Biology Reference
In-Depth Information
solvent occurs on the tens or hundreds of nanosecond timescale (
van
Wilderen, Key, Van Stokkum, van Grondelle, & Groot, 2009
). Anisotropy
measurements reveal that the haem-bound CO makes a substantial angle
with the haem normal (ca. 30
), indicating that the protein environments
in FixL impose strain on the haem-bound CO (
Nuernberger et al.,
2011
). This strain, which will be caused by the steric interaction among
Ile215, Ile236, and CO, causes CO to tilt away from the perpendicular ori-
entation and would be a reason of a lower CO-binding affinity for FixL
compared with globins (
Nuernberger et al., 2011
).
Hiruma, Kikuchi, Tanaka, Shiro, and Mizutani (2007)
have studied the
structural changes of
Sm
FixL by means of time-resolved resonance Raman
spectroscopy. They reveal that changes in the time-resolved resonance
Raman spectra occur in three steps. In the step 1, the band intensities of
the
n
(Fe-His),
g
7, and
n
8 modes change in a timescale of 0.2-2
m
s. In step
2, the intensity of the
d
(C
b
C
c
C
d
) band changes in a timescale of 0.8-9
m
s. In
step 3, the
g
7,
n
8, and
d
(C
b
C
c
C
d
) modes change after
>
1 ms.
The
d
(C
b
C
c
C
d
) band is sensitive to the strength of the hydrogen bond of
the haem propionates (
Gottfried et al., 1996
). The frequency of this band is
down-shifted as the strength of the hydrogen bond decreases. The intensity
of the
n
8 mode is correlated with disorder in the orientation of the haem
propionate groups (
Peterson, Friedman, Chien, & Sligar, 1998
). The
g
7
mode is associated with an out-of-plane motion of the methine carbons
of haem, and this mode is observed in the high-spin haems, but not in
the low-spin haems (
Hu, Smith, & Spiro, 1996
). Thus, the intensity change
of the
g
7 mode is indicative of the extent of haem doming.
In the timescale of 10 ns to 10
m
s, different relaxations are observed
between O
2
-dissociated and CO-dissociated species (
Hiruma et al.,
2007
). In this timescale, fast intensity changes of
g
7 and
n
8 modes are
observed only for O
2
-dissociated species. The intensity change of the
d
(C
b
C
c
C
d
) band is faster for O
2
-dissociated species than for
CO-dissociated species. The
n
(Fe-His) band shows the intensity change
for the O
2
-dissociated species but not
for CO-dissociated species
in
Sm
FixLH.
The relaxation of the
n
(Fe-His) band after ligand photo-dissociation is
different between
Sm
FixLH and
Bj
FixLH especially for O
2
-dissociated spe-
cies. In picosecond time-resolved resonance Raman spectroscopy with
Bj
FixLH, the
n
(Fe-His) band is not observed even at a time delay as short
as 0.5 ps after photolysis of O
2
-bound
Bj
FixLH, which is consistent with
ultrafast O
2
rebinding described earlier (
Kruglik et al., 2007
).
