16. Biogenetic Pathways

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16. Biogenetic Pathways

Ametabolic pathway is a seriesof chemical reactions

occurring within a cell, catalyzed by enzymes, resulting

ineithertheformationofametabolicproducttobeused orstoredbythecell,ortheinitiationofanothermetabolic

pathway. Mainly metabolic pathways are concerned with:



Degradingmaterialstakeninasfood



They are required to synthesize specialized molecules

fromthebasiccompounds.

Theseprocesses demonstrate the fundamental unity ofall living matter, and are collectively described as

primary metabolism,with the compounds involved in

the pathways being termed primary metabolites. Primary metabolic pathways, which synthesize, degrade, and

generallyinterconvertcompoundscommonlyencountered inallorganisms.Theyareneededforgeneralgrowthand physiologicaldevelopmentofplantiswidelydistributed innatureandarealsoutilizedasfoodbyman.Typically, primarymetabolites are foundacross all species within

broad phylogenetic groupings, and are produced using the same pathway (or nearly the same pathway) in all these species.

The secondary metabolites such as alkaloids, glycosides,

avonoids,volatileoils,etc.arebiosyntheticallyderived fromprimary metabolites. Secondary metabolites, by

contrast,are often species-specic (or foundin only a smallsetofspeciesinanarrowphylogeneticgroup),and withoutthesecompoundstheorganismsuffersfromonly amild impairment, lowered survivability/fecundity, aestheticdifferences, or else nochange in phenotype at all.Theyareproducedforeasilyappreciatedreasons,e.g. astoxicmaterialsprovidingdefenceagainstpredators,as volatileattractantstowardsthesameorotherspecies,or

as colouring agents to attract or warn other species, but it

islogicaltoassumethatalldoplaysomevitalroleforthe well-beingoftheproducer.

Important features of secondary metabolites



They are unnecessarily restricted to a taxonomic group.



Theyprovide most of the pharmacologically active

natural products. Though secondary metabolites present in lesser quantities, they are main constituents

responsiblefor medicinal utility, hence areexpensive

to produce.



Theyareproductsduetoanyenvironmentalstressesor anyabnormalcondition,orserveasdefensive,protective oroffensivechemicalsagainstmicroorganisms,insects andhigherherbivorouspredators.



Theyalso act as waste or secretory productsof plant metabolismandareofpharmaceuticalimportance.

List of important secondary metabolites

Plant drug Secondary metabolite Activity

Cinnamomum zeylanicum Quinine alkaloids Antimalarial Catharanthus roseus Vincristin and vinblastin Anticancer Taxus species Taxol Anticancer Papaver somniferum Morphine Analgesic Digitalis purpurea Digitalis glycosides Cardiotonics

List of major metabolic pathways

Cellular respiration Glycolysis

Anaerobic respiration Kreb’s cycle/Citric acid cycle Oxidative phosphorylation

Creation of energetic compounds from non-living matter Photosynthesis (plants, algae, cyanobacteria)

Chemosynthesis (some bacteria)

Other pathways occurring in living organisms Fatty acid oxidation (β-oxidation)

Gluconeogenesis HMG-CoA reductase pathway (isoprene prenylation) Pentose phosphate pathway (hexose monophosphate) Porphyrin synthesis (or haeme synthesis) pathway Urea cycle

Basic metabolic pathways

Glycolysis(EMI:Embden-MeyerhofPathway):Glycolysis

represents an anabolic pathway common in both aerobic and anaerobic organisms. Glycolysis is the metabolic

pathwaythat converts glucose, C

,into pyruvate.

6H12O6

Thefreeenergyreleasedinthisprocessisusedtoformthe

high energy compounds, ATP (adenosine triphosphate)

andNADH(reducednicotinamideadeninedinucleotide).

Section 2 Pharmacognosy

269

Section 2 Pharmacognosy

270

Primary and secondary metabolites derived from carbon metabolism in plants

Inter-relationships of biosynthetic pathways leading to secondary constituents in plants

Biosynthesis of Aromatic Compounds

ShikimicAcidPathway:The shikmic acid pathway is a

keyintermediatefromcarbohydrateforthebiosynthesisof C6-C3units(phenylpropanederivative).Theshikimicacid

pathwayconvertssimplecarbohydrateprecursorsderived fromglycolysisandthepentosephosphatepathwaytothe

aromatic amino acids. The shikimic acid pathway is present

inplants,fungi,andbacteriabutisnotfoundinanimals.

Section 2 Pharmacognosy

271

Shikimic Acid Pathway

Amino acids: Aminoacidsoccurinplantsbothintheform

offreestate andasthebasicunits ofproteinsandother metabolites.Theyariseatvariouslevelsofglycolyticand

TCA systems.

Section 2 Pharmacognosy

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Amino acids and its precursors

Amino acids Precursors

Glutamate family α-ketoglutarate Aspartate family Oxaloacetate Alanine–valine–leucine group Pyruvate Serine–glycine group 3-phosphoglycerate Aromatic amino acids Phosphoenolpyruvate

Erythrose-4-phosphate

Formation of Amino Acids

Biosynthesis of Glycosides

Theterm ‘glycoside’ is a verygeneral one which embracesall the many and varied combinations of sugarsandaglycones.Themetabolicprocessofglycoside formationessentially consists of two parts. The first partofbiosynthesisisthereactionsbymeansofwhich varioustypes of aglycones are formed,whereas the otherpart of biosynthesis process takes into account metabolicpathwayinvolvingcouplingofaglyconewith sugarmoiety.Thesynthesisofglycosidesinplantcells

involvesinteractionofnucleotideglycosidesuchasUDPglucosewiththealcoholicorphenolicgroupofasecond compoundaglycone.Theprincipalpathwayofglycoside formationinvolvesthetransferofuridylyl group from uridinetriphosphate(UTP)tosugar-1-phosphateandthe

enzymes catalyzing this reaction are known as uridylyl

transferases.The subsequent reactioncontrolled by enzymaticsystemglycosyltransferasesinvolvestransfer ofsugarfromuridine diphosphate to aglycone moiety resultinginformationofglycoside.

Section 2 Pharmacognosy

273

Biosynthesis of Alkaloids

Alkaloid derived from ornithine: Ornithine is incorporated

intoboth pyrrolidine specifically and asymmetrically intopyrrolidineringoftropanenucleus,theα-carbonof ornithinebecomingtheC1oftropinenucleus.

Alkaloids derived from lysine: In biogenesisof the

anabasine,lupinine,andiso-pelletierineprecursorislysine.

Biosynthesis of Morphine and Codeine

Alkaloids derived from phenylalanine, tyrosine and related amino acids: These amino acids and their

decarboxylationproducts serve as the precursor for

ephedrine, colchicines and opium alkaloids.

Section 2 Pharmacognosy

274