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Publié par | philipps-universitat_marburg |
Publié le | 01 janvier 2009 |
Nombre de lectures | 14 |
Poids de l'ouvrage | 36 Mo |
Extrait
Unusual Building Blocks and Domain Organization of
Non‐Ribosomal Peptide Synthetases
Ungewöhnliche Synthesebausteine und Domänenorganisation von
Nicht‐Ribosomalen Peptidsynthetasen
Dissertation
zur
Erlangung des Doktorgrades
der Naturwissenschaften
(Dr. rer. nat.)
dem
Fachbereich Chemie
der Philipps‐Universität Marburg
vorgelegt von
Matthias Strieker
aus Lingen an der Ems
Marburg an der Lahn, 2009
Vom Fachbereich Chemie
der Philipps‐Universität Marburg als Dissertation
am 01.09.2009 angenommen.
Erstgutachter : Prof. Dr. M. A. Marahiel
(Philipps‐Universität Marburg)
Zweitgutachter : Prof. Dr. C. T. Walsh
(Harvard Medical School, Boston, MA, Vereinigte Staaten von Amerika)
Tag der Disputation: 25.09.2009
ii
Dedicated to my parents
in loving memory
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Abstract
Abstract
The diverse class of non‐ribosomal peptides consists of manifold pharmacologically
important natural products. They are clinically used in antibiotic, antiviral and antitumor
therapy, furthermore some are known immunosuppresants. The biological activity is based
on their structural diversity, as they contain various non‐proteinogenic building blocks and
amino acids of which many are ‐modified. It was shown that the latter are important for
biological activity, but little is known about their biosynthetic origin. In particular, these
building blocks are key determinants of the class of acidic lipopeptide antibiotics and
kutznerides, which are in the focus of this thesis.
To determine the mechanism underlying the biosynthetic origin of the synthetically
challenging ‐hydroxylated asparagine (hAsn) moieties, found in the acidic lipopeptides CDA
and A54145, the corresponding recombinant non‐heme iron (II)/ ‐ketoglutarate dependent
hydroxylases AsnO and LptL have been examined in vitro. Direct hydroxylation of the free
amino acid was observed in both cases, clearly indicating a precursor synthesis pathway. The
crystal structure of one of the two hydroxylases (AsnO) was determined at high resolution
and revealed a substrate induced fit mechanism of the enzyme. Upon addition of
asparagine, a lid‐like region seals the active site and shields it from sterically demanding
substrates, which explains the observed specificity for free asparagine. Furthermore, the
AsnO structure could be seen as an archetype enzyme for non‐heme iron hydroxylases
acting on free amino acids. It was possible to predict amino acid binding residues for
homologous enzymes by 3D modeling.
In order to fully understand the mechanisms of ‐hydroxylated building blocks synthesis, the
hydroxylases KtzO and KtzP, predicted to be responsible for the generation of the two 3‐
hydroxyglutamic acid isomers found in the mixture of antifungal and antimicrobial
kutznerides, were produced recombinantly and analyzed in vitro. Notably, they were found
to work in trans to the assembly line on PCP‐tethered glutamic acid rather than on the free
amino acid. Unexpectedly, as the two isomers are found in approximately equal amounts in
mature kutznerides, KtzO was shown to stereospecifically generate threo‐hydroxyglutamate,
while KtzP catalyzed the formation of the erythro isomer by co‐elution HPLC experiments
with synthetic dabsylated standards. A powerful method that employs non‐hydrolyzable
coenzyme A analogs was developed, which allowed the determination of the kinetic
parameters of enzymes working on PCP‐bound substrates for the first time. Furthermore, a
hitherto unknown mechanism of NRPS assembly line restoration was observed. The
corresponding adenylation (A) domain for glutamic acid activation in the kutzneride NRPS
was found to be corrupted. Herein, it is shown that this lack of a functional A domain is
compensated in trans by a stand‐alone A domain. These findings elucidated the mechanism
for the in trans compensation and the stereospecific hydroxyglutamate generation in detail
and may guide the usage of in trans hydroxylation/compensation enzymes in
biocombinatorial engineering approaches.
In the third part of this work, the acquired knowledge about the mechanisms underlying
enzymatic ‐hydroxylation of amino acids was exploited for the synthesis of the
pharmaceutically relevant ‐hydroxyaspartate. Primarily, this was facilitated by the
structure elucidation of AsnO in which the substrate binding residues were identified. By site
directed mutagenesis, an AsnO variant was generated, which notably did not hydroxylate
the original substrate asparagine, instead it was found to stereospecifically catalyze the
formation of L‐threo‐hydroxyaspartic acid, even in commercially interesting amounts.
Therefore, the AsnO variant is an excellent example for the application of basic research in
order to generate pharmacologically relevant non‐proteinogenic amino acids.
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Zusammenfassung
Zusammenfassung
Die mannigfaltige Klasse der nicht‐ribosomalen Peptide beinhaltet viele pharmakolisch
relevante Wirkstoffe, die in der Klinik in der Antibiotika‐, Krebs‐ und Immunsupp