Environmental Engineering Reference
In-Depth Information
Low potential
electrons derived from anaerobic CO/CO
2
conversion
(
E
0
'
-558 mV) [
28
] are transferred to various electron acceptors in CO-utilizing
microorganisms [
26
,
27
]. Obligate anaerobic sulfate reducers use the high-energy
electrons to reduce sulfate to sulfide (sulfidogenesis) [
29
,
30
], and
Sulfospirillum
carboxydovorans
uses CO to reduce elemental sulfur, dimethylsulfoxide, and
thiosulfate [
31
].
Some anaerobic carboxydotrophs like
Rhodospirillum rubrum
and
Carboxydo-
thermus hydrogenoformans
produce molecular hydrogen (hydrogenogenesis) [
32
,
33
]. Ferredoxins mediate the transfer of electrons gained from CO-oxidation to a
membrane-bound Ni,Fe-hydrogenase, which is the site for proton reduction
and proton translocation across the cytoplasmic membrane. The resulting proton
motive force facilitates synthesis of ATP by ATP synthase [
34
,
35
].
Methanogenic archaea are able to use CO in energy-conserving processes coupled
to distinct metabolic pathways, which overlap each other [
27
].COoxidationbythe
CODH/ACS complex supplies reducing equivalents in methanogens. In the aceticlastic
pathway, CO generated from acetyl-CoA cleavage by the CODH/ACS complex is used
to generate electrons from CO oxidation, producing methane as final product [
36
-
38
]. In the hydrogenotrophic pathway, electrons resulting from CO oxidation are used
to generate H
2
, which is subsequently employed as an electron source to reduce CO
2
to
methane. Finally, CO-derived reducing power serves to generate methane by reduction
of methyl-coenzyme M in the methylotrophic pathway [
37
].
¼
2 Structure and Function of Carbon Monoxide
Dehydrogenases
2.1 Cu,Mo-Containing Carbon Monoxide Dehydrogenases
The structure and function of Cu,Mo-CODHs have been previously reviewed
[
2
,
39
,
40
]. This section will focus on recent breakthroughs in the investigation of
Cu,Mo-CODHs and on similarities and differences of Cu,Mo-CODHs compared to
related molybdenum hydroxylases.
CODHs catalyze the (reversible) oxidation of CO to CO
2
according to equation (
1
):
2H
þ
þ
2e
CO
þ
H
2
O
Ð
CO
2
þ
ð
1
Þ
Cu,Mo-CODHs belong to a large family of pro- and eukaryotic molybdoflavo-
proteins, which share the use of a pyranopterin cofactor in their active site [
41
-
44
].
The pyranopterin cofactor is a heteroaromatic tricyclic ring system composed of a
bicyclic pterin unit and a pyran ring, which is responsible for anchoring the Mo ion
via an enedithiolate moiety at the pyran ring (Figure
2
)[
41
]. In addition to the
pyranopterin cofactor molybdenum hydroxylases typically employ two [2Fe2S]
clusters and a flavin adenine dinucleotide (FAD) molecule as cofactors (Figure
3
),
but there are some variations in cofactor content [
44
,
45
].
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