Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Enzymatic Carbon–Carbon Bond Formation 185
HO
CO 2 Me
HO
OMe O
sulochrin
OH
2Cu 2+ 2Cu +
O
CO 2 Me
O
OMe O
OH
O
CO 2 Me
O
MeO O
OH
Figure 7.38 Reaction catalysed by sulochrin oxidase.
enzyme exhibits a deep blue colour (l max 605 nm) which disappears upon
reduction with ascorbate under nitrogen. These observations suggest that
there are active site Cu 2þ metal ions which accept one electron from the phenol
substrates to generate Cu þ intermediates. The phenoxy radicals thus generated
are relatively stable, and react together as shown in Figure 7.38.
Oxidative phenol coupling is frequently found in pathways responsible
for the biosynthesis of isoquinoline and indole alkaloids in plants. In several
cases the enzymes responsible for the oxidative phenol coupling have been
found to be cytochrome P 450 enzymes (described in Section 6.8). One example
is the enzyme salutaridine synthase, shown in Figure 7.39, involved in
the biosynthesis of morphine in Papaver somniferum. In other cases, the
enzymes responsible for oxidative phenol couplings have been found to be
a-ketoglutarate-dependent dioxygenases (described in Section 6.9), or Xavoprotein
oxidases (described in Section 6.3).
Lignin is a complex aromatic polymeric material which is a major structural
component of woody tissues in plants. In most trees that require a great deal of
structural rigidity lignin constitutes 30–40% of the dry weight of the wood. It is
highly cross-linked and exceptionally resistant to chemical degradation, making
elucidation of its chemical structure very diYcult. However, it is now known to
be heterogeneous in structure, composed of phenylpropanoid (aromatic ring þ
three-carbon alkyl chain) units linked together by a variety of types of carbon–
carbon linkage. Shown in Figure 7.40 are a selection of structural components
found in lignin. Although structurally diverse, the common feature of these
components is that they can all be formed via radical couplings.
MeO
MeO
HO
MeO
OH
reticuline
H
NMe
salutaridine
synthase
P450
NADPH,O 2
HO
Figure 7.39 Reaction catalysed by salutaridine synthase.
H
MeO
O
salutaridine
NMe