REACTIVE OXYGEN SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 137

TABLE 4.3. Numbering of Amino Acid Ligands of the Catalytic Center of the
SORs Represented in the Sequence Alignment and in the Dendogram
Fe Ligands

glu His His His His cys Lys

1Fe-SOR
P. furiosus E14 H16 H41 H47 H114 c111 K15

D. gigas E15 H17 H45 H51 H118 c115 K16

E12 H14 H40 H46 H113 c110 K13

A. fulgidus

N. equitans - H10 H35 H41 H100 c97 K9

T. pendens E24 H26 H51 H57 H113 c110 -

M. acetivorans - H27 H54 H60 H114 c111 K26

T. pallidum E48 H50 H70 H76 H122 c119 K49

2Fe-SOR
C. phytofermentans

E47 H49 H69 H75 H121 c118 K48

D. vulgaris E47 H49 H69 H75 H119 c116 K48

E47 H49 H69 H75 H119 c116 K48

D. desulfuricans

D. baarsii E47 H49 H69 H75 H119 c116 K48

G. uraniireducens E47 H49 H69 H75 H119 c116 K48

A. fulgidus E47 H49 H69 H75 H119 c116 K48

Rd SOR
Uncultured
bacterium

- H53 H83 H89 H137 c134 K52

-, residues not conserved.
Adapted from Pinto et al. [115] with the permission of Elsevier Inc.

and a broad shoulder at 560 nm (Table 4.4). center II has a broad absorption
band at 660 nm, which represents a blue color of 1Fe-SORs or a gray color (a
mixture of blue and pink) for the oxidized 2Fe-SORs and a shoulder at
ca. 330 nm. A sulfur to iron charge transfer transition generates the 660-nm
band [126]. In the reduced form, both types of SORs are colorless. In the half-
reduced state (center I oxidized, center II reduced), both SOR types are pink.
The UV-visible spectral characteristics were used to understand the catalytic
mechanism of SORs using fast kinetic techniques (stopped-flow and pulse
radiolysis).
The reduction potentials are 0 and 190-365 mV for center I and center II,
respectively (Table 4.4). The resulting potentials are similar to reported values
for SODs and sufficient for the reduction of superoxide, which has a potential
of 890 mV ( O /H O
2

− ) at pH 7.0. The large difference between the two centers
allowed a detailed study of the events at the catalytic site without interferences
from the Fecys 4 center. Both 1Fe-SORs and 2Fe-SORs have pH-dependent
equilibria (Table 4.4), important to understand the catalytic activities of SORs.
Using an example of 1Fe-SOR of A. fulgidus , the catalytic cycle of SOR is
represented in Figure 4.7. The first step of the reaction is the formation of the

2 2