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Enzymatic Redox Chemistry 149
NADH
NAD +
flavoprotein
reductase
FAD
FADH 2
R
+ O 2
ferredoxin
dioxygenase
[FeS] red
Fe 2+
[FeS] ox
HO
OH
R
Figure 6.39 Electron transfer in dihydroxylating dioxygenases.
dihydroxylating dioxygenases. Both these classes of enzyme are involved in the
bacterial degradation of aromatic compounds in the environment. As shown in
Figure 6.38, the dihydroxylating dioxygenases catalyse the oxidative conversion
of an aromatic substrate into the corresponding cis-dihydrodiol. The cis-diol is
then oxidised by a class of NAD þ -dependent dehydrogenases into the corresponding
aromatic diol, or catechol. Oxidative cleavage of the aromatic ring is
then carried out in one of two ways. Cleavage of the carbon–carbon bond
between the two hydroxyl groups and insertion of both atoms of dioxygen is
catalysed by non-haem iron(III)-dependent intradiol dioxygenases. Alternatively,
cleavage of a carbon–carbon bond adjacent to the two hydroxyl groups
and insertion of two oxygen atoms can be catalysed by non-haem iron(II)-
dependent extradiol dioxygenases.
The dihydroxylating dioxygenases are multi-component enzymes whose
sub-units are involved in electron transfer. Two electrons are transferred by a
Xavoprotein reductase sub-unit from NADH to FADH 2 . The two electrons are
then transferred singly to iron–sulphur clusters contained in a ferredoxin subunit.
These electrons are then transferred to the active site of the dioxygenase
sub-unit, as shown in Figure 6.39.
The mechanisms of these non-haem iron-dependent dioxygenases are not
well understood, but all presumably involve activation of dioxygen by electron
transfer from the iron cofactor. In the case of the dihydroxylating dioxygenases,
formation of a four-membered dioxetane intermediate has been proposed,
followed by reduction to generate the cis-diol product, as shown in Figure
6.40. This mechanism would explain the cis-stereochemistry of the product
and the incorporation of 18 O into both hydroxyl groups from 18 O 2 . However,
the formation of dioxetane intermediates from ground state oxygen is endothermic
and therefore apparently unlikely.