Chemistry Reference
In-Depth Information
(a)
A
PCP
C
A
PCP
C
A
PCP
C
OH
OH
OH
[Sfp]
A
PCP
C
A
PCP
C
A
PCP
C
CoA
O
S
SH
SH
3',5'-ADP
H
2
N
O
(b)
NH
O
A
PCP
C
A
PCP
C
A
PCP
C
HN
O
NH
2
SH
SH
SH
[A domain]
[second C domain]
ATP,
cognate substrates
AMP, PPi
(c)
(d)
A
PCP
C
A
PCP
C
A
PCP
C
A
PCP
C
A
PCP
C
A
PCP
C
[first C domain]
S
S
O
S
SH
S
O
S
O
O
H
2
N
O
O
NH
H
2
N
O
NH
2
NH
2
NH
2
NH
2
O
H
2
N
Figure 4.4
Single reactions in NRPSs and their timing. (a) After ribosomal syn-
thesis of the
apo-
enzymes, the PCP domains are postsynthetically modified with 4
-
Phosphopantetheine cofactors by a 4
Ppan transferase, e.g., Sfp. (b) In a second step,
the A domains bind their cognate substrates as well as ATP and form the corresponding
acyl-adenylate intermediates. These are transferred onto the cofactor of the neighboring
PCP domains. (c) The C domains catalyze the condensation of two building blocks. The
specificities of C domains and the affinities of aminoacyl-/peptidyl-
holo-
PCP domains
ensure that no internal start reactions occur: (d) Only after the first condensation domain
has acted does the second C domain seem to process the intermediate. During synthesis,
the growing product chain is continuously translocated toward the C-terminal end of the
enzyme.
data have revealed that amino acid residues at certain positions in the enzyme
determine the specificity of an A domain (13). This result can be explained by
the thereby generated chemical and physical environment of the substrate binding
site. Some A domains, however, are known to have a relaxed substrate specificity.
In these cases, chemically or sterically similar amino acids also are recognized,
processed analogously, and thus found at that very position in the product. For
example, the A domain of the first module of the gramicidin synthetase LgrA
(10) not only activates valine but also activates isoleucine, which is found in 5%
of linear gramicidins extracted from producing strains.
When the first building block has been recognized and activated by the A
domain, the next essential domain comes into play: The peptidyl carrier protein
(PCP) domain. Like the acyl carrier protein (ACP) in fatty acid biosynthesis,
this PCP domain is responsible for keeping the reaction intermediates bound to
