3. CONTENTS
Introduction
Structure of Flavonoids
Classification and Chemistry of Flavonoids
Sources of Flavonoids
Biosynthetic Pathway of Flavonoids
Role of Flavonoids in Plants
Beneficial effects associated with consumption of
Flavonoids
Antioxidant activity of Flavonoids
Role of antioxidants on human health
Mode of action of Flavonoids
Dosage
4. Flavonoids are a group of plant polyphenolic secondary
metabolites showing a common three ring structure (Wiley J.
& Sons, Inc., Publication, 2010).
Widely distributed in different amounts, according to the plant
species, organ, developmental stage and growth conditions.
The Flavonoids have aroused considerable interest recently
because of their potential beneficial effects on human health
as well as their role in plant metabolism.
They have been reported to have antiviral, antitumor, anti-
allergic, anti-platelet, anti-inflammatory and antioxidant
activities.
Introduction
5. Their basic structure is a skeleton of diphenylpropane,
namely, two benzene rings (ring A and B, see figure) linked
by a three carbon chain that forms a closed pyran ring
(heterocyclic ring containing oxygen,the C ring) with
benzenic A ring.
Therefore, their structure is also referred to as C6-C3-C6.
In most cases, B ring is attached to position 2 of C ring, but it
can also bind in position 3 or 4; this, together with the
structural features of the ring B and the patterns of
glycosylation and hydroxylation of the three rings, makes the
flavonoids one of the larger and more diversified groups of
phytochemicals.
Structure of Flavonoids
6. Flavonoids are classified in 6 major subgroups depending on the
carbon of the C ring on which B ring is attached, and the degree
of unsaturation and oxidation of the C ring.
Classification and Chemistry of
Flavonoids.
B ring is linked in position
3 of the ring C-
isoflavones.
B ring is linked in position
4,
Neoflavonoids.
B ring is linked in position
2 ,
flavones, flavonols,
flavanones,
flavanols,catechin,
and anthocyanidin.
open C ring –
Chalcone
Wiley J. & Sons, Inc., Publication, 2010)
15. Phenylpropanoids, act as key chemical modulators of plant
communication with insects and microbes, either as attractants or
repellants , or as phytoalexins against pathogens and herbivores.
Induce root nodulation when excreted by symbiotic nitrogen-fixing
rhizhobia (Mandal et al;2010).
Role Of Flavonoids In Plant Defense
Derivatives of the initial phenylpropanoid scaffold play vital roles
in plant structural integrity, protection against UV radiation and
phytopathogens, internal regulation of plant cell physiology and
signaling. (Koes et al 1994, Shirley, 1996)
Some flavonoids provide stress protection, for example act as
scavengers of free radicals such as reactive oxygen species (ROS), as
well as metal chelating agents that generate ROS. (Mol et al., 1998;
Winkel-Shirley, 2002; Bradshaw and Schemske, 2003)
16. Beneficial effects associated with
consumption of Flavonoids
Reduced risk of cardiovascular diseases.
Reduction in Blood Pressure due to its vasodilatory effect.
Delays or prevents the onset of diseases caused by free
radicals
Best antioxidant
Anti-inflammatory activity
Improvement of endothelial functions
Inhibits LDL oxidation by free radicals
Inhibits platelet Aggregation
Antiviral
Antibacterial
17. Antioxidant activity of Flavonoids
Antioxidants are compounds that protect cells against the damaging effects of
Reactive Oxygen Species(ROS), such as singlet oxygen, superoxide, peroxyl
radicals and peroxy nitrite.
The antioxidative activity of flavonoids is connected with the structure of the
molecule: the presence of conjugated double bonds and the occurrence of
functional groups in the rings.
An imbalance between antioxidants and ROS results in oxidative stress, leading
to cellular damage.
Flavonoids reduce the production of and quench reactive oxygen species
(ROS) through:
-Suppression of singlet oxygen;
-Inhibition of enzymes that generate ROS (cyclooxygenase, lipoxygenase,
monooxygenase, xanthine oxidase);
-Chelating ions of transition metals, which may catalyze ROS production;
-Quenching cascades of free-radical reactions in lipid peroxidation;
-“Re-cycling” of other antioxidants. Due to their low redox potential,
they can reduce strong free radicals such as superoxides, alkyl
radicals, hydroxyl radicals.
18. Oxidative stress has been linked to cancer, aging, atherosclerosis,
ischemic injury, inflammation and neurodegenerative
diseases(Parkinson’s and Alzheimer’s).
Flavonoids may help provide protection against these diseases by
contributing along with antioxidant vitamins and enzymes, to the total
antioxidant defense system of the human body.
Antioxidant flavonoids-
-Quercetin (a flavonol in vegetables, fruit skins, onions)
-Xanthohumol ( a prenylated chalcone in hops and beer)
-Isoxanthohumol ( a prenylated flavanone in hops and beer)
Among all the flavonoids, Quercetin is the most abundant dietary
flavonol, and is potent antioxidant because it has all the right structural
features for free radical scavenging activity.
19. Role of antioxidants on human health
Support kidney function
Improve reproductive function
Maintain good dental health
Improve nervous system functioning
Have anti-aging effect
Protect liver
Support immune system and improve defense power of the body
Reduce obesity
Maintain healthy vision
Offer protection against digestive disorders
Support respiratory system
20. Mode of action of Flavonoids
Flavonoids as antioxidants
The flavones and catechins seem to be the most poerful flavonoids for
protecting the body against ROS. Flavonoids are oxidized by radicals ,
resulting in a more stable, less reactive radical. Because of high reactivity of
hydroxyl group of the flavonoids, radicals are made inactive.
Anti-ulcer effect
Quercetin seems to play very important role in the prevention and treatment
of peptic ulcer. It acts by promoting mucous secretion, thereby serves as
gastroprotective agent, also quercetin has been shown to inhibit the growth
of helicobactor pylori bacterium in in-vito studies.
Anti-atherosclrotic effect
Atherosclerosis is referredto as hardening of arteries.An elevated plasma of
low density lipoprotein (LDL) concentration is a primary risk factor for the
development of atherosclerosis and coronory artery disease. Flavonoids
seem to suppress LDL oxidation and inflammatory progression in the artery
wall.
21. Anti-inflammatory effect
Cyclooxygenase (COX) is an enzyme that plays an important role as
inflammatory mediator and is involved in the release of arachidonic acid,
which is a precursor for biosynthesis of ecosanoids like prostaglandins and
prostacyclin. The release of arachidonic acid can be considered as the
starting point for general inflammatory response. Flavonoids like quercetin
are shown to inhibit the COX pathway.This inhibition reduces the release of
arachidonic acid.
Hepatoprotective activity
Many flavonoids have also been found to posses hepato-protective
activity.e.g. silimarin,apigenin,naringenin,quercetin.
The result of several clinical investigations showed the efficacy and safety of
flavonids in the treatment of hepato-bilary dysfunction and digestive
complaints such as sensation of fullness, loss of appetite, nausea and
abdominal pain.
22. Dosage
Flavonoid intake depends upon the consumtion of fruits,
vegetables and certain beverages, such as red wine, tea and beer.
The high consumption of tea and wine may be most influential on
total flavonoid intake in certain groups of people.
This intake is high as compared to the average daily intake of
other diatery antioxidants like:
-Vitamin C(70 mg)
-Vitamin E( 110 mg)
-Carotenoids (23 mg)
23. (de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and
vegetable phytochemicals: chemistry, nutritional value, and stability. 1th
Edition. Wiley J. & Sons, Inc., Publication, 2010).
B.Winkel-Shirley,Flavonoid biosynthesis. A colorful model for genetics,
biochemistry, cell biology, and biotechnology, Plant Physiology 126
(2001) 485-493.
Winkel-Shirley, B. (2006). “The biosynthesis of flavonoids,” in The
Science of Flavonoids,edE.Grotewold (New York, NY: Springer),71–95.
References
