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Phenolics: types, biosynthesis and functions.
1. SECONDARY METABOLITES -
PHENOLICS
Submitted by,
A. T Milin Sera
Roll no : 1
1st M.Sc. Botany
ST. Teresa’s College, Ekm
Submitted to,
Dr. Elsam Joseph
Associate Professor
ST. Teresa’s College, Ekm
3. INTRODUCTION
• Phenolic compounds are the substances that possess an
aromatic ring bearing one or several hydroxylic groups (– OH)
and their derivatives.
• If the molecule contains only one hydroxylic group it is called
Phenol.
• If the molecule contains two or more hydroxylic groups they
are Polyphenols.
• Plants produce a variety of compounds that contain one or
more phenol groups called phenolics.
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Plant phenolics…
4. • Phenolic compounds are the most widely distributed
secondary metabolites, ubiquitously present in the
plant kingdom.
• Plant phenolics are a chemically heterogeneous group
of nearly 10,000 individual compounds.
—Some are soluble only in organic solvents,
—Some are water-soluble carboxylic acids and glycosides,
—Others are large, insoluble polymers.
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Plant phenolics…
5. OCCURRENCE OF PHENOLICS
• They are usually found as esters or glycosides rather than as free compounds.
• Polyphenols (relatively hydrophilic) usually accumulate in the central vacuoles of
guard cells, epidermal cells and the sub epidermal cells of leaves and shoots.
• Some are found covalently linked to the plant cell wall (lignin) and some found
in waxes (related to lipidic structures) or on the external surfaces (cuticle) of
plant organs.
• Biosynthesis of phenolic compounds occurs at various sites in plant cells, such as
the chloroplasts, the cytoplasm and the endoplasmic reticulum membrane.
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Plant phenolics…
6. • Phenolics compounds can be divided into major groups according to the
number of carbon atoms in their skeleton.
• This type of classification is proposed by Harborne and Simmonds (1964).
CLASSIFICATION OF PHENOLICS
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Plant phenolics…
8. 1. Simple phenolic compounds
• Phenolic compounds that contain one phenol unit (or a derivative of it) are
considered “simple”.
• Fundamentally, they are substituted phenol compounds.
• Simple phenolic compounds have C6 general skeleton representation.
• The phenolics containing 6-10 C-atoms and basic carbon skeletons C6, C6 – C1,
C6 – C2, C6 – C3 and C6 – C4 grouped together as simple phenolic compounds or
simple phenolics.
• In their general structure :-
—The group denoted by “R” is an organic group which could be alkyl, alkenyl, aryl
etc. or hydroxy, alkoxy, amino etc.
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Plant phenolics…
9. —They can be in the ortho (o), meta (m), or para (p) positions of the aromatic ring.
—These descriptors refer, with respect to the position of the hydroxyl group
constituting phenol which is given position 1, to 1,2-, 1,3-, and 1,4-carbon
relationship respectively.
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Plant phenolics…
10. 2. Polyphenols
• Phenolic compounds that contain more than one phenol unit are considered
“polyphenol”.
• Polyphenolic compounds have C15 general skeleton representation.
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Plant phenolics…
11. • Phenolics can be also be classified into 2 groups, the
flavonoids and the non-flavonoids.
• This classification is based on the fact that, all except
flavonoids arise from a common biosynthetic
intermediate, phenylalanine or its close precursor
shikimic acid through shikimic acid pathway.
• In case of flavonoids which have C6 – C3 – C6 carbon
skeleton, one aromatic ring and its side chain arises
from phenylalanine while the other aromatic ring
arises from acetyl-CoA via malonic acid pathway.
CLASSIFICATION
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Plant phenolics…
12. Simple phenolic
• Simple substituted phenol compounds can be
hydroxyphenols or dihydroxybenzenes.
• Examples are catechol (1,2-dihydroxybenzene),
resorcinol (1,3-dihydroxybenzene), and hydroquinone
(1,4-dihydroxybenzene).
• A resorcinol derivative is the hallucinogenic principle of
Cannabis sativa.
• Plant melanins are probably catechol polymers.
• Other simple substituted phenol compounds can also be
dihydroxyphenols or trihydroxybenzenes.
• Examples are pyrogallol, hydroxyquinol and
phloroglucinol.
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Plant phenolics…
13. Phenolic acids
• Phenols that contain a carboxylic acid are termed as phenolic acids.
• If the carboxylic acid functional group is directly bonded to the phenol ring,
the phenolic compound is termed as hydroxybenzoic acid.
• When carboxylic acid functional group and the phenol ring are separated by
two doubly bonded carbons (a C=C bond), phenolic compounds are termed
as hydroxycinnamic acids.
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Plant phenolics…
14. Coumarins
• Coumarins are lactones which are derived from o-hydroxycinnamic acid, by ring
closure between the ortho hydroxyl group and the carboxylic group of the side
chain, after trans-cis isomerization of the side chain.
• In damaged tissues, it can be easily formed from trans o-glucosyloxycinnamic acid,
a constant component of fresh leaves.
• The damage allows access to enzymes which remove glucose and cause trans-cis
isomerization followed by ring closure.
• These reactions results in the formation of the volatile coumarin.
• Examples are umbelliferone, aesculetin and scopoletin.
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Plant phenolics…
15. Flavonoids
• The basic flavonoid structure is the flavan nucleus, containing 15 carbon atoms
arranged in three rings (C6-C3-C6), which are labeled as A, B and C.
• Flavonoids are polyphenolic compounds with the general structure :
• Generally, rings A and C are either mono, di, or trihydroxylated.
• The O-heterocycle B is usually a pyrone ring as in Luteolin but could also
be a pyrlium ring as in delphinidin.
• If ring C is attached to C2 of ring B, the flavonoid is a flavone (as Luteolin),
flavonol (as kaempferol), an anthocyanin (as delphinidin) or a flavanone
(as naringenin).
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Plant phenolics…
16. • If the ring C is attached to C3 of ring B, then the flavonoid is an isoflavone such as
daidzein.
• Chalcones such as chalcone, are a class of flavonoids in which rings A and C are
separated by 3-carbon linear chain rather than a ring.
• The bond between C2 and C3 of ring B is commonly double as in flavones,
flavonols, chalcones and isoflavones.
• However, the C2-C3 bond could be single as in flavanones.
• Flavonoids are the largest group of phenols.
• They are very often seen in epidermis of leaves and fruit skin.
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Plant phenolics…
17. Classification of flavonoids :
• Flavonoid are themselves divided into
six subgroups:
flavones, flavonols, flavanols,
flavanones, isoflavones, and
anthocyanins, according to the oxidation
state of the central C ring.
• Their structural variation in each
subgroup is partly due to the degree
and pattern of hydroxylation,
methoxylation, prenylation, or
glycosylation.
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Plant phenolics…
18. Tannins
• Tannins are known to bind to and precipitate proteins and amino acids.
• They are subdivided into three types; hydrolyzable, condensed and complex.
• Hydrolyzable tannins can be gallotannins or ellagitannins.
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Plant phenolics…
19. Stilbenes
• Stilbenes are phenolic compounds in which two phenol
units are linked by two-doubly bonded carbons.
• They are phytoalexines, produced in response to fungal,
bacterial, viral attack.
• Examples of stilbenes include resveratrol, pterostilbene
and piceatannol.
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Plant phenolics…
Lignans
• Lignans consist of two phenol units linked by four
carbons.
• Examples include matairesinol, secoisolariciresinol and
pinoresinol.
20. Lignins
• Lignins consist of phenol units or phenolic compounds
that are linked with each other by carbon chains.
• Lignins are high molecular weight polymers.
• It is a highly branched polymer of phenylpropanoid
groups that plays both primary and secondary roles.
• Lignin is generally formed from three different
phenylpropanoid alcohols : Coniferyl; Coumaryl; Sinapyl
alcohols.
• Synthesized from phenylalanine via various cinnamic
acid derivatives.
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Plant phenolics…
21. BIOSYNTHESIS OF PHENOLICS
• Phenylalanine is an intermediate in the biosynthesis of most plant phenolics.
• Two basic pathways are involved in biosynthesis of plant phenolics.
• The shikimic acid pathway, participates in biosynthesis of most plant phenolics.
• The malonic acid pathway, an important source of phenolics in fungi and
bacteria, less significance in higher plants.
• The shikimic acid pathway converts simple carbohydrate precursors derived
from glycolysis and the pentose phosphate pathway to the aromatic amino
acids.
• The well-known, broad-spectrum herbicide glyphosate (available commercially
as Roundup) kills plants by blocking a step in this pathway.
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Plant phenolics…
22. • The shikimic acid pathway is present in plants, fungi, and bacteria but is not
found in animals.
• The most abundant classes of secondary phenolic compounds in plants are
derived from phenylalanine via the elimination of an ammonia molecule to form
cinnamic acid.
Plant phenolics…
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23. Shikimic Acid Pathway
• Except flavonoids, all other plant phenolics are biosynthesized in plants from a
common biosynthetic intermediate, phenylalanine or its close precursor
shikimic acid through shikimic acid pathway.
• The starting metabolites in this pathway are erythose-4-phosphate and
phosphoenol pyruvate which are intermediates of pentose phosphate pathway
and glycolysis respectively.
• In the shikimic acid pathway, phosphoenolpyruvate and erthrose-4-phosphate
react in few steps to provide 3-dehydroquinate.
• Dehydration with shikimate dehydrogenase gives 3-dehydroshikimic acid.
BIOSYNTHESIS
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Plant phenolics…
24. • Reduction with NADPH gives shikimic acid.
• 3-dehydroshikimic acid could lead to gallic acid in
several steps.
• Shikimic acid is then converted into chorismic acid
which undergoes Claisen rearrangement to afford
prephenic acid.
• The product is then converted in several steps into
tyrosine.
• The amino acid serves as a central point and a
crucial precursor for the biosynthesis of various
phenolic compounds.
Plant phenolics…
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25. Phenylpropanoid pathway
• Another route toward phenolic compounds, is the phenylpropanoid pathway.
• This route is essentially similar to the shikimic acid pathway until L-
phenylalanine stage where the phenylpropanoid pathway takes form.
• The first step in the synthesis of phenolic compounds from phenylalanine in
plants is deamination of phenylalanine by the enzyme phenylalanine ammonia
lyase (PAL).
• L-Phenylalanine undergoes deamination catalyzed by phenylalanine ammonia
lyase (PAL) enzyme to give cinnamic acid.
• Malonyl-CoA can be produced from acetyl-CoA in the presence of the enzyme
acetyl-CoA carboxylase as follows:
Plant phenolics…
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26. • Hydroxylation followed by conversion to the
Coenzyme A provides p-coumaroyl Coenzyme
A.
• This molecule serves as a central point toward
various phenolic compounds.
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Plant phenolics…
27. FUNCTIONS OF PLANT PHENOLICS
• Phenolic compounds possess a wide range of biological activities such as
antioxidant, anti-inflammatory, and antimicrobial properties.
• Such properties allow phenolic compounds be able to reduce various illnesses
and diseases such as cardiovascular diseases, diabetes, cancer, and
hypertension.
• Therefore, they can be used in pharmaceutical industry as therapeutic agents.
• The antioxidant and antimicrobial properties enable phenolic compounds to
function as food preservatives and additives.
• Thus they have also applications in food industry.
Plant phenolics…
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28. • In addition, phenolic compounds have applications in cosmetic and packaging
industries.
• Some of them act as chemical deterrents against herbivores and pathogens.
• Plant phenolics such as lignins provide mechanical strength to the plants and
have significant protective functions in them.
• Some phenolics play important role in plants in attracting pollinators and fruits
& seeds dispersers.
• Some plant phenolics play important role in allelopathy. Allelopathy is the
influence of chemicals released by one plant species on another plant or animal
with resulting benefits to the species which contains them.
• Helps in absorbing harmful ultraviolet radiation.
Plant phenolics…
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29. REFERENCE
• Badria, F. A., & Blumenberg, M. (2022). Phenolic Compounds: Chemistry,
Synthesis, Diversity, Non-Conventional Industrial, Pharmaceutical and
Therapeutic Applications. BoD–Books on Demand.
• Goodwin, T. W., & Mercer, E. I. (1983). Introduction to plant biochemistry.
• Verma, S. K., & Verma, M. (2008). A textbook of plant physiology, biochemistry
and biotechnology. S. Chand Publishing.
• https://www.slideshare.net/NAGINAFATIMA/plant-phenolic-compounds
• https://www.slideshare.net/ZubyGoharAnsari1/phenolic-compounds
• https://www.slideshare.net/snehaljikamade/sandesh-renew-doc-ppt
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