Biomedical Engineering Reference
In-Depth Information
in the cathode of a fuel cell for reduction of oxygen was by Palmore and Kim [26],
who investigated the reduction of oxygen to water by a solution-phase laccase from
Pyricularia oryzae
using 2,2
-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) as
a diffusional mediator.
The laccases, classed as polyphenol oxidases, catalyze the oxidation of diphenols,
polyamines, as well as some inorganic ions, coupled to the four-electron reduction of
oxygen to water: see Fig. 12.4 for the proposed catalytic cycle. Due to this broad spe-
cifi city, and the recognition that this specifi city can be extended by the use of redox
mediators [27], laccases show promise in a range of applications [28], from biosensors
[29-32], biobleaching [27, 33-35] or biodegradation [36], to biocatalytic fuel cells
[1-3, 18, 26, 37-42].
Laccase was fi rst isolated by Yoshida in 1883 [43] from tree lacquer of
Rhus ver-
nicifera
. Laccases can thus be classifi ed according to their source: plant, fungal or, more
recently, bacterial or insect [44]. The laccase enzyme active site contains four copper ions
classifi ed into three types based upon their geometry and coordinating ligands, denoted
Cu(II)
Cu(II)
A
Cu(II)
Cu(I)
Cu(I)
OH
Cu(I)
OH
H
S
H
2
O
S
Cu(II)
Cu(II)
Cu(II)
Cu(II)
Cu(II)
OH
Cu(II)
Cu(I)
Cu(I)
O
Cu(II)
Cu(I)
H
Cu(II)
Cu(II)
S
B
Cu(II)
Cu(II)
H
H
S
H
H
2
O
H
Cu(II)
Cu(II)
H
2
O
Cu(II)
Cu(II)
OH
OH
Cu(II)
Cu(II)
slow
Cu(II)
Cu(II)
T2
T1
Cu(I)
H
T3
Cu(II)
Cu(II)
resting fully oxidized
Cu(I)
“native intermediate”
S
H
2
O
reduction
Cu(I)
S
2H
Cu(II)
Cu(I)
Cu(II)
Cu(II)
Cu(II)
Cu(II)
Cu(I)
Cu(I)
OH
OH
Cu(II)
Cu(II)
Cu(II)
S
H
S
slow
Cu(I)
O
2
H
2
O
Cu(I)
Cu(II)
Cu(I)
Cu(II)
S
S
Cu(I)
Cu(I)
Cu(II)
Cu(I)
OH
Cu(I)
Cu(II)
Cu(I)
Cu(I)
Cu(II)
Cu(I)
Cu(I)
Cu(I)
Cu(I)
Cu(I)
fully reduced
Cu(I)
Cu(II)
Cu(II)
Cu(II)
OH
Cu(II)
FIGURE 12.4
Proposed catalytic cycle for laccase, where S represents substrate. (From [44], with per-
mission from the American Chemical Society.)
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