Carbon Monoxide. Toxic Gas and Fuel for Anaerobes and Aerobes: Carbon Monoxide Dehydrogenases - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 48

Cu,Mo-CODHs are structurally very similar to other Mo-hydroxylases from
which they differ mainly by the additional Cu ion. However, the first and second
coordination sphere of Mo, including the stereochemistry of the ligands are
practically the same in Cu,Mo-CODHs [ 50 ], xanthine oxidoreductase [ 68 ], and
quinolone 2-oxidoreductase [ 69 ]. Nevertheless, typical Mo-hydroxylases hydroxy-
late C-H groups, whereas Cu,Mo-CODHs oxidize CO, raising the question
on the parallels in their catalytic requirements. A recent comparison of the
orbital contributions found remarkably similar electronic structures for central
intermediates in CO and C-H bond activation [ 67 ]. Cu,Mo-CODH stabilizes the
CO 2 bound intermediate by C-Cu ˃ ! Mo-S π * and Mo-S π ! C-Cu ˃ * charge
transfer, whereas C-H bond activation in xanthine oxidase involves C-H ˃ ! Mo-S

π * charge transfers [ 67 ]. According to the orbital contribu-

tions, Cu,Mo-CODHs employ the Cu(I) ion as a substitute for the hydrogen bound
to C in C-H groups, or in other words, Cu(I)-CO resembles a C-H unit activated for
a nucleophilic attack.
H 2 is chemically unlike CO, but H 2 oxidation by Cu,Mo-CODH also depends on
the presence of Cu(I) [ 50 ]. Accordingly, catalytic H 2 oxidation by Cu,Mo-CODH
was postulated to occur via formation of a copper hydride species in the active site
[ 50 ], which is consistent with recent spectroscopic results on a Mo(V) species
generated by H 2 incubation of Cu,Mo-CODH [ 58 ]. Thus, by extending the Mo

* and Mo-S π ! C-H ˃

S
moiety to a Mo-S-Cu(I) unit, the CO as well as H 2 -oxidizing activities were
implemented into a Mo hydroxylase, a small change with a large effect.

¼

2.2 Monofunctional Ni,Fe-Containing Carbon
Monoxide Dehydrogenases

2.2.1 Function, Distribution, and Overall Structure

Since the discovery and initial isolation of Ni,Fe-CODHs more than three decades
ago [ 70 , 71 ], several reviews summarized the current state of research [ 2 , 72 - 78 ].
Here, we focus on past and present advancements in the field of monofunctional
Ni,Fe-CODHs.
Ni,Fe-CODHs are either monofunctional or bifunctional in a complex with
acetyl-CoA synthase and catalyze the reversible oxidation of CO (equation 1 ).
Monofunctional Ni,Fe-CODHs are used by anaerobic bacteria to employ CO
as energy source and the enzymes isolated from Carboxydothermus hydro-
genoformans ( Ch )[ 34 ] and Rhodospirillum rubrum ( Rr )[ 79 ] have been investi-
gated in some detail. The importance of monofunctional Ni,Fe-CODHs is reflected
in the genome of C. hydrogenoformans , where five different gene clusters contain
structural genes encoding Ni,Fe-CODHs [ 80 ]. Two monofunctional Ni,Fe-CODHs
(CODH I Ch and CODH II Ch ) catalyze the oxidation of CO [ 34 ], whereas a third
bifunctional Ni,Fe-CODH (CODH III Ch ) is found in a stable complex with ACS and
supports autotrophic carbon assimilation (Section 2.3 )[ 81 ].