. Note the heterocyclic nitrogen
ring in berberine.
Latter definition demanded that they should be derived from aminoacids and or these
bases have pharmacological activity. Example
Even in complex alkaloids the aminocids can be noted
Biosynthesis of alkaloids:
The alkaloids of most alkaloids are derived from aminoacids although moieties from
other pathways such as terpenoids often combined. In a number of alkaloids (steroidal
alkaloids) the nitrogen derived other amine sources are added in thefinal step of
biosynthetic pathway. i.e alkaloid skeleton does not come out from amino-acids alone.
The scheme of acyclic alkaloid cocaine is shown below:
Scheme of aromatic alkaloid reticuline is shown below:
Reactions like Decarboxylation , Methylation, Transamination, Aldol Condensation And
Claisen Condensation plays important role in the formation these alkaloids.
plant alkaloids are thus produced from protein and non protein amino acids that
specifically produced in the specific time of plant life cycle and are specifically stored at
specific organ of plant hyoscymine and Nicotine is stored in the leaf vacuoles while the
benzoquinoline A of papaver somniferum is stored in latex vscicles of the fruit. while
some others specifically deposited in stem and or root bark.
Table below shows Biosynthesis and occurrence of alkaloids in some
plants common to flora of Nepal
Amino acids
Ornithine
Alkaloids
Tropane A
Example
Occurrence
Hyscymus niger
Leaf vacuole
Sps organ Root
Hyoscymine
Nictiana
tabacum
Leaf vacuole
Sps organ Root
Lysine
Punica A
steam bark
Lycopodium
clavtum
Lycopodium A
Aspartic acid
Spores
Areca A
Areca catechu
Sps organ
Heart wood
Continued …..
Tryptophan
Claviceps
purpurea
Ergoline A
Clubs / rye
Quinoline
Cinchona
Sps org
steam Bark
Phenylalanine
/Tyrosine
Benzoquinoline
A
Papaveraceae
Papaver
somniferum
Latex vesicles
Protoberberine
Berberis
asiatica
Sps org Bark
and root
Dr.Tirtha Raj bajgai 2110043 (office), 4497764 (Residence)
Sixth semester question:
Full length question
Mid length question
Short length question
2
6
8