Classification of Plant
Secondary Metabolites
Ms M. Mombeshora
HBC 206 Lecture 15
Metabolites
• A substance essential to the metabolism of a
particular organism or to a particular
metabolic process
• Over the centuries humans have relied on
plants for basic needs such as
– food, clothing, and shelter
• All produced or manufactured from plant
matrices(leaves, woods, fibres) and storage
parts (fruits, tubers)
Plant Metabolism
Plant Metabolites
• Plants have also been utilised for additional
purposes, namely:
–
–
–
–
–
–
as arrow and dart poisons for hunting
poisons for murder
hallucinogens used for ritualistic purposes
stimulants for endurance
hunger suppression
medicines
• Chemicals used for these purposes are largely the
secondary metabolites, which are derived
biosynthetically from plant primary metabolites
Plant metabolites
• A plant cell produces two types of
metabolites:
– Primary metabolites
– Secondary metabolites
Primary metabolites
• Involved directly in growth, development and
reproduction
• Examples of primary metabolites:
– carbohydrates
– lipids
– proteins
Secondary metabolites
• Considered as end products of primary
metabolism and not involved in metabolic activity
• Includes:
–
–
–
–
–
–
alkaloids
phenolics
sterols ,steroids
essential oils
lignins
tannins e.t.c
Secondary metabolites
• Play many important roles in plant life e.g:
– involved in defence against herbivores and
pathogens
– regulation of symbiosis
– control of seed germination
– chemical inhibition of competing plant species
(allelopathy)
• An integral part of the interactions of species in
plant and animal communities and the
adaptation of plants to their environment
Secondary metabolites
• Associated with improved nutritive value
• May have beneficial effects on animal health
• Growing interest in the potential healthpromoting effects in human foods
• This has prompted research on their potential
to prevent or treat cancer, circulatory disease,
and viral infection
Secondary metabolites
• Mechanisms by which secondary metabolites
have beneficial effects on health may also be
related to their toxic effects
• The difference between toxicity and beneficial
effects may be dose- and structure-dependent
• The term “plant extract" determines the
part/parts of a plant used for preparing medicine
– for example: leaves, flowers, seeds, roots, barks,
stem....e.t.c
Preparing plant material
• Plants are dried before extraction
• Freed of possible contaminants
Other plants
From diseases
Extraction
• As the term is used pharmaceutically:
– involves the separation of medicinally active
portions of plant from various parts of plants
– Carried out by using selective solvents in standard
extraction procedures
– Selection of the solvent depends on the polarity of
solvent and solubility of the desired components
of the material
Extraction methods
Extraction of Compounds Using Solvents
A. Ultrasonic-Assisted Extraction
B. Microwave-Assisted Extraction (MAE)
General methods of solvent
extraction
• Choosing of the precise method of extraction depend on
texture and water content of the plant material
• Solvents used for the extraction of biomolecules from
plants are chosen based on the polarity of the solute of
interest
• A solvent of similar polarity to the solute will properly
dissolve the solute
• Multiple solvents can be used sequentially in order to limit
the amount of analogous compounds in the desired yield
• The polarity, from least polar to most polar, of a few
common solvents is as follows:
Hexane < Chloroform < Ethyl acetate < Acetone < Methanol <
Water
General methods of solvent
extraction
1. Maceration
• The whole or coarsely powdered crude
powder is placed in a container with the
solvent
• Allowed to stand at room temperature for at
least 3 days with frequent agitation until the
soluble matter has dissolved
General methods of solvent
extraction
2. Hot Continuous Extraction (Soxhlet)
• Finely ground crude powder is placed in a porous
bag or “thimble”
• Thimble is made of strong filter paper, which is
placed in chamber E of the Soxhlet apparatus
• The extracting solvent in flask A is heated, and its
vapour condense in condenser D
• The condensed extractant drips into the thimble
containing the crude powder, and extracts it by
contact
• When the level of liquid in chamber E rises to the
top of siphon tube C, the liquid contents of
chamber E siphon into flask A
A. Ultrasonic-Assisted Extraction
(UAE)
• Used in diverse applications of food-processing
technology to extract bioactive compounds from
plant materials
• Ultrasound, with levels greater than 20 kHz, is
used to disrupt plant cell walls, which helps
improve the solvent’s ability to penetrate the
cells and obtain a higher extraction yield
• Can use a low operating temperature through
processing, maintaining a high extract quality for
compounds
A. Ultrasonic-Assisted Extraction
(UAE)
• Known to be one of the easiest extraction techniques
because it uses common laboratory equipment such as
an ultrasonic bath
• In this technique, a smashed sample is mixed with the
suitable solvent and placed into the ultrasonic bath,
• While temperature and extraction time are controlled
• UAE of various organic and inorganic samples can use a
wide range of solvents
• Common equipment used in ultrasound-assisted
extraction includes an ultrasonic bath and an ultrasonic
probe system
A. Ultrasonic-Assisted Extraction
(UAE)
Disadvantages
• Ultrasonic probe has two main negative properties mainly
related to
• experimental repeatability and reproducibility
Advantages
• Green technology is necessary to protect the environment
from toxic substances
• Extraction of phenolic compounds by ultrasound reduces
the amount of solvent and energy used
• UAE can break down plant tissue and work properly during
the production process and release of active compounds in
solvents with a high efficiency
B. Microwave-Assisted Extraction
(MAE)
• Technique to extract bioactive compounds from a variety of
plants and natural residues
• Microwaves have electromagnetic radiation that occurs at
frequencies between 300 MHz to 300 GHz, and
wavelengths between 1 cm and 1 m
• These electromagnetic waves consist of both an electrical
field and a magnetic field
• The application of microwaves is to heat up objects that
can absorb a part of the electromagnetic energy and
convert it into heat
• Advanced techniques have become available to reduce the
loss of bioactive compound without increasing the
extraction time
B. Microwave-Assisted Extraction
(MAE)
• Used as an alternative to conventional
techniques for the extraction of antioxidants
• Due to its ability to reduce both time,
extraction solvent volume and higher
sensitivity towards target molecules
• The main objective of using MAE is to heat the
solvent and extract antioxidants from plants
with a lesser amount of solvents
Synthesis of secondary metabolites
CO2
Photosynthesis
Primary carbon metabolism
Pentose phosphate
pathway
Phosphoenol pyruvate
TCA Cycle
pyruvate
Acetyl Co-A
Erythrose-4-phosphate
Aliphatic amino acid
Shikimic acid
Aromatic
amino
acids
Nitrogen
containing
compounds
Malonic acid
Pathway
Phenolic
compounds
Mevalonic acid
Pathway
Terpenes
Main Groups of plant secondary
metabolites
A. Terpenoids
B. Nitrogen containing secondary metabolites
(Alkaloids, Non-protein amino acids, Amines,
Cyanogenic glycoside, Glucosinolates,
Alkamides, Lectins )
C. Phenolic compounds
The Terpenoids
• The terpenes, or isoprenoids, are one of the
most diverse classes of metabolites
• ISOPRENE C5 is the basic unit of the
terpenoids
• Production in Plants:
– Flowers
– Leaves
– Fruit
The Terpenoids
• Biological Role(volatile and non volatile):
– Flavour, fragrance, scent
– Antibiotics
– Hormones
– Membrane lipids
– Insect attractants
– Insect anti-feedants
Terpenoids --Important Molecules
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C5-hemiterpenes -e.g. isoprene
C10-monoterpenes -e.g. limonene
C15-sesquiterpene -e.g. abscisic acid (ABA)
C20-diterpene -e.g. gibberellin
C30-triterpene -e.g. brassinosteroids
C40-tetraterpenes -e.g. carotenoids
> carbons -polyterpenes-e.g. ubiquinones,
rubber
Nitrogen containing compounds
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•
•
•
•
Alkaloids (pseudo-, True-, proto-)
Extremely heterogeneous group
Alkali like
Have important pharmacological properties
Further classified in to many groups
Nitrogen containing compounds
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Pyridine alkaloids , e.g. nicotine
Pyrrolidine alkaloids , e.g. stachydrine
Piperidine alkaloids , e.g. coniine
Tropane alkaloids , e.g. atropine
Quinoline alkaloids , e.g. quinine
Isoquinoline alkaloids , e.g. berberine
Quinolizidine alkaloids , e.g. lupinine
Indol alkaloids , e.g. reserpine
Imidazol alkaloids , e.g. pilocarpine
•
Phenylalkylamines:
•
e.g. Ephedrine
CH2 CH
CH3
NH 2
•
Pyridine and piperidine
•
e.g. lobeline, nicotine
N
•
•
N
H
Tropane
e.g. Atropine .
NCH3
OH
•
•
Quinoline
e.g.quinine and
quinidine
N
•
•
•
•
Isoquinoline
e.g. papaverine
Phenantheren
e.g. Morphine
N

Indole
•
e.g.ergometrine
N
H

Imidazole
•
N
e.g. pilocarpine
N

•
Purine
e.g. caffeine
6
1 N
5
7
N
H
8
2
N 4
3
Purine
N
9
Cyanogenic glycosides
• Widely distributed in plants
• Volatile poisons
• e.g. Lotustraline
Glucosinolates
• Contain nitrogen and sulphur
• Volatile toxins
• Strong deterrent
Cyanogenic Glycosides
Non-Protein amino acids
• Found in plants of the family Leguminosae
• Example Canavanine resemble in structure
with arginine
• Thy are not incorporated into proteins
Phenolics
• Plants produce a variety of compounds that
contain one or more phenol groups - called
phenolics
• Thousands of phenolics occur in plants
Phenolics
• Large group of diverse compounds
• Many serve as defense compounds against
herbivores and pathogens
• Some function in support
• Some attract pollinators
• Some absorb UV light
• Some reduce growth of competitors
Tannins
• High molecular weight phenolic compounds
• Widely distributed in plants
• Capable of precipitating animal proteins
Classification of Tannins
Not hydrolysable,
no sugar in molecule
Use of Tannins
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•
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Antioxidant
Antidiarrheal
Antidote for heavy metals poisoning
Treatment of burns, ulcers, inflammations
Astringent to stop bleeding (haemorrhage)
Treatment of Haemorrhoids
Tanning industry
Lignin a complex phenolic
• Primary metabolite - secondary cell wall
component occurs in all vascular plants
• Structural function
• Also protective because deters herbivores due
to its toughness
• Blocks growth of many pathogens because
only small group of fungi can degrade
Flavonoids
• One of the largest classes of phenolics
• Carbon skeleton has 15 carbons with two
benzene rings connected by a 3-C bridge
-C3-
Flavonoids
• The main subclasses of flavonoids are the:
i. flavones
ii. flavonols
iii. flavan-3-ols
iv. isoflavones
v. flavanones
vi. anthocyanidins
Generic structure of major Flavonoid
Anthocyanins
• Colored flavonoids
- red, pink, blue,
purple pigments
• Attract animal
pollinators and
seed dispersers
Anthocyanin
B
- cyanidine
cyanidine 3-glucoside
cyanidine 3-rutinoside
- peonidine
peonidine 3-glucoside
peonidine derivative
Flavonoids
• Involved in such diverse processes as:
– UV protection
– Pigmentation
– stimulation of nitrogen-fixing nodules
– disease resistance
Flavones and Flavonols
• Also flower pigments
• Absorb UV not visible light - not visible to
human eye but visible to many insects maybe be attractants, nectar guides
• Also present in leaves where they protect
against UV damage
• Appear to be involved in legume roots in
attracting N-fixing bacteria
Some applications of
Important plants and their
compounds in biotechnology
Green Tea (Camellia sinensis)
• Polyphenols from leaves
• anti-cancer inhibiting
tumour initiation and cell
proliferation
Wine Grape (Vitis vinifera)
• Contains over 50 different
flavonoid phenolics
including
– Resveratrol
– Catechins
Ginger (Zingiber officinale)
• From the rhizome
• Over 12 compounds with
anti-oxidant activities
greater than vitamin E
• Anti-tumor
Garlic (Allium sativum)
Onion (Allium cepa)
• Organo-sulfur
compounds from leaves
• Anti-carcinogenic and
anti-microbial
• Anti-atherosclerosis
and anti-hypertensive
Soybeans (Glycine max)
• Contains phytoestrogens like isoflavones
• Reduces health risks associated with
menopause: osteoporosis and heart disease in
women