Chemistry Reference
In-Depth Information
Ribosomal
Biosynthesis
Gene
mRNA: Codons
Protein
(a)
Nonribosomal
Biosynthesis
(b)
Gene
NRPS: Modules
Peptide
Figure 4.1
Comparison of ribosomal and nonribosomal peptide synthesis. (a) In the
ribosomal information pathway, the sequence of codons in the mRNA determines the
sequence of amino acids in the protein, whereas (b) the sequence of modules in the nonri-
bosomal peptide synthetases intrinsically determines the primary sequence of the peptide
product.
side product formation caused by diffusion. The catalytic entities responsible
for the incorporation of a distinct building block into the product are called
modules. Each module carries out several chemical steps required for the syn-
thesis of nonribosomal peptides: Recognition of the building block, activation,
covalent attachment, translocation, and condensation. In several cases, additional
modifications are found, such as epimerization [Tyrocidine, (5)], cyclization
[Gramicidin S, (6)], oxidation [Epothilone, (7)], reduction [linear gramicidin
(8)], methylation [Cyclosporin, (9)], and formylation [linear gramicidin, (10)].
In vitro
studies have shown that each module can be subdivided into catalytically
active domains to which the different reactions mentioned above can be assigned
(1-4). Thus, the so-called adenylation (A), peptidyl carrier protein (PCP), and
the condensation (C) domains were identified as being essential to all NRPSs. In
addition, a second group of so-called optional domains exists: the epimerization
(E), cyclization (TE or Cy), oxidation (Ox), reduction (R), N-methylation (Mt),
and formylation (F) domains. Aside from NRPSs themselves, several enzymes are
known to act on some peptides while they are still bound to the synthetase or even
after their release. These modifying enzymes can glycosylate [Vancomycin, (11)],
halogenate [Vancomycin, (11)], or reduce [linear gramicidin, (8)] the peptides
in trans
. With several hundred different building blocks found in nonribosomal
peptide products, it becomes evident that their diversity is vast (Fig. 4.2). This
chapter addresses the biologic background of these secondary metabolites, the
enzymatic machineries of NRPSs, and the chemical reactions catalyzed by their
domains. Furthermore, the possibility of manipulating NRPSs and using certain
domains to produce novel compounds is discussed.
4.1 BIOLOGIC BACKGROUND
Nonribosomal peptides are produced by a large number of bacteria, fungi, and
lower eucaryotes. For most of these compounds, their biologic role is unknown.
One might suspect that these secreted molecules are used for unknown forms
of communication or simply to critically increase the chance of survival for
the producing cell in its habitat, because the metabolic cost of their produc-
tion is enormous. However, the function of some nonribosomal compounds has
