Angewandte
Communications
Chemie
How to cite:
Natural Products
The Mechanism of Dehydrating Bimodules in trans-Acyltransferase
Polyketide Biosynthesis: A Showcase Study on Hepatoprotective
Hangtaimycin
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Minghe Luo , Houchao Xu , Yulu Dong, Kun Shen, Junlei Lu, Zhiyong Yin, Miaomiao Qi,
Guo Sun, Lingjie Tang, Jin Xiang, Zixin Deng, Jeroen S. Dickschat,* and Yuhui Sun*
Abstract: A bioassay-guided fractionation led to the isolation
of hangtaimycin (HTM) from Streptomyces spectabilis
CCTCC M2017417 and the discovery of its hepatoprotective
properties. Structure elucidation by NMR suggested the need
for a structural revision. A putative HTM degradation product
was also isolated and its structure was confirmed by total
synthesis. The biosynthetic gene cluster was identified and
resembles a hybrid trans-AT PKS/NRPS biosynthetic machi-
nery whose first PKS enzyme contains an internal dehydrating
bimodule, which is usually found split in other trans-AT PKSs.
The mechanisms of such dehydrating bimodules have often
been proposed, but have never been deeply investigated. Here
we present in vivo mutations and in vitro enzymatic experi-
ments that give first and detailed mechanistic insights into
catalysis by dehydrating bimodules.
paracetamol overdosage and silymarin, a lipophilic extract
from milk thistle (Silybum marianum) seeds, is administered
in liver injuries by carbon tetrachloride, alcohol, in chronic
viral hepatitis or acute poisonings with Amanita mush-
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4]
rooms. The limited availability of hepatoprotective drugs
raises an urgent need for the discovery of new compounds.
In the process to screen hepatoprotective drug leads,
crude culture extracts from Streptomyces spectabilis CCTCC
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M2017417 revealed an interesting activity by inhibiting the
increase of alanine (ALT) and aspartate aminotransferase
(AST) activities in the human hepatoma cell line HepG2
stimulated with CCl , which are indicators of liver over-
4
[
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function. A subsequent bioassay guided fractionation led to
the isolation of hangtaimycin (HTM, 1) as the active principle
[
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(Figure S1). This known compound was identified by ESI-
HRMS and NMR spectroscopy (Figures 1, S2–S9 and
Table S3). However, a detailed inspection of the NMR
F
or many years Actinomycetes have been a major source for
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9,30
the discovery of bioactive natural products and the develop-
ment of new drugs such as the antibiotic erythromycin, the
immunosuppressant rapamycin, or the antitumor antibiotic
spectra indicated an E configured D
double bond instead
[
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of the published Z configuration, especially because of the
absence of a ROESY correlation between H29 and H30 in
conjunction with the signal multiplicity for H30 (dd, J = 15.0,
[
1,2]
bleomycin.
As an important “detoxification” organ the
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3]
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liver plays an important role in human metabolism. Liver
injury may cause fatigue, coagulopathy, encephalopathy and
liver failure, which are threating peopleꢀs lives, but only a few
hepatoprotective drugs are clinically available for specific
applications, for example, acetylcysteine is used to treat
10.2 Hz). The H-NMR signals for H29 and H31 are over-
lapping with other signals, which prevents a reverse assign-
ment of coupling constants, but J = 15.0 Hz must be relevant
for the coupling over an olefinic double bond (between H29
and H30, thus revealing its E configuration), while J = 10.2 Hz
must be assigned to the coupling between H30 and H31 over
a formal single bond. Besides HTM also a putative degrada-
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[
*] M. Luo, Y. Dong, Dr. K. Shen, J. Lu, M. Qi, G. Sun, L. Tang,
Dr. J. Xiang, Prof. Dr. Z. Deng, Prof. Dr. Y. Sun
Key Laboratory of Combinatorial Biosynthesis and Drug Discovery,
Ministry of Education, and School of Pharmaceutical Sciences,
Wuhan University
tion product was detected by LC-ESI-HRMS ([M+H] m/z
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23.14349, calcd for C H O N m/z 223.14410), which
1
2
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2
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corresponds to the hemiaminal cleavage product HTM222
index 222 referring to its molecular mass, Scheme S1a,
(
Figure 1). This compound was synthesized from l-alanine
(Scheme S1b, Figures S10–S48), and the natural product and
the synthetic material showed identical chromatographic
behaviour and mass spectra (Figure S49). These findings
also confirmed the assigned 2Z,4E configuration in the sorbic
acid amide portion of HTM. HTM222 has been isolated before
No. 185 East Lake Road, Wuhan 430071 (People’s Republic of China)
E-mail: yhsun@whu.edu.cn
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H. Xu, Z. Yin, Prof. Dr. J. S. Dickschat
Kekulꢀ-Institute for Organic Chemistry and Biochemistry, University
of Bonn
Gerhard-Domagk-Straße 1, 53121 Bonn (Germany)
E-mail: dickschat@uni-bonn.de
[8]
from Streptomyces sp. SBI108 and named sarmentosamide
which may also arise by degradation of HTM or a similar
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[
] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
metabolite in this organism. The optical rotation of the
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synthetic compound, [a] = À194 (c 0.28, MeOH), shows the
D
2
D
8
same sign as the natural product, [a] = À141 (c 0.05,
ꢁ
2021 The Authors. Angewandte Chemie International Edition
[8]
MeOH).
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
HTM is the product of a hybrid polyketide synthase/
nonribosomal peptide synthetase (PKS/NRPS). According to
the classical assembly line paradigm of PKS/NRPS, for the
Angew. Chem. Int. Ed. 2021, 60, 1 – 6
ꢁ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
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