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