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406 18 Methyltransferases in Biocatalysis

R
O O
H
N S
N O N
H H H
O O 3 H
HN NH
HN
NH
O
N
N
N N
HOOC S 14a: R = N 3
O N N
NH 2 14b: R = F C
3
OH OH
R
O O
H
N S
N O N
H H H
O O 3 H
HN NH
HN NH 2
N O
N
N N
HOOC S 15a: R = N
O 3
NH 2 15b: R = N N
F 3 C
OH OH
Figure 18.3 SAH-based capture compounds with different attachment positions and photo-
crosslinking groups [58].
a further application of modified cofactors [57]. These compounds can also be
utilized for the determination of the dissociation constant K [58].
D
N-Adenosylaziridine 16 is a SAM analog that has been used as cofactor for DNA-
MTs. The enzyme-catalyzed transformation does not lead to group transfer but to a
sequence-specific coupling of the whole cofactor with DNA [16, 59]. The approach
has been further developed to a powerful application for a one-step DNA labeling
with aziridine cofactors tagged to reporter groups. Depending on DNA-MT, an
applied stringent sequence-specific labeling can be achieved (Scheme 18.9) [60].

18.2.4
Cofactor (Re)Generation

SAM and related cofactors, isolated or produced by total synthesis, are highly
priced and their stoichiometric consumption requires one or even more equiva-
lents. An economically feasible cofactor-dependent biocatalysis thus relies on the
substoichiometric or even catalytic use of the cofactor associated with a cofactor
regeneration system, ideally in a cascaded reaction sequence. Such enzymatic
conversions are assisted by an additional recycling reaction which restores the