SCH 511 Biosynthesis of Fatty Acids - profiles.uonbi.ac.ke · 16 fatty acid, palmitic acid, and...

SCH 511

Dr. Solomon Derese 38

Biosynthesis of Fatty Acids

Saturated thioesters

Saturated fatty acids

Malonyl CoA

Acetyl CoAS

OCoA

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

NHS

O

H

Ser

ON

N

N

N

NH2

OP

O-

O

OP

O-

O

O

OOH

P O-O

-O

N

HO

O

H

N

HS

O

H

ACP

FAS

Coenzyme A

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SCH 511


ACPCE 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)

pant

ethe

ine

chai

nSHSH

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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) KR2) DH3) ER

SH S

ACPCE

O

O

DecarboxylativeClaisencondensation

SH

ACPCE

TRANSLOCATION

O S

OHn

TEO

Biosynthesis of Fatty Acids

AT= Acetyl transferaseMT= Malonyl transferaseCE= Condensing enzyme

ACP= Acyl Carrier ProteinKR = Keto ReductaseER = Enoyl Reductase

DH= DehydrataseTE= Thioesterase

SH

ACPCE

O S

n

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

Steps in the Biosynthesis of Fatty Acids

Step IIS

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

E condensing

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

H CoAA

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 VSyn 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

OACP

S

OOACP

N

CNH2

O

R

HRSH

H

S

OOHACP

HS

-H2O syn-elimination

S

OACP

HS

OACP

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 C18fatty 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 C2starter 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

ACPO

Reduction

S

O

ACP

Starter unit

Extender unit

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Hydnocarpus wightiana (Flacourtiaceae)

SCH 511

Dr. Solomon Derese

Branched-chain Fatty acidsWhile straight chain fatty acids are the mostcommon, branched chain fatty acids have beenfound to occur in mammalian systems.

Branched fatty acids are formed eitherI. By priming the reaction with a branched

starter (e.g 3-Methylbutyric SCoA or 2-methylbutyl SCoA)

O

SCoA

O

SCoA

3-Methylbutyric acid2-Methylbutyric acid64

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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 bondsIt 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 acidsThere 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 routeThe 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.

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S

OH

ACP

cis-double bond

Oleic acid

Desaturase

Anaerobic route to oleic acid

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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 O2to 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

9 6

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

OHH3C

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)

OSACP

O

SACP

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

18:2 (9c, 6c)

SACPO

O

SACP

18:2 (9c, 12c, 15c)-LINOLENIC ACID

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

OARACHIDONIC ACID 20:4 (5c, 8c, 11c, 14c)

ANIMALS

LINOLEIC ACID

w6

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O SACP

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

-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)

w3

SCH 511

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

102014

11

12

13 15

16

17

18

19HOH

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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

OARACHIDONIC ACID

OH

O

EICOSATETRAENOIC ACID

PGE1

OH

O

HO OH

O

HO

PGE2 PGE3

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

Resonance Stabilized

HO O

ARACHIDONIC ACID

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

OH

O

Peroxidase

PGH2 OH

O

OH

OO

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PGH2 OH

O

OH

OO

OH

O

OH

OO

Radical cleavage of cyclic epoxide

OHHO

OH

O

HO

PGF2

OH

O

HO

OH

O

PGE2

OH

HO

O

OH

O

PGD2

+H˙,- H˙

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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

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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 and b indicate the configurationof the two OH groups in a PGF: “” indicates acis diol and “b” indicates a trans diol.

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HO

O

PGD

R1

R2PGE1

OH

O

HO

OH

O

OH

O

HO

PGE2

OH

O

OH

HO

HO

PGF2 PGF1OH

HO

HO

OH

O OH

O

SCH 511 Biosynthesis of Fatty Acids - profiles.uonbi.ac.ke · 16 fatty acid, palmitic acid, and...

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Transcript of SCH 511 Biosynthesis of Fatty Acids - profiles.uonbi.ac.ke · 16 fatty acid, palmitic acid, and...