Biology Reference
In-Depth Information
proximal His77 and distal Met95 as axial ligands (
Gonzalez et al., 2002;
Sasakura et al., 2002
). On the other hand, when the CO-bound form is cre-
ated, Met95 coordinates and stabilizes the ligand in place (
Sato et al., 2002;
Tanaka, Takahashi, & Shimizu, 2007
). Moreover,
Ec
DosP in solution forms
concentration-independent homo-dimers (0.01-1
m
M), and Met95 is
involved also in the intra-dimer interactions (
Kobayashi et al., 2010;
Lechauve et al., 2009
).
Although the gas-binding affinity is cooperative, the affinity of
Ec
DosP
for the binding of CO and NO is significantly low (
K
d
¼
10
m
M) compared
to O
2
(
K
d
13
m
M); therefore, under physiological concentrations, these
two gases are negatively selected (
Delgado-Nixon et al., 2000; Taguchi
et al., 2004
).
Several residues are involved in structural modifications upon O
2
, CO,
and NO binding and, therefore, in signal transduction, that is, Trp53,
Asn84, Arg97, Phe113, and Tyr126 (
El-Mashtoly, Nakashima, Tanaka,
Shimizu, & Kitagawa, 2008; El-Mashtoly, Takahashi, Shimizu, &
Kitagawa, 2007; Ito, Araki, et al., 2009; Tanaka & Shimizu, 2008
). Structural
modifications in the PAS domain are followed by movement in the FG turn,
in the G-strand, and in the HI turn, thus activating the enzymatic domain
(
Ito, Igarashi, & Shimizu, 2009; Park, Suquet, Satterlee, & Kang, 2004
).
Ec
DosC presents an N-terminal globin domain fused to a C-terminal
DGC domain. It is likely one of the most expressed DGCs in
E. coli
(
Sommerfeldt et al., 2009
) and some of the biofilm-regulating pathways that
are under its control have been identified. It regulates the expression of genes
encoding for curli fibre subunits (
Tagliabue, Maciag, Antoniani, & Landini,
2010
) and of the poly-
N
-acetylglucosamine (PNAG) biosynthetic
pgaABCD
operon that controls the production of the exopolysaccharide
PNAG (
Tagliabue, Antoniani, et al., 2010
), thus promoting the formation
of sessile populations of bacteria enclosed in a self-produced polymeric
matrix, namely, the biofilms.
Concerning the characteristics of the globin domain, the proximal and
distal ligands of the haem are His98 and Tyr43, respectively. Tyr43, in par-
ticular, is important for the O
2
specific recognition and stability (
Kitanishi
et al., 2010
). Leu65, also found at the haem distal side, represents a gate that
renders the haem-pocket inaccessible to the environmental water that
would render the haem-iron atom more sensitive to oxidation (
Nakajima
et al., 2012
). In conclusion, Tyr43 and Leu65 together cooperate for creat-
ing the best conditions for a fully reactive and sensitive globin capable of
reversible O
2
binding.
¼
