Investigations of the Efficient Electrocatalytic Interconversions of Carbon Dioxide and Carbon Monoxide by Nickel-Containing Carbon Monoxide Dehydrogenases - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 74

unique active site (called the C-cluster) which is a [Ni4Fe-4S] cluster (or [Ni3Fe-4S])
cubane cluster linked to an extra-cuboidal pendant or 'dangling' Fe. A [4Fe-4S]
cluster (called the B-cluster) is located about 10 Å from each C-cluster, although
these are coordinated by the other subunit. Finally, a single [4Fe-4S] cluster (called
the D-cluster) lying about 10 from each B-cluster and close to the protein surface,

is coordinated by both subunits. The distances suggest immediately that the B-cluster
and D-cluster convey electrons between the C-cluster and an external physiological
redox partner, which is probably a ferredoxin.
Several redox states of the C-cluster involved in the mechanism of CO/CO 2
interconversion by CODH have been identified [ 23 ]. These are known as C ox ,C red1 ,
C int , and C red2 in order of decreasing oxidation level. Various structures of the
active site of CODH II Ch are shown in Figure 3 including two obtained from
enzyme crystallized at different reduction potentials (-320 mV, and -600 mV
with CO 2 present) [ 8 ]. The -320 mV structure (Figure 3a ) shows that an O-donor

Å

Figure 3 The active site of CODH II Ch captured in crystal structures obtained under different
conditions. (a) -320 mV, (b) -600 mV with CO 2 ,(c) -320 mV with cyanide, and (d) CO-reduced
CODH. Two positions are found for the pendant iron atom in the crystal structure, which are
labelled Fe 1a and Fe 1b respectively. The PDB codes are shown in each case. Color codes for atoms:
nickel, light blue; iron, orange; oxygen, red; carbon, green; nitrogen, blue; sulfur, yellow.