Carbon Monoxide. Toxic Gas and Fuel for Anaerobes and Aerobes: Carbon Monoxide Dehydrogenases - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 44

site. The small S subunit harbors two [2Fe2S] clusters and the medium M subunit
binds an FAD molecule non-covalently. The cofactors form a conduit for electron
transfer from the molybdenum ion via the [2Fe2S] clusters to the FAD, from where
electrons can be directly transferred to quinones in the cytoplasmic membrane
[ 53 ]. Arrangement of the cofactors as well as the fold of the three subunits is like
that of other molybdenum hydroxylases [ 42 , 44 ].
The L subunit can be divided into two domains, both interacting with the
MCD cofactor by a network of hydrogen bonds. The characteristic feature of
Cu,Mo-CODHs is a unique active site loop where a cysteine residue forms a
covalent bond to a Cu(I) ion (Figure 3 ).
Figure 3 The structure of Cu,Mo-CODHs. (a) Overall structure of the dimer of trimers, (LMS) 2 .
The L subunit of the right monomer is colored in cyan, the M subunit in orange, and the S subunit
in green. (b) Cofactors of one LMS monomer with shortest distance between the redox active sites
of the cofactors. (c) Active site architecture including residues in the second coordination sphere
of the metals.