comprising (a) specific surface patches to establish contact with specific biomolecules (for cell localisation, integration into metabolic complexes, crosstalk or simple
electron transfer), (b) channels where only (or mainly) the correct substrates come
in and well-determined products come out, as well as, (c) highly defined active
sites, assembled to promote the formation of key transition states and intermediates
and, thus, lower the reaction energy barriers and energy loss. For that, active sites
are built with precise steric features, electrostatic and hydrogen bonding interactions, fine-tuned reduction potentials and pK a values and optimised (and often
synchronised) electron and proton transfer paths. The power and efficiency of
biological catalysis is such that enzymes cascades are the cornerstone of all
metabolic pathways that sustain life on Earth. Hence, there is a growing interest in
making use of all the advantages the “biochemical way” can provide. Numerous
hybrid systems have been (are being) designed to merge the best of the two worlds
—chemical and biochemical—and the CO 2 reduction to formate is no exception.
4.2 Formate Dehydrogenases—Enzymatic Machineries
To interconvert CO 2 and formate, living organisms use formate dehydrogenase
(FDH) enzymes. FDHs are a heterogeneous and broadly distributed group of
enzymes that catalyse the reversible two-electron interconversion of formate and
CO 2 (Eq. 1) [94–101]. These enzymes evolved to take part in diverse metabolic
pathways, being used by some prokaryotic organism to fix (reduce) CO 2 into
formate, while other prokaryotes use FDHs to derive energy, by coupling the
formate oxidation (which has a very low reduction potential value, Eº′(CO 2 /
HCOO
− ) = −0.43 V) to the reduction of several terminal electron acceptors; FDHs
are also broadly used by both prokaryotes and eukaryotes in C1 metabolism.
FDHs can be divided into two major classes, based on their cofactor content and
the consequent chemical strategy used to carry out the formate/CO 2 interconversion.
One class comprises FDHs that have no metal ions or other redox-active centres—the
metal-independent FDHs class [102–109]. These enzymes are widespread, being
found in bacteria, yeasts, fungi and plants, are all (as far as is known)
NAD-dependent and belong to the D-specific dehydrogenases of 2-oxyacids family.
The other class—the metal-dependent FDHs class
3
—comprises only prokaryotic
enzymes that hold different redox-active centres (Table 1) and whose active site
harbours one molybdenum or one tungsten centre (molybdenum-containing FDH
(Mo-FDH) or tungsten-containing FDH (W-FDH), respectively) [94–101, 110–112].
3
It should be noted that the difference between the two FDHs classes is the absence or presence of
redox-active centres. All (so far known) metal-independent FDHs are NAD(P)-dependent. In
contrast, there are some metal-dependent FDHs that use NAD(P)
+ /NAD(P)H as a co-substrate,
while many other use other physiological redox partners (such as membrane quinols,
cytoplasmatic and periplasmatic cytochromes, ferredoxins or coenzyme F 420 ).
36
L. B. Maia et al.
electron transfer), (b) channels where only (or mainly) the correct substrates come
in and well-determined products come out, as well as, (c) highly defined active
sites, assembled to promote the formation of key transition states and intermediates
and, thus, lower the reaction energy barriers and energy loss. For that, active sites
are built with precise steric features, electrostatic and hydrogen bonding interactions, fine-tuned reduction potentials and pK a values and optimised (and often
synchronised) electron and proton transfer paths. The power and efficiency of
biological catalysis is such that enzymes cascades are the cornerstone of all
metabolic pathways that sustain life on Earth. Hence, there is a growing interest in
making use of all the advantages the “biochemical way” can provide. Numerous
hybrid systems have been (are being) designed to merge the best of the two worlds
—chemical and biochemical—and the CO 2 reduction to formate is no exception.
4.2 Formate Dehydrogenases—Enzymatic Machineries
To interconvert CO 2 and formate, living organisms use formate dehydrogenase
(FDH) enzymes. FDHs are a heterogeneous and broadly distributed group of
enzymes that catalyse the reversible two-electron interconversion of formate and
CO 2 (Eq. 1) [94–101]. These enzymes evolved to take part in diverse metabolic
pathways, being used by some prokaryotic organism to fix (reduce) CO 2 into
formate, while other prokaryotes use FDHs to derive energy, by coupling the
formate oxidation (which has a very low reduction potential value, Eº′(CO 2 /
HCOO
− ) = −0.43 V) to the reduction of several terminal electron acceptors; FDHs
are also broadly used by both prokaryotes and eukaryotes in C1 metabolism.
FDHs can be divided into two major classes, based on their cofactor content and
the consequent chemical strategy used to carry out the formate/CO 2 interconversion.
One class comprises FDHs that have no metal ions or other redox-active centres—the
metal-independent FDHs class [102–109]. These enzymes are widespread, being
found in bacteria, yeasts, fungi and plants, are all (as far as is known)
NAD-dependent and belong to the D-specific dehydrogenases of 2-oxyacids family.
The other class—the metal-dependent FDHs class
3
—comprises only prokaryotic
enzymes that hold different redox-active centres (Table 1) and whose active site
harbours one molybdenum or one tungsten centre (molybdenum-containing FDH
(Mo-FDH) or tungsten-containing FDH (W-FDH), respectively) [94–101, 110–112].
3
It should be noted that the difference between the two FDHs classes is the absence or presence of
redox-active centres. All (so far known) metal-independent FDHs are NAD(P)-dependent. In
contrast, there are some metal-dependent FDHs that use NAD(P)
+ /NAD(P)H as a co-substrate,
while many other use other physiological redox partners (such as membrane quinols,
cytoplasmatic and periplasmatic cytochromes, ferredoxins or coenzyme F 420 ).
36
L. B. Maia et al.