nanoscale) and local-specific surface charge/hydrophilicity/hydrophobicity must be
taken into consideration when choosing the materials and its functionalisations.
Having these general points tackled, any FDH could be used to build a device to
promote the CO 2 reduction. The same would be true for whole-cell devices, but
considering in that case the organism whole metabolism (carbon and energy needs).
4.5 Formate Dehydrogenases in Action
The use of enzymes and whole-cells systems to convert CO 2 into VC is growing
exponentially due to the “green” advantages the “biochemical way” can offer,
namely substrate and product specificity (ability to discriminate the substrate in a
complex mixture and to produce only the product of interest) in reactions at ambient
temperature and pressure and neutral pH. Numerous hybrid systems are currently
being exploited to convert CO 2 into formate, following the same master lines as
described in Sect. 3 (Fig. 2). Like electrochemistry, bioelectrochemistry is currently
under intense research, as is reviewed in [221–227] and references herein (below).
Most interest is also being focused on the biophotoreduction of CO 2 , as solar light
represents the most straightforward way to use a RES to convert CO 2 .
Semi-artificial photosynthesis systems have been devised, where enzymes and also
entire metabolic pathways within cells are interfaced with synthetic materials to
develop new solar-to-VC and solar-to-fuel devices, which would not be feasible
with natural or artificial systems alone [228 and references herein (below)]. The
direct CO 2 hydrogenation is also getting enormous attention, mimicking metabolic
pathways, where the formate-hydrogen lyase systems are the most explored
examples, but using also whole-cells systems. Most important are the breakthroughs achieved by exploiting the recently identified metabolic pathways of
acetogens and its dihydrogen-dependent CO 2 reductase enzymes (Sect. 4.2.2.), as is
reviewed by Litty and Müller in this Book [152] and also [153–159] and references
herein (below).
Herein (below), a few promising studies and successful proof of concepts of
FDH-dependent CO 2 reduction to formate and beyond are discussed, to highlight
the power of FDHs and the challenges this CO 2 bioconversion still faces.
One of the most efficient CO 2 reducers so far described (along with the T. kivui
enzyme described below) is a SeCys–W–FDH from the Synthrobacter fumaroxidans that displays an impressive rate of CO 2 reduction of % 2.5 Â 10
3 s
−1
(reported as 900Umg
−1 ; K m
CO2 not determined, assays with 10 mM hydrogencarbonate), with a slightly lower formate oxidation rate (% 1.9 Â 10
3 s
−1 (reported
as 700Umg
−1 ); K m
HCOO− of 40 lM) [229–231]. This enzyme is also a good
electrocatalyst to carry out the electrochemical reduction of CO 2 to formate, using
mild conditions and applying small overpotentials, with a maximum current
density of % 80 lAcm
−2 that corresponds to % 110 s
−1 (from a monolayer of
enzyme) [232]. Intriguingly, while in homogeneous catalysis in solution the CO 2
reduction is slightly faster than the formate oxidation, in the electrochemicalassisted reduction/oxidation is the formate oxidation that is more than 2 times
58
L. B. Maia et al.
taken into consideration when choosing the materials and its functionalisations.
Having these general points tackled, any FDH could be used to build a device to
promote the CO 2 reduction. The same would be true for whole-cell devices, but
considering in that case the organism whole metabolism (carbon and energy needs).
4.5 Formate Dehydrogenases in Action
The use of enzymes and whole-cells systems to convert CO 2 into VC is growing
exponentially due to the “green” advantages the “biochemical way” can offer,
namely substrate and product specificity (ability to discriminate the substrate in a
complex mixture and to produce only the product of interest) in reactions at ambient
temperature and pressure and neutral pH. Numerous hybrid systems are currently
being exploited to convert CO 2 into formate, following the same master lines as
described in Sect. 3 (Fig. 2). Like electrochemistry, bioelectrochemistry is currently
under intense research, as is reviewed in [221–227] and references herein (below).
Most interest is also being focused on the biophotoreduction of CO 2 , as solar light
represents the most straightforward way to use a RES to convert CO 2 .
Semi-artificial photosynthesis systems have been devised, where enzymes and also
entire metabolic pathways within cells are interfaced with synthetic materials to
develop new solar-to-VC and solar-to-fuel devices, which would not be feasible
with natural or artificial systems alone [228 and references herein (below)]. The
direct CO 2 hydrogenation is also getting enormous attention, mimicking metabolic
pathways, where the formate-hydrogen lyase systems are the most explored
examples, but using also whole-cells systems. Most important are the breakthroughs achieved by exploiting the recently identified metabolic pathways of
acetogens and its dihydrogen-dependent CO 2 reductase enzymes (Sect. 4.2.2.), as is
reviewed by Litty and Müller in this Book [152] and also [153–159] and references
herein (below).
Herein (below), a few promising studies and successful proof of concepts of
FDH-dependent CO 2 reduction to formate and beyond are discussed, to highlight
the power of FDHs and the challenges this CO 2 bioconversion still faces.
One of the most efficient CO 2 reducers so far described (along with the T. kivui
enzyme described below) is a SeCys–W–FDH from the Synthrobacter fumaroxidans that displays an impressive rate of CO 2 reduction of % 2.5 Â 10
3 s
−1
(reported as 900Umg
−1 ; K m
CO2 not determined, assays with 10 mM hydrogencarbonate), with a slightly lower formate oxidation rate (% 1.9 Â 10
3 s
−1 (reported
as 700Umg
−1 ); K m
HCOO− of 40 lM) [229–231]. This enzyme is also a good
electrocatalyst to carry out the electrochemical reduction of CO 2 to formate, using
mild conditions and applying small overpotentials, with a maximum current
density of % 80 lAcm
−2 that corresponds to % 110 s
−1 (from a monolayer of
enzyme) [232]. Intriguingly, while in homogeneous catalysis in solution the CO 2
reduction is slightly faster than the formate oxidation, in the electrochemicalassisted reduction/oxidation is the formate oxidation that is more than 2 times
58
L. B. Maia et al.