Lecture 1 
 

Introduction:  

• Phenolic metabolism
• Isoprenoid metabolism
• Special Nitrogen metabolism

 

These lectures will be based on

Chapter 10,11 and 12 of Plant Biochemistry

Edited by J.B.Harborne et.al

Academic press 1997

=================== 

What are phenols?  

Phenols are compound composed of one or more aromatic benzene rings with one or more hydroxyl groups (C-OH). This enormous class includes over 4000 compounds that are chemically distinct from terpenes.

Although the essential oils are often classified as terpenes, many of the volatile chemicals are actually phenolic compounds, such as eucalyptol from Eucalytus, citronellal from Cymbopogon sps oil and thymol from Trachyspermum ,carvacrol thyme and eugenol from Syzygium  
 
 

Three different route leads to plant phenolics  

• Shikimate /arogenate pathway produces aromatic amino acids like phenylalanine, tyrosine and tryptopan. The first one is very important as precursor of phenolics. 

 
 

Several major classes of secondary metabolites such as, flavonoids, coumarins , lignan, stilbenes and benzoic acid are derived from phenylalanine.

Some phenolics such as hydroxybenzoate galate are formed from dehydroshikimate are formed from the intermediates 
 
 
 

Dehydroshikimate 

Some quinones are formed from chrosimate through succinylabenzoate path way 

• Acetate /malonate pathway(polyketide pathway)

 

This pathway leads to some quinones but also to various side chains that forms flavonoids and isoflavonoids. 

• Actate mevlonate pathway: this pathway leads by dehydrogenation of some aromatic terpenoids mostly monoterpenes.

 

Function of phenols

• Plant-Plant Interactions

• Mechanical Support: lignin and suberin provide hard ness to wood so much desired for stem.

• Attraction Pollinators (Pigments and Scent)

• Repellent:

• Resistance to Pathogen (Phytoalexin)

• Protect From Predators: herbivorous

 
 

Major classes of phenols 
 
 

 
 

Summarized image of these compounds can be seen in page 389 of text  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Lecture 2

June 19, 2005 
 
 
 
 
 

Carbohydrate metabolism           Erythrose 4-phosphate             +        Phosphatenolatepyruvate[PEP] 
 
 
 
 
 
 
 

   
 

                                                                                                   

                                                                                                        Shikimate 
 

                                                                                                                 
 
 
 
 
 

        Chorismate  
 
 
 
 
 
 

                                                                

         Prephenate                      Anthranilate 
 
 
 
 
 
 
 
 

                              L-arogenate                    L-tryptophan 
 
 
 
 
 
 
 

                  L-tyrosine        L-phenyalanine 
 

The sequence of reaction needs 11 enzymes  

 

Shikimate pathway 

Over all reaction for the pathway 

 
 
 
 

Lecture 3

June 19,2005 
 

Larsen PO, Onderka DK, Floss HG. [1975] 

14C-labelled shikimic acid and double labelled shikimic acid tritiated stereospicifically at C-6 are incorporated into 3-(3-carboxyphenyl) alanine, 3-(3-carboxy-4-hydroxyphenyl) alanine, phenylalanine, and tyrosine in Reseda lutea L., Reseda odorata L., Iris x Hollandica cv. Prof. Blauw, and Iris x hollandica cv. Wedgwood.

The experiments with 14C-labelled shikimic acid confirm that the aromatic carboxyl groups and rings in 3-(3-carboxyphenyl) alanine and 3-(3-carbocy-4-hydroxyphenyl) alanine derive from the carbocyl group and ring in shikimic acid whereas the experiments with double labelled shikimic acid demonstrate that the pro-6S-hydrogen atom is retained and the pro-6R-hydrogen atom lost in the biosynthesis of 3-(3-carboxyphenyl) alanine, phenylalanine, and tyrosine in the plants used. 3H was located in the ortho-position in the aromatic rings of phenylalanine and tyrosine but in a position para to the alanine side chain of 3- (3-carboxyphenly) alanine. No 3H was found in 3- (3-carboxy-4-hydroxyphenyl) alanine. This supports a derivation of the last two compounds from chorismic acid via isochorismic acid, isoprephenic acid, and 3'-carboxyphenylpyruvic acid and 3'-carbocy-4'-hydroxyphenylphruvic acid. The 3H/14C ratio in 3-(3-carboxyphenyl) alanine was found higher than in the precursor used. This isotope effect must operate by competition between the pathways from isoprephenic acid to 3'-carbocyphenylpyruvic acid and to 3'-carbocy-4'- hydroxyphenylpyruvic acic.

The proposed biosynthetic pathways for the two carboxy-substituted amino acids are in agreement with their distribution patterns in the plant kingdom and suggest that they may derive from minor changes of enzymes involved in the general pathways of aromatic biosynthesis. 
 
 
 
 
 
 

Lecture 3 continued..      

June 21, 2005

Phenylalanine / hydroxycinnamate pathway   

The enzymes involved in the pathway are  

 
 
 
 

+++ For details on PAL page 393

Mechanism:

The elimination of ammonia from -amino acids is a chemically difficult process. While the non-acidic -proton has to be abstracted, the much more acidic ammonium protons must remain untouched to maintain the leaving group ability of this positively charged group.

Histidine and phenylalanine ammonia-lyases (HAL and PAL) possess a catalytically essential electrophilic group which has been believed to be dehydroalanine for 30 years.

Recently [2002], the X-ray structure of HAL has been solved. The electron density was not consistent with dehydroalanine but showed the presence of methylidene imidazolone (MIO) instead. The high electrophilicity of this prosthetic group as well as the geometry at the active site support a previously proposed mechanism involving a Friedel–Crafts-type attack at the aromatic ring of the substrate. Further biochemical evidence for this unprecedented electrophile-assisted ammonia elimination is also presented. Although no X-ray structure of PAL has been published as yet, spectrophotometrical evidence for the presence of MIO has been provided. Based on above, a chemical model for the PAL reaction has been proposed.

    

This part has been taken form www.usm.my/pharmacy/... 
 

Lecture 4

June 24, 2005

In our previous lecture we talked on formation of hydroxycinnamates. This occurs in two forms E-isomer and Z-isomer. Latter is formed either by photochemical or by enzymes. 

Hydroxylases:

Cinnamate hydroxylase ie E-cinnamate,NADPH,Oxygen oxidoreductase &

4-Coumarate3-hydroxylase is monooxygenaes and introduces one molecule of oxygen which split and single atom is introduced in the reaction and second atom is reduced to H₂O by a second oxidizable substrate like NADPH. 

Hydroxylases are of two types: (1) the cytochrome bound P-450 dependent oxygenases like

Cinnamate hydroxylase ie E-cinnamate,NADPH,Oxygen oxidoreductase 

Ferulate 5- hydrxylase 

(2) The soluble phenolase which catalyses introduction of 2^nd hydroxyl group in a phenol. Example of this class is  4-Coumarate3-hydroxylase

NIH shift: This is an intramolecular migration of the proton that is displaced to an adjacent Ortho position. This is named after National Institute of Health [NIH] USA.

This proceeds via arene oxide intermediate. 
 

The phenolase 4-coumarate hydroxylase introduces second hydroxyl ortho (3-position)

To the the hydroxyl group of 4-coumarate.The hydroxylases specifically accept

Hydroxycinnamte conjugate..

This can be seen in case of formation caffeate at the 4-coumaroyl moiety of

4-coumaroyl –CoA or of 5-O-(4-coumaroyl)-shikimate. 

O-methyaltion of caffeate and hydroxyferulate yields ferulate and sinnapate resepectively.

Caffeate/5-hydroxyferulate methyltransferase involvoled uses S-adenosyl-L-methionine. 

The hydroxycinnamates path ways are much important as is used by predominant type

Side chain: 

                                             

1. Condensation (side chain elongation) sequential reaction with three malonyl-CoAs,leads to the formation of flavonoids.Here acetate unit is added and CO₂ liberates.
2. Reduction (NADPH dependent) leads to lignin precursor, Hydroxycinnamyl alcohol.
3. Degradation (side chain shortening) leads to Hydroxybenzoates.
4. Conjugation through attachment of Hydroxy or amino group bearing molecule leads to esters or amides in rare cases attachment to phenolic hydroxyl group leads formation of glycosides.

 
 

Lecture: 5

June 28, 2005

Hydroxybenzoates: Hydroxybenzoate (C6-C₁) seem to be universally distributed in

Plants as 4-hydroxybenzoate, Protocatechulate, vanilate, Gallate, Syringate. They may be soluble conjugated form as well as bound to cell wall. Trihydroxy derivative, Gallate is

Often present as gallotanin.Another trihydroxybenzoate is salicylate ( 2-hydroxybenzoate)

This compound is major constituent of Gaultheria fragarantissima (oil of wintergreen). 

It is likely that there are several pathways leading to individual hydroxybenzoate

in plant.

One pathway is degradation of side chain by removal of acetate. The reaction is

Proposed to proceed via CoA –ester. The second pathway in fig above also can

Not be excluded.

The substitution pattern depends on the hydroxycinnamte precursor.

Hydroxylation and methylation may occur with hydroxybenzoates as in

Phenylalanine/ hydroxycinnamte pathway. 
 

               Postulated Biosynthetic pathways to Hydroxybenzoate

Hydroxybenzoates may also be formed by shikimate /arogenate pathway at 3-hydroxycinnamte point.

Hydroxybenzoate is a precursor for a number of comounds like gallates. Hydroxycinnamate through CoA-ester and may reduce to benzaldehyde which gives cocain.

Lecture 6

June 28,005

Falovonoids: C₁₅ aglycone are occurs in various structural classes according to oxidation state.

They have a central pyran ring.Structures within these classes are modified Hydroxylation and

Methylation. They may also be glcosylated of acylate. The enzymes involved are shown below. 

 

Chalcone synthase ( Malonyl-CoA ;4-coumaroyl-CoA malonyltransferase) is the rate limiting enzyme in flavonoid syntheis .It is because this catalyses the formation of basic C₁₅ and channels Hydroxycinnamte to Flavonoid biosynthesis.

This enzyme M_r 78000-88000 is dimeric protein. pH optimum ranges from 7.5 to 8.5. 
 

Naringen chalcone is the first intermediate monohydoxyl B –ring typically present in all flavonois.

In this reaction Acetate unit is added stepwise to 4-coumaryl –CoA.

A random orientation of Acetate unit forms the “A” ring of the flavonoids. The chalcone synthase uses CoA ester as immediate substrates. And malonyl-CoA is supplied byATP dependent action acetyl CoA carboxylase. 
 
 
 

Chalcone provides the basic skeleton for different class of Flavonoids. 
 
 
 
 
 
 
 

Lecture 7 + 8

June 30, 2005

Substitution of flavanoids  
 
 

Basic substitution patterns of flavanoids are partly determined by mode of action and substrate preference of chalcone synthase (5, 7 hydroxyl group of A ring and 4’ hydroxyl group of B ring) and partly on enzymes of the aglycone pathway.

Further hydroxylation as well as methylations of Hydroxyl group especially on B-Ring is catalyzed by flavanoid specific hydroxyases and O-methyl transferases.

The 3’ and 5’ hydroxylation are catalyzed by cytochrome P-450 enzymes.

Distinct position specific SAM dependent O-methyltranseferases leads to some common

B-Ring methoxylated flavanoids.

The common substitution patterns of naturally occurring flavnoids and anthocyanindins are shown below 
 
 
 

Methyltransferase activities also lead to some rare polymethoxylated flavanoids of

Saxifragaceae.

When sugar molecule is conjugated to a flavone molecule, it is called flavonoidal glycosides. Two major types of glycosides 3-O-glycoside and 3-C –glycosides.

The rutin of fagopyrum esulentum is a 3-O-glycoside and the Vitexin is an 8-C-glucosides. 
 
 

Tannins:

Water soluble plant polyphenols which cause protein precipitation from aqueous solution are called tannins.

They are of two types Hydrolysable and condensed (non-Hydrolysable).

Non-hydrolysable phenols are also called pro-anthocyanidins because they give anthocyanidin on acid hydrolysis.

For our purpose we consider condensed tannins only. They are present in areca nut, catechu etc and are getting more attention as antioxidants.

Flavanoids may bind to each other and form dimmer as in case of amentoflavone.

The linkage is between C-5’ of B-ring and C-8 of A-ring.   The carbonyl group at C-4 is always free. They are rather lipophilic and are locate outside cell wall. 

amentoflavone showing linkage between C-5’ of B-ring and C-8 of A-ring.

Oligo and polymeric flavanoids [condensed tannins] such as pro-anthocyanidins always have bonds between C-4 and C-8.These products accumulates as higher molecular weight [Mr 2000 to 7000].

The monomeric flavanoid subgroups involved anthocyanidin biosynthesis are 3,4-diols, although 4-ols are also frequently found. They form polymerized flavan 3-ols andflavan, respectively.

The  flavan 3-ols units may occur in four isomeric forms although only two of them are commonly found in nature. 
 

The catechins are formed by 2, 3-trans pathway: 

The pathway for epicatechins is not yet confirmed. It is thought that there is a stereospecific capture of the intermediate carbocations or quinone methides by the end products flavan-3-ols. 
 
 
 
 
 
 

Quinones: The common natural quinones are  

Some of the key enzymes leading to the naphthanoate intermediates are described from bacterial sources.

Secondary benzoquinone are constituents of Hyphomyces and Basidiomyces [Fungi].

An example of occurrence of benzoquinone in higher plant is arbutin [Hydroquinone glucoside] in rosaceae and ericaceae.

Naphthoquinones are responsible for pigmentation of colored heartwood an example of its presence is Juglans regia

Anthraquinones are present in Bacteria,Fungi,and lichens; in higher plants they are present in Caesalpiniaceae, polygonaceae, Rhamnaceae and Rubiaceae.

There are at least two structural types of anthraquinones, one bearing hydroxyl group in C-ring and the other bearing hydroxyl group in both A and C ring.

There are two pathway for quinoid structure [1] Polyketide pathway and [2] succinyl benzoate pathway.

The occurrence of both these path way are exemplified by naphthaquinone plumbagin from Plumbago europea / plumbago [?]  anthraquinone emodin in Rhamnus frangula    are  formed by polyketide [ Actate/Malonate derived quinones. 

while naphthaquinone lawsone Impatiens balsamina and anthraquinone alizarin of Rubia tinctoria are derived from succinylbenzoate pathway. 
 
 
 
 

Succinylbenzoate pathway:

In the shikimate pathway [lecture 2] we have come across a compound “chorismate”. The Chorismate is catalyzed by isochorismate  hydroxymutase to isochorismate.The latter in presence of 2-Oxoglutarate and thymine pyrophosphate is converted to 2-succinylbenzoate.This reaction sequence constitutes hitherto unprecedented aromatization process. 
 

The initial steps in the succinylbenzoate pathway have been partially characterized at enzyme level. The metabolite steps beyond SCoA ester is known only in case of formation of Juglone which proceeds via 1, 4-Naphthaquinoate.

The third ring of alizarin is generated by dimethylallylpyrophosphate(DMAPP). 
 
 
 
 
 

Lecture 9+10

July 2, 2005

ISOPRENOIDS are the compounds made up of isoprene units i.e. 5 C atoms;  

They have a general formula of C₅H₁₀. The term “Terpenoid” is an alternative generic name. Both these term are interchangeably used in chemical literature.

They are known since ancient times as ingredients of perfume, soap, flavoring and food colorants.

They are widely distributed in all 94 orders of flowering plants.

Analytical tools such as chromatography and spectrophotometry has led to a general understanding of isoprenoids structure, biosynthesis and properties.

Conolly and Hill [1991] has included 22000 structures in their “Dictionary of Terpenoids”.

Nomenclature and Classification: Nomenclature of isoprenoids reflects number of isoprene units present in it. Isoprene rule proposed by Wallach and latter extended to the biogenetic Isoprene rule by Ruzicka states that each class of Isoprenoid is derived from a single parent compound unique to that class.

These are formed by various cyclizations, rearrangments and even the loss or addition of carbon units from linear arrangements of carbon units.

General Pathway of Isoprenoid biosynthesis:

Originally HMG-CoA sterol pathway was proposed to explain formation sterols in animal kingdom. The side branches shown are seen in plants only where terpenoids characterstics of a particular species. The key to this pathway was discovery of Mevalonic acid (MVA).   
 

Mevalonic acid (MVA) is formed from Acetyl-CoA.In this pathway AcetoacetylCoAthiolase and hydroxymethylallyl pyrophosphate plays vital role. 

The formation of mevalonic acid via mevaldic acid is catalyzed by hydroxymethyl-CoA reductase and requires two molecules of NADPH per molecule of mevalonic acid formed.  

The mevalonic acid in three steps, each requiring one mole of ATP per mole of substrate and gives the biologically “active isoprene”Isopentenyl pyrophosphate (IPP) 

Dihydroxymethylallyl pyrophosphate (DMAPP) acts as prenyl donor to a molecule of isopentenyl pyrophosphate (IPP) and produces geranyl pyrophosphate (GPP).  geranyl pyrophosphate (GPP) acts as prenyl donor to another isopentenyl pyrophosphate to produce Farnesyl pyrophosphate (FPP). Farnesyl pyrophosphate (FPP). Condenses with another molecule of isopentenyl pyrophosphate (IPP) to produce higher prenyl pyrophosphate.Prenyl transferases catalyzes the reaction that produces all –trans prenyl pyrophosphate. 
 
 

In the chain shown below “chain lengthening” is by head to tail condensation  

but the formation of tri and tetraterpenoids involves head to head condensation of farnesyl pyrophosphate geranylgernyl pyrophosphate.. This yields compound like squalene and phytoene. 
 
 

Monoterpenoids are the simplest isoprenoids with C₁₀  structure constructed with two isoprene units.

They are probably everywhere in higher plant kingdom.

They can be classed into four broad structural classes.

Acyclic, cyclopentanoid, cyclohexanoid and irregular monoterpenes

BIOSYNTHESIS OF MONOTERPENOIDS: Geranyl pyrophosphate (GPP) is the precursor of monoterpenoids. 
 

Geranyl pyrophosphate (GPP) is efficiently converted to cyclic monoterpenoids. Involved enzyme Geranyl pyrophosphate synthase( cyclases )has been purified and characterized.

These cyclases (Synthases) are operationally soluble proteins Mr 50 to 100 kDa. More than 20 monoterpenoid cyclases have been identified.

Monoterpene cyclases have similar mechanism as the prenyl transferases.. They involve in an initial ionization of allylic pyrophosphate with an electrophilic attack of allylic carboacation on a double bond.

The cyclic oxygenated monoterpenoids are formed from cyclization products by oxidation and reduction which involves cytochrome P-450 dependent oxidases.

Most of them catalyze the formation of alkenes, but few yield oxygenated products.

It has been shown that (-)-(4S) limonene, the first cyclic terpene formed from GGPP is hydroxylated at C-3 to form Trans isopiperitenol in pipermint and at C-6 to form (-) Trans carveol in spearmint.

Limonene synthase is ( GPP: limonene cyclases is a soluble protein and has been purified from both species of menth. Basic properties have been found to be the same. 

Biosynthesis of pinene has been studied in soluble extract of Salvia officinalis. Three monterpene synthase ( Cyclase) help the formation of α-pinene and  β-pinene. 

   

Biological activities of Monoterpenoids:

1. Monoterpenoids like α-pinene, β-pinene,limonene,citronellol inhibit the growth of Amaranthus retroflexus.
2. Monoterpenoids show antibacterial activity α-pinene, limonene and carvone
3. Monoterpenoids have therapeutic activity.

 
 
 
 
 
 
 
 
 
 
 
 
 
 

Lecture 11

July 5, 2005

Sesquterpenoids are largest class of terpenoids and occur in the entire plant kingdom. Over 100 skeletons of Sesquterpenoids are known with thousands of known compounds.

They commonly occur with monoterpenoids.

Structure elucidation shows them to be derived from three isoprene units (C₁₅).Their structural diversity is remarkable  

They are formed by the condensation of IPP with GPP to yield all-trans-Farnesyl pyrophosphate.

    

The biosynthesis has been proposed based on hypothetical routes with cationic intermediates, with All-trans-FPP,2-cis,6-trans-FPP and nerolidyl pyrophosphate (NPP) as precursor.

Cyclization, hydride shift, rearrangements etc governed by steric and electronic considerations lead to the formation of sesquiterpenoids. 
 
 

Sesquiterpenoid lactone: They are the bitter principles of several herbal remedies.

Over 300 sesquiterpenoid lactones are characterized most of them contain α-methylene-γ-lactone group. Examples are 
 
 

The epoxide function and carbonyl [lactones] cause irreversible alkylation of enzymes and this property probably play an important role in plant’s interaction with other organisms. These functions are also responsible for antibiotic shown by antibiotics.    

Abscisic acid pathway is one of the major course by which these sesquiterpene lactones are formed.

Abscisic acid has structure of sesquiterpenoid and was expected to derive from 3 C₄ units. Theoretically there are two routes for the conversion MVA to ABA.

1. Direct route via Farenesyl pyrophosphate [FPP]   2. Indirect route via cleavage of a C₄₀ carotenoids.

Since 1980’s after experimental evidences the indirect pathway is more accepted.

This path way is via 9-cis violaxanthin to xathoxin to ABA-aldehyde to ABA. 

 

ABA is inactivated by its catabolism through two routes: Oxidation and conjugation. The primary pathway in tomato is 8’-hydroxy ABA Phaseic acid. 
 
 
 
 
 

Diterpenoids are C₂₀ compounds, derived from GGPP, often with skeletal rearrangements. They are found in higher plants and fungi and include the gibberlins. Till date 79 gibberellins are known in higher plants and 27 in fungi.

Gibberellins are divided into

1. Those that retain the full C₂₀ atoms of (-)-ent-kaurene (the C₂₀ GAs example

 

2. Those that have lost one carbon atom (C₂₀) with formation of a 19,10-γ-lactone.

 

================================================ 
 
 
 
 
 
 
 
 
 
 

Biosynthesis of GAs :

Triterpenoids: they are C₃₀ terpenoids formed by head to head condensation of Farnesyl pyrophosphate

   

Over 4000 triterpenoids molecules have been isolated. These triterpenoids have been found to contain over 40 skeleton types but the Pentacyclic ring is the most common: example  

β-amyrin

Phytosterols: Is a member of triterpenoids isoprenoids and are characterized by 3-β-monohydroxy perhydro-1, 2-cyclopentanophenanathrene ring system. Numbering system shown is as per IUPAC-IUB rules [1976] as shown 

Only over 300 sterols are reported from plants. They are mostly alkylated at C₂₄

                 

Note the structure above both in stigmasterols and sitosterol C₂₄ is alkylated. 

        

It is accepted that the step from HMG-CoA to squalene is same in plant and animal kingdom and is catalyzed by squalene synthetase .The epoxidation is facilitated by squalene epoxidase.This reaction also requires O₂ , NADPH,and a cytosolic fraction.

Saponins: are Glycosidic triterpenoids found in the plant plant kingdom are soluble in water and give stable foam [Sapindus]. They have three major classes.

a. Steroidal glycosides; b. Steroid alkaloids glycosides; and c. triterpenoid glycoside. All of them have one or more linear or branched chain of carbohydrates at C₃  

Carotenoids are yet important triterpenoids and contribute color and pigment to plants parts such as leaf, petal, fruits. Over 150 carotenoids are known from plants.

The carotenoid Hydrocarbons are called carotenes  

While those containing oxygen functios are called Xanthophylls examples are lutein ,Violaxanthins and neoxanthins. 
 

The carotenoids are of flower are divided into three main groups

1. Highly Oxygenated such asauroxanthin and falvoxanthins.
2. carotenes with high concentration such as β-carotenes in Narcissus
3. species specific example Crocetins of crocus.

Their biosynthesis is autonomous in most fruit and is responsible for change in color of fruit ripening even after taken out from branches.

In fruits. They are of eight different groups

1. insignificant amount ex strawberry
2. large amount chloroplast carotenoids ex Blue berry
3. Large amount of lycopene and its derivatives ex tomato
4. Large amount of β-carotene and its derivatives in ex peach
5. large amount of carotenoids epoxides ex carambola
6. unusual carotenoids ex capsanthin in red pepper.
7. prolycarpene in ex tangerine
8. apocarotenoids ex persicaxanthins.

Minor classes of carotenoids: They are degraded in vivo to yield nor- or apo –products. Nor refers to loss of methyl group where as apo refers cleavage of larger skeletal fragments. 
 
 
 
 
 
 
 
 
 
 
 

Lecture 12+

July 10, 2005

Special Nitrogen Metabolism: Nitrogen compounds are important for functioning of every plant. Besides, important compounds like purine, pyrimidines and derived nucleosides and nucleotides plant produces a number of secondary metabolites.

• These secondary metabolites function as “alleclochemicals” and as chemical of defense against herbivores and micro-organism. They also function as attractant to insects and animals which help in fertilization as well as dispersion of seed.
• Evidences indicate that many secondary metabolites are important the fitness of plants.
• Nitrogen stores for seed during germination

Besides, 20-protein building amino acids other amino acids such as ornithine is the precursor of these secondary metabolites. Example: Non-protein amino acids and their structural analogue.

 

Biosynthesis of various amino acids:

These Non protein amino acids are formed from protein amino acids: 

Example The Non protein amino acids of garlic and onion are derivatives of cysteine and are not incorporated into protein, rather they are hydrolysed by alliin lyase to allicin, and other product formed is propenylsullfenic acid which spontaneously rearranges to syn-propanethiol S-oxide. Allicin and diallylsulphide display strong antimicrobial activity.  syn-propanethiol S-oxide functions as tear promotion factor. syn-propanethiol S-oxide further hydrolyzes to propionaldehyde , sulfuric acid, and hydrogen sulfide.   

Thus, these non-protein amino acids in Garlic and onion try to defend by release of tear promoting compounds as well as antibacterial compounds.

Amines are yet another Nitrogenous compound. Amines in plants are either aliphatic or aromatic amines. In higher plants aliphatic aldehydes are predominantly aminated by an L-alanine –aldehyde aminotransferase which dependent on pyridoxal phosphate. 
 

Biosynthesis of some aromatic amine are shown below: principal reaction is decarboxylation  

 

Aromatic amine such as tyramine, Dopamine, Phenylethylamine, Hordeine, ephedrine are widely distributed in nature

Example: Dopamine is present in mucuna pururiens , hordeine is present in Hordeum and other members of poa.Ephedrine and nor epinephrine are present in Ephedra gerardiana. 

 

These aromatic amine some time mimic some important neuotransmitters such as  Dopamine and norepinephrine (noradrenaline) or hormone such as epinephrine.   
 
 

Amines play important role of attracting insects which help in fertilization 
 

 

            Role of Amines in the flower Biology [Arum maculatum] 

Monoamine, Diamines and polyamines in the aliphatic series play important role in several plant families play important roles from attractant & repellent. Similarly a number of aromatic amines play roles in plants. Stinging hair of Urtica dioica serotonin and acetylcholine causes stinging repelling the intervening herbivores. The compounds responsible for the stinging action are Histamine, Seratonin, and acetylcholine.  
 

      Stinging hair of Urtica dioica serotonin and acetylcholine causes stinging

Other Nitrogenous compound of prime importance is cynogenic glycosides, and glucosinolates, alkaloids, auxins and cytokinin: 

Here we will discuss the alkaloids as they are among the key compounds in some plants traded from Nepal:   

Earlier the term alkaloids were restricted to plant bases with heterocyclic Nitrogen atom. The exocyclic nitrogen was termed as pseudoalakloids. Note the heterocyclic nitrogen ring in berberine. 

     
 

Latter definition demanded that they should be derived from aminoacids and or these bases have pharmacological activity.  Example  
 
 

Even in complex alkaloids the aminocids can be noted 
 
 
 
 
 
 

Biosynthesis of alkaloids: 
 

The alkaloids of most alkaloids are derived from aminoacids although moieties from other pathways such as terpenoids often combined. In a number of alkaloids (steroidal alkaloids) the nitrogen derived other amine sources are added in thefinal step of biosynthetic pathway. i.e alkaloid skeleton does not come out from amino-acids alone.

The scheme of acyclic alkaloid cocaine is shown below:   
 

Scheme of aromatic alkaloid reticuline is shown below:  

Reactions like Decarboxylation , Methylation, Transamination, Aldol Condensation And Claisen Condensation plays important role in the formation these alkaloids.  

plant alkaloids are thus produced from protein and non protein amino acids that specifically produced in the specific time of plant life cycle and are specifically stored at specific organ of plant hyoscymine and Nicotine is stored in the leaf vacuoles while the benzoquinoline A of papaver somniferum is stored in latex vscicles of the fruit. while some others specifically deposited in stem and or root bark.      

Table below shows Biosynthesis and occurrence of alkaloids in some plants common to flora of Nepal   

 
 
 
 
 
 

Dr.Tirtha Raj bajgai 2110043 (office), 4497764 (Residence)  

Sixth semester question: 

Full length question         2

Mid length question         6

Short length question       8