Biology Reference
In-Depth Information
2.2.3.2 The globin-coupled regulator from Azotobacter vinelandii (AvGReg)
A. vinelandii
globin-coupled regulator (
Av
GReg) is a 472 residues long sol-
uble protein. Hundred and seventy-eight residues at the N-terminus form
the globin domain, and 170 residues at the C-terminus form the DGC
domain. Until now, the cytosolic form of the globin domain (
Av
GReg178)
and an
in vitro
folded full-length form of
Av
GReg have been characterized.
The ligand-binding kinetics have been measured for both
Av
GReg and
Av
GReg178. They show an intriguing
k
off
constant for CO (4 s
1
) that is
more than 200 times faster than for
Sw
Mb (0.019 s
1
), whereas the
k
on
is
comparable (1
10
6
M
1
s
1
10
6
M
1
s
1
for
Av
GReg forms and 0.5
for
Sw
Mb) (
Thijs et al., 2007
).
In the study of Thijs et al., the results of extensive UV-vis, RR, and
continuous-wavelength EPR spectroscopy characterizations have been
shown. Interestingly, the number of haem-iron coordination bonds in
Av
GReg and
Av
GReg178 differs, being six and five, respectively. At first
instance, these data suggested that the presence of the DGC domain influ-
ences the structural conformation of the globin domain.
We have recently expressed and purified the
in vivo
folded soluble form
of
Av
GReg sensor and measured the UV-vis spectra (unpublished data). In
this case, the as-expressed
Av
GReg shows penta coordination of the haem-
iron atom, as originally reported also for the truncated
Av
GReg178. We
therefore can conclude that the presence of the DGC domain does not
change the coordination state of the haem group. Most likely, the reported
difference in coordination is due to an incorrectly folded form of
Av
GReg.
Thijs et al. (2007)
also showed that the haem-pocket conformation in the
presence of an external ligand is comparable in both forms, being rather open
and without direct His-CO stabilization, therefore making a His-dependent
signal transduction unlikely. Significant difference is appreciated in the
protein-haem propionates interactions, thus suggesting a central role of
these chemical groups in the signalling. These data are in accordance with
the presence of the penta coordination in both forms,
Av
GReg and
Av
GReg178.
2.2.3.3 The globin-coupled regulator from Bordetella pertussis (BpeGReg)
Bpe
GReg is the globin-coupled regulator found in
B. pertussis
. It counts for
475 residues, of which 1-155 form the globin domain and 297-475 form the
DGC domain. The region comprising residues 156-296 functions as a linker
and is supposed to be important for the activity regulation of the DGC
domain from the globin domain. The predicted 3D model of the three
