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Enzymatic Redox Chemistry 135
reduced FADH 2 to form a peroxy Xavin adduct, which then acts as an electrophilic
species for attack of the phenol. The mechanism for para-hydroxybenzoate
hydroxylase is shown in Figure 6.19. The structure of p-hydroxybenzoate
hydroxylase is shown in Figure 6.20.
In this case FADH 2 is generated from FAD by reduction with NADPH.
Presumably the mechanism for this reduction involves hydride transfer to either
N-1 or N-5 of the Xavin nucleus. Thus Xavin has the ability to accept two
electrons from NADPH, but then transfer them via two one-electron transfers.
It is apparent from this mechanism why hydroxylation occurs ortho-orpara-to
a phenol, because the phenolic hydroxyl group activates the ortho- and parapositions
for nucleophilic attack.
There is also at least one example of an enzyme in which the Xavin hydroperoxide
intermediate can act as a nucleophile, rather than an electrophile. This
is the enzyme cyclohexanone mono-oxygenase, which catalyses the oxidation of
cyclohexanone to the corresponding seven-membered lactone. This reaction is
analogous to the well-studied Baeyer–Villiger oxidation which uses a peracid to
achieve the same transformation. A likely mechanism for this reaction is shown
in Figure 6.21. Again NADPH is used to generate reduced Xavin for reaction
with oxygen.
R
R H +
R
H
N N O 1-electron
N N O N N O
transfer
NH O 2
NH
NH
N
N
N
H
H
O
O
H O
O H +
O 2 − HO
O 2 C
NADP + O
NADPH
H − BEnz
R H + R
R
N N O N N O N N O
N
NH NADPH
N
NH
H H
O
O
CONH 2
− O 2 C
OH
N
R'
OH
Figure 6.19 Mechanism for p-hydroxybenzoate hydroxylase.
− O 2 C
NH
N
H OH
O
H +
OH
H
O
H BEnz