84
4 Characteristic Properties of Redox Enzymes as Electrocatalysts
ln
Q(θ 0 )
Q(θ 0 = π)
= (π/θ 0 − 1)ln(r/m),
(4.3)
where θ 0 denotes the angle of the edge and r denotes the radius of the edge. For
instance, at an edge with r = 1 nm and θ 0 = 3π/2, Q is intensified a thousand-fold
compared with that at a planar electrode.
Therefore, the kinetics of the electron transfer reaction between an enzyme and
an electrode seems to be affected by the porous electrode surface. The interfacial
electron transfer kinetics must be improved under such an intensified electric field
at the drastically charged edge of the nanostructure. Of course, this situation is valid
even when using inorganic catalysts.
4.4 The Control of the Orientation of Adsorbed Enzymes
by Surface Modification of Electrodes
The first factor is the control of the orientation of enzymes on the electrode surface
by utilizing various kinds of specific interactions [21]. The electrostatic interaction
is frequently utilized in redox enzymes in which the redox site is surrounded by
surface amino acid residues with a positive charge (such as BOD from Myrothecium verrucaria [22, 23]) or with a negative charge (such as an [NiFe] H 2 ase from
Desulfovibrio vulgaris Miyazaki F. [23, 24]). The electrostatic interaction of such
enzymes with the oppositely charged electrode surface improved the performance
of the DET-type reaction [22, 23]. In contrast to such situations, the redox site is
sometimes surrounded by non-charged and hydrophobic peptide. In such cases, the
hydrophobic π–π stacking interaction between the redox active site of the enzyme
and modifiers induced favorable orientation by modifying the electrode surface with
polycyclic aromatic compounds [25, 26]. In addition, the orientation of BOD was
improved by modification with bilirubin as the natural substrate of the enzyme [27],
and methoxy-functionalized electrode improved the DET-type reaction of FDH [28]
that prefers to interact with methoxy-substituent containing quinones [29].
Bilirubin oxidase (BOD)
Electrode modification using BOD has been utilized to improve the orientation of
adsorbed enzymes [27]. Although the adsorbed amount of BOD is decreased by
modification with bilirubin, i.e., the neutral electron donor of BOD, the modification
was effective for improving DET-type bioelectrocatalysis of BOD [27, 30]. This
result demonstrates that the control of the orientation of the adsorbed enzyme is very
effective to improve the performance of the DET-type reaction.
Studies on the effects of electrode modification on the orientation of BOD
showed that, for controlling the orientation of BOD, attractive electrostatic interaction between the modifier and BOD was very effective. Interestingly, effective
modifiers for the orientation of BOD depend on the origin of BOD, e.g., a negatively
4 Characteristic Properties of Redox Enzymes as Electrocatalysts
ln
Q(θ 0 )
Q(θ 0 = π)
= (π/θ 0 − 1)ln(r/m),
(4.3)
where θ 0 denotes the angle of the edge and r denotes the radius of the edge. For
instance, at an edge with r = 1 nm and θ 0 = 3π/2, Q is intensified a thousand-fold
compared with that at a planar electrode.
Therefore, the kinetics of the electron transfer reaction between an enzyme and
an electrode seems to be affected by the porous electrode surface. The interfacial
electron transfer kinetics must be improved under such an intensified electric field
at the drastically charged edge of the nanostructure. Of course, this situation is valid
even when using inorganic catalysts.
4.4 The Control of the Orientation of Adsorbed Enzymes
by Surface Modification of Electrodes
The first factor is the control of the orientation of enzymes on the electrode surface
by utilizing various kinds of specific interactions [21]. The electrostatic interaction
is frequently utilized in redox enzymes in which the redox site is surrounded by
surface amino acid residues with a positive charge (such as BOD from Myrothecium verrucaria [22, 23]) or with a negative charge (such as an [NiFe] H 2 ase from
Desulfovibrio vulgaris Miyazaki F. [23, 24]). The electrostatic interaction of such
enzymes with the oppositely charged electrode surface improved the performance
of the DET-type reaction [22, 23]. In contrast to such situations, the redox site is
sometimes surrounded by non-charged and hydrophobic peptide. In such cases, the
hydrophobic π–π stacking interaction between the redox active site of the enzyme
and modifiers induced favorable orientation by modifying the electrode surface with
polycyclic aromatic compounds [25, 26]. In addition, the orientation of BOD was
improved by modification with bilirubin as the natural substrate of the enzyme [27],
and methoxy-functionalized electrode improved the DET-type reaction of FDH [28]
that prefers to interact with methoxy-substituent containing quinones [29].
Bilirubin oxidase (BOD)
Electrode modification using BOD has been utilized to improve the orientation of
adsorbed enzymes [27]. Although the adsorbed amount of BOD is decreased by
modification with bilirubin, i.e., the neutral electron donor of BOD, the modification
was effective for improving DET-type bioelectrocatalysis of BOD [27, 30]. This
result demonstrates that the control of the orientation of the adsorbed enzyme is very
effective to improve the performance of the DET-type reaction.
Studies on the effects of electrode modification on the orientation of BOD
showed that, for controlling the orientation of BOD, attractive electrostatic interaction between the modifier and BOD was very effective. Interestingly, effective
modifiers for the orientation of BOD depend on the origin of BOD, e.g., a negatively