The majority of synthetically useful ketones can be transformed into the
corresponding chiral secondary alcohols by choosing the appropriate dehydrogenase from the above-mentioned set of enzymes (Fig. 2.15). Other enzymes, which
have been shown to be useful for specific types of carbonyl substrates, are mentioned below.
One general limitation of alcohol dehydrogenases is their inability to convert
sterically demanding ketones bearing bulky groups on both sides. This limitation
was overcome by identification of two special ADHs from Ralstonia sp. DSM 6428
and Sphingobium yanoikuyae DSM 6900 [912]. The former enzyme reduced arylalkyl ketones bearing n-propyl- to n-pentyl chains with excellent Prelogspecificity [913].
The natural role of glycerol dehydrogenase is the interconversion of glycerol and
dihydroxyacetone. The enzyme is commercially available from different sources
and has been used for the stereoselective reduction of α-hydroxyketones
[837]. Glycerol DH has been found to tolerate some structural variation of its
natural substrate – dihydroxyacetone – including cyclic derivatives. An enzyme
from Geotrichum candidum was shown to reduce not only α- but also β-ketoesters
with high selectivity [914].
Enzymes from thermophilic organisms (which grow in the hostile environment
of hot springs with temperatures ranging from 70 to 100
C) have recently received
much attention [915–918]. Thermostable enzymes are not only stable to heat but, in
general, also show enhanced stability in the presence of common protein denaturants and organic solvents. Since they are not restricted to working in the narrow
temperature range which is set for mesophilic, ‘normal’ enzymes (20–40
C), an
influence of the temperature on the selectivity can be studied over a wider range.
For instance, the diastereoselectivity of the HLADH-catalyzed reduction of
3-cyano-4,4-dimethyl-cyclohexanone is diminished at 45
C (the upper operational
limit for HLADH) when compared with that observed at 5
C [919]. On the other
hand, a temperature-dependent reversal of the enantiospecificity of an alcohol
dehydrogenase from Thermoanaerobacter ethanolicus could be achieved when
the temperature was raised to 65
C [920] (compare pp. 75–76).
2.2.3 Reduction of Aldehydes and Ketones Using Whole Cells
Instead of isolated dehydrogenases, which require sophisticated cofactor recycling,
whole microbial cells can be employed. They contain multiple dehydrogenases
which are able to accept nonnatural substrates, all the necessary cofactors and the
metabolic pathways for their regeneration. Thus, cofactor recycling can be omitted
since it is automatically done by the living cell. Therefore, cheap carbon sources
such as saccharose or glucose can be used as auxiliary substrates for asymmetric
reduction reactions. Furthermore, all the enzymes and cofactors are well protected
within their natural cellular environment.
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2 Biocatalytic Applications
corresponding chiral secondary alcohols by choosing the appropriate dehydrogenase from the above-mentioned set of enzymes (Fig. 2.15). Other enzymes, which
have been shown to be useful for specific types of carbonyl substrates, are mentioned below.
One general limitation of alcohol dehydrogenases is their inability to convert
sterically demanding ketones bearing bulky groups on both sides. This limitation
was overcome by identification of two special ADHs from Ralstonia sp. DSM 6428
and Sphingobium yanoikuyae DSM 6900 [912]. The former enzyme reduced arylalkyl ketones bearing n-propyl- to n-pentyl chains with excellent Prelogspecificity [913].
The natural role of glycerol dehydrogenase is the interconversion of glycerol and
dihydroxyacetone. The enzyme is commercially available from different sources
and has been used for the stereoselective reduction of α-hydroxyketones
[837]. Glycerol DH has been found to tolerate some structural variation of its
natural substrate – dihydroxyacetone – including cyclic derivatives. An enzyme
from Geotrichum candidum was shown to reduce not only α- but also β-ketoesters
with high selectivity [914].
Enzymes from thermophilic organisms (which grow in the hostile environment
of hot springs with temperatures ranging from 70 to 100
C) have recently received
much attention [915–918]. Thermostable enzymes are not only stable to heat but, in
general, also show enhanced stability in the presence of common protein denaturants and organic solvents. Since they are not restricted to working in the narrow
temperature range which is set for mesophilic, ‘normal’ enzymes (20–40
C), an
influence of the temperature on the selectivity can be studied over a wider range.
For instance, the diastereoselectivity of the HLADH-catalyzed reduction of
3-cyano-4,4-dimethyl-cyclohexanone is diminished at 45
C (the upper operational
limit for HLADH) when compared with that observed at 5
C [919]. On the other
hand, a temperature-dependent reversal of the enantiospecificity of an alcohol
dehydrogenase from Thermoanaerobacter ethanolicus could be achieved when
the temperature was raised to 65
C [920] (compare pp. 75–76).
2.2.3 Reduction of Aldehydes and Ketones Using Whole Cells
Instead of isolated dehydrogenases, which require sophisticated cofactor recycling,
whole microbial cells can be employed. They contain multiple dehydrogenases
which are able to accept nonnatural substrates, all the necessary cofactors and the
metabolic pathways for their regeneration. Thus, cofactor recycling can be omitted
since it is automatically done by the living cell. Therefore, cheap carbon sources
such as saccharose or glucose can be used as auxiliary substrates for asymmetric
reduction reactions. Furthermore, all the enzymes and cofactors are well protected
within their natural cellular environment.
146
2 Biocatalytic Applications