Biomedical Engineering Reference
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enzymes indicate that they are structurally similar, but unrelated to the Cu, Zn-SOD
proteins [106-114]. The monomers fold into two helix-rich domains with Mn or Fe
bound by two residues from each domain. Dimer contacts occur at an interface bridg-
ing two metal sites that are separated by about 18 Å. In both Fe-SOD and Mn-SOD, the
central metal is coordinated to three histidine nitrogens and one aspartate oxygen, with
several conserved aromatic residues enveloping the active site. While the accurate size
of the SODs is diffi cult to determine, previous works have suggested that Fe-SOD and
Mn-SOD are a little larger than Cu, Zn-SOD [115, 116].
6.4.3 Electrochemistry of SODs
It is known that the direct electron transfer between redox enzymes and proteins and an
electrode is relatively diffi cult to obtain because of the existence of a thick insulating
protein shell around the active sites of the enzymes and proteins [117-119]. Similarly, it
has been a long-standing challenge to realize the direct electron transfer properties of the
SODs. Efforts in this fi eld have been motivated by the facts that the information on direct
electron transfer is very useful in understanding the intrinsic thermodynamic and kinetic
properties of the SODs and, more importantly, in practical development of the SOD-based
third-generation biosensors for O
2
•
. Iyer observed a direct and irreversible oxidation of
Cu, Zn-SOD at a bare Au electrode in phosphate buffer solution (pH 4.0) and suggested
that a conformational change occurs at the active sites via its adsorption on the electrode
surface, which thus facilitated the direct electron transfer [120]. Borsari and Azab and
their coworkers [121, 122] and Wu
et al.
[123, 124] have observed the reversible redox
response of bovine and human Cu, Zn-SOD at an Au electrode in the presence of so-
called promoters for direct electron transfer. Ohsaka
et al.
found that the electron transfer
between a glassy carbon electrode and polyethylene oxide-modifi ed Cu, Zn-SOD could be
successfully accomplished by using methyl viologen as a redox mediator [125].
Recently, much effort has been made on the facilitation of direct electron transfer
of the SODs by self-assembled monolayers (SAMs) confi ned onto Au electrodes. For
instance, Ohsaka
et al.
have formed various kinds of SAMs of alkanethiols onto an Au
electrode and studied the electron transfer properties of the SODs [98]. Here, we will
use the SAM of cysteine as an example to demonstrate the electron transfer of the SODs
promoted by the SAMs of alkanethiols. Figure 6.1 depicts cyclic voltammograms (CVs)
obtained at a cysteine-modifi ed Au electrode (curves a and b) in 25 mM phosphate buffer
containing 0.56mM Cu, Zn-SOD (the concentration used represents that of the Cu
2
or Zn
2
site of Cu, Zn-SOD). For comparison, the CV obtained at a bare Au electrode
(curve c) under the same conditions was also given. As shown, the cysteine-modifi ed
electrode exhibits one pair of well-defi ned voltammetric peaks in the SOD-containing
phosphate buffer (curve a). These redox peaks were not obtained at the bare Au elec-
trode (curve c). This observation suggests that the direct electron transfer between Cu,
Zn-SOD and Au electrode does not occur actually at the bare electrode, but it can be sig-
nifi cantly promoted at Au electrode modifi ed with the SAM of cysteine.
As described above, Cu, Zn-SOD contains one Cu (II) and Zn (II) per monomer sub-
unit, of which both Cu (II) and Zn (II) are redox active. Wang
et al.
have observed two
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