Chemistry Reference
In-Depth Information
7
1
6.5
4
2
6
5
7
3
6
8
5.5
5
9
4.5
7.5
8.5
9.5
10.5
p
K
a
Figure 4.6.
Variation with the p
K
a
of proton donors of log[(
k
cat
/
K
m
)
donor
] for activation
of
E. coli
FeSOD. (
k
cat
/
K
m
)
donor
is the contribution to
k K
cat
/
due to the exogenous
proton donors. conditions are described in greenleaf and Silverman [112]. The
exogenous proton donors were glycine methyl ester (1), 2-bromoethylamine (2),
2-fluoroethylamine (3), 2-aminoethylsulfonic acid (4), benzylamine (5), ethanolamine
(6), glycine (7), 3-aminopropionic acid (8), and ethylamine (9) (adapted from greenleaf
and Silverman [112] with the permission of the American Society for Biochemistry and
Molecular Biology, Inc.).
m
bacteria and archaea [116, 117]. The crystal structure of desulfoferrodoxins
obtained from
Desulfovibrio desulfuricans
[116] has center I ([Fe(Scys)
4
]) and
center II (square pyramidal [Fe(NHis)
4
(Scys)]) sites. center I and center II
perform functions of electron transfer and react with superoxide, respectively
[31, 115]. Neelaredoxins, from
Archaeoglobus fulgidus
, have biofunctional
activities of both SOD and SOR. Desulfoferrodoxins and neelaredoxins have
been mainly isolated in Fe
2+
and Fe
3+
forms, respectively [115]. The numbering
method of amino acid ligands of the catalytic center of the 1Fe-SORs and
2Fe-SORs is given in Table 4.3 [115]. The iron in center I of the 2Fe-SORs is
coordinated to four cysteins in a distorted tetrahedral geometry. The common
site, center II, of both 1Fe-SORs and 2Fe-SORs, has a unique geometry. Iron
in the +2 oxidation state (Fe
2+
) is coordinated in a square-pyramidal geometry
to four nitrogens from histidine-imidazoles and a fifth axial position is occu-
pied by a cystein-sulfur (Table 4.3). generally, it is assumed the sixth axial
position is not occupied by the solvent molecule.
The UV-visible spectroscopy has been applied to study the catalytic activity
of SORs (Table 4.4) [115, 118-125]. center I of 2Fe-SORs has the character-
istics of the Fecys
4
site in
desulforedoxin
, having maxima at 375 and 495 nm
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