Polyketides are a diverse group of secondary metabolites produced through polyketide biosynthesis pathways that are similar to fatty acid synthesis. Polyketide pathways involve the polymerization of acetyl-CoA units to form linear polyketide chains that then undergo cyclization and other modifications to form a variety of aromatic and macrocyclic compounds with antibiotic and other bioactivities. These pathways are carried out by polyketide synthase enzyme complexes that assemble polyketide chains in an ordered fashion through the coordinated activities of ketosynthase, acyltransferase, and other functional domains.

1. 5. Polyketides
RA Macahig
FM Dayrit

2. 5. Polyketides (Dayrit) 2
• Polyketides rank among the largest group of secondary
metabolites in terms of diversity of structure and biological
diversity.
• Polyketide biosynthesis shares some similarities with the
initial steps of fatty acid acetyl polymerization. Like the fats,
the polyketide pathway probably arose early in biological
evolution before the rise of plants.
Introduction
• Polyketides (which literally means “many ketone groups”)
make up a diverse biogenetic group which starts from acetyl-
CoA to form a linear chain without extensive reduction. The
polyketide chain can cyclize to form aromatic rings or undergo
extensive derivatization.

3. 5. Polyketides (Dayrit) 3
Examples of polyketide natural products which illustrate
the wide variety of structures which comprise this group.
CH3
CO2H
OH
HO
orsellinic acid
O
HO
O
OH
OH
alternariol
from the mould Alternaria tenius
O
H3C
O
O
OCH3
CH3O
Cl
OCH3
griseofulvin
from Penicillium griseofulvum
O O
O
O
O
OCH3
aflatoxin B1
from Apergillus species
O
O
OH
CH3
OR
CH3
OR
CH3
O
H3C
HO
H3C CH3
macrolide antibiotic
erythromycin-type
from Streptomyces species

4. 5. Polyketides (Dayrit) 4
• The polyketides have great diversity of structures and
chemical functionalities. These structures range from
saturated macrocyclic lactones (macrolides), which are
unique polyketide metabolites, to various types of aromatic
compounds.
Introduction
• Polyketides occur widely in bacteria, fungi and lichens, but
are of relatively minor occurrence in higher plants.
Bacteria, in particular Actinomycetes and Cyanobacteria,
are prolific sources of polyketides, many of which possess
antibiotic activity. Other significant polyketide producers
are Aspergillus (aflatoxins) and Penicillium and
Streptomyces species (tetracycline antibiotics).

5. 5. Polyketides (Dayrit) 5
Polyketides are produced from poly-acetyl intermediates
(poly-1,3-diketo compounds) which do not undergo complete
reduction, as in the case of the fats. The polyketides then
branch into two major pathways:
Overview of polyketide biosynthesis
1. Aromatic compounds. The reactive 1,3-diketo groups
undergo intramolecular Claisen or lactonization reactions
forming cyclic compounds. Dehydration produces
aromatic compounds.
2. Macrolides. The keto- groups are reduced to alcohols,
which are subsequently dehydrated to form linear
compounds. The final products are macrocyclic esters.
Macrolides generally >12 carbon atoms in the ring.

6. 5. Polyketides (Dayrit) 6
S-CoA
C
CH2
C
CH2
C
CH2
C
H3C
O
O
O
O
a a
b b
c c
d
d
a
8
7
6 5
4
3 2 1
Claisen
6 1
b
Aldol
2 7
c
d
C
CH3
O
OH
HO
OH
xanthoxylin
(a phloroglucinol)
CH3
CO2H
OH
HO
(8)
(1)
(8)
(1)
orsellinic acid
(a resorcinol)
O
HO O
CH2
C
CH3
O
(an -pyrone)
O
O
H3C CH2
CO2H
(1)
(1)
(8)
(8)
(a -pyrone)
O
O
O
S-CoA
O
S-CoA
O
O
O
O
O
O
S-CoA
O
O O S-CoA
O
O
O
Aromatic
polyketides. Major
cyclization pathways
for a tetraketide
followed by
aromatization.

7. 5. Polyketides (Dayrit) 7
Biosynthetic studies on polyketides (Arthur Birch)
4 x
H3C
C
O
O
_
*
*
#
ratio:
14C # 1
--------- = ----
18O * 2
S-CoA
O O O O
* * * *
# # # #
# 1
------ = ----
* 1
O O
OH
S-CoA
O
* *
*
*
#
#
#
#
-H O*
2
#
#
#
#
*
*
*
HO OH
CH3
O
OH
# 4
------ = ----
* 3
orsellinic acid
The elucidation of the
polyketide pathway was
pioneered by Arthur
Birch in 1953. Birch
used 14C and 18O-labeled
acetate which he fed to
microorganisms to
establish the
incorporation pattern
and from this to
postulate the steps in the
biosynthesis of
polyketides.

8. 5. Polyketides (Dayrit) 8
Birch proposal for polyketide biosynthesis:
Overview of polyketide biosynthesis
1. Starting with a starter unit, C2 units are added to form the
polyketide chain (chain assembly).
2. Reduction and/or alkylation of the polyketide chain before
cyclization.
3. Intra- or intermolecular cyclization. (The more common
pathway is intramolecular cyclization.)
4. Secondary processes which modify the intermediate
product after cyclization, such as: halogenation, O-
methylation, C-methylation, reduction, oxidation,
decarboxylation and skeletal rearrangement.

9. 5. Polyketides (Dayrit) 9
Variations in number of C2 units and mode of cyclization
Starting polyketide  Secondary metabolite
O
HO
O
triketide:
o
o
o
tetraketides:
o
o
o
o
o
o
o o
CH3
CO2H
OH
HO
orsellinic acid
(1)
xanthoxylin
(1)
OH
CH3
O
OH
HO
Short-hand
representation of
polyketides:
CH3
S-CoA
O
O
O O
a tetraketide
o
o o
=
o

10. 5. Polyketides (Dayrit) 10
Starting polyketide  Secondary metabolite
OH
CH3
O
HO
CO2H
(1)
curvulinic acid
(Curvularia siddiqui)
(1) O
O
CH3
HO
OH
o
o
o o
o
o o
o
pentaketides:
o
o
OH O
O
HO CH3
(1)
o
o
o
o
o
Variations in number of C2 units and mode of cyclization
hexaketide:
o
o
o
o
o o
O OH
CH3
CH3O
OH O
diaporthin
(Endothia parasitica)

11. 5. Polyketides (Dayrit) 11
Starting polyketide  Secondary metabolite
monocerin
(Helminthosporium
monoceras)
o
o
o o
o o
heptaketide:
o
O
CH3O
OH O
O
CH3O
o
o
o o o
o
o
griseofulvin
(Penicillium
griseofulvum)
O
H3C
O
O
OCH3
CH3O
Cl
OCH3
o
o
o
o
o
o
o
O
CH3
OH
O
HO
HO
alternariol
(Alternaria tenius)
Variations in number of C2 units and mode of cyclization

12. 5. Polyketides (Dayrit) 12
Starting polyketide  Secondary metabolite
octaketide:
o
o
o
o
o o
CH3
HO
OH O
O
CO2H
endocrocin
(Centralia endocrocea)
o o
o
o
o
o
o o
o
o
O
HO
HO
O
O
curvularin
(Curvularia spp.))
Variations in number of C2 units and mode of cyclization
nonaketide:
o
o
o
o
o o
O
O
HO
Cl
O
CH3
O
HO
radiciciol
(Nectaria radiciola)
o o
o
o
o
o
o
o
o o o o
OH
CH3
O
O
CH3O
HO OH
nalgiovensin
(Penicillium
nalgiovensis)

13. 5. Polyketides (Dayrit) 13
Inter- vs.
intramolecular
cyclization:
A. Colletodiol;
B. Use of
labeling
experiments to
distinguish
intra- from
intermolecular
cyclization.
o o o
o o o o
O O
O
O
OH
OH
colletodiol
A. Example of intermolecular cyclization.
B. Use of labeling experiment to distinguish inter- vs. intramolecular cyclization.
o
o
o
o
o
o
o
o o o o
o
o
o
o
[Me*]
OH OH O
*
-CO2
2
-2CO

14. 5. Polyketides (Dayrit) 14
(from: The World of Polyketides, http://linux1.nii.res.in/)
Biosynthesis of
macrolides:
Step-wise
chemical
transformations
and enzymes.

15. 5. Polyketides (Dayrit) 15
Hypothetical
scheme of the
biosynthesis of
phenol
polyketides
on the
Polyketide
Synthase
(PKS)
multienzyme
complex.
multi-enzyme complex
HS HS HS HS HS H3C S-CoA
O
S
O
HS
HS
HS
HS
S-CoA
O
O2C
_
4 x
S
O2C
O
S
O
_
_
S
O2C
O
_
S
O2C
O
_
_
S
O2C
O
S
O2C
O
_
_
S
O2C
O
_
S
O2C
O
_
S
O
o
HS
*
*
*
HS
HS
HS
HS
S
O
O
O
*
O
o
S
O
O
O
O
O
_
base
_
S
O
O
O
OH
O
HO
OH
CO2H
O
*
*
*

16. 5. Polyketides (Dayrit) 16
Polyketide synthase (PKS)
The PKS family share a number of characteristics with the
family of fatty acid synthases (FAS): the PKS is a
multienzyme complex which is arranged so that the stepwise
transformations are carried out sequentially.
Hypothetical model for one type of PKS multienzyme system
which produces 6-methylsalicylic acid and lovastatin. The growing
chain is assembled on two multienzyme complexes.
• ACP: acyl carrying protein
• KS: b-keto acyl synthase
• MAT: malonyl (acyl) transferase
• DH: dehydratase
• ER: enoyl reductase
• KR: keto reductase
• TE: thiol esterase
(from: The World of Polyketides, http://linux1.nii.res.in/)
H3C
O
O
CH3 CH3
O
O OH
Lovastatin
CO2H
H3C OH
6-Methylsalicylic acid

17. The biosynthetic pathway for the
fungal polyketide 6-methylsalicylic
acid (6-MSA). 6-MSA is assembled
from four ketide units (one acetate and
three malonates). 6-MSAS contains the
following domains (in order): KS, MAT,
DH, KR and ACP. These act repeatedly
to catalyse three rounds of chain
extension, carrying out different levels
of reductive processing at each stage.
The first condensation is followed by
reaction with a second equivalent of
malonate extender unit, while the second
condensation is followed by reduction
and dehydration of the newly-formed
keto group. After the third cycle, the
chain undergoes cyclisation, dehydration
and enolisation. The absence of a
thioesterase domain suggests that release
of the chain from the PKS does not
occur by hydrolysis but by an alternative
mechanism which is still not verified.
(Staunton and Weismann, Nat. Prod. Rep., 2001, 18,
380–416)
KS: ketosynthase
MAT: malonyl-acetyl transferase
DH: dehydratase
KR: ketoreductase
ACP: acyl carrier protein

18. 5. Polyketides (Dayrit) 18
Biosynthesis of macrolides on a modular Polyketide Synthase
(PKS) multienzyme complex.
(from: The World of Polyketides, http://linux1.nii.res.in/)

19. 5. Polyketides (Dayrit) 19
Domain organization of the
erythromycin polyketide synthase.
Putative domains are represented
as circles. Each module
incorporates the essential KS, AT
and ACP domains, while all but
one include optional reductive
activities (KR, DH, ER).
The one-to-one correspondence
between domains and biosynthetic
transformations explains how
programming is achieved in this
modular PKS. (Staunton and
Weismann, Nat. Prod. Rep., 2001, 18, 380–
416)

20. 5. Polyketides (Dayrit) 20
Predicted domain organization of the 6-deoxyerythronolide B synthase (DEBS) proteins.
KR indicates the inactive ketoreductase domain. The ruler shows the residue number
within the primary structure of the constituent proteins. The linker regions are also given in
proportion. (Staunton and Weismann, Nat. Prod. Rep., 2001, 18, 380–416)
KS: ketosynthase
AT: acyltransferase
DH: dehydratase
ER: enoyl reductase
KR: ketoreductase
ACP: acyl carrier protein
TE: thioesterase

21. 5. Polyketides (Dayrit) 21
Inactivation of KR5 of DEBS results in the production of erythromycin analogues with
keto groups at the C-5 position. (Staunton and Weismann, Nat. Prod. Rep., 2001, 18, 380–416)
KS: ketosynthase
AT: acyltransferase
DH: dehydratase
ER: enoyl reductase
KR: ketoreductase
ACP: acyl carrier protein
TE: thioesterase

22. 5. Polyketides (Dayrit) 22
Inactivation of ER4 results in an analogue of erythromycin with a double bond at the
expected site. (Staunton and Weismann, Nat. Prod. Rep., 2001, 18, 380–416)
KS: ketosynthase AT: acyltransferase DH: dehydratase
ER: enoyl reductase KR: ketoreductase ACP: acyl carrier protein
TE: thioesterase

23. 5. Polyketides (Dayrit) 23
Domain organization of the rapamycin polyketide synthase
(RAPS). As with the erythromycin PKS there is a co-linearity
between the sequence of modules and the order of biosynthetic
steps. (Staunton and Weismann, Nat. Prod. Rep., 2001, 18, 380–416)

24. 5. Polyketides (Dayrit) 24
What is the link between FAS and PKS?
The PKS system is likely derived from bacterial FAS. Different
PKS pathways in bacteria illustrate the selective evolutionary
advantage that multiple secondary metabolite biosyntheses
confer to individual bacteria and taxonomic kingdoms.
KS: ketoacyl synthase
AT: acyl transferase
DH: dehydratase
ER: enoyl reductase
ACP: acyl carrying protein
Organization of fatty acid synthases (FAS) and polyketide
synthases (PKS). (Jenke-Kodama et al. J Mol Bio Evol 2005)

25. 5. Polyketides (Dayrit) 25
What is the link between FAS and PKS?
Enzymes in a PKS module.
(Jenke-Kodama et al. J Mol Bio
Evol 2005)
Common sequence of reactions performed by FAS and PKS.
KS: ketoacyl synthase
ACP: acyl carrying protein
KR: ketoreductase
DH: dehydratase
ER: enoyl reductase

26. 5. Polyketides (Dayrit) 26
Four proteins comprise the minimal PKS: ketosynthase (KS),
chain length factor (CLF), acyl carrier protein (ACP), and a
malonyl-CoA:ACP transacylase (MAT) which is usually recruited
from fatty acid synthases. Other common enzymes include:
aromatase (ARO) and cyclase (CYC). (Ridley et al., PNAS, 2008,
105:4595-4600)
-
O2C
S-CoA
O
starter unit
min PKS
R
O
O
O
O
O O O
SACP
O
KS-CLF
R
O
O
O
O
O O
SACP
O
OH
15
9
C-9 KR
9
R
O
O
O
HO
O O
SACP
O
OH
ARO
R
O
O
OH
O O
SACP
O
CYC
R
O
O
OH
O
S-ACP
O
5
common aromatic intermediate principal common intermediate
with varying R group
Common enzymes in aromatic polyketides

27. 5. Polyketides (Dayrit) 27
“Deciphering the mechanism for the assembly of aromatic
polyketides by a bacterial polyketide synthase,” Shen and Hutchinson, Proc. Natl.
Acad. Sci. USA, 93, 6600-6604, June 1996.
Acyl-CoA
+
9 Mal-CoA
TcmJKLM
CH3
O
O
O
O
O
O O O
O
SCoA
O
TcmN
unidentified products
aberrant
cy clization
CH3
CO2H
O
OH
OH
OH
OH
HO
TcmF2
TcmF1
CH3
CO2H
OH
OH
O
OH
HO
TcmI
TcmH
CH3
CO2H
OH
OH
O
OH
HO
O
TcmD3
TcmB3
CH3
CO2H
OH
OH
O
OH
CH3O
O
TcmN
Tetracenomycin PKS J K L M N
7 kb
0
The optimal Tcm PKS is a complex consisting of the TcmJKLMN proteins. It is
the integrity of this complex that maximizes the efficiency for the synthesis of
aromatic polyketides from acetyl- and malonyl-CoA.

28. 5. Polyketides (Dayrit) 28
Polyketide modifications: before cyclization and after cyclization (secondary processes). Note:
F: fungi; P: plant refers to the biological system where the process has been studied. The number of 
marks denote frequency of occurrence;  denotes not observed.
Modification Before cyclization After cyclization
(Secondary process)
1. reduction  (F) ?
2. oxidation   (F,P)
3. C-methyation  (F) 
4. O-methylation   (F,P)
5. C-prenylation  (F,P)  (F,P)
6. O-prenylation   (P)
7. C-glycosylation   (P)
8. O-glycosylation   (P)
9. decarboxylation  
10. aromatic radical coupling  
• The various Kingdoms exhibit different characteristics of their PKS
enzymes. In the microbial kingdom, at least three types of PKS
enzymes have been recognized.

29. 5. Polyketides (Dayrit) 29
Reduction and alkylation of the polyketide chain before
cyclization. The polyketide can be reduced to the alcohol
and be subsequently dehydrated to produce the double bond.
The resulting aromatic ring will not have a OH substituent in
the particular position.
S-CoA
O
+ 2 x
S-CoA
O
O2C
_
S-CoA
O O O
1. NADPH
2. -H O
2
S-CoA
O
O
_
S-CoA
O
O2C
O
O
S-CoA
O
o
o
o
CH3
CO2H
OH
6-methylsalicylic acid
from Penicillium urticae

30. 5. Polyketides (Dayrit) 30
Reduction and alkylation of the polyketide chain before
cyclization. The polyketide can be C-alkylated (e.g., with
methyl or isopentyl groups) prior to cyclization although it
may be difficult to determine whether C-alkylation is carried
out before or after cyclization.
o
o
o o
[CH ]
3
3
[CH ]
CH3
OH
H3C
HO
CH3
O
clavatol
CH3
OH
HO
O
CH3
OH
H3C
HO
O

31. 5. Polyketides (Dayrit) 31
Secondary processes: examples of oxidation, decarboxylation and methylation.
6-methylsalicylic acid
CH3
CO2H
OH
[O]
CHO
CO2H
OH
-CO2
CHO
OH
CO2H
OH
HO
[O]
gentisic acid
A. Gentisic acid
B. Fumigatin
fumigatin
[CH ]
CH3
OH
OCH3
HO
HO
2
-CO
CH3
OH
HO
1. [O]
2.
CH3
CO2H
OH
HO
orsellinic acid
3 [O]
CH3
O
OCH3
HO
O

32. 5. Polyketides (Dayrit) 32
Erythromycin, first
isolated from
Streptomyces
erythreus from soil
samples from Iloilo
sent by Abelardo
Aguilar in 1949. It
was first marketed
by Eli Lilly as
Ilosone®.
R.B.Woodward
accomplished its
stereospecific
synthesis in 1981.
It is used for the
treatment of gram-
positive bacterial
infections.
S-CoA
O
S-CoA
CO2H
O
*
*
starter unit
+ 6
o
o
o
o
o
o
o
1
3
5
7
9
11
13
O
O
O
OH
HO
OR3
OR2
OR1
*
1 3
5
7
9
11
13
Erythromycin R1 R2 R3
A OH 1 2
B H 1 2
C OH 1 3
1 : D-desosamine:
2 : L-cladinosine:
3 : L-mycarose:
O
CH3
N(CH3)2
OH
O
HO
CH3
CH3
CH3
O
HO
CH3
CH3
OH

33. 5. Polyketides (Dayrit) 33

34. 5. Polyketides (Dayrit) 34
Intramolecular aromatic radical coupling: biosynthesis of griseofulvin (from a fungus, Penicillium
griseofulvum) involves extensive secondary modification of a heptaketide.
griseofulvin
O
OH
CH3O O
OCH3
O
CH3
Cl
+2 [H]
dehydrogriseofulvin
O
OH
CH3O O
OCH3
O
CH3
Cl
O
OH
CH3
O O
OCH3
O
CH3
Cl
.
.
.
.
O
OH
CH3
O O
OCH3
O
CH3
Cl
[O]
3
+[CH ]
griseophenone A
O
OCH3
CH3
O OH
OCH3
OH
CH3
Cl
+[Cl],
-[H]
O
OH
CH3
O OH
OCH3
OH
CH3
Cl
griseophenone B griseophenone C
O
OH
CH3O OH
OCH3
OH
CH3
3
+2 [CH ]
O
OH
HO OH
OH
OH
CH3
o o
o
o
o
o o

35. 5. Polyketides (Dayrit) 35
Nature of starting unit
Fatty acid synthase (FAS)
H3C
C
SCoA
O
Acetyl CoA
Malonyl CoA
CH2
C
SCoA
O
C
HO
O
CH
C
SCoA
O
C
HO
O
CH3
Methylmalonyl CoA
Polyketide synthase (PKS)
Isobutyryl CoA
SCoA
O
H3C
O
Acetoacetyl CoA
Acetyl CoA
H3C
C
SCoA
O
Hexanoyl CoA, R=C 5H11
Octanoyl CoA, R=C 7H15
OH
O
OH
O
Propionyl CoA
Butyryl CoA
OH
O
SCoA
O
Benzoyl CoA
R1 R2
Cinnamoyl CoA H H
p-Coumaroyl CoA H OH
Caffeoyl CoA OH OH
Feruloyl CoA OH OMe
SCoA
O
R1
R2
N-Methylanthranyloyl CoA
SCoA
O
MeHN
R OH
O
Acetamidoacetyl CoA
SCoA
O
H2N
O

36. 5. Polyketides (Dayrit) 36
Nature of starting unit
Examples of metabolites where the starting unit is not acetyl-CoA. In the case of tetracycline, extensive
secondary processes take place.
o
o o o o
o
o
o
o
o
HO
CO2H
HO OH O
O OH
OH
7S, 9R, 10R--pyrramycinine
CONH2
o
o
o
o
o o o o
o
Cl
OH O OH O
CONH2
OH
OH
H3C
OH
H
N(CH3)2
tetracycline

37. 5. Polyketides (Dayrit) 37
The polyketide metabolites can be classified into five groups:
1. Phenols
2. Quinones
3. Aflatoxins
4. Tetracyclines
5. Macrolide antibiotics
Metabolites from polyketides
1. Phenols
Cyclization and aromatization of polyketides form phenols as
the initial product. In plants however, phenols are also
formed from the shikimate pathway. Therefore, phenols and
their methylated derivatives are common natural products.
Some common phenols are formed via different pathways.
Aromatic compounds

38. 5. Polyketides (Dayrit) 38
2. Quinones
Quinones often occur as the final product from a series of
oxidation reactions on mono- or polycyclic aromatic ring
systems.
The biosynthetic pathway differs in microorganisms and
plants. In microorganisms, quinones arise predominatly via
the polyketide pathway. In plants, however, quinones can
arise via the polyketide or shikimate pathways and
sometimes via the mixed biosynthetic route involving the
ring-formation of an added terpenoid unit. The presence of
multiple pathways to the quinone ring system may reflect the
importance of this type of functionality.
Metabolites from polyketides

39. 39
Overview of biosynthesis
of quinones. Depending
on the organism,
quinones can arise via
the polyketide or
shikimate pathways.
In microorganisms:
[O]
polyketide aromatic compound quinone
In plants:
[O]
polyketide aromatic compound quinone
shikimate aromatic compound
+ terpene
[O]
quinone
(mixed metabolite)
quinone
OH
CO2H
OH
OH
OH
O
O
OH
H
quinones from shikimate + terpene:
quinone from shikimate:
homogentisic acid alkarinin
R
O
O
H
n
ubiquinones: R = H, CH ; n = 4-13
3
• Aromatic metabolites in
microorganisms are likely
to be formed via the
polyketide pathway while
aromatic compounds in
plants are likely to come
from the shikimate
pathway.

40. 5. Polyketides (Dayrit) 40
1,4-Benzoquinone
1,4-Benzoquinone itself is the simplest member of this
group. However, because it is toxic, it is not found in this
form but rather as a protected precursor, such as arbutin, a
glycosylated 1,4-hydroquinone, the reduced form of 1,4-
benzoquinone.
Metabolites from polyketides
O-Glu
OH
Arbutin
Arbutin occurs in the leaves of various
plant species and may be a plant
defense compound. The ability to
detoxify phenols or to store them as
glycosides appears to be a common
characteristic of plants.

41. 5. Polyketides (Dayrit) 41

42. 5. Polyketides (Dayrit) 42
Para-quinone is a toxic
compound which
various organisms use.
A. Various trees secrete
a precursor (arbutin) to
“clear” its surroundings
of competing plants;
B. The bombardier
beetle produces para-
quinone in its collecting
bladder from para-
hydroquinone + H2O2.
A. Plants store precursors of para-quinone in various glycosylated forms.
O-Glucose
OH
arbutin
O-Glu-O-Glu
OH
O-Glu-O-Glu-O-Glu
OH
O
O
para-quinone
(toxic)
B. Para-quinone as a defensive secretion of the bombardier beetle.
lobe
O
O
+ H O
2 2
collecting bladder
explosion
chamber with
enzyme gland
OH
OH
+ H O + heat
2

43. 5. Polyketides (Dayrit) 43

44. 5. Polyketides (Dayrit) 44
Aflatoxins
• The aflatoxins are a group of fungal metabolites which have
closely similar chemical structures, the most evident feature
being two fused furan rings.
• Aflatoxins were first discovered following investigations into
the deaths of turkeys after being being fed mouldy peanuts.
Metabolites from polyketides
O O
O
O
O
OCH3
aflatoxin B1
from Apergillus species

45. 5. Polyketides (Dayrit) 45
Aflatoxins
• Aflatoxins are among the most toxic naturally-occuring
compounds known. They are potent hepatocarcinogens and
cause lesions in the mammalian liver. They are toxic to rats
down to a dose level of 1 g/day.
• Various strains of Aspergillus produce aflatoxins, in
particular, A. parasiticus, A. versicolor and A. flavus.
Aspergillus fungi are usually encountered growing on various
types of organic matter, especially in damp places. They cause
the decay of many stored fruits and vegetables, bread, leather
goods and various fabrics.
• Aflatoxins are one of the major causes of concern in our
copra industry. The European Commission limit is currently set
at 5 ppb.
Metabolites from polyketides

46. 5. Polyketides (Dayrit) 46
o
o
o
o o o o
o o o
decaketide
[O]
HO
OH
O
O OH
OH
O O O
+2[H] +2[H], -H O, +2[H]
2
HO
OH
O
O OH
OH
OH
O
H
+
HO
OH
O
O OH
O
OH
HO
HO
OH
O
O OH
O
O
-H O
2
averufin [O]
[O] HO
OH
O
O OH
OH
OH
O - H
OH
O
-H O
2
O - H
O
OH
OH
O
O
OH
HO
CHO
-C2
OH
OH
O
O
OH
HO
CHO CHO
OH
O
O
OH
HO O O
versicolorin A
[O], Bayer-Villiger
versicolorin B
OH
O
O
OH
HO O O
Aflatoxins
make up a
family of
polyketide
metabolites.
The very
complex
biosynthesis
of aflatoxins
was
elucidated
by George
Büchi.

47. 5. Polyketides (Dayrit) 47
versicolorin A
OH
O
O
OH
HO O O [O]
Bayer-Villiger
OH
O
OH
HO O O
CO2H HO
+2[H]
+2[H],
-CO2
OH
O
OH
HO O
O
OH
O
OH
O O
O
H
H
sterigmatocystin
OH
O
OH
O O
O
H
H
O
[O]
OH
O
OH
O O
O
H
H
O
[O]
[O]
OH
O
O
HO2C
O O
O
H
H
O
OH
O
O O
O
H
H
O
CO2H
O
_
OH
OH
O O
O
H
H
O
CO2H
O
[CH ]
3
-CO ,
+[CH ],
-H O
2
2
3
OCH3
O O
O
H
H
O
O
aflatoxin B1

48. 5. Polyketides (Dayrit) 48
o
o
o
o o o o
o o o
decaketide
[O]
HO
OH
O
O OH
OH
O O O
+2[H] +2[H], -H O, +2[H]
2
HO
OH
O
O OH
OH
OH
O
H
+
HO
OH
O
O OH
O
OH
HO
HO
OH
O
O OH
O
O
-H O
2
averufin [O]
[O] HO
OH
O
O OH
OH
OH
O - H
OH
O
-H O
2
O - H
O
OH
O
HO
-C2
OH
O
HO
CHO CHO

49. 5. Polyketides (Dayrit) 49
OH O OH OH
OH O OH OH
HO
OH
O
O OH
O
O
-H O
2
averufin [O]
[O] HO
OH
O
O OH
OH
OH
O - H
OH
O
-H O
2
O - H
O
OH
OH
O
O
OH
HO
CHO
-C2
OH
OH
O
O
OH
HO
CHO CHO
OH
O
O
OH
HO O O
versicolorin A
[O], Bayer-Villiger
versicolorin B
OH
O
O
OH
HO O O

50. 5. Polyketides (Dayrit) 50
versicolorin A
OH
O
O
OH
HO O O [O]
Bayer-Villiger
OH
O
OH
HO O O
CO2H HO
+2[H]
+2[H],
-CO2
OH
O
OH
HO O
O
OH
O
OH
O O
O
H
H
sterigmatocystin
OH
O
OH
O O
O
H
H
O
[O]
OH
O
OH
O O
O
H
H
O
[O]
[O]
OH
O
O
HO2C
O O
O
H
H
O

51. 5. Polyketides (Dayrit) 51
OH
O
OH
HO O
O
OH
O
OH
O O
O
H
H
sterigmatocystin
OH
O
OH
O O
O
H
H
O
[O]
OH
O
OH
O O
O
H
H
O
[O]
[O]
OH
O
O
HO2C
O O
O
H
H
O
OH
O
O O
O
H
H
O
CO2H
O
_
OH
OH
O O
O
H
H
O
CO2H
O
[CH ]
3
-CO ,
+[CH ],
-H O
2
2
3
OCH3
O O
O
H
H
O
O
aflatoxin B1

52. 5. Polyketides (Dayrit) 52
Biosynthesis of
tetracyclines from
Streptomyces
species.
R=H : tetracycline
R=OH : terramycin
OH O OH O
CONH2
OH
OH
H3C
OH
H
N(CH3)2
R
o
o
o
o
o
o o o o
+2[H] [CH ] [O]
CONH2
3
NH2
OH
HO
CH3
HO OH OH O
OH
[O]
NH2
OH
HO
CH3
HO OH O O
O
NH2
OH
HO
CH3
HO O O O
O
H H
NH2
OH
HO
CH3
HO O O O
O
H
OH
+H O
2
NH2
OH
HO
CH3
HO O O O
O
H
OH
+2[H]
NH2
OH
HO
CH3
HO O O O
H
OH
OH
+[NH ],
+2[CH ]
2
3
NH2
OH
HO
CH3
HO O O O
H
OH
N(CH3)2
A B C D
NH2
OH
HO
CH3
HO O O O
H
OH
N(CH3)2
Cl
D
C
B
A
Cl
OH O OH O
CONH2
OH
OH
H3C
OH
H
N(CH3)2
aureomycin
[Cl]

53. 5. Polyketides (Dayrit) 53
o
o
o
o
o
o o o o
+2[H] [CH ] [O]
CONH2
3
NH2
OH
HO
CH3
HO OH OH O
OH
[O]
NH2
OH
HO
CH3
HO OH O O
O
NH2
OH
HO
CH3
HO O O O
O
H H
NH2
OH
HO
CH3
HO O O O
O
H
OH
+H O
2
NH2
OH
HO
CH3
HO O O O
O
H
OH
+2[H]
NH2
OH
HO
CH3
HO O O O
H
OH
OH
+[NH ],
+2[CH ]
2
3
NH2
OH
HO
CH3
HO O O O
H
OH
N(CH3)2
OH
CH3
H
N(CH3)2
Cl
[Cl]
Biosynthesis of
tetracyclines
from
Streptomyces
species.

54. 5. Polyketides (Dayrit) 54
R=H : tetracycline
R=OH : terramycin
OH O OH O
CONH2
OH
OH
H3C
OH
H
N(CH3)2
R
HO HO OH O O
HO HO O O O
NH2
OH
HO
CH3
HO O O O
O
H
OH
+H O
2
NH2
OH
HO
CH3
HO O O O
O
H
OH
+2[H]
NH2
OH
HO
CH3
HO O O O
H
OH
OH
+[NH ],
+2[CH ]
2
3
NH2
OH
HO
CH3
HO O O O
H
OH
N(CH3)2
A B C D
NH2
OH
HO
CH3
HO O O O
H
OH
N(CH3)2
Cl
D
C
B
A
Cl
OH O OH O
CONH2
OH
OH
H3C
OH
H
N(CH3)2
aureomycin
[Cl]

55. 5. Polyketides (Dayrit) 55
FAS and PKS probably share an evolutionary history. Like
the fats, polyketides also arise from polymerization of
acetyl CoA. The key features and steps are:
1. Alternative starter units are used, in particular in the
formation of tetracyclic antibiotics and macrocylic
lactones.
2. No reduction of the carbonyls, or reduction to alcohol
level only.
3. Cyclization via Claisen displacement or aldol reaction.
There are many modes of cyclization depending on the
chain length.
Summary

56. 5. Polyketides (Dayrit) 56
4. Aromatization often follows with loss of H2O.
5. wider range of compounds are produced: macrocyclic
lactones, phenols, quinones, and polycylic aromatic
compounds.
6. Polyketides are attractive research targets because of
their strong and varied biological activity, the modular
nature of the genetic system and polyketide synthases,
and relatively accessible biosynthetic expression
systems.
Summary