Biology Reference
In-Depth Information
redox state and bind a variety of ligands and has been involved in biological
processes since life appeared on Earth (
Vinogradov & Moens, 2008
).
To remain soluble and active, the haem group needs to be surrounded by a
hydrophobic pocket that is folded in, among others, the globin fold. Modifica-
tions in the redox state, as well as the absence/presence of a ligand, mostly cause
the haem-moiety to undergo structural changes. In the case of multi-domain
haem-based sensors, these modifications are spread all over the haem-binding
structure and represent the signal that is transferred to the transmitter domain.
Four major families of haem-based sensors can be recognized: (i) the
haem-binding Per-Arnt-Sim (PAS) domain, (ii) the CO-sensitive CooA,
(iii) the haem NO-binding domain (HNOB), and (iv) the globin-coupled
sensors (GCSs).
In
Streptomyces avermitilis
and
Frankia
sp., the presence of a chimeric pro-
tein that consists of a C-terminal-truncated globin fused to an N-terminal
cofactor-free monooxygenase has been reported (
Bonamore et al., 2007
).
Given that these sensors have yet to be officially classified among the
haem-based sensors, we propose to divide the GCS family into two sub-
families, that is, 3/3 GCS and 2/2 GCS, and include the
S. avermitilis
and
Frankia
sp. proteins in the latter group.
Alternative sensing chimeric proteins have been identified. They are related
to the known haem-binding sensors for structural or functional reasons, but
cannot be ascribed in the superfamily classification as they are lacking the haem.
For example, globin-coupled proteins in which the sensor domain is a non-
haem globin domain have been identified and characterized in
Bacillus subtilis
,
Moorella thermoacetica
,and
Bacillus anthracis
and represent the first members of an
intriguing class of sensors (
Chen, Lewis, Harris, Yudkin, & Delumeau, 2003;
Quin et al., 2012; Stranzl et al., 2011
). Similarly, a haemerythrin that contains
a di-iron centre but not a haem group, has been shown in
Vibrio cholerae
and
regulates the activity of a diguanylate cyclase (DGC) (
Schaller,Ali,Klose,&
Kurtz, 2012
).
Here, we review the most significant literature on haem-based sensors.
We summarize what is known about the function of these proteins, their
known structures, and the relationship between these two aspects. Along
with this, an evolutionary model is also supported.
2. FUNCTIONS OF THE HAEM-BASED SENSORS
Numerous copies of sensor proteins are present in all kingdoms of life.
For example, the number of GCSs alone, identified to date in Bacteria and
Archea, is 420 (
Vinogradov, Tinajero-Trejo, Poole, & Hoogewijs, 2013
).
