3.1 History of DET-Type Bioelectrocatalysis
59
a non-diffusive and/or synthetic electroactive species on the electrode does not participate in the electron transfer, and (iii) catalytic oxidation or reduction current of the
substrate is observed upon its addition to the system. Therefore, NAD(P)-dependent
enzymes (utilizing ordered mechanisms) are not considered to undergo DET-type
bioelectrocatalysis, while NAD(P)-linked enzymes such as NAD(P)-linked H 2 ase,
and NAD(P)-linked FoDH can undergo DET-type reactions.
Some examples of DET-type reactions reported to data are summarized in Table
3.1. Most of the enzymes undergoing DET-type bioelectrocatalysis have multi-redox
sites. However, some of redox enzymes with single-metallic prosthetic group are
also capable of DET-type bioelectrocatalysis; cyt c peroxidase from yeast [16] and
horseradish peroxidase (HRP) [17] containing one heme b as the catalytic center
catalyze DET-type reduction of H 2 O 2 at potentials close to the formal potential
(~0.7 V vs. Ag|AgCl) of Compound I (i.e. 2-electron oxidized form with oxyferrl
iron).
In addition, ferredoxin-NADP
+ reductase from Chlamydomonas reinhardtii
containing one FAD catalyzes bidirectional NADP
+ -reduction and NADPHoxidation [47]. Cellobiose dehydrogenase type I from Myriococcum thermophilum,
a heterodimeric flavohemoprotein containing an FAD in dehydrogenase subunit and
a heme b in cytochrome subunit, also shows a DET-type reaction at the FAD in the
presence of Ca
2+ , while the enzyme usually undergoes a DET-reaction at the heme
b [50]. DET-type catalytic behaviors were also reported for PQQ-dependent soluble
GDH (PQQ-sGDH) containing non-covalently (and weakly) bound PQQ in each
molecule [49, 51]. These reports support that organic cofactors in redox enzymes
have a possibility to directly communicate with electrodes.
Although there are many reports claiming DET-type bioelectrocatalysis of FADGOD containing a non-covalently bound FAD per each molecule, several authors
have questioned the interpretation [52, 53]. Sakai et al. have shown clear experimental evidences of two mechanisms: DET- and MET-type bioelectrocatalysis by
NAD-linked FoDH containing non-covalently bound FMN, W-pterin, and FeS clusters [54]; one of FeS is the electrochemically active site of the enzyme in DETmechanism, while free FMN liberated from the holoenzyme plays as a mediator in
MET-mechanism. In the case of PQQ-sGDH also, a possibility of MET-type reaction mechanism via free PQQ liberated from PQQ-sGDH could not be ruled out,
because the half-wave potential of the catalytic wave was ~0.4 V more positive than
the averaged peak potential of non-catalytic DET peak of PQQ-sGDH. The difference seems to be too large. In addition, the peak potential of non-catalytic DET
peak of the enzyme was almost identical to that of free PQQ, though spectroelectrochemical measurements indicated that a E
◦ value of PQQ-sGDH was ~0.06 V
more negative than E
◦ of free PQQ (at pH 7.0) [55]. Further research is required to
get a rigid conclusion on DET-type bioelectrocatalysis of PQQ-sGDH. Utilization
of irreversible adduct formation of free PPQ (Sect. 1.2.3) may be utilize to remove
free PQQ throughout the bioelectrocatalytic reaction by PQQ-sGDH.
In construct to the above questioning on DET-reaction of FAD-GOD and PQQsGDH, new concept has been reported to realize DET-reaction of FAD-GOD by
using platinum (Pt) nanoclusters generated near the FAD of the enzyme (Fig. 3.2)
59
a non-diffusive and/or synthetic electroactive species on the electrode does not participate in the electron transfer, and (iii) catalytic oxidation or reduction current of the
substrate is observed upon its addition to the system. Therefore, NAD(P)-dependent
enzymes (utilizing ordered mechanisms) are not considered to undergo DET-type
bioelectrocatalysis, while NAD(P)-linked enzymes such as NAD(P)-linked H 2 ase,
and NAD(P)-linked FoDH can undergo DET-type reactions.
Some examples of DET-type reactions reported to data are summarized in Table
3.1. Most of the enzymes undergoing DET-type bioelectrocatalysis have multi-redox
sites. However, some of redox enzymes with single-metallic prosthetic group are
also capable of DET-type bioelectrocatalysis; cyt c peroxidase from yeast [16] and
horseradish peroxidase (HRP) [17] containing one heme b as the catalytic center
catalyze DET-type reduction of H 2 O 2 at potentials close to the formal potential
(~0.7 V vs. Ag|AgCl) of Compound I (i.e. 2-electron oxidized form with oxyferrl
iron).
In addition, ferredoxin-NADP
+ reductase from Chlamydomonas reinhardtii
containing one FAD catalyzes bidirectional NADP
+ -reduction and NADPHoxidation [47]. Cellobiose dehydrogenase type I from Myriococcum thermophilum,
a heterodimeric flavohemoprotein containing an FAD in dehydrogenase subunit and
a heme b in cytochrome subunit, also shows a DET-type reaction at the FAD in the
presence of Ca
2+ , while the enzyme usually undergoes a DET-reaction at the heme
b [50]. DET-type catalytic behaviors were also reported for PQQ-dependent soluble
GDH (PQQ-sGDH) containing non-covalently (and weakly) bound PQQ in each
molecule [49, 51]. These reports support that organic cofactors in redox enzymes
have a possibility to directly communicate with electrodes.
Although there are many reports claiming DET-type bioelectrocatalysis of FADGOD containing a non-covalently bound FAD per each molecule, several authors
have questioned the interpretation [52, 53]. Sakai et al. have shown clear experimental evidences of two mechanisms: DET- and MET-type bioelectrocatalysis by
NAD-linked FoDH containing non-covalently bound FMN, W-pterin, and FeS clusters [54]; one of FeS is the electrochemically active site of the enzyme in DETmechanism, while free FMN liberated from the holoenzyme plays as a mediator in
MET-mechanism. In the case of PQQ-sGDH also, a possibility of MET-type reaction mechanism via free PQQ liberated from PQQ-sGDH could not be ruled out,
because the half-wave potential of the catalytic wave was ~0.4 V more positive than
the averaged peak potential of non-catalytic DET peak of PQQ-sGDH. The difference seems to be too large. In addition, the peak potential of non-catalytic DET
peak of the enzyme was almost identical to that of free PQQ, though spectroelectrochemical measurements indicated that a E
◦ value of PQQ-sGDH was ~0.06 V
more negative than E
◦ of free PQQ (at pH 7.0) [55]. Further research is required to
get a rigid conclusion on DET-type bioelectrocatalysis of PQQ-sGDH. Utilization
of irreversible adduct formation of free PPQ (Sect. 1.2.3) may be utilize to remove
free PQQ throughout the bioelectrocatalytic reaction by PQQ-sGDH.
In construct to the above questioning on DET-reaction of FAD-GOD and PQQsGDH, new concept has been reported to realize DET-reaction of FAD-GOD by
using platinum (Pt) nanoclusters generated near the FAD of the enzyme (Fig. 3.2)