Phenolics: types, biosynthesis and functions.

Phenolics

1. SECONDARY METABOLITES - PHENOLICS Submitted by, A. T Milin Sera Roll no : 1 1 st M.Sc. Botany ST. Teresa’s College, Ekm Submitted to, Dr. Elsam Joseph Associate Professor ST. Teresa’s College, Ekm

2. Contents INTRODUCTION OCCURRENCE CLASSIFICATION BIOSYNTHESIS FUNCTIONS Plant phenolics…

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 . 1 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. 2 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. 3 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 4 Plant phenolics…

7. Classification Phenolic compounds can generally be classified into simple and polyphenolic compounds. 5 Plant phenolics…

8. 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. 6 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. 7 Plant phenolics…

10. 2. Polyphenols Phenolic compounds that contain more than one phenol unit are considered “ polyphenol ”. Polyphenolic compounds have C15 general skeleton representation. 8 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 9 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. 10 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 . 11 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 . 12 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). 13 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. 14 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. 15 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. 16 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. 17 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. 18 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. 19 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… 20

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 21 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… 22

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… 23

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. 24 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… 25

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… 26

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 27

30. THANK YOU