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398 18 Methyltransferases in Biocatalysis

highly selective regarding the type of substrate and target atom for methylation.
Owing to the great substrate variety, the role of MTs in living organisms is manifold
[17]. In case of non-radical SAM enzymes, the reaction mechanism follows an S 2-
N
type mode and the methylation targets a nucleophilic O-, N-, C-, S-, Se-, As-, or
halide atom (Table 18.1).
Oxygen methylation occurs via transfer of the methyl group to a hydroxyl group
of phenols and riboses, or to carboxyl groups. The best studied enzyme of this
class is catechol-O-methyltransferase (COMT), which attaches a methyl group on a
variety of phenolic compounds including catechol derivatives (Scheme 18.3) [18].
As an indispensable step during catalysis, oxygen needs to be activated to generate
enough nucleophilicity for the transfer of the methyl group. COMT has been
shown to depend on Mg 2+ ions, which replace the proton from the hydroxyl group
to generate a nucleophilic phenolate. The protein l-isoaspartate MT transfers the
methyl group to the carboxyl moiety of l-isoaspartate. Oxygen methylation is also
of great importance for the decoration of antibiotics.
DNA MTs (e.g., M.TaqI, M.EcoRI) transfer the methyl group to the N6 position of
adenine within the respective endonuclease recognition site [19]. M.PvuII is able to
methylate the N4 position of both cytosine and adenine [20]. In the mentioned cases,
the methylation reaction can take place easily because of the high nucleophilicity
of exocyclic nitrogen. There is no need for an active deprotonation because of the
physiological conditions, which keep the attacked nitrogen deprotonated [17]. In
contrast, nitrogen methylation of proteins and small molecules follows a different
mechanistic way. For example, the proteins arginine-N-methyltransferase (PRMT)
and glycine-N-methyltransferase depend on acidic residues for deprotonation of
the attacked substrate nitrogen atom [21].
Compared to oxygen and nitrogen methylation, methyl transfer to carbon atoms
requires more energy for the generation of an intermediate carbanion. However
C–C bond formation is a common event in natural systems, and is also of great
interest for organic synthesis. Prominent members of this type of enzymes are
MTs that connect a methyl group to the C5-position of cytosine nucleobases (e.g.,
M.HhaI) [22]. Mechanistically, the neighboring carbon atom of cytosine has to be
deprotonated by an active site cysteine, resulting in sufficient electron density at the
attacking carbon, to facilitate the reaction. But the C-methylation is not restricted
to aromatic compounds; also aliphatic substrates such as sterol derivatives are
methylated. A well-studied example is sterol-C24 methyltransferases (SMTs) of
plants and fungi [23].
Generally, methylation has a significant impact on the biological activity of a
compound. It results in enhanced lipophilicity, which facilitates transfer through
the membrane barrier and increased bioavailability [24]. Methylation serves often
as a protection step in biological systems and affects the interaction between
functional groups (disruption of intra- and intermolecular hydrogen bonds upon
methylation of –OH, –NH , –NH–, –SH, etc. groups) or intensification of
2
electrostatic interactions.