ISOPRENOIDS=TERPENES=TERPENOIDS - … 308 Dr. Solomon Derese Terpenoids, also known as isoprenoids...
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SCH 308 Dr. Solomon Derese ISOPRENOIDS=TERPENES=TERPENOIDS COMPOUNDS DERIVED FROM 5 CARBON UNITS Biosynthesis of Terpenoids OPP DMAPP 1
Transcript of ISOPRENOIDS=TERPENES=TERPENOIDS - … 308 Dr. Solomon Derese Terpenoids, also known as isoprenoids...
SCH 308
Dr. Solomon Derese
ISOPRENOIDS=TERPENES=TERPENOIDS
COMPOUNDS DERIVED FROM 5 CARBON UNITS
Biosynthesis of Terpenoids
OPP
DMAPP1
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LEARNING OBJECTIVES
• Recognize the role of carbocations interpene biosynthesis
• Describe how carbocations are formed frompyrophosphate precursors
• Describe important reactions ofcarbocations in terpene biosynthesis
• Propose reasonable biosynthetic pathwaysto a variety of terpenes
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Terpenoids, also known as isoprenoidsor terpenes, are naturally occurringorganic compounds constructed fromthe joining together of a five carbonprecursor.
Thus they have C5, C10, C15, C20, C25, C30,C40, Cn (n is more than 40) skeletal.
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They are subdivided, based on the number of C5units used to construct the terpenoids, into:
Hemiterpenes (C5), Monoterpenes (C10),
Sesquiterpenes (C15),
Diterpenes (C20),
Sesterpenes (C25), Triterpenes (C30),
Tetraterpenes (C40)
and Polyterpenes (Cn , n > 40).4
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The terpenes are a structurally diverseand widely distributed family of naturalproducts containing well over 30,000defined compounds identified from allkingdoms of life.
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The majority of terpenes have been isolatedfrom plants where they serve a broad rangeof roles in primary metabolism (includingseveral plant hormones and the mostabundant plant terpenoid, phytol, the sidechain of the photosynthetic pigmentchlorophyll) and in ecological interactions [aschemical defenses against herbivores andpathogens, pollinator attractants,allelopathic agents].
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Chlorophyll
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O
H
Citronella
Monoterpenes [C10]
CO2H
Chrysanthemic acid
fragranceinsecticide
Sesquiterpenes [C15]
Humuleneaphid repellant
Farensene
Examples
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OH
Vitamin A
Diterpenes [C20]
Anti-fungal agent
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OH
HO
Carotenoid C40
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O
O
O
O
O
Anti-malarial drug from Artemisiaannua.
Artemisinin
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BIOSYNTHESIS OF TERPENES
OPP
Dimethylallyl PPDMAPP
Hemiterpenes
C10 Monoterpenes (C10)
IPP
C15 Sesquiterpenes (C15)
C20 Diterpenes (C20)
IPP
IPP
Sesterpenes (C25)C25
C30 Triterpenes (C30) Steroids (C18-C30)
X 2
C40
X 2
Tetraterpenes (C40)
Mevalonic acid Methylerythritol
Phosphate (MEP)
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The five-carbon building blocks of all terpenoids,isopentenyl diphosphate (IPP) and dimethylallyldiphosphate, are derived from two independentpathways localized in different cellularcompartments.
The cytosol localized Mevalonate pathway providesC5 units for sesquiterpene and triterpenebiosynthesis.
The methylerythritol phosphate (MEP ornonmevalonate) pathway, localized in the plastids,is thought to provide IPP and dimethylallyldiphosphate for hemiterpene, monoterpene, andditerpene and tetraterpene biosynthesis.
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CYTOSOL PLASTID
Acetyl CoA
Mevalonic Acid
Sesquiterpenesand
Triterpenes
G3P Pyruvate+
MethylErythritol Phosphate (MEP)
Hemiterpenes, monoterpenes, diterpenes,
and tetraterpene
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THE MEVALONIC ACID PATHWAY
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BIOSYNTHESIS OF MEVALONIC ACID
Three molecules of acetyl-coenzyme A areused to form mevalonic acid. Two moleculescombine initially in a Claisen condensation togive acetoacetyl-CoA, and a third isincorporated via a stereospecific aldoladdition giving the branched-chain ester β-Hydroxy-β-MethylGlutaryl-CoA (HMG-CoA).
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H2C SCoA
O
H
B Enz
H2C SCoA
O
H2C SCoA
O
SCoA
O
O
S
O
CoA
Acetoacetyl CoA
Claisen
The mevalonate pathwaydoes not use malonylderivatives and it thusdiverges from the acetatepathway at the very firststep.
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H3C SCoA
OEnz-SH
H3C SEnz
O :B-Enz
H2C SEnz
O
O
S
O
CoA
H-B-Enz
EnzS SCoA
OHO O
β-Hydroxy-β-MethylGlutaryl-CoA (HMG-CoA)
Stereospecific aldol reaction
In the second step, it should be noted that, onpurely chemical grounds, acetoacetyl-CoA is themore acidic substrate, and might be expected toact as the nucleophile rather than the third acetyl-CoA molecule. The enzyme thus achieves what isa less favourable reaction.
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HMG-CoAEnzS S
CoA
OHO O
HMG-CoA Reductase
2 NADPH
HO OH
OHO
(3R)-Mevalonic Acid
H2O The conversion ofHMGCoA into (3R)-MVA involves atwo-step reductionof the thioestergroup to a primaryalcohol.
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The six-carbon compound MVA istransformed into the five-carbonphosphorylated isoprene units in a series ofreactions, beginning with phosphorylation ofthe primary alcohol group. Decarboxylation /dehydration then give IPP.
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(3R)-Mevalonic Acid
HO P
OH
OH
O ADP2
O OPP
OPPO
H
HO OH
OHO
OPPOPP
H HR S
H+
Isopentenyl pyrophosphate
IPP3,3-Dimethylallyl pyrophosphate
DMAPP21
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IPP is isomerized to the other isopreneunit, DMAPP, by an isomerase enzymewhich stereospecifically removes thepro-R proton (HR) from C-2, andincorporates a proton from water on toC-4. Whilst the isomerization isreversible, the equilibrium lies heavilyon the side of DMAPP.
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OPP
DMAPP
SN1
DMAPP possesses a good leaving group, thediphosphate, and can yield via an SN1 process anallylic carbocation which is stabilized by chargedelocalization.
This generates a reactive electrophile and thereforea good alkylating agent, DMAPP reacts as anelectrophile.
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In contrast, IPP with itsterminal double bond is morelikely to act as a nucleophile,especially towards theelectrophilic DMAPP.
IPP
DMAPPOPP
OPPH H
R S
SN1
Electrophile(Electron deficient)
Nucleophile(Electron rich)
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These differing reactivities are the basisof terpenoid biosynthesis, andcarbocations feature strongly inmechanistic rationalizations of thepathways.
Therefore, terpenoids are synthesized byjoining IPP (a nucleophile) and DMAPP(an electrophile) in a head to tail manner.
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C C
C C
C.
C C
C
C C
.
Head
Tail
Head
Tail
OPP
OPP DMAPP
SN1
OPP
Head
TailHead
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JOINING ISOPRENE UNITS
Head-to-Tail
Tail-to-Tail
Larger terpenoid units dimerize tail-to-tail.Tail to Middle
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Limonene
Terpenes can be shown to be formed fromisoprene units.
Diagram showing howtwo isoprene unitscombine to form thelimonene skeleton.
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Biosynthesis of Hemiterpenes (C5)
O
O OH
OH OPP O
O OH
OH
Peucenin
5,7-Dihydroxy-2-methylchromone
O
O OH
O
OH
Visamminol
Prenylation is a common reaction in secondarymodification of natural products derived fromother metabolic pathways.
C- and O–Prenylationsare very common.
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The monoterpenes are best known ascomponents of the volatile essences offlowers and of the essential oils of herbs andspices, in which they make up as much as 5%of plant dry weight. Monoterpenes areisolated by either distillation or extractionand find considerable industrial use inflavors and perfumes.
Monoterpenes (C10)
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Biosynthesis of Monoterpenes (C10)
Combination of DMAPP and IPP via the enzymeprenyl transferase yields Geranyl PyroPhosphate(GPP).
OPP
OPPHH
R S
OPP
Geranyl Pyrophosphate (GPP)
opp
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This produces a monoterpene diphosphate, GeranylPP (GPP), in which the new double bond is E.
OPP
Geranyl PP (GPP)
E
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OPP
GPP
:OPP PPO PPO
Linalyl PP (LPP)
:OPP
OPP
Neryl PP (NPP)
Z
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Linalyl PP (LPP) and Neryl PP (NPP) areisomers of geranyl PP, and are formed fromgeranyl PP by ionization to the allylic cation,which can thus allow a change inattachment of the diphosphate group (tothe tertiary carbon in linalyl PP) or a changein stereochemistry at the double bond (to Zin neryl PP).
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These three compounds, by relativelymodest changes, can give rise to a rangeof linear monoterpenes found ascomponents of volatile oils used inflavoring and perfumery.
The resulting compounds may behydrocarbons, alcohols, aldehydes, orperhaps esters, especially acetates.
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Acyclic monoterpenes
Open chain monoterpenes are derived from GPP,LPP and NPP by:
Reduction
Oxidation
Dehydration
Using NADPH
Using NADP+
Loss of water
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OPP
GPP
O
Geranial
(Lemon oil)
H
O
Citronellal
(Citronella oil)
H
OH
(Rose oil)Citronellol
OH
GeraniolGeranium oil
H2O1. H2O
1. H2O
1. H2O
2. NADP+3. NADP+
2.NADPH
2. NADPH
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OPP
GPP
PPO
LPP
-Myrcene (Hops)
HO
Linalool (Coriander oil)
H2O
- HOPP
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OPP
GPP
H2O
O
Neral (Lemon oil)
1. H2O
2. NADP+
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Cyclic monoterpenes
Which of these is ready for cyclization?Cyclization would not be expected to occur with theprecursor GPP, the E stereochemistry of the doublebond being unfavorable for ring formation. NPP or LPP,however, do have favorable stereochemistry, andeither or both of these would act as precursors formonocyclic terpenes.
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LPP
PPO
OPP
NPP
Teritary Carbocation
Menthyl or a-terpinylCarbocation
Menthyl or a-terpinylCarbocation is the precursorfor many cyclic monoterpenes.
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- H
Limonnene
OH
a-Terpineol
H2O
OHO
Cineole43
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HH
H HH
Terpinen-4-yl Cation
Phellandryl cation
Wagner-Meerwein Shift
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Phellandryl cation
a-Phellandrene -Phellandrene
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Terpinen-4-yl Cation
- H- H
H2O
a-Terpinene-Terpinene
OH
Terpinen-4-ol
Thujyl cation Sabinene
- H
[O] [H]
Thujone
O
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a-Pinene
- H+
- H+
-Pinene
1,2-A
lkyl S
hift
Fenchylcation
OH
H2O
FencholO
NADP+
Fenchone
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OH
H2O
Borneol
O NADP+
Camphor
Bornyl Cation
Isocamphyl cation
Camphene
- H+
1,2-Alkyl Shift
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a-terpinyl cation
OPP
GPP
OH
O
OPP
OH
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OPP
H2ONADP+
[O]
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Addition of a further IPP unit to geranyldiphosphate (GPP) in an extension ofthe prenyl transferase reaction leads tothe fundamental sesquiterpeneprecursor, farnesyl diphosphate (FPP).
Biosynthesis of Sesquiterpenes (C15)
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Geranyl Diphosphate (GPP)
OPP
OPPHH
R S
OPP
Farnesyl Diphosphate (FPP)
IPP
E,E-Farensyl PP
OPP
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FPP can then give rise to linear and cyclicsesquiterpenes. Because of the increased chainlength and additional double bond, the number ofpossible cyclization modes is also increased, and ahuge range of mono-, bi-, and tri-cyclic structurescan result.
The stereochemistry of the double bond nearestthe diphosphate can adopt an E configuration (asin FPP), or a Z configuration via ionization, asfound with geranyl/neryl PP.
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E,E-Farensyl cation
Nerolidyl cation
E,Z-farnesyl cation
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Germacryl cation
H
Eudesmyl cation
Guaiyl cation
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Humulyl cation
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E,Z-Farnesyl cation
a
H
a
b
H
b
b
Bisabolyl
Carotyl
cis-GermacrylCadinyl
cis-Humulyl
a
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H OH
O
ba
H
O
OH
O
OHH
(-)-a-Bisabolol oxide A
(-)-a-Bisabolol oxide B
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- H
OO
O
[O]
Germacrene AParthenolide
Germacryl cation
Cadinyl cation
- H
a-Cadinene
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- H
Humulyl cation
H
Caryophyllyl cation -Caryophyllene
Humulene
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Diterpenes
The diterpenes arise from geranylgeranyldiphosphate (GGPP), which is formed by additionof a further IPP molecule to farnesyl diphosphatein the same manner as described for the lowerterpenoids.
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OPP
Farnesyl Diphosphate (FPP)
OPPHH
R S
IPP
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Cyclization reactions of GGPP mediated bycarbocation formation, plus the potential forWagner – Meerwein rearrangements, will allowmany structural variants of diterpenoids to beproduced.
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Many different types of monocyclic andpolycyclic diterpenes are encountered.
Acid catalyzed stereospecific cyclization of GGPPand analogues give rise to a variety of diterpeneskeleta.
Two possible modes of cyclization are psssible,leading to a 5a,10 configuration (mostcommonly encountered) or to 5,10aconfiguration at the A:B ring junction.
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In contrast to the cyclization mechanisms shown formonoterpenes and sesquiterpenes, where loss ofdiphosphate generates the initial carbocation, manyof the natural diterpenes have arisen by a differentmechanism.
Carbocation formation is initiated by protonation ofthe double bond at the head of the chain leading toa first cyclization sequence. Loss of the diphosphatelater on also produces a carbocation and facilitatesfurther cyclization.
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GGPP
OPP
H
OPP
H
1
2
3 45
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8
9
10
12
11 13
14
15H
H
aa
A B
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H
H
Decalin( two fused rings)
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Cis-decalin is unstable compared to trans-decalindue to 1,3-diaxial interaction.
H
H
H
H
cis-Decalin trans-Decalin
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OPP
GGPP
OPP OPP
H
H
OPP
H
Labdadienyl PP
SN2
H
H
Pimarenyl cation
H
AA
H
BB
B
HH
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OPP
HH
OH
HO
H
H
OH
OH
Sclareol
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H
H
H H
Ent-kaurene
[O]
H
HO
Ent-kaurenol
[O]
H
HEnt-kaurenal
O
[O]
H
HO
Ent-kaurenoic acid
O
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TRITERPENES (C30)
Triterpenes are not formed by anextension of the now familiar process ofadding IPP to the growing chain. Instead,two molecules of farnesyl PP are joinedtail to tail to yield the hydrocarbonsqualene.
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PPO
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Squalene
O2 NADPH
OSqualene Oxide
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Squalene
O2 NADPH
O Squalene Oxide
H
H
HOHO
HO HO
Protosteryl cation
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HO
Protosteryl cation
H
HH
HO
H
Lanosterol
HOH
HHH
HO
H
Cycloartenol
AnimalsFungi
Plants
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HO
H
H
HH
HO
Protosteryl cation
HO
H
H
H
dammarenyl cation
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HO
H
H
H
dammarenyl cation
H2O
HO
H
H
H
OH
dammarenediols
HO
H
H
H
baccharenyl
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Terpenoids are synthesized by consecutivecondensations of an isoprene unit, isopentenyldiphosphate (IPP), to its isomer, dimethylallyldiphosphate (DMAPP).
Since the discovery of the mevalonate pathway, itwas widely accepted that IPP and DMAPP wereformed only through this pathway in all livingorganisms.
In 1996, Rohmer in France discovered the firstreaction step of the alternative mevalonate-independent pathway for the formation of IPPand DMAPP.
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The five-carbon building blocks of all terpenoids,isopentenyl diphosphate (IPP) and dimethylallyldiphosphate, are derived from two independentpathways localized in different cellularcompartments.
The cytosol localized Mevalonate pathway providesC5 units for sesquiterpene and triterpenebiosynthesis.
The methylerythritol phosphate (MEP ornonmevalonate) pathway, localized in the plastids,is thought to provide IPP and dimethylallyldiphosphate for hemiterpene, monoterpene, andditerpene and tetraterpene biosynthesis.
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