Polyketides and Other Secondary Metabolites Including Fatty Acids and Their Derivatives [INCOMPLETE]

775 Pages·1999·12.1 MB·English

by  Otto Meth-Cohn

1.01
Overview
USHIO SANKAWA
ToyamaMedicalandPharmaceuticalUniversity,Japan
0[90[0 INTRODUCTION 0
0[90[1 FATTYACIDSANDPOLYKETIDES 1
0[90[1[0 EarlyStudiesofPolyketideBiosynthesis 2
0[90[1[1 FattyAcidBiosynthesis 4
0[90[1[2 UnsaturatedFattyAcidsandRelatedCompounds 6
0[90[1[3 ArachidonateCascade 7
0[90[2 POLYKETIDEBIOSYNTHESISINACTINOMYCETESANDFUNGI 09
0[90[2[0 TypeIIAromaticPolyketideSynthetasesinActinomycetes 09
0[90[2[1 TypeIPolyketideSynthaseinActinomycetes 03
0[90[2[2 PolyketideandPolypeptideBiosynthesisinFun‘i 03
0[90[3 BIOSYNTHESISOFNATURALPRODUCTSINVOLVINGTHESHIKIMATEPATHWAY 05
0[90[3[0 Enzymolo‘yandMolecularBiolo‘yoftheShikimatePathway 05
0[90[3[1 BiosynthesisofC(cid:1)C Compounds]CoumarinsandLi‘nans 07
5 2
0[90[3[2 BiosynthesisofFlavonoids 08
0[90[4 BIOSYNTHESISOFCOMPOUNDSCONTAININGSULFUR\AC0PBONDORACNGROUP 19
0[90[5 REFERENCES 11
0[90[0 INTRODUCTION
The term {{polyketides|| is not clearly de_ned\ although the textbooks of chemistry and:or
biosynthesis of natural products describe fatty acids as a subclass of polyketides[0(cid:1)2 The word
polyketides is used in di}erent senses[ One covers a wide range of compounds including fatty
acids while the other is limited to non!fatty acid polyketides[ Sometimes polyketides is applied to
polyketomethyleneintermediatesofaromaticpolyketides[Weusethewordpolyketidesinstinctively
for a limited range of compounds without considering the de_nition that the term polyketides
includesfattyacids[Thisisduetothehistoricalbackgroundofnaturalproductschemistry[Common
fattyacidsareprimarymetabolitesandhavebeenregardedascompoundsforstudybybiochemists[
Non!fattyacidpolyketidesaresecondarymetabolitesandtheyaretheparticularfocusfornatural
productschemists[
Recent investigations on the molecular biology of polyketide biosynthesis have demonstrated
that the genes of fatty acid synthases "FAS# and microbial polyketide synthases "PKS# have a
signi_cant homology and they are recognized to have evolved from the same prototype gene[ The
chain elongation mechanism in the biosynthesis of reduced polyketides such as erythromycin and
rapamycinisbasicallythesameasthatwithFAS[Thedistinctionbetweenprimaryandsecondary
metabolites has disappeared and many chapters of this volume describe the multidisciplinary
approachonthestudiesofbiosynthesisofcorrespondingclassesofcompounds\indicatingthatthis
approach is essential for studies of the biosynthesis of natural products[ It is reasonable therefore
0
1 Overview
that fattyacids areincluded in polyketideswhen theirbiosynthesis is discussedat theenzyme and
genelevels[
The most remarkable feature of natural products is the diversity of chemical structures\ which
re~ects the diversity of enzymes and genes responsible for the biosynthesis of complex structures[
Moleculardiversityisthereasonwhynaturalproductshavebeenthetargetofnewdrugdiscovery[
The advantages of screening natural products to _nd new lead compounds for drug development
are the unpredictability and unexpectedness of the chemical structures found[ Many examples of
complexanddiversestructuresofmicrobialmetabolitesareevidentinthefollowingsections[Toxic
fungalmetabolitesa~atoxinB \patulin\andpenicillicacidaregoodexamplesoffungalpolyketide
0
mycotoxins[Theyaresynthesizedbyextensivemodi_cationsofrathersimplecompounds\orsellinic
acid\ 5!methylsalicylic acid "5MS#\ and norsolorinic acid\ respectively[ They are good examples of
biosynthesis which epitomize the diversity of natural product structures[ Fatty acid derived com!
poundsare alsoextensively modi_edfora varietyofreactions[ Prostaglandins"PG# andjasmonic
acid "JA# are well!known compounds deriving from fatty acids\ linolenic and arachidonic acids
"AA#\respectively[ThestructuresofPGandJAlooksimilar\however\theringformationreaction
is completely di}erent[ Microbial polyketides produced by actinomycetes and _lamentous fungi
have been used for medicinal purposes as antibiotics "erythromycin and tetracycline#\ antitumor
agents "anthracyclinones#\ cholesterol!lowering drugs "pravastin and mevinolin#\ and immu!
nosuppressors "FK 495\ rapamycin#[ They are synthesized from the basic building units by chain
elongation enzymes "PKS# and following modi_cation reactions such as Diels(cid:1)Alder cyclization[
The polypeptide chain elongation reaction of cyclic polypeptide cyclosporin is very similar to
that of macrolide biosynthesis[ The shikimate pathway is well developed in higher plants\ and
phenylalanine\ which is supplied from shikimate pathways\ is the precursor of all C (cid:1)C and C (cid:1)
5 2 5
C (cid:1)C compounds[Phenylalanineammonialyase"PAL#isacharacteristicenzymeofhigherplants
2 5
andresponsiblefortheformationofcinnamicacid\whichistheprecursorofmostabundantplant
constituents\ lignin\ lignans\ coumarins "C (cid:1)C #\ and ~avonoids "C (cid:1)C (cid:1)C #[ PAL is a gateway to
5 2 5 2 5
secondary metabolism from primary metabolism[ The other categories of compounds are rather
miscellaneous and the biosyntheses of compounds containing sulfur\ C0P bonds and CN groups
have been described[ The mechanism of de novo C0S bond formation is di}erent for each
compound\re~ectingthediversityofbiosynthesis[Thisvolumecoversawiderangeofcompounds^
however\itisintendedtofocusonthemultidisciplinaryapproachestoelucidatecomplexanddiverse
reactionsintheirbiosynthesis[
0[90[1 FATTYACIDSANDPOLYKETIDES
Fromtheearlydaysoforganicchemistry\chemistsnoticedthatstraight!chainfattyacidsoccur!
ring in nature consisted of an even number of carbon atoms[ This led to the hypothesis that fatty
acidsareformedbythehead!to!tailcondensationofacetate[Attheendofthenineteenthcentury
Collie proposed a hypothesis that ketene "CH 1C1O# or its equivalent was the building unit of
1
aromatic polyketides[ His hypothesis was demonstrated by the synthesis of phenolic compounds
from so!called polyacetates[3 The concept of a polyketide hypothesis was further elaborated by
Robert Robinson and he compiled a textbook entitled The Structural Relationships of Natural
Productsin0844[4TwoyearsbeforeRobinson|stextbookwaspublished\Birchwrotehis_rstpaper
on the acetate hypothesis in 0842 when he was a professor at Sydney University[5 His concept of
theacetatehypothesisisbasedonthedetailedanalysisofthestructuresofphenolicnaturalproducts\
particularly lichen substances\ depsides\ and depsidones\ as well as ~avonoids and stilbenes[ The
participation of acetate in fatty acid biosynthesis was proved by Lynen in the late 0839s and this
undoubtedlyhadanin~uenceontheconceptoftheacetatehypothesis[Atthattimeitwasbelieved
that in the biosynthesis of fatty acids\ head!to!tail condensation of acetate yielded b!ketoacyl
intermediates and the carbonyl was then reduced to methylene by successive reactions[ The acyl
group was further condensed with another acetate and repeated reaction cycles yielded fatty acid[
On the other hand\ if head!to!tail acetate condensation proceeded without reductive removal of
oxygen\ all the carbonyl groups were retained and thus formed polyketomethylene intermediates
which were cyclized by aldol or Claisen condensation to give resorcinol or phloroglucinol type
phenolics[ This is the theoretical background of the acetate hypothesis[ Scientists engaged in the
structural elucidation of lichen substances noticed that resorcyclic acid homologues\ consisting of
depsidesanddepsidones\hadanoddnumberofcarbonsidechains[Thiswasreasonablyexplained
by the acetate hypothesis[ Shoji Shibata\ who studied in the laboratory of Asahina\ said remi!
Overview 2
niscently\ {{We should have thought seriously how depsides and depsidones were synthesized in
lichens[Everybodyknewthatthesidechainsofdepsidesanddepsidonesconsistedofoddnumbers
of carbons||[ Birch mentioned in his lecture that he could do nothing but use his brain because of
the lack of sta} and facilities when he moved to Sydney from Oxford[ This situation gave him a
timetoseetheobviousacetatehypothesis\asindicatedbythetitleofhisautobiography[6
0[90[1[0 EarlyStudiesofPolyketideBiosynthesis
The_rstpracticalevidencetoverifytheacetatehypothesiswasanincorporationanddegradation
experimenton5MSwith03C!labeledacetatetoprovethelabelingatexpectedcarbons[7Theadvent
of03Cinthebiosyntheticinvestigationsmadeitpossibletoobtainde_niteevidencewhichchanged
thehypothesistoreality[ExtensivestudiesbyBirchandhiscollaboratorsonmanyfungalpolyketides
rigorously established that acetate was the building unit of phenolic polyketides[ The presence of
polyketomethyleneintermediateswasprovedbytheisolationoftetraaceticacidlactonesfromthe
culturesofatropoloneproducingfungusPenicilliumstipitatumwhenitwasculturedinthepresence
ofethionine\aninhibitorofmethionine[8Theintermediateenroutetostipitatonicacid\atropolone\
was shown to be methylorsellinic acid\ formed by the introduction of a methyl group from meth!
ionine to the polyketomethylene intermediate "Figure 0#[ Cyclization a}orded methylorsellinic
acid\ which underwent ring expansion and oxidation to give the seven!membered tropolone ring[
Tetraaceticacidlactonea}ordedorsellinicacidundermildalkalineconditions\thusleadingtothe
acetatehypothesisbeingprovedbybiochemicalandbiomimeticexperimentsinthe0859s[
Incontrast\fattyacidbiosynthesiswasstudiedbybiochemistsusingabiochemicalapproachand
thecharacterizationofenzymesinvolvedinFASwasthemainstreamofresearch[Thebasicbuilding
unit of fatty acid biosynthesis was soon shown to be not acetic acid but acetyl CoA[ This is an
activatedformofacetateinalllivingorganismsandservesastheprecursoroffattyacids\phenolic
polyketides\ and isoprenoids[ During the investigations of FAS with crude enzyme extracts\ in
addition to acetyl CoA and NADPH\ other cofactors were found to be essential for the reaction[
They were soon identi_ed as biotin and carbonate[ This led to the _nding that acetyl CoA was
converted into malonyl CoA by acetyl CoA carboxylase and is the true C chain elongation unit[
1
TheschemeoffattyacidbiosynthesisinvertebrateFASisillustratedinFigure1andinmoredetail
inChapter0[91[TheparticipationofmalonylCoAwassoonprovedinpolyketidebiosynthesisby
feedingexperimentswith03C!labeledmalonateanditactedasachainelongationunit[Insomecases
03C!labeled diethylmalonate gave higher incorporation ratios because of its better permeability
through the cell membrane[ As it appears in Figure 1\ acetyl CoA is a {{starter\|| the starting unit\
and malonyl CoA is the chain elongation unit\ the {{extender[|| Ketoacyl synthase "KS# mediates
decarboxylativecondensation betweenSp!acyl andSc!malonylgroups whicharetransferred from
acetylandmalonylCoAsbyacyltransferase"AT#togiveb!ketothioester[Reductionoftheb!keto
groupbyketoreductase"KR#followedbydehydration"DH#andfurtherenoylreduction"ER#gives
saturatedacylthioester[RepetitionofthecycleleadstotheformationofC orC saturatedacyl
05 07
thioesters\whicharecleavedbythioesterase"TE#toa}ordfreefattyacids[
In contrast to the ample data on FAS\ from bacteria to mammals\ up until recently 5!methyl!
salicylicacidsynthase"5MSase#wasthesolePKScharacterizedandpuri_ed[09\005MSaseisasingle
polypeptideofhighmolecularweight"ca[169kDa#\containingdomainsofKS\AT\DH\KR\and
acyl carrier protein "ACP#[ The steps of reduction and dehydration of one carbonyl group are
involved in the reaction and it requires NADPH[ When NADPH was omitted from the reaction
mixture an incomplete derailed product was released from the enzyme as triacetic acid lactone
"TAL#[Thisledtothede_nitionofthetimingofcarbonylreductionattheC stage^theschemeof
5
5MSbiosynthesisisillustratedinFigure2[Twoaromatichydrogenatomsof5MSarederivedfrom
malonylCoA[Investigationswithchiralmalonaterevealedthatoneofthetwohydrogensderived
fromtheproRandtheotherfromtheproSprecursor"Chapter0[03#[Themodesofbiosynthesisof
FASandPKSclearlyshowamarkedsimilarity[
The structural diversity of microbial polyketides were attractive targets for biosynthetic studies
by natural product chemists[ Radioisotopes\ 03C and 2H\ were the only available tools for inves!
tigations on biosynthesis in the early days[ In late 0859s\ tracer studies with 02C and its NMR
detectionweredevelopedintohighlysophisticatedmethodology[Inparticular\incorporationexperi!
mentswith02C(cid:1)02Cdouble!labeledacetateunambiguouslyclari_edthelabelingpatterns"Figure3#[
The_rst experimenton mollisinbiosynthesis byCary\ Seto\and Tanabeclearly demonstratedthe
labeling pattern of intact acetate units[01 The advantage of double!labeled acetate was not only in
3 Overview
OH OH OH O HO O
CO2H CO2H
HO HO OH
O
6-Methylsalicylic acid Orsellinic acid Norsolorinic acid
O
O O
MeO
O
O
HO
O
O
O
O OH MeO O
Patulin Penicillic acid Aflatoxin B1
Mycotoxins and their precursors
O CH O O O
3 7
O OH O OH O OH
HO OH CO2H HO OH CO2H MeO O COH
2
C3H7 Cp
Lecanoric acid Divaricatic acid Lobaric acid
Depsides and Depsidone in Lichens
Me+ (methionine)
O O
OH
O O
MeCO– x 4
2
O O OH
OC OC COH
2
Presence of
ethionine
O
OH
OH OH
OH–
O
HO
O
O O OH
O
COH O
2
Tetraacetic acid lactone Orsellinic acid Stipitatonic acid
Isolation of tetraacetic acid lactone by ethionine inhibition
Figure 0 Mycotoxinsandtheirprecursors[Productionoftetraaceticacidlactonebyethionineinhibitionin
tropoloneproducingfungus\Penicilliumstipitatum\andbiomimeticcyclizationa}ordingorsellinicacid[
determining the labeling patterns of intact acetate units but also detecting 02C signals even if the
incorporation of acetate was very poor because 02C(cid:1)02C coupled signals appeared as satellites not
hindered by the 0[0) 02C natural abundance[ Double!labeled acetate was extensively used in
polyketide biosynthesis[ For example\ the labeling patterns of rubrofusarin and norsolorinic acid
were assigned and the folding patterns of polyketo!chains could not be clari_ed without using
Overview 4
Figure 1 CatalyticcycleofvertebrateFASanditsgeneconstruction[
incorporation experiments with double!labeled acetate[02 Incorporation of double!labeled acetate
intoscytalonerevealeda0]0ratiooftwodi}erentlabelingpatterns\indicatingthatasymmetrical
intermediate\ tetrahydroxynaphthalene\ was reduced in either of the two benzene rings in equal
proportion[03SplittingNMR signalsby02C(cid:1)02Ccouplingobservedin thecompoundslabeledwith
doublelabeledacetateindicatedthepresentof02Catadjacentcarbons[Thismeansinformationon
adjacent atoms can be obtained by measuring the 02C NMR of compounds labeled with double!
labeledacetate[Themethodwassoonextendedtothecombinationof02Cwith1H04and07O\05and
the02CNMRspectrumgavecoupledand:orshiftedsignalsandthesiteoflabelingcouldbeeasily
detected[
Enzymes involved in polyketide biosynthesis were particularly di.cult to characterize^ this is
clear from the fact that 5MSase was the only aromatic polyketide synthetase puri_ed and char!
acterized[ This problem was overcome by the advent of gene cloning and functional expression
techniques in this area^ however the characterization of enzymes of fungal polyketide synthases is
stillratherlimited[
0[90[1[1 FattyAcidBiosynthesis
The structure of FAS has been well characterized by gene cloning and enzymatic studies[ Fatty
acidsareconstituentsofthecellmembranesofalllivingorganismsexceptforsomearchaebacteria\
5 Overview
Figure 2 Reactionof5MSaseanditsgeneconstruction[
and FAS is present in all living organisms from bacteria to human beings[ FASs are divided into
two major classes\ type I and type II\ according to their protein constructions[ FASs of bacteria
and plants belong to type II in which each enzyme is an independent protein as it appears in the
geneclustermapofE[coli"Figure4#[TypeIIFASenzymeproteinsformamultienzymecomplex
tocatalyzefattyacidsynthesisinlivingcells[Incontrast\vertebratetypeIFASconsistsofasingle
polypeptideof1499aminoacidswithamolecularweightof169kDa[Itsnativeandfunctionalstate
Overview 6
O O O O
O O O
SX O SX
O O
O O
O O
O
OH O HO O
OH OH O
HO OH
MeO O
O
Rubrofusarin Norsolorinic acid
O HO
HO HO O HO O HO
SX
OH
O HO OH HO OH HO OH
Tetrahydroxynaphthalene Scytalone
Figure 3 Labelingandcyclizationpatternsinfungalpolyketides[
is an a dimer[ Yeast and fungal FAS are also type I^ however\ they consist of two proteins and
1
formadodecamera b whentheyfunctionasactiveFAS[Eachenzymeandcatalyticdomainwas
5 5
identi_ed by the comparison of amino acid sequences with known motifs of FAS proteins[ The
aminoacidsequencesofcorrespondingcatalyticsitesareretainedfrombacteriatovertebratesand
thewholeaminoacidsequencesoftheregionswereassignedtocorrespondingfunctionaldomains
from limited catalytic site sequences[ Acyl!carrier proteins "ACP# possess 3?!phosphopantetheine
moiety at a serine residue which is the key SH group\ acting as a swinging arm to carry growing
acylgroups[GeneconstructionsoftypeIandtypeIIFASsandPKSsaresummarizedinFigure4[
FattyacidswithnonstraightchainsarebiosynthesizedeitherbyusingnonacetylCoAstarterunits
ormodi_cationofdoublebondstocyclopropanerings[v!Cyclopentyl\cyclohexyl\andcycloheptyl
fattyacidsarebiosynthesizedwithcorrespondingstarters\althoughthebiosynthesisofthesecyclic
acylCoAsareuniqueprocesses"seeChapter0[92#[
0[90[1[2 UnsaturatedFattyAcidsandRelatedCompounds
Excessfattyacidsproducedortakenfromfoodsarestoredastriglyceride[Saturatedfattyacids
aredegradedbyb!oxidationtoyieldacetylCoAwhichisusedasanenergysourceinmitochondria[
Unsaturated fatty acids are formed by the dehydrogenation of saturated fatty acids in various
organismsandthemajorunsaturatedfattyacidsinplantsthatwetakeasfoodsareoleic\linoleic\
andlinolenicacids[Theseunsaturatedfattyacidsarenotonlyconstituentsofphospholipidbutare
also the precursors of a wide variety of bioactive natural products[ Leguminous plants have been
used by farmers as biological fertilizers[ This is because the legume plants accumulate nitrogen in
nodules\formedonroots[NoduleformationisacomplexprocessandthesoilbacteriaRisobiumis
responsiblefornoduleformation[Flavonoidsarechemotactic!factorsforRisobiumandlipochitins
areessentialintheformationofsymbioticnodules[Nodgenes\associatedwiththenoduleformation
ofRizobium\haveahomologywithStreptomycesPKSgenes[NodEandNodFarenowclari_edto
be genes for the biosynthesis of a unique trans!unsaturated fatty acid of lipooligosaccharide\
lipochitin[Thisisatypicalexampleofnaturalproducts\exchangingsignalsbetweenmicroorganism
andplant[Thismeansthat{{conversation||amongorganismsinnatureisatthechemicallevel"see
Chapter0[02#[Thesimplealdehyderesponsibleforthe{{greenodor||ofplantleavesisderivedfrom
linoleicandlinolenicacidsbyperoxidationwithlipoxygenasefollowedbylyasereaction[Jasmonic
acid "JA# and its methyl ester are known as the fragrance of jasmine "see Figure 5#[ JA has been
recognizedasresponsiblefortuberformationinpotatoes\yams\andonions[Italsocausesgrowth
inhibition\ senescence\ and leaf abscission and also acts as a signal transmitter in elicitor treated
7 Overview
Figure 4 GeneconstructionsoftypeIandtypeIIFASsandPKSs[
cellsinducingtheproductionofphytoalexins[JAisnowcharacterizedastheseventhplanehormone\
the others being auxin\ cytokinin\ gibberellin\ ethylene\ abscicic acid\ and brasinolide[ The cyclo!
pentane structure of JA seemed to indicate that its synthesis is analogous to prostaglandins^
however\thereactionmechanismiscompletelydi}erent[Peroxidationoflinolenicacidfollowedby
dehydrationa}ordsalleneepoxide\whichgeneratestheoxypentadienylcationandcyclizestogive
theoxocyclopenteneskeleton"seeChapter0[94#[Prostaglandinsofmarineorganismsareproduced
viatheoxypentadienylcation[Aconsiderablenumberofcompoundsarederivedfromunsaturated
fatty acids by oxygenation and bromocation!induced cyclization "see Chapters 0[97 and 0[01#[ A
polyene"yne#alkenederivedfromunsaturatedfattyacidsistheprecursorofacyclicbromoetherof
marine algae[ Laurencin is formed from hydroxyacetylenic alkene by bromocation!induced cycli!
zation catalyzed by haloperoxidase[ A unique heptadiene ring of ectocarpene is formed by lipox!
ygenase!induced radical reaction followed by electrocyclic reaction[ Ectocarpene is a signal
substance in the sexual cycle of algae and acts as an attractant pheromone of gametes[06 Another
exampleofasignalsubstanceisAfactorwhichisformedbyaldolcondensationbetweenbranched
shortfattyacidandglycerolderivativesandinducesantibioticproductioninStreptomyces[
0[90[1[3 ArachidonateCascade
Among the compounds derived from fatty acids\ prostaglandins "PGs# and other eiconsanoids
areamongthemostimportant_ndingsofthetwentiethcentury[ThehistoryofPGscanbetraced
back to the 0829s and after a long dark period of no progress\ the chemistry and biology of PGs
~ourished in the 0859s[ The physiological functions of PGs were clari_ed using chemically syn!
thesized PGs\ since the amount of PGs obtained from tissues is minuscule[ Aspirin had been used
asanantipyretic\analgesic\andantiin~ammatorydrugfromtheendofnineteenthcenturywithout
Overview 8
CO2H
CO2H
Chaulmoogric acid w -Cyclohexylundecanoic acid
H
OH
O
CO2H
O H
w -Cycloheptylundecanoic acid OH
A factor
OOH
Lipoxygenase
Leaf aldehyde
CO2H CO2H
Linolenic acid
Green order compound
–O O
O
CO2H CO2H CO2H
12-Oxo-phytodienoic acid (oxopentadiene cation) Allenoxide
O
CO2H Br O
OAc
Jasmonic acid
Laurencin Ectocarpene
O
CO2H
–O
OH
15-b PGA2 CO2H
CO2H
OAc Arachidonic acid
O
HO
OAc HO
O
Clavulone OH
N O
H
nod E, F product O
HO
OH
Ac-HN O
O
HO 2-3
Lipooligosacharide
OH
Ac-HN O
OH
Figure 5 Fatty acid related compounds and biosynthesis of leaf aldehyde\ jasmonic acid\ and marine
prostaglandins[
09 Overview
knowingwhatthetargetofaspirinwas[In0860Vanediscoveredthatnonsteroidalantiin~ammatory
drugs such as aspirin and indomethacin exhibited their action by inhibiting prostaglandin biosyn!
thesisincyclooxygenase"COX#reaction[AspirinisauniquedruginitsinhibitoryactivityofCOX[
Aspirin inhibits COX irreversibly by acetylating a serine residue near catalytic center[ The X!ray
crystal structure of COX clearly demonstrated the mechanism of action of nonsteroidal anti!
in~ammatory drugs[07 The discovery of thromboxane "TX# and prostacyclin "PGI # from platelet
1
and blood vessel endothelial cells is another achievement in prostaglandin studies[ Thromboxane
A actstoinduceplateletcoagulation\whileprostacyclininhibitsplateletcoagulation[Theopposite
1
activitiesofTXandPGI answerthesimplequestionastowhyblooddoesnotcoagulateinblood
1
vessels[TXandPGI aresynthesizedbythecorrespondingsynthasefromPGH producedbyCOX
1 1
in platelets and endothelial cells[ The sensitivities of COX are di}erent in both cells and platelet
COX is more sensitive to aspirin[ Moreover\ aspirin inhibits COX irreversibly and TX is not
synthesized de novo in platelets since platelets have no nucleus\ whereas in endothelial cells COX
can be synthesized leading to the formation of PGI [ This selectivity led to the administration of
1
low!doseaspirintopatientswhosu}erfrommyocardialinfarctiontopreventplateletcoagulation
whichtriggersthrombosis[MorerecentlyinducibleCOXwasfoundinvariouscellsandwasfound
toberesponsibleforin~ammation[ThenewCOXwasnamedasCOX1andthepreviouslyknown
COX as COX0[ COX0 is present in almost all tissues and plays important physiological roles[
Selective inhibition of COX1 is the target in the development of new antiin~ammatory drugs "see
Chapters0[96and0[98#[
Arachidonicacidalsoservesasaprecursorofleukotrienes"LT#\originallyfoundasslowreacting
substances of anaphylaxis "SRSA#[ After a long blank as with PG studies\ SRSA!like compounds
were detected in leukocytes and their structures _nally determined by total synthesis[ Since these
eiconsanoids were found in leukocytes\ they are called leukotrienes[ The _rst enzyme of PG\ TX\
andPGI synthesisisCOX\whilearachidonate4!lipoxygenaseisthe_rstenzymereactionleading
1
totheLTgroup[Metabolismfromarachidonateiscalledthearachidonatecascade\whichisnamed
after metabolic ~ow like a cascade falling with several separation streams "see Figures 6 and 7#[
StudiesonPGandLThaverecentlyfocusedontheirreceptors[Multipletypesofreceptors\speci_c
tovarioustissuesandcells\arenowthetargetsfordrugdevelopment[Themolecularevolutionof
arachidonate cascade enzymes and PG and LT receptors are summarized in Chapter 0[09[ The
structure of platelet activating factor "PAF# resembles those of phospholipids\ however the ether
linkage is unique to PAF[ Its biosynthesis looks simple\ but the reactions taking place in PAF
biosynthesisareinterestingandreasonablefromthepointofviewoforganicchemistry"seeChapter
0[00#[
0[90[2 POLYKETIDEBIOSYNTHESISINACTINOMYCETESANDFUNGI
Microorganisms produce natural products with diverse structures[ Following the discovery of
penicillin and streptomycin\ antibiotics have been used as medicines and tools to study biological
phenomena[Themoleculardiversityofmicrobialmetabolitesposedachallengetoorganicchemists
todeterminetheircomplexstructures"seeFigure8#[Theirstructuraldiversityandcomplexityhave
beenchallengingproblemsinbiosyntheticinvestigationsformanyyearsandvarioussophisticated
methodshavebeendevelopedduringinvestigationsontheirbiosynthesis[
0[90[2[0 TypeIIAromaticPolyketideSynthetasesinActinomycetes
Thewaveofprogressinmolecularbiologyoverwhelmedexistingmethodologiesandgenecloning
has become the most common and essential technique for the investigation of the biosynthesis of
polyketides[ The methodology of polyketide biosynthesis in Streptomycetes at the gene level has
been established by the great contribution of David Hopwood[ His longstanding study on the
geneticsofActinomycetesledtothefruitfuldevelopmentofmolecularbiologyinthe0879s[Cloning
and identi_cation of genes involved in the biosynthesis of actinorhodin had a tremendous impact
on the biosynthesis of polyketides "see Figure 09#[ Although actinorhodin itself is not used for
medicinalpurposes\itisthe_rstActinomycetespolyketidestudiedatthegenelevel[Theapproach
developedbyHopwoodisbasedonthepreparationofmutantsinwhichactinorhodinbiosynthesis
was blocked in Streptomyces coelicolar and a restoration of actinorhodin production by comp!
lementationwithshuttlevectorscontainingrandomfragmentsofgenesfromwildtypeS[coelicolar[