Other reaction efficiently catalysed by enzymes is the reversible reduction of
CO 2 to CO, by carbon monoxide dehydrogenases (CODH), although its main
reaction is the CO oxidation to CO 2 , found in bacteria and archaea, involving two
electrons and two protons. This enzyme participates in the Wood-Ljungdahl
pathway of Acetyl-CoA synthesis together with the acetyl-CoA synthase
(ACS) enzyme [21, 22], being called as bifunctional CODH. It can also be found
without ACS, called unifunctional CODH, in some anaerobic bacteria that use CO
as source of growth and energy. These were isolated from anaerobes, such as, R.
rubrum or C. hydrogenoformans, in which Ni was found in the active catalytic site.
CODH is a homodimer (approx. 130 kDa) containing five metal-sulfur clusters.
The active centre (the C-cluster) contains a Ni coordinated with four Fe ions, in a
[NiFe 3 S 4 ] cubane-like structure with an extra-cubane Fe bond to the Ni by a sulfide
bridge [23–25] for which these enzymes are frequently named as Ni-CODHs. In
some aerobic bacteria, Mo (in a molybdopterin structure) is found in the active
centre with a structure (and properties) similar to the one found in Mo enzymes,
such as xanthine oxidase, among others [25, 26]. The enzyme also integrates other
FeS clusters that play a role in the electron transfer pathway [27]. Ni-CODHs have
been the subject of intense research recently aiming its integration in CO 2 mitigation systems, biofuels and H 2 production, due to its catalytic properties and the
ease to purify and express them, although they also present an important disadvantage, namely its sensitivity to O 2 [22, 27–29].
Formate dehydrogenase enzymes (FDHs) are a particular interesting class of
enzymes that have been considered to have huge potential to integrate systems
using CO 2 . These enzymes have the ability to catalyse the reversible 2 electrons
reduction of CO 2 to formate, a reaction economically very relevant [3, 30–32].
FDHs can be divided into two classes, namely the metal-dependent (found in
prokaryotes) and metal-independent enzymes (NAD-dependent, found in bacteria,
fungi and plants), whether they present or not metal ions (Mo, W) in their active
site, respectively [33, 34]. So far, metal-dependent enzymes have shown more
activity towards the CO 2 reduction than the metal-independent [35], albeit the later
has been more extensive studied, probably due to the fact of being easier to get
(some are commercially available). In the metal-dependent enzymes mononuclear
enzymes, Mo or W ion is coordinated in the active site by four sulfur atoms of two
pyranopterin cofactors, S-Cysteine (or Se-Cysteine) and one additional S atom [33,
36]. The electrochemical control of the FDH catalysis towards CO 2 reduction has
been intensively pursued and some studies have already shown the feasibility of its
integration in CO 2 mitigation systems [37] or other bioelectrochemical systems
(BES). This topic will be further developed in the next sections.
Nitrogenases are well-known enzymes involved in nitrogen fixation found
largely in bacteria and archaea. The main catalytic reaction (reduction of N 2 to
NH 3 ) has a chemical parallel in the high used industrial Haber–Bosch reaction, but
contrarily to this later, nitrogenase works at room temperature and normal 1 atm of
pressure being much more effective than the industrial reaction [38, 39]. Nitrogenase is composed by a complex that comprises two proteins, one containing the
catalytic MoFe centre (where Mo may be replaced by V or Fe), with structure
Carbon Dioxide Utilization—Bioelectrochemical Approaches
87
CO 2 to CO, by carbon monoxide dehydrogenases (CODH), although its main
reaction is the CO oxidation to CO 2 , found in bacteria and archaea, involving two
electrons and two protons. This enzyme participates in the Wood-Ljungdahl
pathway of Acetyl-CoA synthesis together with the acetyl-CoA synthase
(ACS) enzyme [21, 22], being called as bifunctional CODH. It can also be found
without ACS, called unifunctional CODH, in some anaerobic bacteria that use CO
as source of growth and energy. These were isolated from anaerobes, such as, R.
rubrum or C. hydrogenoformans, in which Ni was found in the active catalytic site.
CODH is a homodimer (approx. 130 kDa) containing five metal-sulfur clusters.
The active centre (the C-cluster) contains a Ni coordinated with four Fe ions, in a
[NiFe 3 S 4 ] cubane-like structure with an extra-cubane Fe bond to the Ni by a sulfide
bridge [23–25] for which these enzymes are frequently named as Ni-CODHs. In
some aerobic bacteria, Mo (in a molybdopterin structure) is found in the active
centre with a structure (and properties) similar to the one found in Mo enzymes,
such as xanthine oxidase, among others [25, 26]. The enzyme also integrates other
FeS clusters that play a role in the electron transfer pathway [27]. Ni-CODHs have
been the subject of intense research recently aiming its integration in CO 2 mitigation systems, biofuels and H 2 production, due to its catalytic properties and the
ease to purify and express them, although they also present an important disadvantage, namely its sensitivity to O 2 [22, 27–29].
Formate dehydrogenase enzymes (FDHs) are a particular interesting class of
enzymes that have been considered to have huge potential to integrate systems
using CO 2 . These enzymes have the ability to catalyse the reversible 2 electrons
reduction of CO 2 to formate, a reaction economically very relevant [3, 30–32].
FDHs can be divided into two classes, namely the metal-dependent (found in
prokaryotes) and metal-independent enzymes (NAD-dependent, found in bacteria,
fungi and plants), whether they present or not metal ions (Mo, W) in their active
site, respectively [33, 34]. So far, metal-dependent enzymes have shown more
activity towards the CO 2 reduction than the metal-independent [35], albeit the later
has been more extensive studied, probably due to the fact of being easier to get
(some are commercially available). In the metal-dependent enzymes mononuclear
enzymes, Mo or W ion is coordinated in the active site by four sulfur atoms of two
pyranopterin cofactors, S-Cysteine (or Se-Cysteine) and one additional S atom [33,
36]. The electrochemical control of the FDH catalysis towards CO 2 reduction has
been intensively pursued and some studies have already shown the feasibility of its
integration in CO 2 mitigation systems [37] or other bioelectrochemical systems
(BES). This topic will be further developed in the next sections.
Nitrogenases are well-known enzymes involved in nitrogen fixation found
largely in bacteria and archaea. The main catalytic reaction (reduction of N 2 to
NH 3 ) has a chemical parallel in the high used industrial Haber–Bosch reaction, but
contrarily to this later, nitrogenase works at room temperature and normal 1 atm of
pressure being much more effective than the industrial reaction [38, 39]. Nitrogenase is composed by a complex that comprises two proteins, one containing the
catalytic MoFe centre (where Mo may be replaced by V or Fe), with structure
Carbon Dioxide Utilization—Bioelectrochemical Approaches
87