Carbon Monoxide. Toxic Gas and Fuel for Anaerobes and Aerobes: Carbon Monoxide Dehydrogenases - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 54

modeled as a CO molecule [ 100 ]. The modeled CO molecule has a similarly bent
coordination as the CN ligand in the CODH/ACS Mt -CN complex. However, CO
had not been added to the crystals of the CODH component and the authors
suggested it to be a stable adduct left from enzyme isolation, where it would need
to be stably bound for several days. However, a stable Ni-CO complex is difficult to
reconcile with the rapid equilibrium binding of CO to the active site of Ni,Fe-
CODHs and their high turnover numbers [ 101 ].
Additional information on the reactivity of cluster C became available when the
structure of CODH II Ch incubated with n-butyl isocyanide (nBIC) was solved. nBIC
inhibits Cu,Mo-CODH [ 50 ] and acts as a rapid binding competitive inhibitor and
slow-turnover substrate of Ni,Fe-CODH [ 102 ]. nBIC reacted with the OH x ligand
of Fe 1 , yielding the product n-butyl isocyanate bound to cluster C [ 95 ]. Several H
bonds with the OH x ligand of Fe 1 as well as with the side chains of Lys 563 and His 93
stabilize the bound reaction product. In summary, studies of inhibited enzyme states
as well as incubation with slow turnover substrates revealed valuable information
on possible substrate binding modes and the stabilization of reaction intermediates.
Structural insights were recently complemented by electrochemical studies of
inhibited states of CODH II Ch [ 89 ]. An overview of key findings from electrochem-
istry can be found in Chapter 4 of this volume.
2.2.5 Mechanism of Reversible Carbon Dioxide Reduction at Cluster C
A reaction mechanism for reversible CO oxidation/CO 2 reduction has been
deduced by combining insights from high-resolution crystal structures of CODH
II Ch [ 94 ] with insights into the electronic structure from spectroscopic studies
[ 86 - 88 , 90 ] (Figure 10 ).
In the active state for CO oxidation (C red1 ), cluster C has a hydroxyl ligand
bound to ferrous Fe 1 (1). In this state, CO may either bind in the apical coordination
site and relocate to the equatorial site at the Ni ion, or it binds directly to the
equatorial position. Hydrogen bonds with His 93 stabilize the bound CO and increase
the polarization of the Ni-bound carbonyl, preparing the carbon atom of CO for a
nucleophilic attack by the OH x group (2). The resulting carboxylate bridges Ni and
Fe 1 and is stabilized by electrostatic and H-bonding interactions with Lys 563 and
protonated His 93 (3). Addition of a water molecule may assist in provoking the
release of CO 2 and two protons. The newly added water molecule is bound to Fe 1
and cluster C is in the two electron reduced C red2 state (4). Transfer of two electrons
via cluster B restores cluster C and closes the catalytic cycle. Advances during the
last years defined the major steps of the mechanism [ 94 , 95 , 98 ]. To further refine
the mechanism a combined spatial and electronic structure description of all states
will be necessary.