Chemistry Reference
In-Depth Information
laboratory,
41
and even more recently, enterococcal vanA-resistance has been found
in clinical isolates of S. aureus strains.
42
2.5
BIOSYNTHESIS
The investigation of the glycopeptide biosynthesis has been a main focus of many
research groups over the past 5 years, and currently, a relatively detailed picture
already exists of the biosynthetic assembly. As a consequence, this topic has also
been the subject of several review articles, which give an overview of the current
status of research.
8,9
The sequencing of the chloroeremomycin biosynthesis gene cluster in 1998
allowed the putative gene functions for the assembly of glycopeptide antibiotics
to be deduced for the first time.
43
Until now the biosynthesis gene clusters of sev-
eral other glycopeptide producers have been sequenced: balhimycin (Amycolatopsis
balhimycina),
44
teicoplanin (Actinoplanes teichomyceticus),
45-47
A40926 (Nono-
muraea species),
48
A47934 (Streptomyces toyocaensis),
49
and complestatin (Strep-
tomyces lavendulae).
50
The peptide synthetase genes found in the gene clusters of
all sequenced glycopeptide producers indicate a peptide assembly by nonribosomal
peptide synthetases (NRPS).
51,52
The biosynthetic assembly of glycopeptide anti-
biotics can be subdivided into three parts. The first stage comprises the assembly
of building blocks, which are the nonproteinogenic amino acids and the vancosa-
mine sugars. The second stage is the peptide assembly on the NRPS and finally the
tailoring by P450-dependant oxygenases, glycosylation, and N-methylation. In the
case of teicoplanin, the amino sugar is acylated with fatty acids. The biosynthesis of
vancomycin-type glycopeptides has been investigated by two different approaches.
One approach is the overexpression of biosynthetic proteins followed by protein
characterization and the conversion of substrates. The other approach is the inacti-
vation of biosynthetic genes followed by the chemical characterization of accumu-
lated biosynthesis intermediates. Both approaches have been used in a
complementary way to shed light on the different stages of glycopeptide assembly.
The protein overexpression approach was successful for the early and late stages,
namely the building block assembly, the glycosylation, and N-methylation reactions.
53
The biosyntheses of the five aromatic core amino acids 4-hydroxyphenylglycine
(Hpg),
54,55
3,5-dihydroxyphenylglycine (Dpg),
54,56-59
and b-hydroxytyrosine
(Hty)
60-62
are well investigated. However, the central structural features exclusively
characteristic for glycopeptide antibiotics are the three side-chain cyclizations with
the AB, C-O-D, and D-O-E rings. The characterization of a linear heptapeptide,
which was obtained from gene inactivation of P450-monooxygenases of the balhi-
mycin (Scheme 2-1) producer Amycolatopsis balhimycina, shed light on the nature
of putative peptide intermediates and the likely course of aglycon formation.
44,63
This heptapeptide intermediate aready showed b-hydroxylation, chlorination, as
well as D-Leu, D-Hpg, and L-Dpg as the structural features being present before
side-chain cyclization. Subsequent contributions determined a sequence of ring
closure reactions (Scheme 2-6) in the order: (1) C-O-D, (2) D-O-E, and (3)
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