MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 85

dithionate, the FeMo-protein P-cluster and D cluster are ESR silent. Oxidation of
FeMo-protein P-clusters induced the ESR signal associated with S = 7/2. The ESR
spectrum of the

cluster exhibits the signal characteristic of S = 3. The ESR spectrum

of the cluster contains the signals of the spin states S = 5/2 and S = 1/2.

s isolated in the presence of dithionate, the FeMoco exists in a semi-reduced state
with a plausible assignment of metal atoms to the state as nine six
ferrous and one ferric The reduction center of the FeMoco substrates is a
cluster of the new type containing both Mo and Fe. In this cluster each Fe—S—Fe group
binds two 4Fe—3S and

A

Mo—3Fe—3S subclusters as bridges and forms two 4Fe—4S
faces. The residue is localized at a short distance from one of these faces (Fig.
3.3-3.4) (Rees and Howard, 2000; Christiansen et al., 2000). Site-directed substitutions
allow the identification residues around the Fe-Mo-cofactor (Fig. 3.4) (Fisher et al.,
2000; Christiansen, et al., 2000). As an example, after the replacement of proton donor
with glutamine residues, the enzyme still reduces acetylene to ethylene but not
dinitrogen though the latter is bound to FeMoco. The stopped-flow IR spectroscopy and
ENDOR have demonstrated that CO, which is an inhibitor of the reduction, can bind
to the cofactor (Newton et al., 1995; Christie et al., 1996; George et al., 1997).

1