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Lecture 14:SECONDARY METABOLITES

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The world is a healthier, more colorful, tasty, and fragrant placebecause of the phenolics that plants produce.

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Chapter 24. Buchanan et al. (eds. 2015)

LEARNING OUTCOMESStudents, after mastering materials of the presentlecture, should be able

1. to explain secondary metabolites in plants.

2. to explain more detail terpenoids including.biosynthesis.

3. to explain more detail cyanogenic glucosidesincluding biosynthesis.

4. to explain more detail alkaloids includingbiosynthesis

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LECTURE OUTLINE

I. INRODUCTION1. Definition2. Classification

II. TERPENOIDS1. Definition2. Classification3. Functions4. Biosynthesis

III. ALKALOIDS1. Definition2. Functions3. Biosynthesis

IV. PHENOLICS1. Definition2. Functions4. Biosynthesis

V. CYANOGENICGLUCOSIDES

1. Definition2. Classification3. Functions4. Biosynthesis

Notes. Due to time limitation, discussion will befocused on Terpenoids, Alkaloids & Phenolics, butcyanoglucoside materials are presented in theappendix

I. INTRODUCTION1. Definition Plant secondary metabolites, also referred to as

natural products or specialized metabolites, constitutean enormously rich reservoir of chemical biodiversity.

The secondary metabolites have been the subject ofresearch since 1850s for their practical utility as dyes,polymers, fibers, glues, oils, waxes, flavors,fragrances, drugs, insecticides, and herbicides.

Today it is recognized that natural products haveimportant ecological roles in plants, and their studyhas transitioned into the realm of modern biology.

In the absence of a valid distinction between primaryand secondary metabolites based upon structure orbiochemistry, a functional definition is used herein.

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Secondary products are defined as substances thatinfluence communication between the plant and itsenvironment, and primary products as substancesthat participate in nutrition and essential metabolicprocesses in the plant.- In comparison with primary metabolites, which are essential

to plant growth and development, secondary metaboliteshave internal roles in plants and also are integral to thecommunication of a plant with its environment.

The interaction between a plant and its environmenttakes many forms.1. It can be an accumulation of pigments in flower petals, or a

release of volatile chemicals by flowers to attract pollinators.2. It can be the release of volatiles by a leaf damaged by a

grazing caterpillar to attract predatory wasps in a tritrophicinteraction, or the production of bitter or toxic chemicals thatserve as antifeedants.

3. It can also be the release by roots of secondary metabolitesinto the rhizosphere to attract beneficial soil microorganisms.

2. Classification More than 200,000 diverse chemical structures have

been identified. However, primary and secondary metabolites cannot

be readily distinguished by their precursor molecules,chemical structures, or biosynthetic origins.- For example, the diterpenes kaurenoic acid and abietic acid

are formed by a similar sequence of related enzymaticreactions (Fig. 24.1).

- Kaurenoic acid is an essential intermediate in the synthesisof gibberellins

- Abietic acid is a resin component (a secondary metabolite)largely restricted to members of the Fabaceae and Pinaceae.

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- The essential amino acid proline is a primary metabolite,whereas the C6 analog pipecolic acid (Fig. 24.1) isconsidered an alkaloid and, thus, a secondary metabolite.

- Even lignin, the essential structural polymer of wood andsecond only to cellulose as the most abundant organicsubstance in plants, has been considered a natural productrather than a primary metabolite.

FIGURE 24.1 Kaurenoic acidand proline are primarymetabolites, whereas theclosely related compoundsabietic acid and pipecolic acidare considered secondarymetabolites.

Plant natural products can be divided into severalmajor groups based on their chemical structures, withthe best studied being the terpenoids, cyanogenicglucosides and glucosinolates, alkaloids, andphenolics.

There is no fixed, commonly agreed upon system forclassifying secondary metabolites. Based on theirbiosynthetic origins, plant secondary metabolites canbe divided into three major groups:- Flavonoids and allied phenolic and polyphenolic compounds,- Terpenoids and- Nitrogen-containing alkaloids and sulphur-containing

compounds.

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II. TERPENOIDS

1. DefinitionWhat are terpenoids Terpenoids, also referred to as isoprenoids, are

organic chemicals derived from five-carbon units,sometimes called isoprene (C5H6) units, assembledand modified in thousands of ways.

The term of isoperene units is based on the fact someterpenoids decompose to give off the gas isoprene atelevated temperatures (Fig. 24.2).

Fig. 24.2 The structure of the basic five-carbon (C5) units of terpenoids. Isopentane(left) is a branched C5 hydrocarbon, andisoprene (right) is a gas formed when certainterpenoids decompose.

Terpenoids (isoprenoids) represent the largest andmost diverse class of chemicals among the myriadcompounds produced by plants (Tholl, 2015).

The terms “terpenoid” and “terpene” originate fromthe word turpentine (“terpentin” in Germany) becausesome of the first terpenes described were isolatedfrom turpentine (a fluid from the resin of live trees).

Terpenoids can be thought of as modified terpenes,wherein methyl groups have been moved or removed,or oxygen atoms added. Terpenes [(C5H8)n] arehydrocarbons resulting from the condensation ofseveral 5-carbon isoprene [(C5H8)n or CH2=C(CH3)CH=CH2] units.

Terpenoids contribute to- the scent of eucalyptus (-pinene)

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- the flavors of cinnamon (cinnamaldehyde and trans-cinnamaldehyde), cloves,(Caryophyllene, β-caryophyllene)and ginger (Zingiberene, a monocyclic sesquiterpene),

- the yellow color in sunflowers,(-carotene, tetraterpenoids)and the red color in tomatoes (lycopene, bright redcarotene).

Well-known terpenoids include- citral, present in the oils of several plants,- menthol, obtained from several plants (corn mint,

peppermint and others),- camphor, derived from the wood of the camphor tree

(Cinnamomum camphora),- salvinorin A, found in the plant Salvia divinorum,- cannabinoids found in cannabis,- ginkgolide and bilobalide found in Ginkgo biloba, and- cucurminoids found in turmeric and mustard seed.

2. Classification Although terpenoids are found in all living organisms,

they reach their greatest structural and functionaldiversity in plants.

Many terpenoids have a well‐characterized function inbasic plant growth and development and so can beclassified as primary, rather than secondarymetabolites.

Terpenoids—with over 30,000 members described todate— take the honors for being the largest class ofsmall molecule natural products in plants.

The classification of terpenoids has a long history,and the 10‐carbon terpenoids were once thought tobe the smallest naturally occurring representatives ofthis class.

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Hence, these were called monoterpenes, and thename persists.

The designation of the 10‐carbon terpenoids asmonoterpenes (one terpene) leads to the name of- five‐carbon terpenoids as hemiterpenes (half terpenes),- 15‐carbon terpenoids as sesquiterpenes (one and

one‐half terpenes),- 20‐carbon terpenoids as diterpenes (two terpenes),- 30‐carbon terpenoids as triterpenes (three terpenes),

and so on.

3. Functions Ecological Roles. Most of the thousands of

terpenoids produced by plants have no discernibleroles in growth and development and are, therefore,secondary metabolites.

Many of these are volatile compounds or constituentsof essential oils, resin, latex, and waxes. Theseterpenoids, once thought of as metabolic wasteproducts, actually serve a suite of ecological roles,and- can defend against herbivores and pathogens, inhibit

germination and growth of competing plants, or attract animalsthat disperse pollen and seeds.

Many terpenes form sticky, oily, smelly, or irritatingmixtures (Fig. 24.3), so it is easy to appreciate theirdefensive properties.

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(A) The resin of conifers comprises mainly monoterpenes (C10) andditerpenes (C20). When the tree is damaged and resin ducts are severed, thesmelly monoterpenes quickly evaporate from the extruded resin, leavingbehind the sticky diterpenes, which become polymerized. (B) Themonoterpene‐rich oil of peppermint (Mentha sp.) is stored in glandular hairs onthe surface of leaves and other organs. Shown in this cutaway view of a youngleaf is a single glandular hair, about 0.5 mm in diameter, on the leaf surface.(C) Plant latex is a milky fluid that is made up of an emulsion of proteins,natural products, and other cellular constituents that often contains diterpenes,triterpenes, or higher order terpenes, including rubber. Stored in ducts knownas laticifers, it is exuded after injuries, such as that caused by herbivory.Source: (A) Reiderer, University of Würzburg, Germany; (B, C) Wittstock &Gershenzon (2002). Curr. Opin. Plant Biol. 5:300–307.

Fig.E 24.3 Terpenoids can be usedfor plant defense. Thesecompounds often form oily, sticky,or irritating mixtures that deterherbivore feeding and pathogeninvasion.

For example, the triterpene azadirachtin (Fig. 24.4)from the seed oil of the Asian neem tree (Azadirachtaindica) is a powerful insect feeding deterrent andexerts a variety of toxic effects on insects as well.

Fig. 24.4 Structures of some economicallyimportant terpenes. Azadirachtin is atriterpene (C30) derivative from the seed oilof the Asian neem tree (Azadirachta indica)used in insect pest control. Taxol, aditerpene (C20) derivative, is from the yewtree (Taxus brevifolia) and used in cancertreatment. Digitoxigenin is an aglycone ofdigitoxin, a steroid glycoside used to treatheart disease. Artemisinin is asesquiterpene from Artemisia annua usedagainst malaria.

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Utility For Humans. Terpenoids play important rolesin human society (used as flavors and fragrances offoods, beverages, soaps, perfumes, toothpaste, andother products).

Some find use as industrial materials (resins andrubber) or pigments (carotenoids), and yet others areinsecticides that are of value because of their lowtoxicity to humans and lack of persistence in theenvironment (azadirachtin and pyrethrins).

Many terpenoids are also of nutritional orpharmaceutical significance, including vitamins A, D,E, and K as well as some well‐known drugs (Fig.24.4):1. Taxol is a diterpene from yew (Taxus brevifolia) that has

become a blockbuster drug for the treatment of ovarian andbreast cancer,

2. Artemisinin is a sesquiterpene from Artemisia annua usedagainst malaria, and

3. Cardenolides (triterpenes, such as digitoxigenin) extractedfrom foxglove (Digitalis lanata) are prescribed to millions ofpatients for the treatment of heart disease.

4. Carotenoids are also becoming more popular as“neutraceuticals” (foods with health benefits) believed to helpprevent cancer, heart disease, and macular degeneration.

3. Biosynthesis The five‐carbon isopentane or isoprene units that

make up terpenoids are usually joined in a“head‐to‐tail” fashion (Fig. 24.6).

This pattern was first recognized in the late 1800sand updated in the 1930s, but other types of reactionscould occur in terpenoid formation because of thechemical mechanisms involved.

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Fig. 24.6 Terpenoids are formed from the fusion of five‐carbonisopentane units (isoprene units), which are usually joined in ahead‐to‐tail fashion. The division of terpenoids into such units helps tovisualize their biosynthetic assembly, although extensive metabolicrearrangements may complicate this task. Myrcene and menthone aremonoterpenes (C10—two C5 units); (E)‐β‐farnesene andamorpha‐4,11‐diene are sesquiterpenes (C15—three C5 units); andpimaric acid is a diterpene (C20—four C5 units).

However, all terpenoids were proposed to be formedfrom a “biological” isoprene unit. These units can bejoined in head‐to‐tail, head‐to‐head, and by varioushead‐to‐middle fusions, and substantial structuralrearrangement can occur during biosynthesis.

In such modified terpenoids, it may be difficult todiscern the original organization of the isoprene units(Fig. 24.6).

The biosynthesis of terpenoids can be convenientlydivided into four stages :(i) the synthesis of the biological five‐carbon isoprene unit,(ii) repetitive condensations of the five‐carbon unit to form a

series of larger and larger prenyl diphosphates,(iii) conversion of prenyl diphosphates to the basic terpenoid

skeletons, and

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(iv) further modifications to the basic skeletons, includingoxidation, reduction, isomerization, conjugation, and othertransformations.

The biosynthesis of basic 5‐carbon unit of terpenoidsis represented by isopentenyl diphosphate anddimethylallyl diphosphate.- These intermediates are synthesized in plants by two

completely different routes that are spatially separated butexchange intermediates: the mevalonate pathway and themethylerythritol 4‐phosphate (MEP) pathway.

The mevalonate pathway, demonstrated initially inyeast and mammals and well‐characterizedsequence, begins with the condensation of threemolecules of acetyl‐CoA in two steps to form thesix‐carbon compound, 3‐hydroxy‐3‐methylglutaryl‐CoA (HMG‐CoA) (Fig. 24.7).

Fig. 24.7 The mevalonate pathway for the formation of isopentenyldiphosphate, the basic five‐carbon unit of terpenoid biosynthesis. Synthesisof each isopentenyl diphosphate unit requires three molecules of acetyl‐CoA.The mevalonate pathway occurs in the cytosol of plant cells.

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The discovery of a second route for producing thebasic five‐carbon building blocks of terpenoidbiosynthesis, a route completely distinct from themevalonate pathway, is one of the most excitingadvances in plant biochemistry in recent years was(Fig. 24.8).

This route begins with pyruvate and glyceraldehyde3‐phosphate and is usually named for its secondintermediate, 2C‐methyl‐d‐erythritol 4‐phosphate(MEP).

The existence of a second, nonmevalonate pathwayto isopentenyl diphosphate in plants had beensuspected for many years based on the poorincorporation of mevalonate into certain types ofterpenoids.

Fig. 24.8 The MEP pathway, a second route for the formation ofisopentenyl diphosphate in plants, occurs in plastids.

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The second stage of terpenoid biosynthesis involvesfusion of the basic C5 building blocks, isopentenyldiphosphate and dimethylallyl diphosphate, into largerprenyl diphosphates and other metabolicintermediates (Fig. 24.9).- Dimethylallyl diphosphate, as the smallest prenyl diphosphate

with an allylic double bond, is a primer to which varyingnumbers of isopentenyl diphosphate units can be added insequential chain elongation steps. Thus, isopentenyldiphosphate and dimethylallyl diphosphate condense inhead‐to‐tail fashion to form the allylic C10 compound, geranyldiphosphate.

- Another molecule of isopentenyl diphosphate may thencondense head‐to‐tail with geranyl diphosphate to generatethe C15 allylic diphosphate, farnesyl diphosphate. Additionof a further isopentenyl diphosphate gives the C20geranylgeranyl diphosphate.

farnesyl diphosphate, the precursor of sesquiterpenes (C15), andgeranylgeranyl diphosphate, the precursor of diterpenes (C20). To maketriterpenes (C30), two farnesyl (C15) units are combined, while to maketetraterpenes (C40), two geranylgeranyl (C20) units are joined.

Fig. 24.9 The majorsubclasses of terpenoids arebiosynthesized from the basicC5 building blocks,isopentenyl diphosphate andits isomer, dimethylallyldiphosphate, which is formedfrom isopentenyl diphosphatevia isopentenyl diphosphateisomerase. Reactionscatalyzed byprenyltransferases condensevarying numbers ofisopentenyl diphosphate anddimethylallyl diphosphateunits to geranyl diphosphate,the precursor ofmonoterpenes (C10),

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(B) The antimalarial drug artemisinin was originally isolated from Artemisiaannua. Much effort has been devoted to breeding A. annua with higherartemisinin levels. In addition, much of the artemisinin pathway has beenengineered into microbes to increase production. Source: (A) Golden RiceHumanitarian Board, www.goldenrice.org; (B) HarroBouwmeester,Wageningen University, The Netherlands.

Fig. 24.16 (A)Metabolic engineeringof plant terpeneproduction has yielded avariety of rice (“goldenrice”, here compared toordinary rice on the left)with high levels of avitamin A precursor,β‐carotene, to helpreduce vitamin Adeficiency in manyrice‐eating countries.

III. ALKALOIDS1. Definition The term alkaloid, coined in 1819 in Halle (Saale)

Germany, finds its origin in the Arabic name al‐qali,the plant from which soda was first isolated.

Alkaloids were originally defined as pharmacological-ly active, nitrogen‐containing basic compounds ofplant origin.- After 200 years of alkaloid research, this definition no longer

encompasses the entire alkaloid field, but in many cases it isstill appropriate. Well-known alkaloids include morphine,strychnine, quinine, ephedrine, and nicotine.

- Alkaloids are not unique to plants, and have also beenisolated from numerous animal sources. The alkaloidmorphine, for example, has been detected in mammals andis synthesized de novo in mouse.

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Many of the alkaloids that have been discovered arenot pharmacologically active in mammals, and someare neutral rather than basic in character, despitethe presence of a nitrogen atom in the molecule.

2. Uses of Alkaloid For much of human history, alkaloid‐containing plant

extracts have been used as ingredients in potions andpoisons.

In the eastern Mediterranean, use of the latex of theopium poppy (Papaver somniferum) can be tracedback at least to 1400–1200 bc.

The Sarpagandha root (Rauwolfia serpentina) hasbeen used in India since approximately 1000 bc.

Ancient people used medicinal plant extracts aspurgatives, antitussives, sedatives, and treatmentsfor a wide range of ailments, including snakebite,fever, and insanity.- During his execution in 399 bc, the philosopher Socrates

drank an extract of coniine‐containing hemlock (Coniummaculatum; Fig. 24.27).

- In the last century bc, Queen Cleopatra used extracts ofhenbane (Hyoscyamus), which contains atropine (Fig. 24.28),to dilate her pupils and appear more alluring to her malepolitical rivals.

Alkaloid‐containing plants were mankind’s original“materia medica,” and many are still in use today asprescription drugs (Table 24.1).

One of the best‐known prescription alkaloids is theantitussive and analgesic codeine from the opiumpoppy, Papaver somniferum (Fig. 24.29).

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Fig. 24.27 (A) The piperidine alkaloid coniine, the first alkaloid to besynthesized, is extremely toxic, causing paralysis of motor nerve endings.(B) In 399 bc, the philosopher Socrates was executed by consuming anextract of coniine‐containing poisonous hemlock. This depiction of the event,“The Death of Socrates,” was painted by Jacques‐Louis David in 1787.Source: (B) The Metropolitan Museum of Art, New York, NY.

Fig. 24.28 Structure of theanticholinergic tropane alkaloidatropine from Hyoscyamus niger.

FIGURE 24.29 (A) Structures of the alkaloids codeine and morphinefrom the opium poppy Papaver somniferum. Asymmetric (chiral)carbons are highlighted with red dots. (B) The frog Bufo marinusaccumulates a considerable amount of morphine in its skin. (C)Structure of diacetyl morphine, commonly known as heroin. Source: (B)Rickl, Universität München, Germany; previously unpublished.

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3. Biosynthesis Until the mid‐20th century, our view of plant alkaloid

biosynthesis was based on biogenic hypotheses. In the 1950s, alkaloid biosynthesis became an

experimental science, as radioactively labeled organicmolecules became available for testing hypotheses.

These early precursor‐feeding experimentsestablished that alkaloids are, in most cases, formedfrom L‐amino acids (e.g., tryptophan, tyrosine,phenylalanine, lysine, and arginine), either alone or incombination with a steroidal, secoiridoid (e.g.,secologanin) or other terpenoid‐type moiety.

One or two transformations can convert theseubiquitous amino acids from primary metabolites tosubstrates for species‐specific alkaloid metabolism.

Although we do not thoroughly understand how mostalkaloids are made by plants, several systems canserve as examples of the types of building blocks andenzymatic transformations that have evolved inalkaloid biosynthesis.

Ajmalicine (Fig. 24.39), an L‐tryptophan‐derivedmonoterpenoid indole alkaloid, was the first alkaloidfor which biosynthesis was clarified at the enzymelevel.

In plants, the biosynthesis of ajmalicine and morethan 1800 other monoterpenoid indole alkaloidsbegins with the decarboxylation of the L‐amino acidtryptophan by tryptophan decarboxylase to formtryptamine.

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Fig. 24.39Strictosidine, theproduct oftryptamine andsecologanin, is theprecursor for manyspecies-specificalkaloids.

Then tryptamine, by action of strictosidine synthase,is stereospecifically condensed with the secoiridoidsecologanin (derived in multiple enzymatic steps fromgeraniol) to form 3α(S)‐strictosidine. Strictosidine canthen be enzymatically permutated in aspecies‐specific manner to form a multitude ofdiverse structures (Fig. 24.39).

The elucidation of the enzymatic formation ofajmalicine laid the groundwork for analysis of morecomplex biosynthetic pathways, such as thoseleading to two other l‐tryptophan‐derivedmonoterpenoid indole alkaloids, ajmaline (Fig. 24.40)and vindoline.

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Fig. 24.40 The biosynthesis of the antiarrhythmic monoterpenoid indole alkaloidajmaline in Rauwolfia serpentina from tryptamine and secologanin is one of thebest-understood alkaloid biosynthetic pathways. Several of the enzymes havebeen crystallized, and their atomic structures have been determined (Section24.13.2).

IV. PHENOLICS1. Definition The transition of early vascular plants to a terrestrial

habitat was successful in large part because of thedevelopment and elaboration of a diverse group ofcompounds known generally as “phenolics.”

Although the majority of phenolics are structuralcomponents of cell walls, a vast array are the toxinsand antifeedants of plant defense, coloring agents offlowers and fruits, aroma complement of plant organs(such as flower scents and fruit flavors), andantioxidants of wood, bark, and seeds.

Phenolics (phenols) are a class of chemicalcompounds consisting of a hydroxyl group (—OH)bonded directly to an aromatic hydrocarbon group.

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Plant phenolics vary greatly in size and complexity,but all generally possess (or are derived fromcompounds that once possessed) an aromatic arene(phenyl) ring with at least one hydroxyl groupattached (Fig. 24.49).

The phenolic hydroxyl group isacidic compared to other hydroxylgroups because it resides on anarene ring, which can readilystabilize a deprotonated oxygensubstituent.

As a result, plant phenolics are reactive and wellsuited to being the building blocks of large polymers,such as lignins or suberins, and in the generation ofa large number of compounds that play importantroles in many aspects of plant biology.

Phenolic compounds can be classified in variousways, for example according to the functional groupsattached to the phenol, or based on the number ofphenol units in the molecule.

Major subclasses of phenolic compounds include theflavonoids, anthocyanidins, isoflavones, chalcones,stilbenes, coumarins and furanocoumarins, monolig-nols and lignans, naptha‐ and anthraquinones, anddiarylheptanoids.

2. Uses of Phenolics Human needs. Human societies would not exist in

their present form without the properties imparted byphenolic compounds (Table 24.3).

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- Leather, which throughout early human history enabled morerapid transportation and thereby better trade (and war), hasbeen produced for millennia by treating raw animal hides with“tannins,” either gallate esters or condensed tannins.

- The wheel was perfected because of the properties of wood,the strength and durability of which can be attributed to thelignin polymers and lignans, flavonoids, and hydrolyzabletannins.

- Much of the world relies on wood for building as well as a fuelto provide warmth.

Source of energy. We are now turning to woodproducts as a source of lignocellulosic materials forbiofuel production.

As alternatives to dwindling natural resources aresought, plants, based in part on the phenolics thatthey contain, are among the best choices forrenewable industrial feedstocks and materials.

Medicines. Human health is also intimately tied tothis class of compounds, as many medicinalcompounds are derived or inspired by plantphenolics.- Examples include the lignan podophyllotoxin from May apple

(Podophyllum peltatum), which is derivatized to formteniposide, etoposide, and etophos, used to treat a numberof cancers.

- The diarylheptanoid curcumin from turmeric (the curry spice),which has potent anti‐inflammatory properties and is usedworldwide to treat arthritis and other inflammatory diseases;and the stilbene resveratrol from red grapes and wine, whichhas been demonstrated to improve heart health and detercancer development.

The world is a healthier, more colorful, tasty, andfragrant place because of the phenolics that plantsproduce.

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QUESTIONS1. What is secondary metabolites based on functional definition?2. How does a plant interact with its environment? is3. What are the main constituents of conifer resin?4. What is taxol?5. What are the biosynthetic pathways of basic 5‐carbon unit of

terpenoids?6. What is atropine?7. What is codeine?8. What are the early precursors of alkaloids?9. What is strictosidine?10. What is11. What is

12. What is

3. Biosynthesis Phenolic compounds account for approximately 40% of

organic carbon in plants and are derived primarily fromphenylpropanoid and phenylpropanoid-acetatebackbones (Fig. 24.50), although related biochemicalpathways, such as those leading to “hydrolyzable” tannins,also contribute.

Most phenolics originate from the phenylpropanoid and thephenylpropanoid‐acetate pathways. Both pathways share thefirst three biochemical steps, the core phenylpropanoid pathway(Fig. 24.52).

These steps begin with an L‐phenylalanine precursor and endwith the formation of 4‐coumaroyl CoA, a reactive compound thatcan participate in further reactions that lead to the wealth ofcompounds that are the plant phenolics.- The phenylpropanoid and phenylpropanoid‐acetate pathways

produce products that ameliorate the challenges that facedearly land plants.

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3. Biosynthesis Phenolic compounds account for approximately 40% of

organic carbon in plants and are derived primarily fromphenylpropanoid and phenylpropanoid-acetatebackbones (Fig. 24.50), although related biochemicalpathways, such as those leading to “hydrolyzable” tannins,also contribute.- The phenylpropanoid and phenylpropanoid‐acetate pathways

produce products that ameliorate the challenges that facedearly land plants.

- These challenges persist and provide selection pressure tomaintain the phenylpropanoid pathway in plants (Table 24.2).

Evolution of the phenylpropanoid‐acetate pathwayhelped overcome challenges such as damaging UVirradiation which was faced by plants even beforethey left the water.

Most phenolics originate from the phenylpropanoidand the phenylpropanoid‐acetate pathways. Bothpathways share the first three biochemical steps, thecore phenylpropanoid pathway (Fig. 24.52). Thesesteps begin with an L‐phenylalanine precursor andend with the formation of 4‐coumaroyl CoA, a reactivecompound that can participate in further reactions thatlead to the wealth of compounds that are the plantphenolics.

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This pathway leads to production of a large anddiverse class of compounds called flavonoids, whichhas more than 5,000 members.

Flavonoids, such as quercetin (Fig. 24.50), consist ofa basic three‐ring core structure that is modified toproduce subclasses (Fig. 24.51), such as theanthocyanins (pigments), proanthocyanidins orcondensed tannins (feeding deterrents and woodprotectants), isoflavonoids (active in plant defensiveand signaling), and flavones and flavonols(anti‐inflammatory agents in animals).

The most basic flavonoids—the flavones, flavonols,and flavanones—are widespread in the plant kingdomand typically absorb damaging UV rays.

Modifications to the C ring are responsible for creation of new classes ofcompounds, whereas modifications to the A and B rings lead to diversificationwithin the compound class.

Fig. 24.50 Phenylpropanoidand phenylpropanoid-acetateand skeletons andrepresentative plantcompounds based on thosestructures. Flavonoids, suchas quercetin, typically havethree interconnected rings attheir core, the A, B, and Crings.

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Fig. 24.51 Basic structures of phenylpropanoid‐acetate‐derived subclassesof flavonoids.

http://leavingbio.net/TheStructureandFunctionsofFlowers%5B1%5D.htm

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V. CYANOGENIC GLYCOSIDES

1. Definition Cyanogenic glycosides (Fig. 24.17) are

β‐glycosides of ‐hydroxynitriles (cyanohydrins), andcharacterized by their ability to liberate hydrogencyanide (HCN) when hydrolyzed by β‐glycosidases.

This process (cyanogenesis) typically occurs whenplant tissue containing cyanogenic glycosides isdisrupted, for example, when bitten, chewed, oringested by animals or insects.

Cyanogenic glycosides are important components ofplant defense against generalist herbivores due totheir bitter taste and release of toxic HCN upon tissuedisruption.

(B) Cyanogenesis. Cyanogenic glycosides are formed from a limited number ofl‐amino acids and converted to cyanogenic glycosides with E‐oximes andα‐hydroxynitriles as key intermediates. Upon hydrolysis of the cyanogenicglycoside by the action of β‐glycosidases and α‐hydroxynitrilases, toxichydrogen cyanide (HCN) and ketones or aldehydes are produced.

Fig. 24.17 (A) Generalstructure of cyanogenicglycosides. The sugarresidue is always ad‐glucose that is joinedby an O‐β‐d‐glucosyllinkage (referred to morespecifically, then, ascyanogenic glucosides),but additional sugarspresent may be different.

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Cyanogenic glycosides are derived from five proteinamino acids (Val, Ile, Leu, Phe, and Tyr) and thenonproteinogenic amino acid cyclopentenyl glycine(Fig. 24.18).

Cyanogenic glycosides are widely distributed amongmore than 2,600 different species of pteridophytes,gymnosperms, and angiosperms.

Whereas pteridophytes and gymnosperms containcyanogenic glycosides derived from aromatic aminoacids, angiosperms may contain cyanogenicglycosides derived from either aliphatic or aromaticamino acids (Figs. 24.18 and 24.19).

Many crops, including sorghum (Sorghum bicolor),cassava (Manihot esculentum), and barley(Hordeum vulgare), are cyanogenic;

Fig. 24.18 Structures of selected cyanogenic glycosides (a-hydroxynitrileglycosides).

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Plants and insects have coevolved for the last ≈400 million years, and bothgroups of organisms produce toxic secondary metabolites like cyanogenicglucosides to defend themselves. A few arthopod families also accumulatecyanogenic glucosides, for example, Zygaena larvae that has evolved theability to either sequester cyanogenic glucosides from Lotus plants or, if rearedon acyanogenic Lotus, to synthesize cyanogenic glucosides de novo.

Fig. 24.19 Evolutionarytrees depicting theevolution of arthropodsand plants and thepresence of differentclasses of defensecompounds, includingcyanogenic glycosides.

2. Functions Cyanogenic glycosides are widely distributed among

more than 2,600 different species of pteridophytes,gymnosperms, and angiosperms.

Cyanogenic glycosides acting as plant defensecompounds is one of the most notable functions ofcyanogenic glycosides in plants.

The defense process depends on the activation ofcyanogenic glycosides by β‐glycosidases to releasetoxic volatile HCN, as well as a ketone or aldehyde, tofend off herbivores and pathogens (Fig. 24.17).

In addition to serving as defense compounds,cyanogenic glycosides serve many other functionsthat afford an advantage at certain developmentalstages or following specific environmental challenges.

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3. Biosynthesis The biosynthesis of cyanogenic glycosides is

catalyzed by two groups of enzymes: cytochromeP450 enzymes and UDP‐glycosyltransferases.

The pathway for cyanogenic glycoside biosynthesiswas first elucidated in sorghum (Sorghum sp.) usingbiochemical approaches.

Sorghum contains the tyrosine‐derived cyanogenicglucoside dhurrin, and the dhurrin biosyntheticpathway involves the intermediates N‐hydroxytyro-sine, N,N-dihydroxytyrosine, (E)‐ and (Z)‐p‐hydroxyl-phenylacetaldoxime, p‐hydroxyphenylacetonitrile,and p‐hydroxymandelonitrile (Fig. 24.20).

The enzymes of cyanogenic glycoside biosynthesisare organized within a dynamic metabolon(macromolecular enzyme complex) (Fig. 24.22).

Fig. 24.20 Biosynthesis anddegradation of thetyrosine‐derived cyanogenicglycoside dhurrin in Sorghumbicolor. The enzymescatalyzing the different stepsare shown. Each cytochromeP450 enzyme catalyzes morethan one step in the pathway.These steps are indicated byred (CYP79A1) and blue(CYP71E1). Degradation ofdhurrin occurs when the planttissue is disrupted, and canproceed nonenzymatically. TheNADPH cytochrome P450oxidoreductase (POR)provides reducing power fromNADPH in single electrontransfer steps.

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Subsequent studies in other cyanogenic speciesinvolved metabolomics, proteomics, andtranscriptomics, genome sequencing, heterologousexpression of candidate genes, and natural variationand mutants.

They have shown the pathway for cyanogenicglycoside biosynthesis in these species also utilizescytochrome P450 enzymes and UDP‐ glycosyltrans-ferases and follows the same sequence oftransformation of intermediates as identified insorghum.

Fig. 24.22 Metabolon formation in cyanogenicglycoside synthesis. The two cytochrome P450enzymes (CYP79A1 and CYP71E1) andUDP‐glucosyltransferase (UGT85B1) interact tochannel intermediates in the biosyntheticpathway directly into formation of dhurrin.

Sequence comparisons, however, indicate that thegenes encoding the biosynthetic enzymes in differentcyanogenic plant species are not necessarilyorthologous (genes in different species that evolvedfrom a common ancestral gene by speciation).

When the genes encoding the pathway forcyanogenic glycoside formation were identified in L.japonicus, S. bicolor, and cassava (M. esculentum), itbecame apparent that the genes were clusteredwithin all three species (Fig. 24.21).

This indicates a strong evolutionary mechanismpromoting this type of genomic organization.However, the clusters differ substantially in size, genedensity, and with respect to the additional genespresent.

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Fig. 24.21 Clustering of cyanogenic glucoside biosynthetic genes in thegenomes of Lotus japonicus, Sorghum bicolor, and Manihot esculenta. Thegenes are positioned differently in the clusters and with large differences inthe spacing. Arrows indicate orientation of functional genes. CYP79 genes areshown in pink, CYP71E and CYP736 genes in green, and UGT85 genes inblue.