Biology Reference
In-Depth Information
Additionally, hydroxamate-type siderophores were identified to be
secreted by the coastal marine cyanobacteria
Synechococcus
sp. PCC 7002
(
Armstrong & van Baalen, 1979
), which were later on characterized as the
amphiphilic hydroxamate-type siderophores synechobactin A-C. These are
composed of a citric acid backbone, related to schizokinen, but different
in the length of their fatty acid tail attached to the second α-hydroxamate
group (Fig.
3
.3B;
Ito & Butler, 2005
). The fatty acid tail is thought to
enhance the affinity of the siderophore for the bacterial membrane surface.
Similar to many other Fe(III) chelators, synechobactin is photoreactive: the
ligand undergoes a light-induced charge transfer reaction leading to oxida-
tive cleavage resulting in a hydrophilic peptide fragment and a fatty acid tail
fragment. In the process, bound Fe(III) is reduced to Fe(II) (
Barbeau, Rue,
Trick, Bruland, & Butler, 2003
). The physiological significance of the pho-
toreactivity of siderophores remains to be established.
Certain cyanobacteria can produce catecholate-type siderophores, simi-
lar to anachelin produced by
Anabaena cylindrica
. It coordinates iron with
one catecholate and one 2-hydroxyphenyl-oxazoline system. Its structure
is unusual, combining a polyketide, a central tripeptide consisting of L-Thr,
D-Ser, and L-Ser and an alkaloid building block (
Gademann & Bethuel,
2004a
). Initially, two forms of anachelin were isolated; anachelin H, contain-
ing a terminal salicylamid and anachelin 1 containing a terminal oxazoline
ring (
Beiderbeck, Taraz, Budzikiewicz, & Walsby, 2000
), while a subsequent
study identified anachelin 1 and an isomeric form thereof (anachelin 2)
instead of anachelin H (Fig.
3
.3C;
Itou, Okada, & Murakami, 2001
). It was
suggested that the only biologically active compound is anachelin H and
that the oxazoline ring of anachelin 1 and 2 are formed under dehydrating
conditions during purification. Anachelin H has a compact folded confor-
mation as determined by NMR (
Gademann & Bethuel, 2004a
).
Siderophores have been recently introduced into biotechnology research
(
Gademann, Kobylinska, Wach, & Woods, 2009
). For example, a synthetic
anachelin chromophore was coupled to metal oxide surfaces linked via
PEG to the antibiotic vancomycin producing a non-fouling antimicrobial
surface. The usage of these surfaces for antimicrobial implants such as cath-
eters and stents was suggested (
Gademann et al., 2009
).
2.2. Siderophore Synthesis
At least three different systems were reported to be involved in siderophore
synthesis in cyanobacteria; two of them belonging to the non-ribosomal
peptide synthetases (NRPSs), which catalyse the peptide bonds between
