Chemistry Reference
In-Depth Information
of small carboxylic acids, such as acetate and malonate. As long ago as 1953,
Birch realized that polyketide biosynthesis is related to fatty acid biosynthesis
and some of the earliest applications of radioisotopes to natural product biosyn-
thesis were to fungal polyketide metabolites, where the ease of fermentation and
isolation of metabolites in pure form, and relatively efficient uptake of simple
labeled precursors facilitated the work. In more recent years, fungal metabolites
in general, and polyketides in particular, were the focus of the rapidly expanding
applications of stable isotope labeling in the 1970s and 1980s beginning with
incorporations of singly
13
C-labeled precursors with analysis of regiospecifity of
labeling being greatly facilitated by the contemporaneous development of Fourier
Transform methods and their application to
13
C NMR. The application of dou-
bly
13
C-labeled precursors led to the concept of bond labeling, which allowed
inter alia
the mode of cyclization of linear polyketide precursors into polycyclic
molecules, bond fragmentation, and rearrangements processes to be detected for
the first time through analysis of the resulting
13
C-
13
C coupling patterns. This
method was then rapidly followed by applications of isotope induced shifts in
13
C
NMR which allowed indirect detection of
2
Hand
18
O labels and the use of direct
2
H NMR. These new methods (2) allowed stereochemistry and regiochemistry
of labeling to be detected and in particular permissible levels of oxidation and
reduction in otherwise undetectable biosynthetic intermediates to be determined.
Along with similar work with bacterial polyketides, this method laid the basis
for the ideas of the processive mode of polyketide biosynthesis to be established.
It was a major step because it changed fundamentally the idea that polyketide
chains were assembled in their entirety and then subjected to necessary reductive
modifications to the simple, in retrospect, idea that these changes occur con-
comitant with chain elongation rather than post-elongation. Although it is still
not uncommon for these classic “Birch” fully oxygenated polyketide intermedi-
ates to be invoked it is now evident that in most polyketide metabolites, these
have no reality. The concept of processive polyketide assembly and modification
brought polyketide biosynthesis even closer to the process of fatty acid biosyn-
thesis in which full reductive processing in each cycle of chain condensation and
elongation is the norm. The rapid developments in understanding of the molecu-
lar genetics of polyketide biosynthesis particularly in bacteria in the 1990s were
fully consistent with the processive mode. Understanding of the genetics of fun-
gal polyketide biosynthesis still lags behind that of bacterial polyketides and the
remainder of this chapter will provide a brief overview of current understanding.
6.2.1 Polyketide Assembly
The basic assembly cycle for both polyketide and fatty acid biosynthesis is shown
in Fig. 6.1 in which a starter unit, normally acetate is transferred to the ketosyn-
thase (KS) or condensing enzyme which catalyzes a decarboxylative condensation
with malonate, bound after after malonyl tranferase (MT) catalyzed malonylation
to the acyl carrier protein (ACP). During fatty acid biosynthesis, the resulting
β
-ketothiolester (
32
) is subjected to additional chemical processing while attached
