86
4 Characteristic Properties of Redox Enzymes as Electrocatalysts
heme c revealed that one heme c moiety (the first one from the N-terminal) does not
participate in DET-type bioelectrocatalysis [42]. Therefore, the variant FDH, lacking
the heme c moiety, shows a high DET-type bioelectrocatalytic activity [43].
DET-type bioelectrocatalysis of FDH has been studied using a SAM-modified
electrode [3]. SAM formed by 2-mercaptoethanol was found to be more effective for
DET-type bioelectrocatalysis of FDH compared to charged and hydrophobic SAMs.
The terminal hydroxy group changes the hydrophobicity of the electrode surface. The
coexisting surfactant that will be used for solubilizing FDH forms a bilayer on the
hydrophilic electrode surface. Therefore, the FDH seems to be firmly embedded in
the surfactant bilayer on the electrode surface to communicate with the electrode [44].
The natural electron acceptor of FDH is ubiquinone, although FDH was able
to donate an electron to various mediators based on the linear free energy relationship (LFER). However, ubiquinone derivatives were found to react with FDH
with a much higher activity than expected in accordance with LFER [29]. Certain
specific interactions of FDH with the structure of ubiquinone (2,3-dimethoxy-1,4benzoquinone) were expected; thus, we investigated the effects of the surface modifier on DET-type bioelectrocatalysis of FDH at the KB electrode and found that
the methoxy group in the modifier improved DET-type bioelectrocatalysis of FDH.
Analysis of the catalytic wave based on Eq. (3.19) revealed that modification with
2,4-dimethoxyaniline reduced the 2βr as values from 2.1 to 1.2 for the orientation of
FDH. This indicates that the methoxy-substituent-functionalized surface increases
the effective orientated FDH at the electrode surface [28].
In conclusion, nanostructures of the electrode surface are very important and
probably essential for DET-type bioelectrocatalysis by redox enzymes. Important
functions of nanostructures are the curvature effect of the mesoporous structure,
the enhanced electric double layer effect at the edge of the microporous structure,
and specific interaction between the electrochemically active site of enzymes and
electrode surface as well as the mass transfer effect of the macroporous structure.
Several specific interactions are very critical to improve the orientation of enzymes.
The electrode surface can be suitably tailored for each redox enzyme in the near future
by a better understanding of the effects described in this chapter. Protein engineering
is essential and effective to suitably interact enzymes with nanostructured electrode
surfaces.
4.5 DET-Type Bi-Way Bioelectrocatalysis
Progress has been made in tailor-made tuning of electrode surfaces for several kinds
of enzymes to improve the DET-type bioelectrocatalytic performance by considering the above factors, and the number of redox enzymes that provide DET-type
bioelectrocatalytic waves is growing. Among such DET-type redox enzymes, it can
be noticed that several enzymes display single catalytic wave of bi-directional interconversion of oxidized and reduced substrates, in which the catalytic wave sharply
intersects with the potential axis at the zero-current–potential (E i = 0 ) that is identical
4 Characteristic Properties of Redox Enzymes as Electrocatalysts
heme c revealed that one heme c moiety (the first one from the N-terminal) does not
participate in DET-type bioelectrocatalysis [42]. Therefore, the variant FDH, lacking
the heme c moiety, shows a high DET-type bioelectrocatalytic activity [43].
DET-type bioelectrocatalysis of FDH has been studied using a SAM-modified
electrode [3]. SAM formed by 2-mercaptoethanol was found to be more effective for
DET-type bioelectrocatalysis of FDH compared to charged and hydrophobic SAMs.
The terminal hydroxy group changes the hydrophobicity of the electrode surface. The
coexisting surfactant that will be used for solubilizing FDH forms a bilayer on the
hydrophilic electrode surface. Therefore, the FDH seems to be firmly embedded in
the surfactant bilayer on the electrode surface to communicate with the electrode [44].
The natural electron acceptor of FDH is ubiquinone, although FDH was able
to donate an electron to various mediators based on the linear free energy relationship (LFER). However, ubiquinone derivatives were found to react with FDH
with a much higher activity than expected in accordance with LFER [29]. Certain
specific interactions of FDH with the structure of ubiquinone (2,3-dimethoxy-1,4benzoquinone) were expected; thus, we investigated the effects of the surface modifier on DET-type bioelectrocatalysis of FDH at the KB electrode and found that
the methoxy group in the modifier improved DET-type bioelectrocatalysis of FDH.
Analysis of the catalytic wave based on Eq. (3.19) revealed that modification with
2,4-dimethoxyaniline reduced the 2βr as values from 2.1 to 1.2 for the orientation of
FDH. This indicates that the methoxy-substituent-functionalized surface increases
the effective orientated FDH at the electrode surface [28].
In conclusion, nanostructures of the electrode surface are very important and
probably essential for DET-type bioelectrocatalysis by redox enzymes. Important
functions of nanostructures are the curvature effect of the mesoporous structure,
the enhanced electric double layer effect at the edge of the microporous structure,
and specific interaction between the electrochemically active site of enzymes and
electrode surface as well as the mass transfer effect of the macroporous structure.
Several specific interactions are very critical to improve the orientation of enzymes.
The electrode surface can be suitably tailored for each redox enzyme in the near future
by a better understanding of the effects described in this chapter. Protein engineering
is essential and effective to suitably interact enzymes with nanostructured electrode
surfaces.
4.5 DET-Type Bi-Way Bioelectrocatalysis
Progress has been made in tailor-made tuning of electrode surfaces for several kinds
of enzymes to improve the DET-type bioelectrocatalytic performance by considering the above factors, and the number of redox enzymes that provide DET-type
bioelectrocatalytic waves is growing. Among such DET-type redox enzymes, it can
be noticed that several enzymes display single catalytic wave of bi-directional interconversion of oxidized and reduced substrates, in which the catalytic wave sharply
intersects with the potential axis at the zero-current–potential (E i = 0 ) that is identical