REACTIVE OXYGEN SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 130

Figure 4.4. Rate constants for O − dismutation by wild-type and various arginine 143
mutants of human scuZnSOD as a function of pH (20 mM phosphate, 10 mM formate,
5 µM EDTA). The charge on the new residue is indicated in parentheses. WTSOD,
wild-type superoxide dismutase (adapted from Abreu and cabelli [31] with the permis-
sion of Elsevier Inc.). See color insert.

Overall O : 2

•−

+ 2 H

+

→ + O H O . (4.21)

2 2 2 2

The catalytically active metal of SOD can be iron, manganese, copper, or
nickel [87]. The catalytic pH dependence activities of cu, Zn-SOD, manganese
superoxide dismutase (MnSOD), and Ni-SOD disproportionate to O •− are
shown in Figure 4.4 [31]. The diffusion-controlled rates were determined,
which did not vary in the pH range of 5.0-9.5. Significantly, the catalytic rates
were several orders of magnitude faster than the noncatalytic rates (Fig. 4.4).
This influences the half-lives ( t 1/2 ) of the dismutation of superoxide. For
example, t 1/2 of the superoxide dismutation without SOD is 10 seconds at pH 7
for 10 6 M O /H[ ] . However, in the presence of the SOD enzyme concen-

tration of 10 −9 M, t 1/2 of the enzymatic dismutation of superoxide is over an
order of magnitude faster than the spontaneous reaction.
Several studies have been carried out to understand the cu-Zn-SOD-
catalyzed dismutation of superoxide (Eqs. 4.22 and 4.23) [88-93]. Most studies
were performed using pulse radiolysis as a result of its clean and rapid genera-
tion of HO /O
2

− •− •

2 2

• •− :

2