Chemistry Reference
In-Depth Information
mixture of both stereoisomers always can be detected when the substrate bound
to the enzyme is analyzed, which is indicative for either a nonstereoselective or
a reversible reaction. Once the epimerized substrate undergoes the subsequent
condensation reaction, only the species with an inverted stereocenter is found in
the elongated product. Thus, the C domain only processes the inverted species.
In some rare cases, the C domain also exhibits epimerization activity besides its
normal function, and it is then called the “dual C/E” domain [Arthrofactin, (20)].
Another structural feature often found in NRPS products is N-methylated
amide bonds. The domain that introduces this C
1
unit, the so-called methyl-
transferase (Mt) domain, is situated between the A and the PCP domain (21).
By consumption of
S
-Adenosyl-methionine, the
-amino group of the acceptor
substrate is methylated before condensation with the donor.
In the case of linear gramicidin, the N-terminus of the nonribosomal peptide
carries a formyl group (10). Just like in the bacterial ribosomal synthesis, only
a formylated first building block is processed additionally by the corresponding
enzymatic machinery. Thus, one can find a distinct formylation (F) domain at
the very N-terminus of the synthetase. Another formylated NRPS product is
coelichelin whose N-terminal ornithine residue is believed to be N
δ
-formylated
in
trans
by a formyltransferase genetically associated with the NRPS (17). Formyl-
tetrahydrofolate is used as source of the formyl group by these enzymes.
The essential condensation domain mentioned above can, in some cases, not
only condensate but also catalyze a side-chain cyclization. It is then called
cyclization (Cy) domain. The cyclization is initiated by a nucleophilic attack
of the side-chain heteroatom on the carbonyl group of the amide bond formed
by the same domain. When water is eliminated, a stable pentacycle is inte-
grated into the peptide chain without altering the rest of the backbone. The
nucleophiles known to be reactants in these Cy domain reactions are either thre-
onine/serine [mycobactin A, (22)] or cysteine [bacitracin, (23)]. The former leads
to (methyl-)oxazoline heterocycles, whereas the latter gives rise to thiazoline-
like units. Another domain sometimes associated with this heterocyclization is
the oxidation (Ox) domain [Epothilone, (7)]. It is located between A and PCP
domains, and it catalyzes the oxidation of oxa/thiazoline intermediates, which
leads to oxazoles or thiazoles, respectively.
α
4.3 TECHNIQUES FOR THE PRODUCTION OF NOVEL
NONRIBOSOMAL PEPTIDES
With a constantly growing number of pathogenic bacterial strains resistant to the
known antibiotics, the demand for novel antibiotics or, more generally speaking,
therapeutic agents is evident. Because many NRPS products already have such
activities and their chemical and structural diversity is so huge, efforts have been
made to use NRPSs to broaden the known spectrum of therapeutics. In this
section, the possibilities of using NRPS machineries or parts of them to produce
new bioactive compounds are addressed.
