Biology Reference
In-Depth Information
As typical for haem-proteins, HemAT reversibly binds diatomic gases
(e.g. O
2
, CO, NO) (
Hou, Freitas, et al., 2001
), and the O
2
affinity of
HemAT-
Bs
is calculated to be similar to the one of
Sw
Mb, namely,
K
d
882 nM) (
Aono et al., 2002
).
Different techniques were used to collect complementary structural data,
proving that HemAT is capable of discriminating among gaseous ligands.
Indeed, it ensures an unambiguous response that occurs after signal transduc-
tion exclusively with O
2
as haem distal ligand.
A network of hydrogen bonds that involves the haem-bound O
2
and
Thr95, and His86 and haem 6-propionate causing conformational changes
is essential for the transduction of the signal from the sensor domain to the
signalling domain (
El-Mashtoly, Gu, et al., 2008; Yoshimura et al., 2006
).
Moreover, changes at the proximal side of the haem group, with the forma-
tion of a hydrogen bond between Tyr133 and the axial His123, are also
linked to signal transduction upon O
2
ligand binding (
Yoshida, Ishikawa,
Aono, & Mizutani, 2012; Yoshimura, Yoshioka, Mizutani, & Aono, 2007
).
Using CO as ligand, the presence of an additional binding site at the
proximal side of the haem group has been shown. In this respect, Tyr133
assumes a critical role in gating a tunnel that connects the two haem cavities,
proximal and distal (
Pinakoulaki et al., 2006
). This system of tunnel and
binding cavities suggests the possibility of fluctuating movements of the
ligand between the two environments that might be crucial for determining
O
2
affinities and, therefore, the dynamics of the aerotactic response.
After these residue-specific observations, structural changes involving
helical movements have been proved (
El-Mashtoly, Gu, et al., 2008;
El-Mashtoly et al., 2012
). Indeed, spectroscopic evidence is available on
the central role of the B- and G-helices displacement, specifically triggered
by O
2
binding, which leads to signal transduction (
El-Mashtoly, Gu, et al.,
2008; El-Mashtoly et al., 2012
).
The crystal structures of HemAT-
Bs
in the liganded and unliganded
forms have been determined and reveal a homo-dimeric organization of
the sensor (
Zhang & Phillips, 2003
). The dimerization interface involves
the helices G and H, which form an antiparallel four-helical bundle, part
of the Z-helix, and the B-C corner of the globin domain (
Fig. 1.2
). It
has also been noted that the asymmetry of the HemAT-
Bs
structure
increases, going from the liganded to the unliganded form. Along with this
observation, the authors propose a signalling transduction model in which
the unliganded state is energetically unfavourable. When O
2
diffuses from
the environment into the cytoplasm, one of the two subunits of the dimer
¼
719 nM (where
Sw
Mb
K
d
¼
