18
1 Redox Proteins and Bioelectrocatalysis
throughout mammalian tissues. Much of the manganese present functions as cofactors of several redox enzymes: pyruvate carboxylase, SOD, arginase, glutamate
synthetase. Vanadium is involved in nitrogenase and haloperoxidases.
1.3 Bioelectrocatalysis
It is well known that redox enzymes are extremely efficient catalysts, but it has also
been established that redox enzymes can work as efficient electrocatalysts. Over the
last four decades, redox enzymes have received much attention for use in the coupling
of the enzyme reactions with non-specific electrochemical reactions. The coupled
reaction is known as bioelectrocatalysis. Several review articles have been published
[7, 42–56] and they can be helpful for readers with interest in this field.
We would like to propose an important concept in the coupling that the electrode
reaction is not hydride ion transfer, but electron transfer, therefore molecules that can
communicate with electrodes should have the electron transfer ability. In addition, the
essential features of redox enzymes for realizing the coupling are that most of redox
enzymes except NAD(P)-dependent dehydrogenases show low substrate specificity
for either substrate. The natural substrates with low specificity have the electron
transfer ability, and can be replaced with artificial redox compounds (and in some
cases with electrodes). For example, the natural electron acceptor of FDH is CoQ 10
[57]; however, it can be replaced with several other quinones, inorganic oxidants, and
several electrodes [58], because the electron donating site is a heme in the enzyme.
Similar situations are observed even in oxidases and peroxidases. For instance, O 2
as the natural electron acceptor of FAD-dependent glucose oxidase (GOD) can be
replaced with several organic and inorganic oxidants, because the electro donating
site in the enzyme is FAD that can undergo both electron and hydride ion transfers.
On the other hand, bilirubin as the natural electron donor of bilirubin oxidase (BOD)
can be replaced with several organic and inorganic reductants, and several electrodes
[59], because the electro accepting site is T1Cu in the enzyme. Several organic and
inorganic reductants (as well as electrodes) can work as electron donors of HRP [60],
of which the electrodonatig site is a protoheme.
In these manners, most redox enzyme reactions can be coupled with electrode
reactions via (artificial) redox compounds (called mediators) that shuttle electrons
between enzymes and electrodes. This reaction is known as mediated electron transfer
(MET)-type bioelectrocatalysis (Fig. 1.16). Since NAD(P)(H) cannot communicate with electrode in a reversible way (or with low over potentials), NAD(P)dependent enzymatic reactions have to be coupled with electrode reactions by using
redox mediators that can transfer both hydride ion and electron, such as flavins,
quinones (especially o-quinones), and phenothiazines (such as Meldola’s blue).
NAD(P)-dependent enzymatic reactions can also be coupled with electrode reactions by flavoproteins such as diaphorase and ferredoxin NADP
+ reductase. Microbes
and organelles (containing such redox enzymes) also work as electrocatalysts in
MET-type bioelectrocatalysis.
1 Redox Proteins and Bioelectrocatalysis
throughout mammalian tissues. Much of the manganese present functions as cofactors of several redox enzymes: pyruvate carboxylase, SOD, arginase, glutamate
synthetase. Vanadium is involved in nitrogenase and haloperoxidases.
1.3 Bioelectrocatalysis
It is well known that redox enzymes are extremely efficient catalysts, but it has also
been established that redox enzymes can work as efficient electrocatalysts. Over the
last four decades, redox enzymes have received much attention for use in the coupling
of the enzyme reactions with non-specific electrochemical reactions. The coupled
reaction is known as bioelectrocatalysis. Several review articles have been published
[7, 42–56] and they can be helpful for readers with interest in this field.
We would like to propose an important concept in the coupling that the electrode
reaction is not hydride ion transfer, but electron transfer, therefore molecules that can
communicate with electrodes should have the electron transfer ability. In addition, the
essential features of redox enzymes for realizing the coupling are that most of redox
enzymes except NAD(P)-dependent dehydrogenases show low substrate specificity
for either substrate. The natural substrates with low specificity have the electron
transfer ability, and can be replaced with artificial redox compounds (and in some
cases with electrodes). For example, the natural electron acceptor of FDH is CoQ 10
[57]; however, it can be replaced with several other quinones, inorganic oxidants, and
several electrodes [58], because the electron donating site is a heme in the enzyme.
Similar situations are observed even in oxidases and peroxidases. For instance, O 2
as the natural electron acceptor of FAD-dependent glucose oxidase (GOD) can be
replaced with several organic and inorganic oxidants, because the electro donating
site in the enzyme is FAD that can undergo both electron and hydride ion transfers.
On the other hand, bilirubin as the natural electron donor of bilirubin oxidase (BOD)
can be replaced with several organic and inorganic reductants, and several electrodes
[59], because the electro accepting site is T1Cu in the enzyme. Several organic and
inorganic reductants (as well as electrodes) can work as electron donors of HRP [60],
of which the electrodonatig site is a protoheme.
In these manners, most redox enzyme reactions can be coupled with electrode
reactions via (artificial) redox compounds (called mediators) that shuttle electrons
between enzymes and electrodes. This reaction is known as mediated electron transfer
(MET)-type bioelectrocatalysis (Fig. 1.16). Since NAD(P)(H) cannot communicate with electrode in a reversible way (or with low over potentials), NAD(P)dependent enzymatic reactions have to be coupled with electrode reactions by using
redox mediators that can transfer both hydride ion and electron, such as flavins,
quinones (especially o-quinones), and phenothiazines (such as Meldola’s blue).
NAD(P)-dependent enzymatic reactions can also be coupled with electrode reactions by flavoproteins such as diaphorase and ferredoxin NADP
+ reductase. Microbes
and organelles (containing such redox enzymes) also work as electrocatalysts in
MET-type bioelectrocatalysis.