Biosynthesis - Aromatic Polyketides, Isoprenoids, Alkaloids - F. Leeper, J. Vederas (Springer, 2000) WW

242 Pages·2000·3.31 MB·English

Preface

The present book is the second of two volumes that provide state of the art
expert reviews of central topics in modern natural products chemistry and
secondary metabolism.Using specific examples,the previous volume emphasiz-
ed two revolutions in experimental techniques that completely transformed the
field of natural products chemistry from what it was in the 1950s.These were the
use of stable isotopes in conjunction with modern NMR and mass spectrometry,
and more recently, the development of molecular biological techniques to
identify, purify and manipulate the enzymes responsible for the intricate
series of steps to complex natural compounds.The previous volume specifically
covered the use of isotopes in biosynthetic research and the formation of
enzyme cofactors, vitamin B12 and reduced polyketides.
This second volume describes the application of the same approaches (isotope
methodology and molecular biology) to the biosynthesis of aromatic (unreduced)
polyketides,enzymes responsible for cyclization of terpenoids (isoprenoids),and
biochemical generation of selected classes of alkaloids (prenylated tryptophan,
tropane, pyrrolizidine). The knowledge of the metabolic pathways and the tech-
niques to elucidate them opens the door to combinatorial biosynthesis as well as
to the production of targeted pharmaceutical agents utilizing a combination of
chemistry, molecular biology and protein biochemistry. Recent advances suggest
that it may soon be possible to rationally manipulate biochemical pathways to
produce any target molecule, including non-natural variants, in substantial
quantity. Genetically modified organisms containing mix-and-match combina-
tions of biosynthetic enzymes (natural and/or mutated) will allow formation of
large numbers of new compounds for biological evaluation.In addition,the avail-
ability of vast arrays of specialized enzymes in pure form may provide new
reagents for combinatorial chemistry and parallel synthesis in drug discovery
programs.
In the current volume, Ben Shen begins with a review of the assembly of un-
reduced polyketides leading to aromatic compounds. The current understand-
ing of the functions and interactions of the enzymes involved is gradually pro-
viding the rules for designing new compounds of this class as well as affording
a basis for biosynthesis of flavonoids via chalcone synthases. In chapter 2,
Edward Davis and Rodney Croteau describe the terpenoid synthases responsible
for formation of the huge array of mono-, sesqui- and diterpenes (over 30,000
terpene derivatives are known).In particular,detailed structural and functional
evaluation of four representative terpene synthases is provided. In the third

chapter, Robert Williams, Emily Stocking and Juan Sanz-Cervera review the bio-
synthesis of prenylated indole alkaloids and related substances derived from
tryptophan. Many of these compounds are potent mycotoxins that contaminate
food, but some, such as the ergot alkaloids (e.g. ergotamine, ergonovine) see
extensive application in medicine. In chapter 4 Thomas Hemscheidt describes
the current state of knowledge on the biosynthesis of tropane and related alka-
loids, including cocaine. This well illustrates the difficulties that can be faced in
elucidating the sequence of reactions involved in a biosynthetic pathway,
especially when the intermediates are unstable and produced in very low
amounts. The last chapter, by Thomas Hartmann covers the pyrrolizidine alka-
loids. It not only describes the chemistry of the biosynthetic pathway but also
gives an account of the physiology and ecology involved in the distribution and
elaboration of the alkaloids within the producing plant, in the insects which eat
the plants, and even in the animals which eat the insects. This shows us that for
many natural products an understanding of how they are made is only a part of
the whole story.

Cambridge, January 2000
Finian J. Leeper
John C.Vederas

VIII
Preface

Aromatic polyketides differ from other polyketides by their characteristic polycyclic aromatic
structures. These polyketides are widely distributed in bacteria, fungi, and plants, and
many of them are clinically valuable agents or exhibit other fascinating biological activities.
Analogous to fatty acids and reduced polyketide biosynthesis,aromatic polyketide biosynthe-
sis is accomplished by the polyketide synthases that catalyze sequential decarboxylative con-
densation between the starter and extender units to yield a linear poly-b-ketone intermediate.
The latter undergoes regiospecific reduction,aromatization,or cyclization to furnish the poly-
cyclic aromatic structures, which are further modified by tailoring enzymes to imbue them
with various biological activities. This review begins with a brief discussion on the architec-
tural organizations among various polyketide synthase genes and genetic contributions to
understanding polyketide synthases. It then presents a comprehensive account of the most
recent advances in the biochemistry and enzymology of bacterial, fungal, and plant polyke-
tide synthases,with emphasis on in vitro studies.It concludes with a cautious summary of the
so-called design-rules to guide rational engineering of polyketide synthases for the synthesis
of novel aromatic polyketides.

Keywords. Aromatic polyketides, Bacterial polyketide synthase, Engineered biosynthesis,
Fungal polyketide synthase, Plant polyketide synthase

1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3

2
Aromatic Polyketide Synthase Genes . . . . . . . . . . . . . . . . .
8

2.1
Bacterial Polyketide Synthase . . . . . . . . . . . . . . . . . . . . . .
8
2.2
Fungal Polyketide Synthase . . . . . . . . . . . . . . . . . . . . . . .
9
2.3
Plant Polyketide Synthase . . . . . . . . . . . . . . . . . . . . . . . .
10

3
Polyketide Synthase Biochemistry and Enzymology . . . . . . . . .
11

3.1
Bacterial Type II Polyketide Synthase . . . . . . . . . . . . . . . . .
11
3.1.1 Phosphopantetheinyl Transferase . . . . . . . . . . . . . . . . . . . .
12
3.1.2 Acyl Carrier Protein . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
3.1.3 Malonyl CoA:Acyl Carrier Protein Transacylase . . . . . . . . . . .
16
3.1.4 b-Ketoacyl Synthase . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
3.1.5 Polyketide Aromatase and Cyclase . . . . . . . . . . . . . . . . . . .
21
3.1.6 In Vitro Reconstitution of Type II Polyketide Synthase . . . . . . . .
23
3.2
Fungal Polyketide Synthase . . . . . . . . . . . . . . . . . . . . . . .
28

Biosynthesis of Aromatic Polyketides

Ben Shen

Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616,
USA. E-mail: