Biomedical Engineering Reference
In-Depth Information
Rapamycin is one of the few examples of a molecule for which two unique
approaches have been successfully employed for the modification of two different
positions by precursor-directed biosynthesis. As yet, the simultaneous application of
these two approaches in the production of a single molecule has yet to be reported, but
it is the logical extension of thework and will allow a great deal of structural flexibility
with minimal synthetic effort.
14.4.1.3. Avermectin Avermectin
, a macrolide that exhibits insecticidal and
antiparasitic activities, is natively produced by a type 1 PKS in
S. avermitilis
. During
the exploration of the biosynthesis of this compound, feeding of labeled precursors
revealed that the C25 positions of avermectins A and B were derived from isoleucine
and valine, respectively. It was hypothesized that these amino acids served as
precursors to the CoA thioesters of
S
-2-methylbutyric acid and isobutyric acid
that were known to be fundamental building blocks for these molecules [18].
In preparation for the use of precursor-directed biosynthesis for the discovery of
novel analogues, a mutant strain of
S. avermitilis
was produced in which the
branched-chain 2-oxoacid dehydrogenase was inactivated and the bacterium was
thus unable to produce the aforementioned CoA thioesters. More than 800 commer-
cially available carboxylic acids were fed to the blocked mutant in an effort to
determine the range of molecules that would be accepted by the avermectin PKS. This
vast screen led to the successful incorporation of over 40 unnatural carboxylic acids,
each leading to an analogue of each of avermectins A1, B1, A2, and B2 [19]
(Scheme 14.3).
9
OMe
HO
OMe
O
O
Wild-type precursor
production removed from
blocked mutant
H
O
O
O
R
1
O
O
NH
2
O
O
O
HO
HO
OH
H
O
O
R
1
HO
R
2
Avermectin A
(R
2
= OMe)
Avermectin B
(R
2
2
= OH)
9
10
11
O
S-CoA
S. collinus
S
12
13
14
SCHEME 14.3
Production of avermectin analogues, including the commercially available
doramectin
11
.
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