Biology Reference
In-Depth Information
2003
). These proteins show an
a
-helical fold and bind the haem group,
either covalently to a His residue or non-covalently when the His is
not present.
The haem-binding domain can be fused to an MCP or be a single-
domain protein. HNOBs are encoded by operons that contain genes for
the HNOB-associated (HNOBA) protein, another component of the sig-
nalling system (
Iyer et al., 2003
).
Therefore, HNOBs sense intracellular NO presence and provide
answers by activating MCPs, PDEs, or DGCs
(
Gilles-Gonzalez &
Gonzalez, 2005; Iyer et al., 2003
).
2.3. Haem-based sensors with enzymatic function
A new class of haem-based sensors is represented by chimeric proteins in
which the transmitter domain is not directly involved in any of the previ-
ously described activities. These sensors mediate different enzymatic reac-
tions in response to a stimulus.
Although to date this class only counts for two members, we believe that
other molecules might be present in nature that still have to be characterized.
2.3.1 The truncated globin-coupled sensors
Similarity searches identified the existence of truncated globin-coupled sen-
sors (tGCSs) in
S. avermitilis
and
Frankia
sp. that are coupled to antibiotic
biosynthesis monooxygenases (
Bonamore et al., 2007
). These sensors differ
from the known GCS for the presence of the 2/2 truncated globin (trHb) in
place of the classical 3/3 globin domain, and for the position of this domain
at the C-terminus of the sensor.
The preferred form in which the tGCSs are found in solution is homo-
dimeric. The sequence alignment, the spectroscopic investigation, and the
ligand-binding properties highlight resemblance to the group II trHb.
Although the link between the trHb domain and the quinones redox activity
of the monooxygenase domain remains unclear, the haem group remains in
the reduced O
2
-bound state during the catalysis (
Bonamore et al., 2007
).
Further experiments are needed to unveil
the physiological
role of
these sensors.
2.4. Haem-based sensors with unknown function
The role of some haem-based sensors is not yet known. Similarity
approaches have been used to predict the function of their transmitter
