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Enzymatic Redox Chemistry 143
R
H
N
N
NH 2
O 2
H
N
N
NH 2
NH
N
N
N
R N
H
R N
H
O
O
H
O
OH
O
O
O
Fe II Fe II
H
N
Fe IV
N
2 H
NH 2
OH
2 H
Fe II
O
+
1 H
2 H
Fe II
O
2 H
+
− O 2 C
NH 3
+
R
R
R
Figure 6.28 Mechanism for phenylalanine hydroxylase involving iron(IV)-oxo species.
[2Fe2S]
[4Fe4S]
SEnz
Enz-S
Fe
S
S
Fe
S-Enz
EnzS
Fe
EnzS
S
S
S
Fe
Fe
Fe
S
SEnz
Figure 6.29 Iron–sulphur clusters.
another electron carrier or to the active site of a redox enzyme. We shall meet
one such example in Section 6.10. Redox potentials for ferredoxins are typically
in the range 0:2 to 0:6 V (see Figure 6.1), implying that these are strongly
reducing biological redox agents.
6.8 Metal-dependent mono-oxygenases
Enzymatic hydroxylation of organic molecules is one of the remarkable
examples of enzymatic reactions for which there is little precedent in organic
chemistry. We have seen above how Xavin is able to utilise dioxygen to carry out
the hydroxylation of a phenolic aromatic ring. However, more remarkably there
is a class of mammalian enzymes that are able to catalyse the speciWc hydroxylation
of unactivated alkanes. These enzymes are known as the P 450 monooxygenases,
due to the presence of a haem cofactor which upon treatment with
carbon monoxide gives a characteristic UV absorbance at 450 nm. At the centre