Chemistry Reference
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H30Q did not show much variation in
k
25
and
k
26
but increased the rate sig-
nificantly of the protonation off of the bound
O
•−
in human enzymes (Table
4.2). Other possible residues involved in the dissociation of the complex in
human enzymes were E162 and E66. E162D, E162A, F66A, and F66L had a
decreased
k
27
value in comparison with the wild-type (WT) MnSOD. Values
of
k
27
for these mutants were similar to the values determined for prokaryotic
enzymes. Overall, the rate constants determined in Table 4.2 assist in the
understanding of the mechanistic differences in eukaryotic versus prokaryotic
MnSODs [31, 100, 102, 105].
4.1.4.2 Iron Superoxide Dismutase.
FeSOD, a prokaryotic enzyme, was dis-
covered in some bacterial cells and in the cytosol area of plants [31]. Most of
the structural and mechanistic studies have been performed on FeSOD
obtained from
E. coli
. The structures of FeSODs are dimers in which each iron
active site contains a single iron atom bonded to three histidines, one aspartate,
and one water molecule [106]. The coordinated water molecule involves a
hydrogen bond (H-bond) with an aspartate ligand and another with the con-
served active site, glutamine 69 (gln 69). The H-bonding network plays an
important role to determine the reactivity of the Fe site.
A ping-pong mechanism is displayed in Equations (4.28) and (4.29), sug-
gested in the dismutation of
O
•−
by FeSOD [107]:
Fe SOD O
3
+
+
•−
→
Fe SOD O
2
+
+
(4.28)
2
2
2
+
•−
+
3
+
Fe SOD O
+
(
+
2
H
)
→
Fe SOD H O
+
.
(4.29)
2
2
2
In this mechanism, Fe
III
is reduced by superoxide through the binding of
O
•−
to Fe (Eq. 4.28) [107, 108]. Reaction (4.29) indicates second-sphere binding
of the next superoxide molecule, which results in the oxidation of Fe
II
and the
formation of hydrogen peroxide. Electron and proton transfers are coupled
together during the activity of FeSOD, and in particular, the transfer of a
proton to superoxide in Equation (4.29) determines the thermodynamic fea-
sibility [108]. The involvement of the coordinated water molecule may be a
redox-coupled proton acceptor and may possibly be donating one of the
protons in peroxide [109].
The decay of
O
•−
in the presence of FeSOD, purified from marine bacterium
Photobacterium leiognathi
, was determined to be first order and was propor-
tional to the concentration of FeSOD [107]. The second-order rate constant
decreased from 6.1 × 10
8
/M/s to 1.3 × 10
8
/M/s with an increase in pH from 6.2
to 10.1. Similar results were also observed in FeSOD from the
filamentous
cyanobacterium Nostoc
Pcc 7120 [110]. However, the rate constants ranged
from 5.3 × 10
9
/M/s (pH 7) to 4.8 × 10
6
10
9
/M/s (pH 10). The decay study using
FeSOD from
E. coli
followed Michaelis-Menton kinetics (Eq. 4.30) [108]:
- [
d
O /
•−
]
dt
=
k
[
O
•−
]
+
2
k
[
O
•−
]
2
+
2
[
FeSOD TN O /
]
[
•−
] (
K
+ O
•−
]).
(4.30)
[
2
fir
2
sec
2
2
m
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