REACTIVE OXYGEN SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 135

The TN (turnover number) was determined to be independent of pH. Based
on the concentration of Fe, the rate constant was ∼2.6 × 10 4 /s at 25°c. The
values of K m (Michaelis-Menton constant) were independent below pH 8 but
increased by 10-fold above pH 10. K m had a value of ∼80 µM, which suggests
the modification of Equation (4.28). Thus, reaction (4.28) may be followed by
reaction (4.31):

Fe SOD O

2 +

+

•−

 [

Fe SOD- O

2 + •−

] + ( 2 H

+

) → Fe SOD H O

3 +

+ . (4.31)

2 2 2 2

The saturation process was not observed with all studied FeSODs [110].
FeSOD also goes through a parallel peroxidative mechanism (Eqs. 4.32 and
4.33). Reaction (4.32) is faster than reaction (4.33) [111]:

Fe SOD H O

3 +

+ → Fe SOD HO H

2 +

+ +

+

(4.32)

2 2 2

Fe SOD H O

2 +

+ ( + 2 H

+

) → Fe SOD OH OH

3 + … .

+

−

. (4.33)

2 2

A study on the activation of E. coli FeSOD was performed by the addition
of primary amines [112]. The saturation kinetics was carefully observed for
most of the studied amines. K m was calculated as 100 ± 20 µM under the
studied conditions. Activation of the proton transfer pathway was explored by
seeking a linear relationship in the free energy plot of the apparent second-
order rate constant of the activation and the p K a of the amines (Fig. 4.6). The
linearity in the plot confirmed the activation of the proton transfer. The slope
of the plot was calculated as 0.50 ± 0.07, indicating the position of the transi-
tion state was between the reactants and products. The results suggest an
occurrence of proton transfer in a ternary complex of amine with the enzyme-
bound peroxide dianion. Furthermore, the activation by amines was uncom-
petitive with respect to superoxide [112].
The redox equilibria of FeSOD involve the inhibition by anions, N − and F − ,
which bind directly to the Fe 3+ ion [108]. complexes of FeSOD and MnSOD
have thus been compared [113]. crystal structures of N − complexes with
Fe 3+ SOD and Mn 3+ SOD showed N − bound to the Fe 3+ -to-coordinating-N
vector in the Fe 3+ SOD is more linear, while N − engages in a H-bond with Tyr34
in the Mn 3+ SOD [106]. The electron paramagnetic resonance (EPR) spectros-
copy showed the fluoride ion binds to Fe 2+ SOD outside the first coordination
sphere. In the case of Mn 2+ SOD, the fluoride ion joins the first coordination
sphere [114].

4.1.4.3 Iron Superoxide Reductase (FeSOR). Based on the number of
metal centers, two types of FeSORs have been classified: neelaredoxins (1Fe-
SORs) and desulfoferrodoxins (2Fe-SORs) [115]. In FeSORs, both iron sites
are near the molecular surfaces and are exposed to the solvent. comparatively,
metal centers in FeSODs are located inside the protein. Desulfoferrodoxins
are a homodimeric nonheme iron protein found in some sulfate-reducing