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416 18 Methyltransferases in Biocatalysis

O
O
N
O
O
O N
37

O O
O O

38 36
Figure 18.6 Natural products containing a methylenedioxo motif. Safrol 37, piperine 38,
sanguinarine 36.

progress, severe limitations still apply for in vitro systems. With the advent of
synthetic biology, live whole-cell biotransformation has become a more favorable
alternative, leaving the regeneration to the hosts’ (improved) biosynthetic machin-
ery where possible.
Integration with chemical or enzymatic processes for the production of MT
substrates (premodification) is often connected to more established biocatalytic
processes and therefore most examples can be found in this category.
Postmodification of MT products is very limited. This limitation, however, is
natural, as in most cases methylation is the final modification at that specific
position of a molecule, with one exception: oxidation to form methylenedioxo
bridges from catechols (such as in the flavoring substances and alkaloids depicted
in Figure 18.6). Unfortunately, this process is not well exploited in biotechnology,
probably because chemical formation of this moiety is ready, easy, and cheap
through the instantaneous reaction of catechols with formaldehyde. The enzymatic
route is cumbersome especially because of the oxidation step [100], and few of
the relevant cytochromes are known [101]. Considering the whole process with
downstream purification, and so on, in vitro processes to methylenedioxo bridges
are also much less environmentally friendly than chemical ones. This is not true
anymore, however, if such a process requires excellent chemo- or regioselectivity, or
if it is required as part of an engineered biosynthetic pathway in a living organism
that is not likely to tolerate larger concentrations of formaldehyde.
In contrast to the biocatalytic potential of the MTs, which already have been
applied in enzyme cascades, that of a number of enzymes with exceptional
substrate specificity or catalytic properties has not been fully exploited. This is espe-
cially true for C-methyltransferases, which catalyze the challenging Friedel–Crafts
methylation of phenols [102], tyrosine, or coumarine derivatives (M. Tengg et al.,
unpublished results), or antibiotics such as actinomycin [103].
In general, discovery of new enzymes might facilitate the synthesis of a variety of
industrially relevant methylated products, natural or artificial. For example, an OMT
from Mycobacterium marinum that has been characterized recently converts fatty