the molybdenum center in these enzymes is coordinated by two pyranopterin
molecules; the first two enzymes have monomeric structures [228, 247, 248], while
perchlorate reductase is a heterodimeric protein and contains Fe-S clusters (four
[4Fe-4S] and one [3Fe-4S]) in addition to the molybdenum cofactor (Table 1 and
Fig. 10d–f) [249].
DMSO reductase catalyzes the reduction of DMSO to dimethyl sulfide (DMS).
As a common organic solvent used in numerous industrial and laboratory applications, DMSO can be found in wastewaters; besides, it is also naturally occurring
in beverages and foodstuff. Given its unpleasant smell in high concentrations,
DMSO detection is important in beverage quality control, as well as, in wastewater
monitoring [216]. As for perchlorate reductase, the enzyme is involved in the first
two steps of perchlorate breakdown to chloride and O 2 (ClO 4
−
!ClO 3
−
!ClO 2
−
!Cl
− +O 2 ) [249]. The contamination of groundwaters with perchlorate (that can derive from solid rocket propellant manufacturing for example)
can have significant adverse defects in animal development and human health
[217]. Finally, TorA is involved in the two-electron reduction of TMAO to
trimethylamine (TMA) [247]. Because of its potential as a disease biomarker,
TMAO quantification is relevant for clinical diagnosis [85]. Biosensors based on
these three enzymes follow similar approaches. In the reported devices MV is used
for mediated signal transduction and immobilization on electrode surfaces relies on
non-conduction materials, such as Nafion and dialysis membranes [85, 216, 217,
222, 250]. The TorA biosensor also displayed direct electrocatalytic currents for
TMAO reduction (Fig. 11b), however, the activity was about 80 times higher in the
presence of the redox mediator [85]. Different methods have been employed to
avoid O 2 background currents when operating these biosensors: addition of the
GOx/Cat O 2 removal system in the case of DMSO and TMAO biosensors [85, 250]
and of cysteine hydrochloride with the perchlorate reductase-based device [217].
The glutathione/glutathione disulfide ratio (GSH/GSSG) is an important indicator of cellular oxidative stress. The accumulation of the oxidized compound,
GSSG, has been associated with several pathological conditions (e.g., diabetes and
Alzheimer’s disease), making it an important target in the clinical analysis [251]. In
vivo, the enzyme glutathione reductase and NADPH are responsible for maintaining cellular GSH/GSSH equilibrium. Each monomer of the homodimer enzyme
contains a FAD cofactor, while the active site consists of a disulfide bond formed
by two cysteine residues (Fig. 10g) [252]. Several glutathione reductase-based
amperometric biosensors have been reported in the past three decades. The electrocatalytic detection of GSSG by the immobilized enzyme is typically mediated by
viologen redox mediators or NADPH. Glutathione reductase was shown to retain its
activity while immobilized on thiolated self-assembled monolayers, electropolymerized PPy films, or chitosan membranes [251, 253, 254]. More recently,
nanostructured interfaces based on MWCNTs and gold nanoparticles (GNPs) have
allowed lowering the detections limits and increasing biosensor’s lifetimes [207,
255]. The devices have been used for the analysis of blood and plasma samples, as
well as, in vivo measurements in rat livers [207, 251].
340
T. Monteiro et al.
molecules; the first two enzymes have monomeric structures [228, 247, 248], while
perchlorate reductase is a heterodimeric protein and contains Fe-S clusters (four
[4Fe-4S] and one [3Fe-4S]) in addition to the molybdenum cofactor (Table 1 and
Fig. 10d–f) [249].
DMSO reductase catalyzes the reduction of DMSO to dimethyl sulfide (DMS).
As a common organic solvent used in numerous industrial and laboratory applications, DMSO can be found in wastewaters; besides, it is also naturally occurring
in beverages and foodstuff. Given its unpleasant smell in high concentrations,
DMSO detection is important in beverage quality control, as well as, in wastewater
monitoring [216]. As for perchlorate reductase, the enzyme is involved in the first
two steps of perchlorate breakdown to chloride and O 2 (ClO 4
−
!ClO 3
−
!ClO 2
−
!Cl
− +O 2 ) [249]. The contamination of groundwaters with perchlorate (that can derive from solid rocket propellant manufacturing for example)
can have significant adverse defects in animal development and human health
[217]. Finally, TorA is involved in the two-electron reduction of TMAO to
trimethylamine (TMA) [247]. Because of its potential as a disease biomarker,
TMAO quantification is relevant for clinical diagnosis [85]. Biosensors based on
these three enzymes follow similar approaches. In the reported devices MV is used
for mediated signal transduction and immobilization on electrode surfaces relies on
non-conduction materials, such as Nafion and dialysis membranes [85, 216, 217,
222, 250]. The TorA biosensor also displayed direct electrocatalytic currents for
TMAO reduction (Fig. 11b), however, the activity was about 80 times higher in the
presence of the redox mediator [85]. Different methods have been employed to
avoid O 2 background currents when operating these biosensors: addition of the
GOx/Cat O 2 removal system in the case of DMSO and TMAO biosensors [85, 250]
and of cysteine hydrochloride with the perchlorate reductase-based device [217].
The glutathione/glutathione disulfide ratio (GSH/GSSG) is an important indicator of cellular oxidative stress. The accumulation of the oxidized compound,
GSSG, has been associated with several pathological conditions (e.g., diabetes and
Alzheimer’s disease), making it an important target in the clinical analysis [251]. In
vivo, the enzyme glutathione reductase and NADPH are responsible for maintaining cellular GSH/GSSH equilibrium. Each monomer of the homodimer enzyme
contains a FAD cofactor, while the active site consists of a disulfide bond formed
by two cysteine residues (Fig. 10g) [252]. Several glutathione reductase-based
amperometric biosensors have been reported in the past three decades. The electrocatalytic detection of GSSG by the immobilized enzyme is typically mediated by
viologen redox mediators or NADPH. Glutathione reductase was shown to retain its
activity while immobilized on thiolated self-assembled monolayers, electropolymerized PPy films, or chitosan membranes [251, 253, 254]. More recently,
nanostructured interfaces based on MWCNTs and gold nanoparticles (GNPs) have
allowed lowering the detections limits and increasing biosensor’s lifetimes [207,
255]. The devices have been used for the analysis of blood and plasma samples, as
well as, in vivo measurements in rat livers [207, 251].
340
T. Monteiro et al.