[MoFe 7 S 9 C](R-homocitrate), (R can be C or N), called M-cluster (also known as
FeMo cofactor) and a [Fe 8 S 7 ] cluster (P-cluster) and a [4Fe-4S] cluster in a second
protein [38, 40–42]. It was found that besides N 2 , nitrogenases can catalyse the
reduction of small carbon molecules (with multiple bonds) such as acetylene or
cyanide, among many others [41, 43]. Interestingly, nitrogenases have also shown
activity towards CO 2 and CO reduction. These enzymes were shown to reduce CO 2
to CO, formate (or formic acid) and to methane [44], displaying rates towards
formic acid 10 times higher than to the two other products. This activity has made
to regain the interest in nitrogenases as potential enzymes to integrate CO 2 mitigation and added-value compounds production systems.
Other enzymes have been considered aiming the same goal. These usually take
advantage of other reactions in which CO 2 is a by-product of reactant and may be
integrated systems in combination with other enzymes. One example is the malic
enzyme (also known as malate dehydrogenase, MDH) whose catalytic reaction is
the oxidation of malic acid to pyruvate releasing CO 2 . This reaction is reversible
being able to promote CO 2 fixation and production of malic acid, a compound with
many industrial applications [45]. Some studies have also shown the feasibility of
this reaction using HCO 3
− as model compound to achieve carbon fixation from it.
This reaction, however, shows a low conversion rate (less than 40%) [46]. Other
example was the use of isocitrate dehydrogenase to reduce CO 2 in oxoglutaric acid
using electrochemistry. This reaction requires a large overpotential although conversions are reasonable (around 80%) [47].
2.2 Redox Properties of CO 2 -Related Enzymes
CO 2 -related enzymes use in electrochemical systems presupposes that their electrochemical properties are known allowing its electrochemical control. Highly
complex enzymes, such as RuBisCO, are not easily handled and/or electrochemically studied. RuBisCO behaviour is known to be dependent on the oxidation of
cysteines, with its activity decaying with the loss of cysteine sulfhydryl groups and
subsequent conformational changes [48, 49]. This change from active to an inactive
enzyme with the oxidation of the different cysteines corresponds to an apparent
redox potential of −308 mV and further oxidation at −299 mV and implies conformational critical changes [50]. Although the redox control of RuBisCO has been
considered an important factor that could help to improve the process efficiency
[51], the experimental difficulties seem to have hindered the study of the isolated
enzyme, though some approaches using it combined with other enzymes and
structures have been reported. For instance, RuBisCO has been immobilized in
multi enzymatic self-assembled synthetic amphiphilic peptide nanostructures aiming to improve the enzyme stability and activity [52]. CAs are also usually used in
immobilized systems, with improved stability comparing with the free enzyme, to
increase the CO 2 hydration, taking advantage of the enzyme high turnover rate
[53–55]. Very often CAs are used in association with other enzymes, such as
phosphoenolpyruvate carboxylase or formate dehydrogenase enhancing the
88
C. M. Cordas et al.
FeMo cofactor) and a [Fe 8 S 7 ] cluster (P-cluster) and a [4Fe-4S] cluster in a second
protein [38, 40–42]. It was found that besides N 2 , nitrogenases can catalyse the
reduction of small carbon molecules (with multiple bonds) such as acetylene or
cyanide, among many others [41, 43]. Interestingly, nitrogenases have also shown
activity towards CO 2 and CO reduction. These enzymes were shown to reduce CO 2
to CO, formate (or formic acid) and to methane [44], displaying rates towards
formic acid 10 times higher than to the two other products. This activity has made
to regain the interest in nitrogenases as potential enzymes to integrate CO 2 mitigation and added-value compounds production systems.
Other enzymes have been considered aiming the same goal. These usually take
advantage of other reactions in which CO 2 is a by-product of reactant and may be
integrated systems in combination with other enzymes. One example is the malic
enzyme (also known as malate dehydrogenase, MDH) whose catalytic reaction is
the oxidation of malic acid to pyruvate releasing CO 2 . This reaction is reversible
being able to promote CO 2 fixation and production of malic acid, a compound with
many industrial applications [45]. Some studies have also shown the feasibility of
this reaction using HCO 3
− as model compound to achieve carbon fixation from it.
This reaction, however, shows a low conversion rate (less than 40%) [46]. Other
example was the use of isocitrate dehydrogenase to reduce CO 2 in oxoglutaric acid
using electrochemistry. This reaction requires a large overpotential although conversions are reasonable (around 80%) [47].
2.2 Redox Properties of CO 2 -Related Enzymes
CO 2 -related enzymes use in electrochemical systems presupposes that their electrochemical properties are known allowing its electrochemical control. Highly
complex enzymes, such as RuBisCO, are not easily handled and/or electrochemically studied. RuBisCO behaviour is known to be dependent on the oxidation of
cysteines, with its activity decaying with the loss of cysteine sulfhydryl groups and
subsequent conformational changes [48, 49]. This change from active to an inactive
enzyme with the oxidation of the different cysteines corresponds to an apparent
redox potential of −308 mV and further oxidation at −299 mV and implies conformational critical changes [50]. Although the redox control of RuBisCO has been
considered an important factor that could help to improve the process efficiency
[51], the experimental difficulties seem to have hindered the study of the isolated
enzyme, though some approaches using it combined with other enzymes and
structures have been reported. For instance, RuBisCO has been immobilized in
multi enzymatic self-assembled synthetic amphiphilic peptide nanostructures aiming to improve the enzyme stability and activity [52]. CAs are also usually used in
immobilized systems, with improved stability comparing with the free enzyme, to
increase the CO 2 hydration, taking advantage of the enzyme high turnover rate
[53–55]. Very often CAs are used in association with other enzymes, such as
phosphoenolpyruvate carboxylase or formate dehydrogenase enhancing the
88
C. M. Cordas et al.