and direct oxidation of dihydrogen, that is, without the intervention of an external
electron-transfer protein or molecule, as reviewed by Litty and Müller in this Book
[152] and also [153–159]. The dihydrogen-dependent CO 2 reductase of the acetogen
A. woodii is a tetramer (abcd), holding one FDH-like subunit comprising one
molybdenum and one [4Fe–4S] centres, where CO 2 is reduced; the necessary electrons are transferred intramolecularly from an iron–iron hydrogenase-like
(Fe/Fe-Hase) subunit (second active site), via two small electron-transfer subunits
(each with four [4Fe–4S] centres) (Fig. 8) [153]. A tungsten-containing homologue
enzyme is found in Thermoanaerobacter kivui [156].
A further example of the “plasticity” of FDH-like proteins is provided by Nformyl-methanofuran dehydrogenases (FMFDH) that also have two physically
separated active sites: one catalyses the reduction of CO 2 to formate, which is then
intramolecularly transferred to the second active site, where it is condensed with
methanofuran to form N-formyl-methanofuran (Eq. 4) [140, 160, 161]. The
FMFDHs are even more complex than FDHs and the enzyme from the methanogen
M. wolfeii is a tetramer of (abcdex) units, whose CO 2 -reducing subunit shares the
tungsten and [4Fe–4S] centres, as well as, the protein fold of the W-FDHs and
Mo-FDHs (Fig. 9).
In contrast to the structural and organisational diversity, the active site of all
presently known metal-dependent FDHs and FMFDH is very well conserved [94–
101, 110–112, 140]. In the oxidised form, the active site harbours one molybdenum
ion (in the case of Mo-FDHs and Mo-FMFDHs) or one tungsten ion (in W-FDHs
and W-FMFDHs) coordinated by the cis-dithiolene (–S–C = C–S–) group of two
pyranopterin cofactor molecules (Fig. 10), as is characteristic of this family of
mononuclear molybdenum and tungsten enzymes [97, 110–112, 162–165]. The
metal first coordination sphere is completed by one terminal sulfido group
(Mo/W = S)
5 plus one sulfur or selenium atom from a cysteine or selenocysteine
residue (Mo/W–S(Cys) or Mo/W–Se(SeCys)) (abbreviated as Cys–Mo–FDH,
Cys–W–FDH, SeCys–Mo–FDH and SeCys–W–FDH), in a distorted trigonal
prismatic. Noteworthy, there is no apparent relation (as far as is presently known)
between the metal and the bound amino acid residue (examples of the four combinations Cys–Mo–FDH, Cys–W–FDH, SeCys–Mo–FDH and SeCys–W–FDH are
known for long; Table 1) or the enzyme activity. The active site also comprises two
other residues that are strictly conserved to all known FDHs and FMFDHs and are
thought to be crucial to the catalytic cycle (as discussed below), one arginine and
one histidine (this linked (C-terminal side) to the selenocysteine or cysteine that
coordinates the molybdenum or tungsten ion) [116, 117, 119, 130].
5
Although initially thought to be an oxygen [116], it is now unambiguously established that this
terminal atom it is a sulfur, in both Mo-FDHs and W-FDHs, as well as in FMFDH, as established
by X-ray crystallography and XAS [117, 140, 166]. In addition, it was already identified the
sulfotransferase that, in conjunction with the IscS cysteine desulfurase, catalyses the insertion of
this ligand in the active site [167–169].
Carbon Dioxide Utilisation—The Formate Route
43
electron-transfer protein or molecule, as reviewed by Litty and Müller in this Book
[152] and also [153–159]. The dihydrogen-dependent CO 2 reductase of the acetogen
A. woodii is a tetramer (abcd), holding one FDH-like subunit comprising one
molybdenum and one [4Fe–4S] centres, where CO 2 is reduced; the necessary electrons are transferred intramolecularly from an iron–iron hydrogenase-like
(Fe/Fe-Hase) subunit (second active site), via two small electron-transfer subunits
(each with four [4Fe–4S] centres) (Fig. 8) [153]. A tungsten-containing homologue
enzyme is found in Thermoanaerobacter kivui [156].
A further example of the “plasticity” of FDH-like proteins is provided by Nformyl-methanofuran dehydrogenases (FMFDH) that also have two physically
separated active sites: one catalyses the reduction of CO 2 to formate, which is then
intramolecularly transferred to the second active site, where it is condensed with
methanofuran to form N-formyl-methanofuran (Eq. 4) [140, 160, 161]. The
FMFDHs are even more complex than FDHs and the enzyme from the methanogen
M. wolfeii is a tetramer of (abcdex) units, whose CO 2 -reducing subunit shares the
tungsten and [4Fe–4S] centres, as well as, the protein fold of the W-FDHs and
Mo-FDHs (Fig. 9).
In contrast to the structural and organisational diversity, the active site of all
presently known metal-dependent FDHs and FMFDH is very well conserved [94–
101, 110–112, 140]. In the oxidised form, the active site harbours one molybdenum
ion (in the case of Mo-FDHs and Mo-FMFDHs) or one tungsten ion (in W-FDHs
and W-FMFDHs) coordinated by the cis-dithiolene (–S–C = C–S–) group of two
pyranopterin cofactor molecules (Fig. 10), as is characteristic of this family of
mononuclear molybdenum and tungsten enzymes [97, 110–112, 162–165]. The
metal first coordination sphere is completed by one terminal sulfido group
(Mo/W = S)
5 plus one sulfur or selenium atom from a cysteine or selenocysteine
residue (Mo/W–S(Cys) or Mo/W–Se(SeCys)) (abbreviated as Cys–Mo–FDH,
Cys–W–FDH, SeCys–Mo–FDH and SeCys–W–FDH), in a distorted trigonal
prismatic. Noteworthy, there is no apparent relation (as far as is presently known)
between the metal and the bound amino acid residue (examples of the four combinations Cys–Mo–FDH, Cys–W–FDH, SeCys–Mo–FDH and SeCys–W–FDH are
known for long; Table 1) or the enzyme activity. The active site also comprises two
other residues that are strictly conserved to all known FDHs and FMFDHs and are
thought to be crucial to the catalytic cycle (as discussed below), one arginine and
one histidine (this linked (C-terminal side) to the selenocysteine or cysteine that
coordinates the molybdenum or tungsten ion) [116, 117, 119, 130].
5
Although initially thought to be an oxygen [116], it is now unambiguously established that this
terminal atom it is a sulfur, in both Mo-FDHs and W-FDHs, as well as in FMFDH, as established
by X-ray crystallography and XAS [117, 140, 166]. In addition, it was already identified the
sulfotransferase that, in conjunction with the IscS cysteine desulfurase, catalyses the insertion of
this ligand in the active site [167–169].
Carbon Dioxide Utilisation—The Formate Route
43