Biology Reference
In-Depth Information
Table 8.4 Values of kinetic parameters for O
2
and CO binding to penta and
hexacoordinated Hbs.
k
on
O
2
(
m
M
1
s
1
)
k
off
O
2
(s
1
)
k
on
CO
(
m
M
1
s
1
)
k
off
CO
(s
1
)
P
50
O
2
(Torr)
haem protein
Reference
Hexacoordinated
Ph-2
/2HbO
(8
C)
0.9
1.0
0.35
-
1
Russo et al.
(2013)
Ph
-2/2HbO
(25
C)
4.2
0.69
(slow)
12.0 (fast)
-
-
Giordano
et al. (2011)
Ngb
170
0.7
40
-
6.8
(0.9 S-S)
Uzan et al.
(2004)
Pentacoordinated
Sperm whale
Mb
14
12
0.51
0.019
0.51
Springer et al.
(1994)
M. tuberculosis
HbO
*
0.11
0.0014 0.014
0.004
-
Ouellet et al.
(2003)
(80%)
(78%)
(79%)
(60%)
0.85
0.0058 0.18
0.0015 -
(20%)
(22%)
(21%)
(40%)
*
The relative percentage of the two rate constants are reported in parentheses.
Olson, & Phillips, 1994
). The relatively fast CO-dependent kinetic
process is unusual for a TrHbs of group II; the second-order rate constant
of the fast phase for carbonylation of
M. tuberculosis
HbO is in the range
of 10
5
M
1
s
1
, whereas the second-order rate constant of the slow phase
is in the range of 10
4
M
1
s
1
(
Ouellet et al., 2003
). Thus the proteins dis-
plays two conformations that greatly differ in the ligand association rate,
suggesting that they may switch between two distinct functional levels.
At 8
C, 85% of the CO bimolecular recombination occurs on the ms
timescale at a rate similar to that of a 3/3 Mb (3.5
10
5
M
1
s
1
), whereas
the remaining kinetic component is faster (10
7
M
1
s
1
)(
Russo et al.,
2013
), as observed at 25
C(
Giordano et al., 2011
;
Table 8.4
).
Hexacoordination of the haem-Fe atom of
Ph
-2/2HbO via distal Tyr is
only partial (the Tyr equilibrium affinity is close to 1), indicating a weak inter-
action between Tyr and the Fe atomunder atmospheric pressure (
Russo et al.,
2013
). The fast binding and dissociation of Tyr from the Fe atom can be a
molecular event
that
triggers
the shift of
the globin between two
