Biology Reference
In-Depth Information
Voordouw, 1992
). DcrA from
D. vulgaris
consists of an N-terminal trans-
membrane helix, periplasmic PAS, another transmembrane helix, HAMP,
cytoplasmic PAS, and MCP domains (
Deckers & Voordouw, 1994a,
1994b
). Though its physiological effector signal is not obvious at present,
it is supposed to act as a chemotaxis signal transducer protein sensing oxygen
concentration and/or redox potential (
Fu & Voordouw, 1997; Fu, Wall, &
Voordouw, 1994; Yoshioka et al., 2005
).
Though the sequence identity of the periplasmic domain of DcrA
(DcrA-N) from
D. vulgaris
with the PAS domains in GSU0935 and
GSU0582 is low, the c-type haem-binding motif (Cys-x-x-Cys-His) is con-
served among them (
Fig. 7.13
). DcrA contains a c-type haem covalently
bound to Cys in this motif with a thioether bond (
Fu et al., 1994;
Yoshioka et al., 2005
). In the resonance Raman spectra, DcrA-N shows
the
d
(C
b
C
a
C
b
) and
n
(C
a
-S) bands that are characteristic bands for c-type
haems containing covalent thioether bonds (
Desbois, 1994; Hu, Morris,
Singh, Smith, & Spiro, 1993; Yoshioka et al., 2005
).
The ferric haem in DcrA-N is a mixture of a 6-coordinate, low-spin and
6-coordinate, high-spin states, which is revealed by resonance Raman spec-
trum showing two
n
2 (1568 and 1580 cm
1
) and
n
3 (1481 and 1509 cm
1
)
bands (
Yoshioka et al., 2005
). Ferric DcrA-N shows a similar UV-vis spec-
trum to that of M80A variant of cytochrome c with the Soret band at
400 nm (
Brem & Gray, 1993
). Ferrous DcrA-N shows the
n
2 and
n
3 bands
at 1593 and 1495 cm
1
, respectively, indicating that the ferrous haem in
DcrA-N is a 6-coordinate, low-spin state (
Yoshioka et al., 2005
). Reso-
nance Raman and UV-vis spectra of DcrA-N reveal the coordination struc-
tures of the haem as shown in
Fig. 7.12
B. The His in the Cys-x-x-Cys-His
motif is the proximal ligand of the haem in DcrA. Given that DcrA is a redox
sensor, the ligand exchange between H
2
O (or OH
) and Met upon the
change in the oxidation state of the haem will play an important role for
intramolecular signal transductions because the coordination/dissociation
of the sixth ligand may cause conformational changes in the distal haem
pocket. Though the sixth axial ligand in the ferrous haem in DcrA-N is
not identified, Met61 will be a candidate of the axial ligand of the ferrous
haem as are the cases of GSU0935 and GSU0582 because Met 61 is con-
served at the corresponding positions of Met60 in GSU0935 and
GSU0582 (
Fig. 7.13
).
While a stable O
2
-bound form is not produced because an autoxidation
takes place upon the reaction of ferrous DcrA with O
2
, CO is bound to the
haem in DcrA to form the CO-bound DcrA. CO-bound DcrA-N shows
