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Enzymatic Redox Chemistry 151
Dioxetane intermediates were also proposed for the intradiol and extradiol
catechol dioxygenases, based on 18 O 2 incorporation experiments. However,
recent evidence suggests that 1,2-rearrangements (similar to the Baeyer–Villiger
oxidation of ketones) of peroxy intermediates are taking place in these reactions
leading to anhydride and lactone intermediates respectively, as shown in Figure
6.41. The non-haem iron(II) centre of the extradiol catechol dioxygenases, the
dihydroxylating dioxygenases, and the a-ketoglutarate dioxygenases, is ligated
by a common motif of two histidine residues and one carboxylate group from a
glutamic acid or aspartic acid residue. This tridentate motif seems to be especially
used in Nature for activation of dioxygen and organic substrates by nonhaem
iron(II). It is illustrated in Figure 6.42 in the case of protocatechuate
4,5-dioxygenase.
Many of the redox reactions in the second half of this chapter have little
precedent in organic chemistry, although in some cases inorganic model complexes
have been prepared which mimic the action of metallo-enzymes. Thus,
utilising only a small selection of organic redox coenzymes and metal redox
cofactors as electron carriers, an extraordinary range of enzyme-catalysed
oxidation/reduction chemistry is possible.
Problems
(1) Work out the redox potential diVerences for the following enzymatic reactions,
using the data in Figure 6.1. In example (c) what can you deduce
about the redox potential of the enzyme-bound Xavin
(a)
O
alcohol dehydrogenase
OH
NADH
NAD +
(b)
R
O
SCoA
enoyl reductase
R
O
SCoA
NADH
NAD +
(c)
R
O
SCoA
acyl CoA dehydrogenase
flavin, cytc ox
R
O
SCoA
(2) In the enoyl reductase reaction illustrated in Problem 1(b), incubation of
4S- 2 H-NADPD with enzyme and crotonyl CoA gives no incorporation
of deuterium in the butyryl CoA product. Incubation of 4R- 2 H-NADPD
with enzyme and crotonyl CoA gives 3R- 2 H-butyryl CoA. Incubation
of enzyme, NADH and crotonyl CoA in 2 H 2 O gives 2S- 2 H-butyryl