SCH 511 Secondary Metabolites Derived from Acetate · SCH 511 Dr. Solomon Derese LEARNING...

SCH 511

Dr. Solomon DereseDr. Solomon Derese

Secondary Metabolites Derived

from Acetate

Acetyl CoA

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SCH 511

Dr. Solomon Derese

LEARNING OBJECTIVES• Recognize the source, structure and function in

biological systems of fatty acids

• Identify the roles of acetyl coenzyme A andmalonyl coenzyme A in biosynthesis of acetatederived secondary metabolites.

• To understand how even-numbered, odd-numbered and branched fatty acids arebiosynthesized

• Describe how unsaturated fatty acids are producedfrom saturated fatty acids

• Describe how fatty acids can undergo furthermodification.

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Dr. Solomon Derese

• To understand the role of essential fatty acidsin animals.

• Recognize the structure of natural productsderived from the polyketide pathway.

• Describe the primary process in whichpolyketides are biosynthesized.

• Recognize the various cyclization pathwaysencountered in the formation of polyketides.

• Describe how secondary processes give furtherstructural diversification.

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Dr. Solomon Derese 4

CO2 + H2O O

OHOH

HOHO

HOPhotosynthesis

Glucose

Glycolysis

Phosphoenol pyruvatePyruvateAcetyl coenzyme A

Malonyl coenzyme A

CO2

PO

CO2

O

SCoA

O

SCoA

O

CO2

Fatty Acids & PolyketidesProstaglandinsPolyacetylenes

Aromatic compounds.

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Dr. Solomon Derese

S

O

CoA

or

Saturated thioestersPolyketo thioesters

Saturated fatty acidsAromatic compounds or other Polyketide derived metabolites

Unsaturated fatty acids

Malonyl CoA

Acetyl CoA

Prostaglandins Polyacetylenes5

S

O

CoA

O

n-1 S

O

CoA

CH2H3C CH2

n-2SCoA

O O O

S

O

CoAO

O

Hn

FASPKS

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Biosynthesis of Fatty Acids

Saturated thioesters

Saturated fatty acids

Malonyl CoA

Acetyl CoA

Fatty Acid Synthase

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Fatty acids are usually encountered innature as their ester derivatives and theseester derivatives are collectively known aslipids, a term which recognizes theirinsolubility in water.

Fatty acids are alkanoic acids and themajority of naturally occurring fatty acidshave straight-chains possessing an evennumber of carbon atoms.

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Naturally occurring fatty acids (Lipids) are almostentirely straight chain aliphatic carboxylic acids.The broadest definition includes all chainlengths, but most natural fatty acids are C4 toC22, with C18 being the most common.

CH3CH2-(CH2CH2)n-CH2CO2H

The most abundant fatty acids, the saturatedfatty acids have the general formula (exampleoctadecanoic acid (n=7)):

aω

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Occurrence and function of fatty acids

Fatty acids, esterified to glycerol, are the mainconstituents of oils and fats. Storage fats (seed oilsand animal adipose tissue) consist chiefly (98%) oftriglycerides.

Triglyceride

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Animal fats contain a high proportion of glyceridesof saturated fatty acids and tend to be solids,whilst those from plants and fish containpredominantly unsaturated fatty acid esters andtend to be liquids.

Saturated fatty acid

Cis unsaturated fatty acid

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The biological function fatty acids are:

I. Storage of metabolic fuel,

II. Protective coatings on skin, fur, feathers,

leaves, etc

III. Cell membrane component

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I. Storage of metabolic fuel

Glycerides act as an energy store in biologicalsystems; they can be broken down into their fattyacid constituents which can subsequentlyundergoes a process termed as b-oxidation.

In b-oxidation, the fatty acid is broken down, twocarbon atoms at a time, into acetyl coenzyme Aunits to provide energy.

Triglycerides are a highly concentrated store ofenergy (9 kcal/g).

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b-Oxidation of fatty acids

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II. Protective coatings on skin, fur, feathers, leaves

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III. Cell membrane component

Fatty acids are alsothe major componentof the cell membrane.

A cell is composed of nucleic acids,proteins, and other biochemicalssurrounded by a membrane.

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Dr. Solomon Derese

A typical membrane-forming lipid isphosphatidyl choline.

17

HydrophilicHydrophobic

The membranes completely enclose theircontents, and so cells have a defined inside andoutside.

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The ubiquity of many of the commonfatty acids and the vital roles they play,puts them into the class of primarymetabolites.

It is only the more unusual oruncommon fatty acids that can beconsidered as true secondarymetabolites.

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Naturally occurring fatty acids share acommon biosynthesis. The chain is builtfrom two carbon units, and cis doublebonds are inserted by desaturaseenzymes at specific positions relative tothe carboxyl group.

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A large number of fatty acids varying in chainlength and unsaturation result from this pathway.

This results in even-chain-length fatty acids with acharacteristic pattern of methylene interrupted cisdouble bonds. A typical example of a fatty acid islinoleic acid.

Linoleic acid

Methylene separated by cis-double bonds

O

O

H

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Fatty acids with trans or non-methylene-interrupted unsaturation also occur naturally.

Vaccenic acid (18:1) (11t))

Examples

Rumenic acid (18:2 (9t, 11c))

O

O

H

O

O

H

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Systematic names for fatty acids are toocumbersome for general use, and shorteralternatives are widely used.

Two numbers separated by a colon give,respectively, the chain length and number ofdouble bonds: octadecenoic acid with 18carbons and 1 double bond is therefore 18:1.

Nomenclature of fatty acids

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The position of double bonds is indicated in anumber of ways: explicitly, defining the positionand configuration; or locating double bondsrelative to the methyl or carboxyl ends of the chain.

Double-bond position relative to the methyl end isshown as n-x or x, where x is the number ofcarbons from the methyl end. The n-system is nowpreferred, but both are widely used. The positionof the first double bond from the methyl end isdesignated x.

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Linoleic acid

18:2 (9c, 12c)

9Z,12Z-Octadecadienoic acid

18:2 n-6 18:2 ω6

18:2 ∆9,12

Example

O

O

H1

9

12

18

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The terms cis and trans, abbreviated c and t, areused widely for double-bond geometry; as withonly two substituents, there is no ambiguity thatrequires the systematic E/Z convention.

1

912

cis double bond abbreviated as “c”

Acetylinic bond (abbreviated as “a”)

2 , 12a)(9c[18 carbons: 2 points of unsaturation (cis double bond at position 9,acetylinic bond at the 12-position)

Crepenynic acid

18

18 :

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Malonyl CoA

Acetyl CoAS

O

CoA

S

O

CoAO

O

Hn

n-1 S

O

CoA

CH2H3C CH2

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Sources of Acetyl CoA

Acetyl CoA is formed in the cell bydegredation of fatty acids (b-oxidation). Fattyacids esters are energy stores which duringmetabolism yield acetyl CoA and energy.

A.b-Oxidation of fatty acids

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b-Oxidation

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a

b

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b

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b

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C6H12O6

O

O

OH

Pyruvic acidGlucose

CoASH

O

SCoA

Acetyl coenzyme A

NAD

B. Decarboxylation of pyruvic acid which isobtained via glycolysis of glucose alsoyields acetyl CoA.

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Synthesis of Malonyl CoA

The formation of Malonyl coenzyme A takesplace in two steps:

Step I:Carboxylation of biotin (involving ATP)

HO O

OATP

HO OP

O

Coupling to ATP ‘hydrolysis’ provides energy todrive carboxylation process.

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NHN

S

HH

O

OH

ON'-Carboxybiotin

HO

O

NHHN

S

HH

O

OH

OBiotin (Cofactor)

HO OP

O

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Step II: Transfer of the carboxyl group to Acetyl CoAto form Malonyl CoA

Keto-enol tautomerizationS

O

CoAS

OH

CoA

NHN

S

HH

O

OH

ON'-Carboxybiotin

O

HO

S

O

CoAOH

O

Malonyl-CoA

Acetyl CoA

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Acetyl-CoA carboxylase

S

O

CoAS

O

CoAOH

OHCO3 ATP

BIOTIN

Acetyl CoA Malonyl CoA

Synthesis of Malonyl CoA

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Malonyl CoA

Acetyl CoAS

O

CoA

S

O

CoAO

O

Hn

n-1 S

O

CoA

CH2H3C CH2

Sourceb-oxidation of FAGlycolysis of glucose

Carboxylation of acetyl CoA

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The processes of fatty acid biosynthesisare known to be catalyzed by amultienzyme complex known as FattyAcid Synthase (FAS).

Fatty acid synthase is arranged around acentral Acyl Carrier Protein (ACP), whichcontains a protein bound pantetheinechain [Condensing Enzyme (HSEcondensing)]similar to the long chain of Coenzyme A,with six distinct catalytic centers.

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OP

O-

O

ON

HO

O

H

N

HS

O

H

Ser

ON

N

N

N

NH2

OP

O-

O

OP

O-

O

O

O

OH

P O-O

-O

N

HO

O

H

N

HS

O

H

ACP

FAS

Coenzyme A

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ACP

CE MT

KR

DHER

TE

AT

AT= Acetyl transferaseMT= Malonyl transferaseCE= Condensing enzyme

ACP= Acyl Carrier ProteinKR= Keto ReductaseER= Enoyl Reductase

DH= DeHydrataseTE= ThioEsterase

Fatty Acid Synthase (FAS)

pan

teth

ein

ech

ainSH

SH

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Before acetyl CoA and malonyl CoA can be used inbiosynthetic reactions, they have to be attached tothe fatty acid synthase .

In this reaction the – SCoA group of acetyl CoAand malonyl CoA are substituted by the thiolgroups (SH) of HSEcondensing and ACP, respectively,of FAS.

The thiol groups are behaving as nucleophiles andthe SCoA group as a leaving group in anucleophilic acyl substitution reaction.

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Acetyl-CoA and malonyl-CoA themselves arenot involved in the condensation step: theyare converted into enzyme-bound thioesters,Acetyl-SEcondensing and Malonyl S-ACP,respectively.

Once they are anchored to the fatty acidsynthase, a carbon-carbon bond formationreaction occurs.

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The intermediates in fatty acid synthesis arecovalently linked to the Acyl Carrier Protein (ACP).

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O

CoAS

SH SH

ACPCE

Fatty Acid Synthase (FAS)

‘molecular machine’

S SH

ACPCE

O

AT

SH S

ACPCE

O

REDUCTION1) KR

2) DH

3) ER

SH

ACPCE

TRANSLOCATION

O S

OHn

TEO



ACP= Acyl Carrier ProteinKR = Keto ReductaseER = Enoyl Reductase

DH= DehydrataseTE= Thioesterase

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Step I

Transfer of the acetyl group of acetyl CoA to theCondensing Enzyme (HS-Econdensing).

Steps in the Biosynthesis of Fatty Acids

Step II

S

O

CoA + HS-Econdensing S

O

Econdensing + CoASH

Transformation of Malonyl CoA into Malonyl SACP.

+S

O

CoAO

O

H

Malonyl CoA

HS-ACPS

O

ACPO

O

H + CoA-SH

Malonyl SACP

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Step III

Condensation of acetyl Econdensing with MalonylSACP.

S

O

ACP

O

+

S

O

Econdensing

HSEcondensing

S

O

ACPO

O

H

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It has been shown that the carboxylation of acetylCoA goes with retention of configuration but thecondensation with Malonyl SACP goes withinversion of configuration.

S

O

E

HCO3- ATP

Biotin

(Retention of configuration)

S

O

HH

HCoA

A

B C

S

O

HH

ACPO

OH

B C

S

O

HH

ACPO

BC

(Inversion of configuration)

HS-ACP

codensing

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Step IV

Stereospecific reduction mediated by NADPHproducing 3-(R)-hydroxy intermediate exclusively.

S

OO

ACP

N

CNH2

O

R

HHs

NADPH

H+

S

OOH

ACP

N

CNH2

O

R

NADP

HS

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Step V

Syn elimination produces a 2-(E-enoyl)-ACPderivative, a trans-alkene.

S

OOH

ACPH

H

S

O

ACP- H2O

2-(E-enoyl)-ACP

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Step VI

The cycle is completed by further reductionmediated by NADPH to produce a saturated acyl-SACP.

S

O

ACPNADPH

HS

O

ACP

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Ketone → Methylene - Reduction

S

O

ACP

S

OO

ACP

N

CNH2

O

R

HRSH

H

S

OOH

ACP

HS

-H2O syn-elimination

S

O

ACP

HS

O

ACP

Achieved in 3 steps:

KR

DH

ER

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This process terminates when the chain reachesC16 or C18, yielding palmitic or stearic acid, or theirthiol esters.

Repetition of this cycle (steps II to VI) utilizing thenewly formed acyl intermediate in place of acetylSCoA, leads to the lengthening of the carbonchain by two carbon atoms every cycle, to yieldacyl-species of the general formulaMe(CH2CH2)nCH2COSACP.

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It is probable that as the chain lengthapproaches C16 - C18, the active site thiolof the condensing enzyme has greateraffinity for an acetyl-SACP species.

That is, steric or electronic effects hinderaccess acyl substrates bigger than C16 -C18 to the active site, and termination ofthe chain results.

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Me(CH2CH2)nCH2COSEcondensing

H2O

Me(CH2CH2)nCH2CO2H

Extension of the chain beyond C18 does occurbut the ultimate chain length is rarely greaterthan C22-C24, except in higher plants wherethe chain of up to C30 are encountered.

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The combination of one acetate starter unit withseven malonates would give the C16 fatty acid,palmitic acid, and with eight malonates the C18

fatty acid, stearic acid.

Note that the two carbons at the head of the chain(methyl end) are provided by acetate, notmalonate, whilst the remainder are derived frommalonate, which itself is produced bycarboxylation of acetate.This means that all carbons in the fatty acidoriginate from acetate, but malonate will onlyprovide the C2 chain extension units and not the C2

starter group.

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Step I Step II

Step IV

Step VStep VI

Step III

S

O

CoA

HS-Econdensing

S

O

Econdensing

Malonyl-SACP

HCO3 ATP

Biotin

HS-ACP

S

OOH

ACP

NADPH + H+

NADP

S

O

ACP

NADPH + H+

NADPS

O

ACP

S

OO

ACP

S

O

HO

O

ACP

S

O

HO

O

CoA

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The pathway can be visualized as a cyclic process inwhich the acetyl primer undergoes a series ofClaisen condensation reactions with seven malonylextender molecules and, following eachcondensation, the [b-carbon of the b-3-ketoacylmoiety formed is completely reduced by a three-step ketoreduction-dehydration-enoyl-reductionprocess.

The saturated acyl chain product of one cyclebecomes the primer substrate for the followingcycle, so that two saturated carbon atoms areadded to the primer with each turn of the cycle.

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Odd numbered fatty acids

Other acyl CoA moieties (e.g. proinoyl SCOA,with three carbons) may also function asstarter units in place of the usual starteracetyl SCoA.

The linear combination of acetate C2 unitsexplains why the common fatty acids arestraight chained and possess an even numberof carbon atoms.

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The biosynthesis of fatty acids thatpossesses an odd number of carbon atoms isessentially similar to the biosynthesis oftheir even numbered counter parts.

The synthesis of odd numbered fatty acidsinvolve the use of the same extender unit,malonyl SACP, with an odd numberedstarter unit.

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It would appear that it is the availability of unusualstarters that determines whether normal orabnormal fatty acids are produced, rather than arequirement for special enzymes.

S

O

Econdensing

Proinoyl SEcondensing

S

O

ACPO

O

H

Malonyl SACP

S

O

ACP

OReduction

S

O

ACP

Starter unit

Extender unit

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Hydnocarpus wightiana (Flacourtiaceae)

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Dr. Solomon Derese

Branched-chain Fatty acids

While straight chain fatty acids are the mostcommon, branched chain fatty acids have beenfound to occur in mammalian systems.

Branched fatty acids are formed either

I. By priming the reaction with a branchedstarter (e.g 3-Methylbutyric SCoA or 2-methylbutyl SCoA)

O

SCoA

O

SCoA

3-Methylbutyric acid2-Methylbutyric acid

64

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II. By using an alkylated malonyl SCoA extenderunit.

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Unsaturated fatty acids

The majority of naturally occurring unsaturatedfatty acids are in the C18 series. Acids shorter thanC14, or higher than C22, are rare. Somerepresentative examples are:

Palmitoleic acid

O

O

H

CO2H

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Oleic acid

O

O

H

CO2H

O

OH

Linoleic acid

O

OH

g-Linoleic acid

(>80% of oil in olive oil)

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Geometry of double bonds

It will be noted that most of the double bondsin the examples given possess the cis (Z) -stereochemistry while trans double bonds arefound in fatty acids, they occur more rarely.

The cis double bond has an important biologicalsignificance: by introducing a “bend” in thealkyl chain it prevents the hydrophobic chainsof the acyl groups in fats, phosphoglyceridesform compact aggregates maintaining thefluidity of fat depot and cell membranes.

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Saturated fatty acid

Trans unsaturated fatty acid

Cis unsaturated fatty acid

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The typical pattern in polyunsaturatedfatty acids is to have methylene groupsflanked by two double bonds “amethylene – interrupted” pattern ofunsaturation.

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Biosynthesis of unsaturated fatty acids

There are two common routes towards thebiosynthesis of unsaturated fatty acids dependingon the organism:

II. Aerobic route (in animals and plants)

- An absolute requirement for oxygen

I. Anaerobic route (occurs in some bacteria)

- Proceed in the absence of oxygen

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The aerobic process is by far the mostcommon, and operates in yeasts, certainbacteria, algae, higher plants and vertebrates.

The anaerobic pathway is mainly confined toanaerobic bacteria.

The aerobic route directly introduces a doublebond into a fatty acid precursor by a processknown as oxidative desaturation, which isregulated by desaturase enzymes.

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I. Anaerobic route

The fact that unsaturated fatty acids aresynthesized in the absence of oxygen isapparent from their wide spread occurrencein anaerobic bacteria. However, onlymonounsaturated fatty acids aresynthesized.

Dehydrogenation occurs during chainelongation.

SCH 511

Dr. Solomon Derese 74Oleic acid

Desaturase

Anaerobic route to oleic acid

SCH 511


This is an anaerobic route as nooxidation is required (the doublebond is already there — it just has tobe moved) and is used byprokaryotes such as bacteria.

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Production of unsaturated fatty acids(insertion of double bonds) requiresmolecular oxygen. In an oxidation step,hydrogen is removed and combined with O2

to form water.

II. Aerobic route

Dehydration occurs after the required chainlength of fatty acid is formed leading tomono- and poly-unsaturated fatty acids.

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This is apparently accomplished by synelimination of a vicinal pair of pro-R hydrogenatoms, resulting in the formation of a cis-doublebond exclusively.

The double bond is usually introduced between C-9and C-10.

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Example

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Most organisms possess a ∆9-desaturase enzymethat introduces a cis double bond into a saturatedfatty acid at C-9.

The position of further desaturation thendepends very much on the organism.

Animals always introduce new double bondstowards the carboxyl group.

Non-mamalian enzymes tend to introduceadditional double bonds between the existingdouble bond and the methyl terminus.

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g-Linoleic acidLinoleic acid

In plants

In animalsO

OH

O

OH

9

12

96

Oxidative Desaturation

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∆9

∆12∆15∆5

∆6

Higher plants

Lower plants

Animals

Insects

CO2H1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

1618

17

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The first double bond introduced into asaturated acyl chain is generally in the ∆9position so that substrates for furtherdesaturation contain either a ∆9 double bondor one derived from the ∆9 position by chainelongation.

Just like ∆9 desaturation that inserts the firstdouble bond, further desaturation is anoxidative process requiring molecularoxygen.

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Animal systems cannot introduce doublebonds beyond the C-9 position. Thus,second and subsequent double bonds arealways inserted between an existing bondand the carboxyl end of the acyl chain,never on the methyl side.

Plants, on the other hand, introduce secondand third double bonds between theexisting double bond and the terminalmethyl group.

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Consequently, double bonds are found at the C-9,C-6, and C-5 positions as a result of desaturation inanimals, at the C-9, C-12 and C-15 positions inplants, and at the C-5, C-6, C-9, C-12 and C-15positions in insects and other invertebrates.

n(H2C) (CH2)m

O

OH

H3C

9

Animals: Further unsaturationoccurs primarily in this region

Plants: Further unsaturationoccurs primarily in this region

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Fatty Acid Modifications

These modifications include:

I. Chain elongation to give longer fatty acids.

II. Desaturation, giving unsaturated fatty acids.

The biosynthesis of fatty acids mainly yieldstraight chain saturated fatty acids with sixteenand/or eighteen carbons. Other fatty acids areobtained by structural modifications of thesestraight chain saturated acids.

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O SACP

STEARIC ACID

OLEIC ACID 18:1 (9c)

O

SACP

O

SACP

LINOLEIC ACID 18:2 (9c, 12c)

18:2 (9c, 6c)

SACPO

PlantsFungi

Desaturationtowards carboxyl terminus

Animals

All Organisms

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The most common mono unsaturated FA inanimals are oleic acid (18:1(9c)) andpalmitoleic acid (16:1(9c)). Fatty AcidDesaturase enzymes in animals can onlyintroduce double bond up to C-9.

Thus the important unsaturated fatty acidsLinoleic (with C-9 and C-12 double bonds)and Linolenic acids (with C-9, C-12 and C-15double bonds) can not be biosynthesized inanimals including humans.

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They must be obtained from the diet. Plantshave the enzymes necessary to synthesizethese acids. Such fatty acids are described asEssential Fatty Acids (EFA).

Linolenic acid, an omega-3 fatty acid, andlinoleic acid, an omega-6 fatty acid are bothfound in soybean oil and other types of plantoils.

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Linoleic acid is the starting material from whichthe body makes arachidonic acid, the precursor forprostaglandins, the hormone like substance thatregulates a wide range of body functions, includinggrowth, wound healing and epidermal health.

Animals can metabolize these two fatty acidsobtained from the diet to form longer and moreunsaturated PUFAs to meet their metabolic needs.Since these two fatty acids must be obtained fromthe diet, they are considered to be essential fattyacids.

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O SACP

18:2 (9c, 12c)

ANIMALS

g-LINOLEIC ACID

SACP

O

18:3 (6c, 9c, 12c)

Chain extension byreaction with malonate

SACP

O

DIHOMOg-LINOLEIC ACID 20:3 (8c, 11c, 14c)

SACP

O

ARACHIDONICACID 20:4 (5c, 8c, 11c, 14c)

ANIMALS

LINOLEICACID

6

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O SACP

18:3 (9c, 12c, 15c)

a-LINOLENIC ACID

SACP

OANIMALS

18:4 (6c, 9c, 12c, 15c)STEARIDONIC ACID

Chain extension byreaction with malonate

SACP

O

20:4 (8c, 11c, 14c, 17c)EICOSATETRAENOIC ACID

SACP

20:5 (5c, 8c, 11c, 14c, 17c)

O

DEASTURASE

DEASTURASE

EICOSAPENTAENOIC ACID (EPA)

3

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Dr. Solomon Derese 93SCoA

O

Desaturation

Sterculic acid

SCoA

O

S

Ad

CH3

R

SAM

Oleoyl CoA

SCoA

O

CH2H

Electrophilic addition reaction

SCoA

O

NADPH

Tuberculosteraic acid

SCoA

ODihydrosterculic acid

Further Structure Modification

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Prostaglandins

They are now known to occur widely inanimal tissues, but only in tiny amounts, andthey have been found to exert a wide varietyof pharmacological effects on humans andanimals.

The prostaglandins are a group of modifiedC20 fatty acids first isolated from humansemen and it was recognized that they weresynthesized in the prostate gland (hence thename).

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They are active at very low, hormone-likeconcentrations and can regulate blood pressure,contractions of smooth muscle, gastric secretion,and platelet aggregation.

It is thought that they are moderators of hormoneactivity in the body, a theory that explains theirfar reaching biological effects.

Imbalances in prostaglandins can lead to nausea,diarrhea, inflammation, pain, fever, menstrualdisorders, asthma, ulcers, hypertension,drowsiness or blood clots.

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The basic prostaglandin skeleton is that of a cyclizedC20 fatty acid containing a cyclopentane ring, a C7side-chain with the carboxyl function, and a C8 side-chain with the methyl terminus.

Basic skeleton of Prostaglandin

C7

C8

OH

OH

12

3

4

5

6

7

8

9

1020

14

11

12

13 15

16

17

18

19H

OH

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Naturally occurring prostaglandins contain acyclopentane ring, a trans double bond between C-13 and C-14, and a hydroxyl group at C-15.

The prostaglandins are produced by mostmammalian cells.

Prostaglandins are biosynthesized from three fattyacids, dihomo-g-linolenic (20:3 (8c,11c,14c)),arachidonic (20:4 (5c,8c,11c,14c)), andeicosapentaenoic (20:5 (5c,8c,11c,14c,17c)) acid,which yield prostaglandins of the 1-, 2-, and 3-series,respectively.

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These three fatty acids are obtained from theessential fatty acids linoleic and linolenic acid.

OH

O OH

O

ARACHIDONIC ACID

OH

O

EICOSATETRAENOIC ACID

PGE1

OH

O

HO OH

O

HO

PGE2PGE3

OH

OOH

O

OH

O

HO

OH

O

DIHOMOg-LINOLEICACID

The numerical subscripts indicate the number ofcarbon-carbon double bonds in the side chains.

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Biosynthesis of Prostaglandins

Biallylic hydrogen

O O

Biallylic free radical

OH

O

OH

O

OH

O

Oxygen is adiradical

Re

son

ance

Stabilize

d

HO O

ARACHIDONICACID

OH

O

H

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A methylene flanked by doublebonds on both sides is susceptibleto free radical oxidation; freeradical reaction allows addition ofO2 and formation of peroxideradical.

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OH

O

O

O

OH

O

O

O

OHO

PGG2O

O

OH

O

O

H

O

Peroxidase

PGH2 OH

O

OH

O

O

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PGH2 OH

O

OH

O

O

OH

O

OH

O

O

Radical cleavage of cyclic epoxide

OHHO

OH

O

HO

PGF2a

OH

O

HO

OH

O

PGE2

OH

HO

O

OH

O

PGD2

+H˙,- H˙

SCH 511


Prostaglandins are named in accordance withthe format PGX, where X designates thefunctional groups of the five-membered ring.PGAs, PGBs, and PGCs all contain a carbonylgroup and a double bond in the five-memberedring. The location of the double bonddetermines whether a prostaglandin is a PGA,PGB, or PGC.

R2

O

R2

O

R2

O

PGA PGB PGC

R1 R1R1

SCH 511


The letters following the abbreviation PGindicate the nature and location of the oxygen-containing substituents present in thecyclopentane ring.

PGDs and PGEs are b-hydroxyketones, andPGFs are 1,3-diols. A subscript indicates thetotal number of double bonds in the sidechains, and a and b indicate the configurationof the two OH groups in a PGF: “a” indicates acis diol and “b” indicates a trans diol.

SCH 511


HO

O

PGD

R1

R2

PGE1

OH

O

HO

OH

O

OH

O

HO

PGE2

OH

O

OH

HO

HO

PGF2a PGF1a

OH

HO

HO

OH

O OH

O

SCH 511


SCH 511


Polyketo thioesters

Aromatic compounds or otherPolyketide derived metabolites

Acetyl SEcondensing

n-2SACP

O O O

SACP

O

O

O

Hn

S

O

Econdensing

Malonyl SACPPKS

SCH 511


The biosynthesis of fatty acids and polyketides isbasically the same except that the β-keto group isgenerally not completely reduced out in thebiosynthesis of polyketide derive compounds.

SACP

OO

This gives rise to huge structural diversity basedaround a 1,3-oxygenation pattern and cyclization togive aromatic compounds.

SCH 511


O

CoAS

SH SH

ACPCE

Polyketide Synthase (PKS)

‘molecular machine’

S SH

ACPCE

O

AT

ACPCE

O

CoAS O

O

MT

SO S O

O

O

H+

SH S

ACPCE

O

REDUCTION

1) KR2) DH3) ER

O

SH S

ACPCE

O

O

Decarboxylative

Claisen

condensation

SH

ACPCE

TRANSLOCATION

O S

O

SH

ACPCE

O S

n

nO

O

OHn

TEOOO

Biosynthesis of Polyketides


ACP= Acyl Carrier ProteinKR= Keto ReductaseER= Enoyl Reductase

DH= dehydrataseTE= thioesterase

±Differencebetween FA and PKbiosynthesis

SCH 511


Unlike fatty acids, polyketides are NOTBiosynthesised by humans – only by plants,microorganisms (bacteria) & fungi.

The biosynthesis of polyketides is catalyzed by theenzyme PolyKetide Synthase (PKS).

S

O O

Econdensing

O S

O O

ACPH

O S

O O

ACPH

O O

S

O

ACP

O O O O

SACP

Such compounds which possess a chain ofalternating ketone and methylene groups are calledpolyketides.

SCH 511


EXAMPLES OF POLYKETIDE DERIVED SECONDARY METABOLITES

OH O OHOH

O

CONH2

OH

NCH3

H

H3C

HH3C OH

O

OO

O OOHO

O

OH

HO

Emodin

O

OOH

Plumbagin

Eugenone

O

O

OH

HO

O

OH

Rebelomycine

OHHO

O

O

H

Orsellinic acid

Tetracycline (Antibiotic)

SCH 511


n-2SACP

O O O1,3-Diketo polymer

The poly-β-keto ester is very reactive, andthere are various possibilities forintramolecular Claisen or Knoevenagelcondensation reactions.

SCH 511


Methylenes flanked by two carbonylsare activated, allowing formation ofcarbanions/enolates and subsequentreaction with ketone or ester carbonylgroups, with a natural tendency to formstrain-free six membered rings.

SCH 511


O O

R R'

HO O

H

R R'

Keto TautomerEnol tautomer Enol tautomer

H-Bonding

Favored structure

The Chemistry of 1,3-DicarbonylsKeto-Enol Tautomerism

There are two important condensation reactions of1,3-Dicarbonyls which are relevant to polyketidechemistry; Knoevenagel and Claisen condensationreactions.

SCH 511


The Knoevenagel Condensation reaction

O OH

R OEt

O O

R OEt

1,3-Dicarbonyl compund

O

R2

R1

Aldehyde/Ketone

O OH

R OEt

R2R1O

Proton transfer

O O

R OEt

R2R1OH

HO O

R OEt

R2R1A new C-C

double bond

SCH 511


A condensation reaction between 1,3-dicarbonyl compounds andaldehydes/ketones resulting in the formationof a new carbon-carbon double bond with aloss of water is called the KnoevenagelCondenation reaction.

SCH 511


The Claisen Condensation reaction

O OH

R OEt

O O

R OEt

1,3-Dicarbonyl compund

O

EtO

R1

Ester

O OH

R OEt

OR1

- H+

O O

R OEt

OR1

The condensation reaction of 1,3-dicarbonyls withesters is called Claisen condensation reaction.

SCH 511


O O

R OEt

O

EtO R1

Ester

H H

O O

R OEt

OR1

In principle, a thioester could replace an estergiving the same product but with loss of thiol(RSH) rather than an alcohol. This is importantin biosynthesis as nature often works withthioesters.

The Claisen Condensationreaction

SCH 511


Polyketide Cyclisations

Formation of Unsaturated Products

Polyketide can cyclize to obtainvarious classes of natural productsthrough condensation reactions ofthe Knoevenagel and Claisen types.

SCH 511


S

O

Econdensing

SACP

O

O

O

H3

O O O O

SEnz

SCH 511


The polyketo ester formed from four acetate units(one acetate starter group and three malonatechain extension units) is capable of being folded inat least two ways, A and B.

A B

O O O O

SEnz

O

O

O O

SEnz

OO

O SEnz

O

SCH 511


O

O

O O

SEnz

AO

OH

O

OOH

H

O

OH

O

O

Knoevenagelreaction

Dehydration favored by formationof conjugated system

O

OH

OH

HO Enolization

Orsellinic acid

Enolization favored by formation of aromatic ring.

SCH 511


OO

O SEnz

O

B

OO

OO

SEnz

Claisenreaction

OO

O O

Re-formation of carbonylpossible by expulsion ofleaving group

EnolizationOOH

HO OH

Phloracetophenone

SCH 511


A distinctive feature of an aromatic ring systemderived through the acetate pathway is that severalof the carbonyl oxygens of the poly-β-keto systemare retained in the final product. These end up onalternate carbons around the ring system.

Of course, one or more might be used in forming acarbon–carbon bond, as in orsellinic acid.

Nevertheless, this oxygenation on alternate carbonatoms, a meta oxygenation pattern, is usuallyeasily recognizable, and points to the biosyntheticorigin of the molecule.

SCH 511


SEnz

O O O O O O O

O

O

O

O

O

OS

O

Enz

HO

O

OH

OHO

Alternariol

O

O

O

OSEnz

O

HO

O

OH

OHOEnzS

H

SCH 511


The biosynthesis of compound Alternariolinvolves two Knoevenagel-typecondensations and in the last step shown,an ester linkage is formed between aphenolic hydroxy group and the thioestergroup. Such cyclic esters are called lactones.

SCH 511


SEnz

O O O O O O O

O

O

O

O

O

OS

O

Enz

O

O

O

OSEnz

O

HO

O

OH

OHOEnzS

H

HO

O

OH

OHO

Alternariol

SCH 511


Secondary Structural ModificationsDuring Polyketide Cyclisations

The structural variety of polyketide-derived naturalproducts is increased enormously by secondarystructural modifications.

We have already seen such an example in thebiosynthesis of alternariol in which an ester linkagehas been created. The formation of these esterlinkages can be considered as secondarymodifications after cyclization of the polyketideshas occurred.

SCH 511


I. MethylationII. DecarboxylationIII. ReductionIV. Oxidation

There are many types of secondarymodification which can occur to polyketide-derived natural products. Four commonmodifications which we will consider are:

SCH 511


I. Alkylation

O and C-methylation reaction can occur inbiological systems using S-Adenosinemethionine(SAM).

Methylation

S

Ad

CH3

RS-AdenosineMethionine (SAM).

SCH 511


OH

O-P director

OH

CH3

C-Methylation

S

Ad

CH3

R

OH

S

Ad

CH3

R

C-Methylation

O

O-Methylation

S

Ad

CH3

R

SCH 511


Example

OOO

O

O

SEnz

O O

OMe

OMe

MeO

Eugenone

3 SAMClaisen

O O

OH

OH

HO

OO OOH OOH

CH3

SCH3R2

R1

OO

CH3

SCH 511


II. Decarboxylation

OH

O

O

R

R

b-Keto carboxylic acid

Decarboxylation is very common in biosyntheticand organic reactions.

b-Keto carboxylic acid decarboxylates readily.

OH

R

R O

R

R

a

b

SCH 511


OHHO

OH

O

OHO

O

O

H

o-Hydroxy carboxylc acid

CO2

OHHO

SCH 511


III. Reduction

Ketone reduction followed by dehydration is oftenused as a method for introducing a double bondand we have already met examples of this processin the biosynthesis of unsaturated fatty acids.

O

:H- (from NADPH)

Ketone

OH+ OH

H H Cis

Reduction and Dehydration

SCH 511


Example

SCH 511


IV. Oxidation

a. One common biosynthetic oxidation is theconversion of a methyl group which is directlyattached to a benzene ring into its correspondingcarboxylic acid i.e. Ar-CH3 > ArCO2H.

CH2OH

H

O

OH

O

[O]

[O]

[O]

SCH 511


The oxidation of a methyl groupwhich is attached to an aromaticneed not necessarily proceed to thecarboxylic acid level of oxidation; analcohol could be formed i.e. Ar-CH3 >ArCH2OH, even an alhehyde (ArCHO).

SCH 511


b. Oxidation benzene into phenol (Ar-H toAr-OH). These reactions are catalysed byenzymes known as monooxygenases (so-called because they introduced oneoxygen atom from oxygen).

SCH 511


Quinone

c. A phenol is oxidized to a quinone using theenzyme Dehydrogenase (- H2).

OH

OH

Dehydrogenase

O

O

OH

OH

Dehydrogenase

O

O

Examples

SCH 511


O

HO CH2OH

O

Shanorelin

O

SO

Enz

O

O

2 X SAM

O

SO

Enz

O

O

OH

HO

O OH

Knoevenagel

Enolization

Hydroxylation

OH

HO CH2OH

OH

Decarboxylation

Methyl Oxidation

OH

HO CH2OH

Example

Dehydrogenase

SCH 511


d. Phenolic oxidative coupling

OH

[O]

12

3

4

O

12

3

4

Phenoxy raidcal

O

12

3

4

O

12

3

4

O

12

3

4

Phenols are oxidised to their correspondingphenoxy radicals.

The unpaired electron in a phenoxy radical can bedelocalised over the oxygen atom, the carbon atomat the 2-position (ortho) and the carbon atom atthe 4-position (para).

SCH 511


Once phenoxy radicals have been generated, theydimerise by pairing the unpaired electron of onephenoxy radical with the unpaired electron of asecond phenoxy radical.

OO

H

H

OHOH

Coupling

Aromatization

C-C Bond

O

O

SCH 511


OO

O

OH

OH

O

Aromatization

C-O Bond

SCH 511


OO

Aromatization

O O

H

HO OC-O Bond

SCH 511


ExampleO O O

O

O O

OSEnz

OH O OH

HO OHOH

OH O

OH

O

MeO

OMe

H

MeOOH

MeO O

O

O

MeO

MeO

Cl

O

O

O

OMe

OH

O•O•

MeO

OMeOOH

OO•

MeO

OMeO

Griseofulvin, A natural productwith fungicidal activity isolatedfrom the mould of Peniciliumgriseofulvum.

SAM

[o]

SAM

NADPH Chlorination

SCH 511


Biosynthesis of Anthraquinones

SEnz

O O O O O O O O

SCH 511


O O O

O

O O O

SEnz

O

O

O O O

CO2H

Hypothetical

intermediate I

HO

O O O

CO2H

Hypotheticalintermediate II

HO

O O O

CO2H

Hypotheticalintermediate III

1. Knoevengel1. Knoevengel

1. Knoevengel2. NADPH

2. NADPH

Enolization [O]

HO

OH O OH

CO2H

O

EndocrocinOH O OH

O

ChrysophanolOH O OH

O OH

Islandicin

Enolization[O]

- H2OEnolization[O]-CO2H -CO2H

- H2O

SCH 511


SCH 511


O O O

O

O O OSEnz

O OH OH O

HO

OSEnz

O

O

SEnz

HO

OH OH O

OH

O

OH

HO

OH OH O

OH

HO

OH OH O

OH

Antrochyrsone

AntrochyrsoneCarboxylic acid

SCH 511


Hypericin is being investigated for its antiviralactivities, in particular for its potential activityagainst HIV.

Biosynthesis of Hypericin

Hypericum perforatum(Guttiferae)

SCH 511


HO

OH O OH

Emodin anthrone

Tautomerization

HO

OH OH OH

-H

[O]

HO

OH O OH

HO

OH O OH

SCH 511


HO

OH O OH

OH

OHOOH

Oxidativecoupling

Emodin dianthrone

HO

OH O OH

OH

OHOOH

[O]

HO

OH O OH

OH

OHOOH

HO

OH O OH

OH

OHOOH

[O][O]

ProtohypericinHypericin

SCH 511

Dr. Solomon Derese

BIOSYNTHESIS OF KNIPHOLONE

Polyketide derived secondary metabolite

Anthraquinone (Chrysophanol)

Acetylphloroglucinol

Phenolic OxidativeCoupling

SCH 511

Dr. Solomon Derese

NADPH- H2O

KnoevengelCondensation

[O]

Chrysophanol

SCH 511

Dr. Solomon Derese

ClaisenCondensation

Aromatization

Acetylphloroglucinol

SAM

SCH 511

Dr. Solomon Derese

SCH 511

Dr. Solomon Derese

Aromatization

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SCH 511 Secondary Metabolites Derived from Acetate · SCH 511 Dr. Solomon Derese LEARNING...

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