4.4 The Control of the Orientation of Adsorbed Enzymes …
85
charged modifier was shown to be effective for orienting BOD from Myrothecium
verrucaria (MvBOD) [22, 30], while a positively charged modifier was effective for
BOD from Bacillus pumilus [30]. The entire charge of MvBOD was negative at pH
7; however, the surface of the BOD around the T1 copper site was positively charged
at pH 7. The effectiveness of the negatively charged modifier for orienting MvBOD
demonstrates that the local charge of the surface of the enzyme or the dipole in the
enzyme are crucial for controlling the orientation of the adsorbed enzyme. Therefore, MvBOD was shown to be effectively orientated on the electrode surface by
modifying negatively charged aromatic compounds.
Both small molecules and nanomaterials are effective modifiers of the electrode
for DET-type bioelectrocatalysis of BOD. The modified gold nanoparticles [12, 14,
31, 32] and carbon nanotubes [30, 33–35] have been shown to be effective scaffolds
for DET-type bioelectrocatalysis of BOD.
Hydrogenase
Hydrogenase is a unique enzyme that catalyzes bi-directional reactions of hydrogen
oxidation and proton reduction. Membrane-bound [NiFe] hydrogenases (MBH 2 ase)
provide high DET-type bioelectrocatalytic activity [36, 37]. Moreover, MBH 2 ase has
several redox centers such as [NiFe] cluster and iron-sulfur clusters (FeS) called the
proximal, medial, and distal. The electronic communication between MBH 2 ase and
electrode generally occurred at distal FeS [38, 39]. To improve the performance of
DET-type bioelectrocatalysis of MBH 2 ase, we studied the effects of modification of
the electrode surface.
Carbon nanotubes have been shown to be effective scaffolds of DET-type bioelectrocatalysis of MBH 2 ases from Desulfovibrio fructosovorans and Aquifex aeolicus
[40, 41]. Because the shortening process of nanotubes improved DET-type activity,
the edge parts or defects on the surface of carbon nanotubes seem to be conducive for
DET-type bioelectrocatalysis. Moreover, the chemical interaction between MBH 2 ase
and modifiers has been investigated. The adsorption of thiol on the gold surface forms
a functional self-assembled monolayer (SAM). The SAM formed by a short and positively charged thiol was most effective for DET-type bioelectrocatalysis of MBH 2 ase
from Aquifex aeolicus [41].
Furthermore, the positively charged modifier was found to be effective in the
DET-type bioelectrocatalysis of MBH 2 ase from Desulfovibrio vulgaris Miyazaki F.
This experimental observation agrees with the fact that the distal FeS is located on
the negatively charged surface of the MBH 2 ase. The increase in ionic strength in the
measurement buffer decreases with the DET-type activity of the MBH 2 ase adsorbed
at p-phenylenediamine-modified electrodes. Moreover, the effect of the ionic strength
has been found to support the expectation that the electrostatic interaction between the
MBH 2 ase and electrode controls the orientation of the adsorbed MBH 2 ase [23, 24].
d-Fructose dehydrogenase (FDH)
FDH is a heterotrimeric enzyme for which one subunit containing three heme c
moieties is the most electrochemically active site [29] and exhibits an extremely
high DET-type bioelectrocatalytic activity. The mutations on the axial ligands of each
85
charged modifier was shown to be effective for orienting BOD from Myrothecium
verrucaria (MvBOD) [22, 30], while a positively charged modifier was effective for
BOD from Bacillus pumilus [30]. The entire charge of MvBOD was negative at pH
7; however, the surface of the BOD around the T1 copper site was positively charged
at pH 7. The effectiveness of the negatively charged modifier for orienting MvBOD
demonstrates that the local charge of the surface of the enzyme or the dipole in the
enzyme are crucial for controlling the orientation of the adsorbed enzyme. Therefore, MvBOD was shown to be effectively orientated on the electrode surface by
modifying negatively charged aromatic compounds.
Both small molecules and nanomaterials are effective modifiers of the electrode
for DET-type bioelectrocatalysis of BOD. The modified gold nanoparticles [12, 14,
31, 32] and carbon nanotubes [30, 33–35] have been shown to be effective scaffolds
for DET-type bioelectrocatalysis of BOD.
Hydrogenase
Hydrogenase is a unique enzyme that catalyzes bi-directional reactions of hydrogen
oxidation and proton reduction. Membrane-bound [NiFe] hydrogenases (MBH 2 ase)
provide high DET-type bioelectrocatalytic activity [36, 37]. Moreover, MBH 2 ase has
several redox centers such as [NiFe] cluster and iron-sulfur clusters (FeS) called the
proximal, medial, and distal. The electronic communication between MBH 2 ase and
electrode generally occurred at distal FeS [38, 39]. To improve the performance of
DET-type bioelectrocatalysis of MBH 2 ase, we studied the effects of modification of
the electrode surface.
Carbon nanotubes have been shown to be effective scaffolds of DET-type bioelectrocatalysis of MBH 2 ases from Desulfovibrio fructosovorans and Aquifex aeolicus
[40, 41]. Because the shortening process of nanotubes improved DET-type activity,
the edge parts or defects on the surface of carbon nanotubes seem to be conducive for
DET-type bioelectrocatalysis. Moreover, the chemical interaction between MBH 2 ase
and modifiers has been investigated. The adsorption of thiol on the gold surface forms
a functional self-assembled monolayer (SAM). The SAM formed by a short and positively charged thiol was most effective for DET-type bioelectrocatalysis of MBH 2 ase
from Aquifex aeolicus [41].
Furthermore, the positively charged modifier was found to be effective in the
DET-type bioelectrocatalysis of MBH 2 ase from Desulfovibrio vulgaris Miyazaki F.
This experimental observation agrees with the fact that the distal FeS is located on
the negatively charged surface of the MBH 2 ase. The increase in ionic strength in the
measurement buffer decreases with the DET-type activity of the MBH 2 ase adsorbed
at p-phenylenediamine-modified electrodes. Moreover, the effect of the ionic strength
has been found to support the expectation that the electrostatic interaction between the
MBH 2 ase and electrode controls the orientation of the adsorbed MBH 2 ase [23, 24].
d-Fructose dehydrogenase (FDH)
FDH is a heterotrimeric enzyme for which one subunit containing three heme c
moieties is the most electrochemically active site [29] and exhibits an extremely
high DET-type bioelectrocatalytic activity. The mutations on the axial ligands of each