Biology Reference
In-Depth Information
et al., 2012
). The bond distance between the N atom of CN
and the NE2
atom of Trp283 is 2.9
˚
, suggesting that the haem-bound CN
interacts
with the side chain of Trp283. Given that CN
is a model of O
2
,
Trp283 will form a hydrogen bond with the haem-bound O
2
, which is
the first example of O
2
-binding haem proteins in which a Trp residue is
involved for the stabilisation of the oxy state as a hydrogen bond donor.
The formation of the hydrogen bond between Trp283 and the haem-
bound O
2
will cause a conformational change of the
b
5 strand upon O
2
binding because Trp283 is located in the C-terminal region of the
b
5 strand.
Once the PAS domain in Aer2 senses O
2
by binding it to the haem, the sig-
nal of O
2
sensing should be transduced to the C-terminal MCP domain. As
the
b
5 strand is directly linked to the
a
4 helix composing a linker between
the PAS and di-HAMP domains, the conformational change of the
b
5 strand
could cause a change in the conformation of the di-HAMP domain, which
will result in a conformational change of the MCP domain. This signal trans-
duction would play an important role for the functional regulation of Aer2
in response to O
2
.
Recently,
Airola et al. (2013)
report the crystal structure of the PAS
domain (residues 173-289) of Aer2 in the ferric state. Comparing the struc-
tures of the CN
-bound and ligand-free forms reveals some structural
differences. The indole ring of Trp283 rotates ca. 90
in the absence of
the external ligand and Leu264 contacts towards the haem iron to occupy
the position where CN
binds. These changes are accompanied with the
movement of
b
3,
b
4, and
b
5 strands. Haem itself also shifts up towards
Leu264 by 1.5-2.0
˚
upon ligand binding (
Airola et al., 2013
). These
changes will be involved in the intramolecular signal transductions upon
O
2
binding to the haem.
6.3. MCP containing a PAS domain with c-type haem
6.3.1 GSU0935 and GSU0582
Geobacter sulfurreducens
has the ability to oxidise organic compounds to CO
2
with Fe
3
þ
or other metal ions as the electron acceptor. Genome sequence
analyses reveal that there are 10 sequences containing at least one c-type
haem-binding motif (Cys-X
2
4
-Cys-His), which are parts of proteins anno-
tated as signal transduction/chemotaxis proteins: five are annotated as being
sensor histidine kinases (GSU0059, GS2916, GSU1302, GSU2816,
and GSU2314), two as chemotaxis signal transducer proteins (GSU0935
and GSU0582), two as cytochrome
c
family proteins (GSU0303 and
GSU0591), and one as a HAMP/GAF/HD-GYP protein (GSU2622)
