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discriminate its physiological effector among them. There will be two strat-
egies in principle for discriminating a physiological gas molecule from
others. First, if only the physiological effector is able to bind to the haem,
the discrimination can be achieved. Second, if there are specific interactions
between the haem-bound ligand and surrounding residues only upon bind-
ing of a physiological effector, it can be used for discrimination of the cog-
nate physiological effector. The haem-containing PAS family of sensor
proteins adopts the second strategy with some modifications in individual
proteins. In these systems, the binding of the cognate physiological effector
to the haem results in the reconstruction of the hydrogen-bonding network
among the haem, haem-bound ligand, and surrounding residues along with
conformational changes in the haem pocket. Dynamic exchange of the axial
ligand upon redox change of the haem or upon binding of an external ligand
can also be a trigger for conformational changes of several haem-containing
PAS domains. Conformational changes induced by sensing the cognate
effector are crucial processes for the regulation of the sensor proteins. As
described in this chapter, spectroscopic and structural studies of the isolated
PAS domains give the detailed information of what and how conformational
changes occur in the sensor domains upon sensing the effector. However,
structural studies of sensor proteins in full-length remain to be achieved.
To fully understand the structural and functional relationships of the
haem-containing PAS family, it is required to determine the structures at
“on-state” and “off-state” in the full-length protein.
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