Biology Reference
In-Depth Information
domain, but functional studies, for example, knock-out as well as over-
expressing strains, are needed to fully understand the biological role that
these molecules play in the bacterial organism.
2.4.1 The GCS from Geobacter sulfurreducens and Geobacter
metallireducens
All theGCSsmentioned thus far are soluble proteins. Exceptions to this rule are
the GCSs identified in two Gram-negative
d
-Proteobacteria,
G. sulfurreducens
(
Gs
GCS) and
G. metallireducens
(
Gm
GCS).
G. metallireducens
is an obligate
anaerobe and the first organism found to oxidize organic compounds using
iron oxide as the final electron acceptor (
Lovley et al., 1993
).
G. sulfurreducens
displays the same energy-producing pathway, but, on the
other hand, it has been found not to be an obligate anaerobe; therefore, it is
also able to grow with O
2
as terminal electron acceptor (
Lin, Coppi, &
Lovley, 2004; Methe et al., 2003
). With respect to this O
2
tolerance, the
engagement of the identified transmembrane GCS is yet to be proved.
Both
Gs
GCS and
Gm
GCS have been predicted to display the classical
globin-like-sensing domain coupled to a transmembrane transmitter domain
with yet unknown function (
Freitas et al., 2003
). A putative function as
S
-nitrosoglutathione reductase has been ascribed to both transmitter
domains (
Freitas et al., 2005
). Therefore,
Gs
GCS and
Gm
GCS might be
involved in regulating intracytoplasmatic NO levels and in protecting the
organism against nitrosative stress. On the other hand, it has been reported
that
Gs
GCS expression levels are higher when the bacterium is grown under
Fe(III)-reducing conditions, suggesting that it might also have a role in an
optimal Fe(III) reduction (
Aklujkar et al., 2013; Ding et al., 2006
). None-
theless, experimental evidences of the real function are yet to be shown.
Very few structural data are available for these GCSs. So far, only the
globin domain of
Gs
GCS (
GS
GCS
162
) has been investigated in this direc-
tion (
Desmet et al., 2010; Pesce et al., 2009
). In the reported crystal struc-
ture,
Gs
GCS
162
displays a modified 3/3 globin fold with an extra Z-
a
-helix
preceding the N-terminal A-helix and with the complete loss of the D-helix
(
Pesce et al., 2009
). The molecules are disposed forming homo-dimers at the
G-H interface, as previously reported also for HemAT-
Bs
(
Zhang &
Phillips, 2003
) and
Methanosarcina acetivorans
protoglobin (
Ma
PGB)
(
Freitas et al., 2004
), but it is yet to be proved whether this quaternary assem-
bly plays any role in signal transduction.
Gs
GCS
162
is the first example of bis-His haem coordination that involves
proximal His(93)F8 and distal His(66)E11 as axial
ligands
(
Desmet
