Chemistry Reference
In-Depth Information
For a long time, a major limitation for applications using the FDH from
C. boidinii
was its inability to regenerate NADP
+
, thus being limited to the regeneration of NAD
+
only. An elegant solution of this problem has been recently found by the Hummel group,
thus expanding the application range of FDH-based cofactor regeneration also to
NADP
+
-dependent ADHs [210]. As such an ADH, the highly effi cient ADH from
L.
kefi r
[211,212] was chosen. The key step is the integration of an additional enzymatic
step within the cofactor-regeneration cycle, namely the pyridine nucleotide transhy-
drogenase (PNT)-catalyzed regeneration of NADPH from NADP
+
under consumption
of NADH forming NAD
+
[210]. The concept is graphically shown in Scheme 6.54,
exemplifi ed for the synthesis of (
R
) - phenylethanol ((
R
) -
133
) .
OH
H
3
C
HCO
2
NADP
NAD
(
R
)-
133
Formate
dehydrogenase
(
NADH-dependent
)
Pyridine
nucleotide
transhydrogenase
(
R
)-alcohol
dehydrogenase
(
NADPH-dependent
)
O
NADH
NADPH
CO
2
132
Scheme 6.54.
A further effi cient option for recycling the cofactor NAD(P)H, which is oxidized
during the reduction process, is based on the use of a glucose dehydrogenase (GDH).
Therein, D-glucose is oxidized to D-gluconolactone, while the oxidized cofactor
NAD(P)
+
is reduced to NAD(P)H (which is the required reducing agent for the reduc-
tion process). Since D-gluconolactone is subsequently hydrolyzed under the formation
of D-gluconic acid (as its sodium salt at neutral pH), this reaction can also be regarded
as an irreversible step, thus shifting the whole reaction into the direction of the desired
alcohol product. Although some preparative synthetic applications by means of isolated
enzymes are known [213-215], most of the reported applications of GDH-coupled cofac-
tor regeneration in asymmetric reduction are based on the use of recombinant whole-cell
systems. Notably, industrial applications of this recombinant whole-cell technology
based on an ADH and a GDH have already been reported in particular by Kaneka
Corporation and Degussa AG (now Evonik-Degussa GmbH). Some selected examples
of this technology are given in the following.
The proof of principle and pioneering work for a biocatalytic reduction using a GDH-
coupled cofactor-regeneration process has been done by Wong and coworkers [213,214].
The corresponding enzymatic reduction of ketones in the presence of different types of
ADHs such as ADHs from horse liver, yeast, and
Thermoanaerobium brockii
gave the
