2.5.4 Aldehyde Oxidase and Xanthine Dehydrogenase
These two enzymes are found in the cytosol of the liver (and other tissues) and
also oxidize substrates (Panoutsopoulos et al., 2004; Rajagopalan, 1997), with
a very different stoichiometry and mechanism:
RHþH 2 O!ROHþ2e
The two electrons may be transferred to an oxidized pyridine nucleotide or
to O 2 (yielding H 2 O 2 ).
These enzymes contain flavin, a molybdenum center, and Fe–S clusters in
the ratio 1 : 1 : 4. The pathway of electron flow is from Mo to Fe–S to FAD.
N
H
N
NH
N O
O
O
O
H
NH 2
N
H
N
NH
N O
O
O
H
OH
NH 2
H
N
N
NH
N O
O
OH
NH 2
••
+
•• ••
••
FIGURE 2.6 Proposed mechanism of hydroxylation of activated aromatic ring by
FMO (Frederick et al., 1982).
OXIDATION ENZYMES 23